Self Curing Concrete 6cc38
This document was ed by and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this report form. Report r6l17
Overview 4q3b3c
& View Self Curing Concrete as PDF for free.
More details 26j3b
- Words: 7,865
- Pages: 44
Department of CIVIL Engineering, A.I.T. Chikmagalur
SYNOPSIS External self curing of concrete. By:
2010
1. SYNOPSIS
In the present situation there is a need for the search of alternate materials in the place of water for curing not only to save water for the sustainable development of the environment, but also to promote indoor and outdoor construction activities even in remote areas where there is scarcity of water. If no any curing measure is taken for concrete against natural setting or hardening, substantial water losses will occur due to fast water evaporation, thus prevents cement from hydration and leads to dusting or drying crack on concrete surface. In such case, the strength and durability of concrete will be reduced. After placing of concrete, curing shall be duly required to avoid water loss and drying crack. Curing of concrete by which the concrete, after laying, is kept moist for some days is essential for the development of proper strength and durability. IS 456-2000 recommends a curing period of 7 days for ordinary Portland cement concrete, and 10 to 14 days for concrete prepared using mineral ixtures or blended cements. But, being the last act in the concreting operations, it is often neglected or not fully done. Consequently, the quality of hardened concrete suffers, more so, if the freshly laid concrete gets exposed to the environmental conditions of low humidity, high wind velocity and high ambient temperature. To avoid the adverse effects of neglected or insufficient curing, which is considered a universal phenomenon, concrete technologist and research scientists in various countries including India, are working on the development of self-curing concrete. The present work deals with the performance of self curing concrete, its advantages, applications. Concrete is said to be self cured, if it is able to retain its water content for the development of its strength. This work explains the performance of self curing concrete by the application of wax based curing compound. In the present work, investigation is carried out on 53 grades O.P.C to study the strength behavior and other parameters of concrete prepared from above mentioned cement. An attempt has been made to compare different parameters. The physical properties of cement, fine aggregate & coarse aggregate are determined in accordance with BIS specifications. Sieve analysis of fine aggregate & coarse aggregate is also carried out. On fresh concrete workability related tests such as slump test, compaction factor test and on hardened concrete strength related tests such as compressive strength test and split tensile strength test were conducted in accordance with BIS specifications .The grade of concrete chosen for investigation is M25 of w/c 0.55. The concrete mix design is carried out according to BIS specifications The result of the investigation clearly indicates that self curing can be adopted economically in places where there is scarcity of water and in remote places without compromising with the performance characteristics of concrete including durability. Department of CIVIL Engineering, A.I.T. Chikmagalur
2
CHAPTER 1 GENERAL INTRODUCTION Construction industry use lot of water in the name of curing. The days are not so far that all the construction industry has to switch over to an alternative curing system, not only to save water for the sustainable development of the environment but also to promote indoor and outdoor construction activities even in remote areas where there is scarcity of water. Curing is the process of controlling the rate and extent of moisture loss from concrete during cement hydration. It may be either after it has been placed in position (or during the manufacture of concrete products), thereby providing time for the hydration of the cement to occur. Since the hydration of cement does take time – days, and even weeks rather than hours – curing must be undertaken for a reasonable period of time. If the concrete is to achieve its potential strength and durability. Curing may also encom the control of temperature since this affects the rate at which cement hydrates. The curing period may depend on the properties required of the concrete, the purpose for which it is to be used, and the ambient conditions, i.e. the temperature and relative humidity of the surrounding atmosphere. Curing is designed primarily to keep the concrete moist, by preventing the loss of moisture from the concrete during the period in which it is gaining strength. Curing may be applied in a number of ways and the most appropriate means of curing may be dictated by the site or the construction method. Curing is the maintenance of a satisfactory moisture content and temperature in concrete for a period of time immediately following placing and finishing so that the desired properties may develop. The need for adequate curing of concrete cannot be overemphasized. Curing has a strong influence on the properties of hardened concrete; proper curing will increase durability, strength, watertightness, abrasion resistance, volume stability, and resistance to freezing and thawing and deicers. Exposed slab surfaces are especially sensitive to curing as strength development. And freeze-thaw resistance of the top surface of a slab can be reduced significantly when curing is defective. When Portland cement is mixed with water, a chemical reaction called hydration takes place. The extent to which this reaction is completed influences the strength and durability of the concrete. Freshly mixed concrete normally contains more water than is required for hydration of the cement; however, excessive loss of water by evaporation can delay or prevent adequate hydration. The surface is particularly susceptible to insufficient hydration because it dries first. If temperatures are favorable, hydration is relatively rapid the first few days after concrete is placed; however, it is important for water to be retained in the concrete during this period, that is, for evaporation to be prevented or substantially reduced. With proper curing, concrete becomes stronger, more impermeable, and more resistant to stress, abrasion, and freezing and thawing. The improvement is rapid at early ages but continues more slowly thereafter for an indefinite period. Department of CIVIL Engineering, A.I.T. Chikmagalur
3
Covering the concrete with an impermeable membrane after the formwork has been removed. By the application of a suitable chemical Curing agent (wax etc). Curing by continuously wetting the exposed surface thereby preventing the loss of moisture from it. Ponding or spraying the surface with water is methods typically employed.
Concrete can be kept moist (and in some cases at a favorable Temperature) by three curing methods: 1. Methods that maintain the presence of mixing water in the concrete during the early hardening period. These include ponding or immersion, spraying or fogging, and saturated wet coverings. These methods afford some cooling through evaporation, which is beneficial in hot weather. 2. Methods that reduce the loss of mixing water from the surface of the concrete. This can be done by covering the concrete with impervious paper or plastic sheets, or by applying membrane-forming curing compounds. 3. . Methods that accelerate strength gain by supplying heat and additional moisture to the concrete. This is usually accomplished with live steam, heating coils, or electrically heated forms or pads.
Ponding and Immersion On flat surfaces, such as pavements and floors, concrete can be cured by ponding. Earth or sand dikes around the perimeter of the concrete surface can retain a pond of water. Ponding is an ideal method for preventing loss of moisture from the concrete; it is also effective for maintaining uniform temperature in the concrete. The curing water should not be more than about 11°C (20°F) cooler than the concrete to prevent thermal stresses that could result in cracking. Since ponding requires considerable labor and supervision, the method is generally used only for small jobs. The most thorough method of curing with water consists of total immersion of the finished concrete element. This method is commonly used in the laboratory for curing concrete test specimens. Where appearance of the concrete is important, the water used for curing by ponding or immersion must be free of substances that will stain or discolor the concrete. The material used for dikes may also discolor the concrete. As shown in Fig: 01
Department of CIVIL Engineering, A.I.T. Chikmagalur
4
Fig. 01: Ponding method of water curing
Fogging and Sprinkling Fogging (Fig. 02) and sprinkling with water are excellent methods of curing when the ambient temperature is well above freezing and the humidity is low. A fine fog mist is frequently applied through a system of nozzles or sprayers to raise the relative humidity of the air over flatwork, thus slowing evaporation from the surface. Fogging is applied to minimize plastic shrinkage cracking until finishing operations are complete. Once the concrete has set sufficiently to prevent water erosion, ordinary lawn sprinklers are effective if good coverage is provided and water runoff is of no concern. The cost of sprinkling may be a disadvantage. The method requires an ample water supply and careful supervision. If sprinkling is done at intervals, the concrete must be prevented from drying between applications of water by using burlap or similar materials; otherwise alternate cycles of wetting and drying can cause surface crazing or cracking.
Fig. 02: Fogging minimizes moisture loss during and after Placing and finishing of concrete
Department of CIVIL Engineering, A.I.T. Chikmagalur
5
Wet Coverings Fabric coverings saturated with water, such as burlap, cotton mats, rugs, or other moisture-retaining fabrics, are commonly used for curing (Fig. 03). Treated burlaps that reflect light and are resistant to rot and fire wet, moisture-retaining fabric coverings should be placed as soon as the concrete has hardened sufficiently to prevent surface damage. During the waiting period other curing methods are used, such as fogging or the use of membrane forming finishing aids. Care should be taken to cover the entire surface with wet fabric, including the edges of slabs. The coverings should be kept continuously moist so that a film of water remains on the concrete surface throughout the curing period. Use of polyethylene film over wet burlap is a good practice; it will eliminate the need for continuous watering of the covering periodically rewetting the fabric under the plastic before it dries out should be sufficient. Alternate cycles of wetting and drying during the early curing period may cause crazing of the surface. Wet coverings of earth, sand, or sawdust are effective for curing and are often useful on small jobs. Sawdust from most woods is acceptable, but oak and other woods that contain tannic acid should not be used since deterioration of the concrete may occur. A layer about 50 mm (2 in.) thick should be evenly distributed over the previously moistened surface of the concrete and kept continuously wet. Wet hay or straw can be used to cure flat surfaces. If used, it should be placed in a layer at least 150 mm (6 in.) thick and held down with wire screen, burlap, or tarpaulins to prevent its being blown off by wind. A major disadvantage of moist earth, sand, sawdust, hay, or straw coverings is the possibility of discoloring the concrete.
Fig. 03: Lawn sprinklers saturating burlap with water keep the concrete continuously moist. Intermittent sprinkling is acceptable if no drying of the concrete surface occurs.
Department of CIVIL Engineering, A.I.T. Chikmagalur
6
Impervious Paper Impervious paper for curing concrete consists of two sheets of Kraft paper cemented together by a bituminous adhesive with fiber reinforcement. Such paper, conforming To AASHTO M 171, is an efficient means of curing horizontal surfaces and structural concrete of relatively simple shapes. An important advantage of this method is that periodic additions of water are not required. Curing with impervious paper enhances the hydration of cement by preventing loss of moisture from the concrete (Fig.04).As soon as the concrete has hardened sufficiently to prevent surface damage, it should be thoroughly wetted and the widest paper available applied. Edges of adjacent sheets should be overlapped about 150 mm (6 in.) and tightly sealed with sand, wood planks, pressure-sensitive tape, mastic, or glue. The sheets must be weighted to maintain close with the concrete surface during the entire curing period. Impervious paper can be reused if it effectively retains moisture. Tears and holes can easily be repaired with curing-paper patches. When the condition of the paper is questionable, additional use can be obtained by using it in double thickness. In addition to curing, impervious paper provides some protection to the concrete against damage from subsequent construction activity as well as protection from the direct sun. It should be light in color and no staining to the concrete. Paper with a white upper surface is preferable for curing exterior concrete during hot weather.
Fig.04: Impervious curing paper is an efficient means of curing horizontal surfaces.
Plastic Sheets Plastic sheet materials, such as polyethylene film, can be used to cure concrete (Fig. 05). Polyethylene film is a lightweight, effective moisture retarder and is easily applied to complex as well as simple shapes. Its applications the same as described for impervious paper. Curing with polyethylene film (or impervious paper) can cause patchy discoloration, especially if the concrete contains calcium chloride and has been finished by hard steel troweling. This discoloration is more pronounced
Department of CIVIL Engineering, A.I.T. Chikmagalur
7
when the film becomes wrinkled, but it is difficult and time consuming on a large project to place sheet materials without wrinkles. Flooding the surface under the covering may prevent discoloration, but other means of curing should be used when uniform color is important. Polyethylene film should conform to AASHTO M 171, which specifies a 0.10-mm (4-mil) thickness for curing concrete, but lists only clear and white opaque film. However, black film is available and is satisfactory under some conditions. White film should be used for curing exterior concrete during hot weather to reflect the sun’s rays. Black film can be used during cool weather or for interior locations. Clear film has little effect on heat absorption.
Fig. 05: Polyethylene film is an effective moisture barrier for curing concrete and easily applied to complex as well as simple shapes. To minimize discoloration, the film should be kept as flat as possible on the concrete surface.
Membrane-Forming Curing Compounds Liquid membrane-forming compounds consisting of waxes, resins, chlorinated rubber, and other materials can be used to retard or reduce evaporation of moisture from concrete. They are the most practical and most widely used method for curing not only freshly placed concrete but also for extending curing of concrete after removal of forms or after initial moist curing. However, the most effective methods of curing concrete are wet coverings or water spraying that keeps the concrete continually damp. Curing compounds should be able to maintain the relative humidity of the concrete surface above 80% for seven days to sustain cement hydration. Membrane-forming curing compounds are of two general types: clear or translucent; and white pigmented. Clear or translucent compounds may contain a fugitive dye that makes it easier to check visually for complete coverage of the concrete surface when the compound is applied. The dye fades away soon after application. On hot, sunny days, uses of white-pigmented compounds are base slab of a two-course floor. Similarly, some curing Compounds may affect the adhesion of paint to concrete floors. Curing compound manufacturers should be consulted to determine if their product is suitable for the intended application. Curing compounds should be uniform and easy to maintain in a thoroughly mixed solution. They should not sag, run off peaks, or collect in grooves. They should form a tough film to withstand early Department of CIVIL Engineering, A.I.T. Chikmagalur
8
construction traffic without damage, be no yellowing, and have good moistureretention properties. Caution is necessary when using curing compounds containing solvents of high volatility in confined spaces or near sensitive occupied spaces such as hospitals because evaporating volatiles may cause respiratory problems. Applicable local environmental laws concerning volatile organic compound (VOC) emissions should be followed. Curing compounds should conform to AASHTO M 148. A method for determining the efficiency of curing compounds, waterproof paper, and plastic sheets is described in AASHTO T 155. ASTM C1151, discontinued in 2000, also evaluates the effectiveness of curing compounds. Curing compounds with sealing properties are specified under ASTM C 1315. As shown in Fig:06
Fig. 06: Liquid membrane-forming curing compounds should be applied with uniform and adequate coverage over the entire surface and edges for effective, extended curing of concrete.
Steam Curing Steam curing is advantageous where early strength gain in concrete is important or where additional heat is required to accomplish hydration, as in cold weather. Two methods of steam curing are used: live steam at atmospheric pressure (for enclosed cast-in-place structures and large precast concrete units) and high-pressure steam in autoclaves (for small manufactured units). Steam curing at atmospheric pressure is generally done in an enclosure to minimize moisture and heat losses. Tarpaulins are frequently used to form the enclosure. Application of steam to the enclosure should be delayed until initial set occurs or delayed at least 3 hours after final placement of concrete to allow for some hardening of the concrete. However, a 3- to 5- hour delay period prior to steaming will achieve maximum early strength. Steam temperature in the enclosure should be kept at about 60°C (140°F) until the desired concrete strength has developed. Strength will not increase significantly if the maximum steam temperature is raised from 60°C to 70°C (140°F to 160°F). Steam-curing temperatures above 70°C (160°F) should be avoided; they are uneconomical and may result in damage. It is recommended that the internal Department of CIVIL Engineering, A.I.T. Chikmagalur
9
temperature of concrete not exceed 70°C (160°F) to avoid heat induced delayed expansion and undue reduction in ultimate strength. Use of concrete temperatures above70°C (160°F) should be demonstrated to be safe by test or historic field data. SELF CURING OF CONCRETE Self curing concrete is the one which can cure itself by retaining its moisture content. A concrete can made to self cure by adding curing ixtures or by the application of curing compounds.
Advantages of self curing 1. Reduces autogenously cracking. 2. Largely eliminates autogenously shrinkage. 3. Reduces permeability. 4. Protects reinforcement steel. 5. Provide greater durability. 6. Increases the early age strength. 7. Improved rheology. 8. Lower maintenance. 9. Higher performance. 10. Doesn’t adversely effects finish ability.
TYPE OF CEMENT Ordinary Portland cement (O.P.C) This is by far the most common cement in use. This is the basic type of cement which is used on large scale in all general types of construction works. The details regarding the composition and properties of this type of cement are given in IS: 269. This cement is irably suitable for use in general concrete constructions where there is no exposure to sulphates in the soil or ground water. These cements are available in different grades viz. 33, 43 and 53 grade. 43 grade O.P.C. In these types of cements, the 28days cement strength is expected to have a minimum value of 43 Mpa. 53 grade O.P.C. In this type of cement, the 28 days cement strength is expected to have a minimum value of 53 Mpa.
Department of CIVIL Engineering, A.I.T. Chikmagalur
10
The use of high grade cement should not be taken for granted to yield high grade [strength] concrete as the strength of concrete depends on the mixture of cement, sand, coarse aggregate and water. In fact, the cement’s grade has no relationship to the strength of concrete. It is possible to produce concrete of wideranging strength using a particular grade of cement. Moreover the “grade” has nothing to do with quality; increase in the grade does not increase the quality of the cement. Every structure has to satisfy the requirement of strength and durability. Strength is the ability of the structure to withstand load. Durability refers to the period of trouble free life. A structural cement of concrete may possess high strength, but may deteriorate sooner than expected, making it a material of poor quality. Here the quality is with reference to concrete and not that of the cement. A grade of cement can be said to be of good quality if the concrete made with it satisfies both strength and durability requirements. The strength requirements [i.e. the strength of concrete] is satisfies by choosing the proper amount of cement, limiting the amount of water, consolidating the mixture well and curing the hardened concrete as long as possible. Durability on the other hand depends on the several factors that are attributable both to the material and to the exposed environment. During a recent survey made in Chennai, the only grades of cement freely available was found to be grade 53 and grade 43 was available on special order only and grade 33 was not found available.
Department of CIVIL Engineering, A.I.T. Chikmagalur
11
CHAPTER 2 REVIEW OF LITERATURE Construction industry is growing like day by day even in remote areas and deserted regions. Even India and other countries are facing lot of problems in supplying drinking water to their citizens. Hence construction industries are under pressure in finding out alternative curing methods of curing concrete. The objective of curing is to keep concrete saturated are nearly saturated as possible until the water fill the space in the fresh cement paste have been sequentially reduced by the products of hydration of cement [09]. Curing of concrete is complex phenomenon where the controlling process is hydration of cement [10]. Hydration is an essential process of hardened concrete but some micro cracking can occur as a consequence of hydration especially in HSC. The literature reports that the different compressive strength trends displayed by cement paste and concrete specimen suggest that the presence of aggregate is influencing the behavior of self curing concrete. In the last 100 years there has been placed a certain amount of long lasting good performing and economical concrete. However the deficiencies of much of concrete have been obvious. In the last 75 years there have been great amount of knowledge generated how to make the concrete better by best means of curing ACI committee 308 has studied the subjects since BRYANT MATHER called the industry’s attention to “SELF CURING”. Water retention of concrete containing self curing agents is investigated. Concrete weight loss and internal relative humidity measurements with time were carried out, in order to evaluate the water retention of self curing concrete. Non evaporable water at different ages was measured to evaluate the hydration. Water transport through concrete is evaluated by measuring absorption %, permeable voids %, and water sorptivity and water permeability. The water transport through self curing concrete is evaluated with age. The effect of the concrete mix proportions on the performance of self curing concrete were investigated, such as cement content and w/c ratio. To achieve good cure, excessive evaporation of water from a freshly cast concrete surface should be prevented. Failure to do this will lead to the degree of cement hydration being lowered and the concrete developing unsatisfactory properties. Curing can be performed in a number of ways to ensure that an adequate amount of water is available for cement hydration to occur. However, it is not always possible to cure concrete satisfactorily. This paper is concerned with achieving optimum cure of concrete without the need for applying conventional curing methods. Department of CIVIL Engineering, A.I.T. Chikmagalur
12
The guide to curing concrete is being re written to recognize the value of external curing as an adjunct to internal curing. Curing period is important for concrete in which it attains its maximum strength. Normally all structures made of concrete are cured for a period of 28 days by the application of water. At present, meeting the requirements of drinking water is a global issue. Amidst of this situation. Construction industry is growing rapidly. The scarcity of water is forcing the construction industry to switch over to a curing method which can cure their structures without the use of water. Self curing concrete is the only solution for these problems. Looking at the demand of drinking water in all the metropolitan cities, construction of highways and air fields in remote places where water curing is practically not possible, deserted areas through out the globe, etc. …forced to study on the curing compounds and their performance. Curing compounds are liquids which are usually sprayed directly on to the concrete surface and they are an efficient and cost effective means of curing concrete and may be applied to freshly placed concrete. When used to cure fresh concrete, the timing of the application of the curing compound is critical for maximum effectiveness. They should be applied to the surface of concrete after it has been finished. As soon as free water on the surface has evaporated and there is no water seen visible. They may also be used to reduce the moisture loss from concrete after initial moist curing or removal of form work. In case of columns and beams the application is done after the removal of formwork on the horizontal surface; the curing compound is applied upon the complete disappearance of all bleeding water. After spraying, no further application of water or other material is necessary to ensure continued curing. The concrete surface should not be disturbed until it has sufficient strength to bear surface loads. The applied film should not be walked on before it is fully dry and care should be taken to ensure that the film is not broken. In case the concrete surface has dried, the surface should be sprayed with water and thoroughly wetted and made fully damp before curing compound is applied. The container of curing compound should be well stirred before use. It takes nearly 10 to 15 minutes for its drying and drying and it forms a thin water proofing film on the surface. The fundamental conclusion shows that the efficiency test indeed is significant and worthwhile test, yielding very reasonable test conclusion. Also at an age other than 7 days, a good correlation can be found between the compressive strength and the evaporation.
Department of CIVIL Engineering, A.I.T. Chikmagalur
13
B.Mangaiarkarasa and S.R.Damodaraswamy conducted compressive test on concrete cubes of 150mm size, the curing compound used is CONCURE WB. Based on the results of the investigation, they concluded the following. Self curing concrete develops higher compressive strength then the water cured concrete in three days. Self curing concrete using wax based curing has an average efficiency of 84.0%. Self cured concrete satisfies serviceability conditions also. Self curing concrete can be practiced in pre fabrication units and in place of water scarcity as well as exposed weather condition. Whenever there is difficulty in water curing, self curing concrete will be very economical in remote areas as well as in water scarcity area. By adapting self curing compound concrete the sustainable development of environment is maintained there is no doubt that self curing concrete play a vital role and dominate the construction industry in future.
Department of CIVIL Engineering, A.I.T. Chikmagalur
14
CHAPTER 3 EXTERNAL SELF CURING Introduction: External self curing concrete is the one which can cure itself by retaining its moisture content. Concrete can made to self cure by the application of curing compounds on the surface of the concrete. The curing compound is applied by means of brushing or spraying. CURING COMPOUND The curing compound used is “CONCURE WB” which is a product of Fosroc chemicals. CONCURE WB is water based concrete curing compound based on a low viscosity wax emulsion. It is supplied as a white emulsion which forms a clear film on drying. When first applied to a fresh cementitious surface the emulsion breaks to form a continuous, non-penetrating white coating. This dries to form a continuous clear film which provides a barrier to moisture loss, ensuring more efficient cement hydration, improved durability and reduced shrinkage. Specifications of CONCURE WB Base –wax Shelf life – 12 months Coverage – 3.5 to 5 m2/litre Cost - Rs.90/litre Specific gravity-0.98 at 25 °C Features:
single application no other curing necessary easy and safe spray application endures hard wearing surface
Application procedure The curing compound is applied by brush or by spraying while the concrete is wet. In case of columns and beams the application is done after the removal of formwork. On the horizontal surface, the curing compound is applied upon the complete disappearance of all bleeding water. After spraying, no further application of water or other material is necessary to ensure continued curing. The concrete surface should not be disturbed until it has sufficient strength to bear surface loads. The applied film should not be walked on before it is fully dry and care should be taken to ensure that the film is not broken.
Department of CIVIL Engineering, A.I.T. Chikmagalur
15
In case the concrete surface has dried, the surface should be sprayed with water and thoroughly wetted and made fully damp before curing compound is applied. The container of curing compound should be well stirred before use. It takes nearly 10 to 15 minutes for its drying and drying and it forms a thin water proofing film on the surface. As shown in Fig: 07.
Fig: 07: Application of external self curing compound. Uses of external self curing concrete As a spray applied membrane to retain moisture in concrete for effective curing. Suitable for all general concreting applications and of particular benefit for large area concrete surfaces, such as airport runways, roads and bridgeworks. It is also suitable for piece works. Where, it is difficult to curing. Advantages of external self curing concrete Improved curing of concrete enhances cement hydration and provides a more durable concrete. Control of moisture loss improves surface quality, reducing permeability, producing a hard wearing; dust free Surface and minimizing potential for surface cracking and shrinkage. Fugitive colour provides visual guide during application. Water based, therefore, non-flammable.
Department of CIVIL Engineering, A.I.T. Chikmagalur
16
Spray application reduces labour costs and eliminates the need for alternative curing systems. CHAPTER 4 TEST ON INGREDIENT MATERIALS Introduction: The present investigation is carried out to study the behavior of normal and self curing cured concrete and ingredient material by using O.P.C. 53 grade cement. The tests were carried out at the Civil Engineering Laboratory of Civil Engineering Department, AIT Chikmagalur. Materials used and their properties: It is well known that strength of the concrete is dependent on the properties of its ingredients. The materials used in the present investigations are as follows. • • •
53 grade O.P.C. River sand as fine aggregate Quarried and crushed stone as coarse aggregate
Test on cement: The specific gravity, normal consistency, initial setting time, final setting time and compressive strength of cement were found as per B.I.S specifications. The results are tabulated in tables. Test on fine aggregate: 53 grade O.P.C. was used throughout the investigations. The cement was tested according to B.I.S. specifications to determine its various properties. The overall quantity of cement required for the investigation was procured in a single lot and stored in the appropriate manner. The specific gravity of natural sand was found according to the norms of the Indian Standards and was used throughout in preparing the required mix of concrete. Results are tabulated in table. Sieve analysis of the fine aggregate was also carried out as per the B.I.S. specifications to determine the grading zone. The results of sieve analysis are tabulated in table.
Department of CIVIL Engineering, A.I.T. Chikmagalur
17
Tests on coarse aggregate: The specific gravity of crushed stone aggregates of 20mm and downsize was found according to the norms of Indian standards and were used for all concrete mixes. The results are tabulated in table. Sieve analysis of the coarse aggregate was also carried out as per B.I.S. specifications. The results are tabulated in table. Specimen details: Two types of specimens namely cubes and cylinders were cast. Cubes were used for compressive strength test and Cylinders for split tensile strength test. Curing of the test specimens: The specimens are stored in the laboratory atmosphere for 24 hours from the time of adding water to the ingredients. Temperature was maintained at 270 ± 20C. The specimens were removed from the moulds after 24 hrs. And then kept immersed in clean water for the required age. The water in the tank was changed every week and the temperature was maintained constant. Testing of the specimens: The testing was carried out according to I.S. specifications. Table 1 Results of test on cement Particulars 1 2 3
Type of cement Normal Consistency Specific Gravity Setting time (in min) (a) Initial setting time (b) Final setting time
references 53 grade O.P.C. 34.00 % 2.85 80 min. 460 min.
Department of CIVIL Engineering, A.I.T. Chikmagalur
------------IS:269-1958 IS:269-1976 IS: 269-1976 Should more Than 30 min. Should not be More than 600min.
18
Table 2 Sl No.
Particulars
Compressive Strength in N/mm2(MPa)
--
Type of cement
3 days
1
53 grade O.P.C.
30.2
7 days 45.7
28days 59.8
Compressive Strength of Cement 70.6mm X 70.6mm X 70.6mm= Size of the specimen
Table 3 Properties of the aggregate used Sl Particulars No. 1 Specific gravity 2 Fineness modulus 3
Fine Coarse Aggregate Aggregate 2.50 2.60 2.85
Grading zone Zone II
7.13
20 mm and Downsize
Department of CIVIL Engineering, A.I.T. Chikmagalur
Reference IS:2386(partIII)1963 Is;2836(partIII)1963 IS:383-1970 IS:460-1962 IS 383-1963
19
Table 4 Sieve analysis of fine aggregate Weight of sample taken =1000gm Sl.No 1 2 3 4 5 6 7
IS sieve size 10mm 4.75mm 2.36mm 1.18mm 600μ 300 μ 150 μ
Weight retained(gm) 0 24 32 180 394 282 84
Cumulative Wt. retained 0 24 56 236 630 912 996
Cumulative % Wt. retained 0 2.40 5.60 23.60 63.00 91.20 99.60 ∑=285.40
% finer 100.00 97.60 94.40 76.40 37.00 8.80 0.40
Calculations: Fineness modulus = 285.40/100 =2.85
Table 5 Sieve analysis of coarse aggregate Weight of sample taken =2000gms IS sieve Sl.No size
Wt. retained
Cumulative Wt.retained
Cumulative % wt.retained
%finer
1 2 3 4 5 6 7 8 9 10
0 0 560 1152 276 09 03 0 0 0
0 0 560 1712 1988 1997 2000 2000 2000 2000
0 0 28.00 85.60 99.40 99.85 100.00 100.00 100.00 100.00 ∑ =712.85
100 100 72.00 14.40 0.60 0.15 0.00 0.00 0.00 0.00
80mm 40mm 20mm 10mm 4.75mm 2.36mm 1.18mm 600 300 150
Fineness modulus =712.85/100 =7.13
Department of CIVIL Engineering, A.I.T. Chikmagalur
20
CHAPTER 5 TESTS ON FRESH CONCRETE Measurement of workability of concrete: Workability is defined as the amount of useful internal work necessary to achieve full compaction. It is also defined as the ease with which concrete can be placed and degree to which it resists segregation. It is also given a new definition which includes all the essential properties of the concrete in plastic condition i.e. mixing ability, transportability, modulability and ease compaction. Some of the important factors that affect the workability of concrete are, • • • •
Relative quantities of paste and aggregate. Plasticity of the paste itself. Maximum size and grading of aggregate. Shape and surface characteristics of aggregate particle.
Consistency of the concrete is an important component of workability and refers in a way to the wetness of concrete. However it must be assumed that the wetter the mix more workable it is. If a mix is too wet, segregation may occur resulting in honey combing, excessive bleeding and sand streaking on the formed surfaces. On the other hand if the mix is too dry it may be difficult to place and compact and segregation may occur because of lack of cohesiveness and plasticity of the plastic. In the present investigations workability connected with physical quantity is correlated by slump test, compaction factor test and Vee-Bee consistometer test. The merit of Vee-Bee test is that it simulates atleast in some respects, the compaction of concrete by vibration in practice. Slump test: Slump test gives an idea about consistency of concrete mix and indirectly measures workability of the concrete. Depending on the slump values of concrete can be classified into different categories as per IS: 1199-1959. Classification of concrete Stiff Poorly mobile Mobile Cast mix
Slump value (mm) 0 10-30 40-150 >150
In this investigation work, slump test was conducted for all types of cement chosen for grades M25 and the results are tabulated in the below table
Department of CIVIL Engineering, A.I.T. Chikmagalur
21
Compaction factor test: The compaction factor is defined as the ratio of weight of partially compacted concrete to weight of fully compacted concrete. This test mainly deals with the amount of energy required to compact a particular mix of concrete, which is a measure of workability. The test gives a rough idea of workability of the given concrete mix. Depending upon the compaction factor test values the concrete mix can be classified into different categories as per IS: 1199-1959.
Compaction factor
Quantity of mix
0.95
Good
0.92
Medium
0.85
Bad(poor)
The results of compaction factor test are tabulated in below table
Table 6 Slump test values of concrete mixes
Sl.No.
Grade of concrete
w/c ratio
Slump (mm)
1
M25 O.P.C.
0.55
90
Department of CIVIL Engineering, A.I.T. Chikmagalur
22
CHAPTER 6 TESTS ON HARDENED CONCRETE Details of the standard specimens: Two types of specimens namely cubes and cylinders were cast. Cubes were used for compression strength test and cylinders for split tensile strength test.The details of the standard specimens used in the investigation are shown in table. Table 7 Details of the standard specimens: Type of test
Type of specimen
Dimensions (mm)
Compression test
Cubes
150 x 150 x 150
Split tension test
Cylinder
100 ᴓ x 200 depth
Test for compressive strength: The specimens were removed from the curing tank and its surfaces are cleaned with cotton waste. They were tested in wet condition in a Compression Testing Machine. The rate of loading was maintained at 140 kg/cm2/minute as per the requirements given in the code of practice (IS: 516-1969). Three specimens of 150mm cubes were tested for required age and the average value of compressive strength was calculated. The results of compressive strength test were tabulated in table. Table 7
Average Compressive strength of conventionally cured concrete: Compressive strength, MPa (N/mm2)
Type of cement OPC 53 grade
3 days 14.07
7 days 25.77
Department of CIVIL Engineering, A.I.T. Chikmagalur
28 days 33.48
23
Table 8
Average Compressive strength of external self cured concrete: Compressive strength, MPa (N/mm2)
Type of cement
OPC 53 grade
3 days
7 days
28 days
17.18
27.12
34.59
Test for Split Tensile Strength: The tests were performed while they were in wet condition in a Compression Testing Machine. Three specimens were tested and the mean value was computed and the results were tabulated in table. Table 9
Average Split tensile strength of conventionally cured concrete: Split tensile strength, MPa (N/mm2)
Type of cement
OPC 53 grade
3 days 1.01
7 days 1.17
28 days 1.91
Table 10
Average Split tensile strength of external self cured concrete: Split tensile strength, MPa (N/mm2)
Type of cement
OPC 53 grade
3 days 1.06
7 days 1.30
Department of CIVIL Engineering, A.I.T. Chikmagalur
28 days 2.31
24
CHAPTER 7 Interpretation of test results
Setting time of cement: The initial and final setting time of the cement (53 grade O.P.C) are shown in the table (1). Workability of concrete: The workability test results with respect to slump test are tabulated in table(6) Compressive strength of concrete: Normally compressive strength of the concrete is a measure of quality of concrete for a particular mix. The results of the compressive strength are tabulated in table 7 & 8 and variations are shown in fig.
Comparison between the compressive strength of conventional cured concrete And external self cured concrete.
1) Ordinary Portland cement: From the test results it is observed that the 3, 7 and 28 day’s compressive strength of external self cured concrete is more than that for conventional cured concrete. It is observed that the increase in compressive strength of external self cured concrete is about 22.10% at the age of 3 days, 5.24% at the age of 7 days and 3.32% at the age of 28 days as compare to conventional cured concrete.
Department of CIVIL Engineering, A.I.T. Chikmagalur
25
Splitting tensile strength of concrete: Split tensile strength is the method of determining tensile strength of concrete. The results and variations of split tensile strength are tabulated in table 9 & 10. And variations are shown in fig.
Comparison between the split tensile strength of conventional cured concrete And external self cured concrete.
1) Ordinary Portland cement: From the test results it is observed that the 3, 7 and 28 days split tensile strength of external self cured concrete is greater than that for conventional cured concrete. It is observed that the increase in split tensile strength of external self cured concrete is about 4.95% at the age of 3 days, 11.11% at the age of 7 days and 20.94% at the age of 28 days as compare to conventional cured concrete.
Department of CIVIL Engineering, A.I.T. Chikmagalur
26
TABLES
Department of CIVIL Engineering, A.I.T. Chikmagalur
27
COMPRESSIVE STRENGTH OF SELF CURED CONCRETE: TYPE OF CEMENT
3 DAYS
7DAYS
28 DAYS
O.P.C.
26.57
41.1
47.46
BLENDED CEMENT
22.62
39.55
51.67
Department of CIVIL Engineering, A.I.T. Chikmagalur
28
SPLITING TENSILE STRENGTH OF SELF CURED CONCRETE: TYPES OF CEMENT
3 DAYS
7 DAYS
28 DAYS
O.P.C.
2.32
3.74
3.43
BLENDED CEMENT
2.3
3.76
3.36
Department of CIVIL Engineering, A.I.T. Chikmagalur
29
CHARTS
Department of CIVIL Engineering, A.I.T. Chikmagalur
30
Compressive strength in Mpa
FIG 1:COMPRESSIVE STRENGTH OF CONVENTIONAL CURED CONCRETE 120 100 80
P.P.C O.P.C
60 40 20 0 3
7
28
AGE IN DAYS
FIG 2: SPLITTING TENSILE STRENGTH OF CONVENTIONAL CURED CONCRETE
Split tensile strength inMpa
12 10 8 P.P.C. O.P.C.
6 4 2 0 3
7
28
Age in days Department of CIVIL Engineering, A.I.T. Chikmagalur
31
FIG 3:FLEXURAL STRENGTH OF CONVENTIONAL CURED CONCRETE
flexural strength in Mpa
14 12 10 8
P.P.C
6
O.P.C
4 2 0 3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
32
Department of CIVIL Engineering, A.I.T. Chikmagalur
33
FIG 4:COMPRESSIVE STRENGTH OF SELF CURED CONCRETE
Compressive strength in Mpa
120 100 80 P.P.C
60
O.P.C
40 20 0 3
7
28
Age in days
Split tensile strength in Mpa
FIG 5:SPLITTING TENSILE STRENGTH OF SELF CURED CONCRETE 8 7 6 5
P.P.C
4
O.P.C
3 2 1 0 3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
34
FIG 6:FLEXURAL STRENGTH OF SELF CURED CONCRETE
Flexural strength in Mpa
10 9 8 7 6
P.P.C
5 4
O.P.C
3 2 1 0 3
7
28
Age in days
Compressive strength in Mpa
FIG 7:COMPRESSIVE STRENGTH OF CONVENTIONAL AND SELF CURED CONCRETE 120 100 80
SELF CURED
60
CONVENTION AL CURED
40 20 0 3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
35
Split tensile strength in Mpa
FIG 8:SPLITTING TENSILE STRENGTH OF CONVENTIONAL AND SELF CURED CONCRETE OF O.P.C 10 9 8 7 6 5 4 3 2 1 0
self cured conventional cured
3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
36
FIG 9:FLEXURAL STRENGTH OF CONVENTIONAL AND SELF CURED CONCRETE OF O.P.C
Flexural strength in Mpa
12 10 8
self cured
6
conventional cured
4 2 0 3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
37
FIG 10:COMPRESSIVE STRENGTH OF CONVENTIONAL AND SELF CURED CONCRETE OF BLENDED CEMENT
Compressive strength in Mpa
120 100 80
self cured
60
conventional cured
40 20 0 3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
38
Split tensile strength in Mpa
FIG 11:SPLITTING TENSILE STRENGTH OF CONVENTIONAL AND SELF CURED CONCRETE OF BLENDED CEMENT 10 9 8 7 6 5 4 3 2 1 0
self cured conventional cured
3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
39
FIG 12:FLEXURAL STRENGTH OF CONVENTIONAL AND SELF CURED CONCRETE OF BLENDED CEMENT
Flexural strength in Mpa
12 10 self cured
8 6
conventional cured
4 2 0 3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
40
DESIGN CONCRETE MIX (As per IS:10262-2009)
DESIGN BY IS 10262-2009 METHOD Design stipulations: • • • •
Characteristic compressive strength required in the field at 28 days Maximum size of aggregates Degree of quality control Type of exposure
25N/mm2 20mm Good Mild
Test data for materials Cement used Grade of cement Specific gravity of cement Specific gravity of fine aggregate Specific gravity of coarse aggregate Slump value Maximum size of aggregate Fine aggregate falls into
: Ordinary Portland cement : 53 grade : 2.85 : 2.50 : 2.60 : 90mm : 20mm : Zone-II
Department of CIVIL Engineering, A.I.T. Chikmagalur
41
Target mean strength of concrete: fck1 = fck + (1.65 x S) = 25 + (1.65 x 4.0) = 31.60 N/mm2. fck1 = Target average compressive strength at 28 days, fck= Characteristic compressive strength at 28 days, s = Standard deviation From table no.01 of IS10262-2009, S=4 N/mm2 Selection of water cement ratio: From table no. of IS456-2000 maximum, W/C ratio =0.55 (For mild exposure condition) Selection of water content: From table 2 of IS10262-2009 for 20mm nominal maximum size aggregate and fine aggregate conforming to grading zone 2 and for 25-50mm slump range maximum water content per cubic meter of concrete =186 kg. Estimated water content for 90mm slump = 186 + (6/100) x 186 = 197 Kg/m3 Calculation of cement content: W/C ratio = 0.55 Cement content = 197/0.55 . = 358.18 Kg/m3 From table no.05 of IS456-2000 minimum cement for mild exposure condition = 300Kg/m3 Therefore 358.183 Kg/m3 >300Kg/m3 Hence ok Proportions of volume of coarse aggregate and fine aggregate content Department of CIVIL Engineering, A.I.T. Chikmagalur
42
From table 3, volume of coarse aggregate corresponding to 20 mm size aggregate and fine aggregate zone2, for W/C ratio of 0.50=0.62 Here, W/C ratio = 0.55, therefore W/C ratio is increased by 0.05 correspondingly the volume of coarse aggregate is lower by 0.01 Therefore, the volume of coarse aggregate = 0.61 Volume of fine aggregate = 1-0.61=0.39
Mix calculation The mix calculation per unit volume of concrete shall be as follows. a) Volume of concrete = 1 m3 b) Volume of cement= mass / (specific gravity x 1000) = 358.18 / (2.85 x 1000) = 0.126 m3 c) Volume of water = mass / (specific gravity x 1000) = 197 / (1 x 1000) = 0.197 m3 d) Volume of aggregate = 1 - 0.126 - 0.197 = 0.677 m3 e) Mass of coarse aggregate = volume of aggregate x proportion of aggregate x specific gravity of aggregate x 1000 = 0.677 x 0.61 x 2.60 x 1000 = 1073.72 Kg f) Mass of fine aggregate = volume of aggregate x proportion of aggregate x specific gravity of aggregate x 1000 = 0.677 x 0.39 x 2.50 x 1000 = 660.08 Kg Mix proportion for 1 m3 concrete: W/C ratio
Water In litre
Cement In Kg
Fine aggregate In Kg
Department of CIVIL Engineering, A.I.T. Chikmagalur
Coarse aggregate in Kg 43
0.55
197
358.18
660.08
1073.72
1
1.84
3.00
Department of CIVIL Engineering, A.I.T. Chikmagalur
44
SYNOPSIS External self curing of concrete. By:
2010
1. SYNOPSIS
In the present situation there is a need for the search of alternate materials in the place of water for curing not only to save water for the sustainable development of the environment, but also to promote indoor and outdoor construction activities even in remote areas where there is scarcity of water. If no any curing measure is taken for concrete against natural setting or hardening, substantial water losses will occur due to fast water evaporation, thus prevents cement from hydration and leads to dusting or drying crack on concrete surface. In such case, the strength and durability of concrete will be reduced. After placing of concrete, curing shall be duly required to avoid water loss and drying crack. Curing of concrete by which the concrete, after laying, is kept moist for some days is essential for the development of proper strength and durability. IS 456-2000 recommends a curing period of 7 days for ordinary Portland cement concrete, and 10 to 14 days for concrete prepared using mineral ixtures or blended cements. But, being the last act in the concreting operations, it is often neglected or not fully done. Consequently, the quality of hardened concrete suffers, more so, if the freshly laid concrete gets exposed to the environmental conditions of low humidity, high wind velocity and high ambient temperature. To avoid the adverse effects of neglected or insufficient curing, which is considered a universal phenomenon, concrete technologist and research scientists in various countries including India, are working on the development of self-curing concrete. The present work deals with the performance of self curing concrete, its advantages, applications. Concrete is said to be self cured, if it is able to retain its water content for the development of its strength. This work explains the performance of self curing concrete by the application of wax based curing compound. In the present work, investigation is carried out on 53 grades O.P.C to study the strength behavior and other parameters of concrete prepared from above mentioned cement. An attempt has been made to compare different parameters. The physical properties of cement, fine aggregate & coarse aggregate are determined in accordance with BIS specifications. Sieve analysis of fine aggregate & coarse aggregate is also carried out. On fresh concrete workability related tests such as slump test, compaction factor test and on hardened concrete strength related tests such as compressive strength test and split tensile strength test were conducted in accordance with BIS specifications .The grade of concrete chosen for investigation is M25 of w/c 0.55. The concrete mix design is carried out according to BIS specifications The result of the investigation clearly indicates that self curing can be adopted economically in places where there is scarcity of water and in remote places without compromising with the performance characteristics of concrete including durability. Department of CIVIL Engineering, A.I.T. Chikmagalur
2
CHAPTER 1 GENERAL INTRODUCTION Construction industry use lot of water in the name of curing. The days are not so far that all the construction industry has to switch over to an alternative curing system, not only to save water for the sustainable development of the environment but also to promote indoor and outdoor construction activities even in remote areas where there is scarcity of water. Curing is the process of controlling the rate and extent of moisture loss from concrete during cement hydration. It may be either after it has been placed in position (or during the manufacture of concrete products), thereby providing time for the hydration of the cement to occur. Since the hydration of cement does take time – days, and even weeks rather than hours – curing must be undertaken for a reasonable period of time. If the concrete is to achieve its potential strength and durability. Curing may also encom the control of temperature since this affects the rate at which cement hydrates. The curing period may depend on the properties required of the concrete, the purpose for which it is to be used, and the ambient conditions, i.e. the temperature and relative humidity of the surrounding atmosphere. Curing is designed primarily to keep the concrete moist, by preventing the loss of moisture from the concrete during the period in which it is gaining strength. Curing may be applied in a number of ways and the most appropriate means of curing may be dictated by the site or the construction method. Curing is the maintenance of a satisfactory moisture content and temperature in concrete for a period of time immediately following placing and finishing so that the desired properties may develop. The need for adequate curing of concrete cannot be overemphasized. Curing has a strong influence on the properties of hardened concrete; proper curing will increase durability, strength, watertightness, abrasion resistance, volume stability, and resistance to freezing and thawing and deicers. Exposed slab surfaces are especially sensitive to curing as strength development. And freeze-thaw resistance of the top surface of a slab can be reduced significantly when curing is defective. When Portland cement is mixed with water, a chemical reaction called hydration takes place. The extent to which this reaction is completed influences the strength and durability of the concrete. Freshly mixed concrete normally contains more water than is required for hydration of the cement; however, excessive loss of water by evaporation can delay or prevent adequate hydration. The surface is particularly susceptible to insufficient hydration because it dries first. If temperatures are favorable, hydration is relatively rapid the first few days after concrete is placed; however, it is important for water to be retained in the concrete during this period, that is, for evaporation to be prevented or substantially reduced. With proper curing, concrete becomes stronger, more impermeable, and more resistant to stress, abrasion, and freezing and thawing. The improvement is rapid at early ages but continues more slowly thereafter for an indefinite period. Department of CIVIL Engineering, A.I.T. Chikmagalur
3
Covering the concrete with an impermeable membrane after the formwork has been removed. By the application of a suitable chemical Curing agent (wax etc). Curing by continuously wetting the exposed surface thereby preventing the loss of moisture from it. Ponding or spraying the surface with water is methods typically employed.
Concrete can be kept moist (and in some cases at a favorable Temperature) by three curing methods: 1. Methods that maintain the presence of mixing water in the concrete during the early hardening period. These include ponding or immersion, spraying or fogging, and saturated wet coverings. These methods afford some cooling through evaporation, which is beneficial in hot weather. 2. Methods that reduce the loss of mixing water from the surface of the concrete. This can be done by covering the concrete with impervious paper or plastic sheets, or by applying membrane-forming curing compounds. 3. . Methods that accelerate strength gain by supplying heat and additional moisture to the concrete. This is usually accomplished with live steam, heating coils, or electrically heated forms or pads.
Ponding and Immersion On flat surfaces, such as pavements and floors, concrete can be cured by ponding. Earth or sand dikes around the perimeter of the concrete surface can retain a pond of water. Ponding is an ideal method for preventing loss of moisture from the concrete; it is also effective for maintaining uniform temperature in the concrete. The curing water should not be more than about 11°C (20°F) cooler than the concrete to prevent thermal stresses that could result in cracking. Since ponding requires considerable labor and supervision, the method is generally used only for small jobs. The most thorough method of curing with water consists of total immersion of the finished concrete element. This method is commonly used in the laboratory for curing concrete test specimens. Where appearance of the concrete is important, the water used for curing by ponding or immersion must be free of substances that will stain or discolor the concrete. The material used for dikes may also discolor the concrete. As shown in Fig: 01
Department of CIVIL Engineering, A.I.T. Chikmagalur
4
Fig. 01: Ponding method of water curing
Fogging and Sprinkling Fogging (Fig. 02) and sprinkling with water are excellent methods of curing when the ambient temperature is well above freezing and the humidity is low. A fine fog mist is frequently applied through a system of nozzles or sprayers to raise the relative humidity of the air over flatwork, thus slowing evaporation from the surface. Fogging is applied to minimize plastic shrinkage cracking until finishing operations are complete. Once the concrete has set sufficiently to prevent water erosion, ordinary lawn sprinklers are effective if good coverage is provided and water runoff is of no concern. The cost of sprinkling may be a disadvantage. The method requires an ample water supply and careful supervision. If sprinkling is done at intervals, the concrete must be prevented from drying between applications of water by using burlap or similar materials; otherwise alternate cycles of wetting and drying can cause surface crazing or cracking.
Fig. 02: Fogging minimizes moisture loss during and after Placing and finishing of concrete
Department of CIVIL Engineering, A.I.T. Chikmagalur
5
Wet Coverings Fabric coverings saturated with water, such as burlap, cotton mats, rugs, or other moisture-retaining fabrics, are commonly used for curing (Fig. 03). Treated burlaps that reflect light and are resistant to rot and fire wet, moisture-retaining fabric coverings should be placed as soon as the concrete has hardened sufficiently to prevent surface damage. During the waiting period other curing methods are used, such as fogging or the use of membrane forming finishing aids. Care should be taken to cover the entire surface with wet fabric, including the edges of slabs. The coverings should be kept continuously moist so that a film of water remains on the concrete surface throughout the curing period. Use of polyethylene film over wet burlap is a good practice; it will eliminate the need for continuous watering of the covering periodically rewetting the fabric under the plastic before it dries out should be sufficient. Alternate cycles of wetting and drying during the early curing period may cause crazing of the surface. Wet coverings of earth, sand, or sawdust are effective for curing and are often useful on small jobs. Sawdust from most woods is acceptable, but oak and other woods that contain tannic acid should not be used since deterioration of the concrete may occur. A layer about 50 mm (2 in.) thick should be evenly distributed over the previously moistened surface of the concrete and kept continuously wet. Wet hay or straw can be used to cure flat surfaces. If used, it should be placed in a layer at least 150 mm (6 in.) thick and held down with wire screen, burlap, or tarpaulins to prevent its being blown off by wind. A major disadvantage of moist earth, sand, sawdust, hay, or straw coverings is the possibility of discoloring the concrete.
Fig. 03: Lawn sprinklers saturating burlap with water keep the concrete continuously moist. Intermittent sprinkling is acceptable if no drying of the concrete surface occurs.
Department of CIVIL Engineering, A.I.T. Chikmagalur
6
Impervious Paper Impervious paper for curing concrete consists of two sheets of Kraft paper cemented together by a bituminous adhesive with fiber reinforcement. Such paper, conforming To AASHTO M 171, is an efficient means of curing horizontal surfaces and structural concrete of relatively simple shapes. An important advantage of this method is that periodic additions of water are not required. Curing with impervious paper enhances the hydration of cement by preventing loss of moisture from the concrete (Fig.04).As soon as the concrete has hardened sufficiently to prevent surface damage, it should be thoroughly wetted and the widest paper available applied. Edges of adjacent sheets should be overlapped about 150 mm (6 in.) and tightly sealed with sand, wood planks, pressure-sensitive tape, mastic, or glue. The sheets must be weighted to maintain close with the concrete surface during the entire curing period. Impervious paper can be reused if it effectively retains moisture. Tears and holes can easily be repaired with curing-paper patches. When the condition of the paper is questionable, additional use can be obtained by using it in double thickness. In addition to curing, impervious paper provides some protection to the concrete against damage from subsequent construction activity as well as protection from the direct sun. It should be light in color and no staining to the concrete. Paper with a white upper surface is preferable for curing exterior concrete during hot weather.
Fig.04: Impervious curing paper is an efficient means of curing horizontal surfaces.
Plastic Sheets Plastic sheet materials, such as polyethylene film, can be used to cure concrete (Fig. 05). Polyethylene film is a lightweight, effective moisture retarder and is easily applied to complex as well as simple shapes. Its applications the same as described for impervious paper. Curing with polyethylene film (or impervious paper) can cause patchy discoloration, especially if the concrete contains calcium chloride and has been finished by hard steel troweling. This discoloration is more pronounced
Department of CIVIL Engineering, A.I.T. Chikmagalur
7
when the film becomes wrinkled, but it is difficult and time consuming on a large project to place sheet materials without wrinkles. Flooding the surface under the covering may prevent discoloration, but other means of curing should be used when uniform color is important. Polyethylene film should conform to AASHTO M 171, which specifies a 0.10-mm (4-mil) thickness for curing concrete, but lists only clear and white opaque film. However, black film is available and is satisfactory under some conditions. White film should be used for curing exterior concrete during hot weather to reflect the sun’s rays. Black film can be used during cool weather or for interior locations. Clear film has little effect on heat absorption.
Fig. 05: Polyethylene film is an effective moisture barrier for curing concrete and easily applied to complex as well as simple shapes. To minimize discoloration, the film should be kept as flat as possible on the concrete surface.
Membrane-Forming Curing Compounds Liquid membrane-forming compounds consisting of waxes, resins, chlorinated rubber, and other materials can be used to retard or reduce evaporation of moisture from concrete. They are the most practical and most widely used method for curing not only freshly placed concrete but also for extending curing of concrete after removal of forms or after initial moist curing. However, the most effective methods of curing concrete are wet coverings or water spraying that keeps the concrete continually damp. Curing compounds should be able to maintain the relative humidity of the concrete surface above 80% for seven days to sustain cement hydration. Membrane-forming curing compounds are of two general types: clear or translucent; and white pigmented. Clear or translucent compounds may contain a fugitive dye that makes it easier to check visually for complete coverage of the concrete surface when the compound is applied. The dye fades away soon after application. On hot, sunny days, uses of white-pigmented compounds are base slab of a two-course floor. Similarly, some curing Compounds may affect the adhesion of paint to concrete floors. Curing compound manufacturers should be consulted to determine if their product is suitable for the intended application. Curing compounds should be uniform and easy to maintain in a thoroughly mixed solution. They should not sag, run off peaks, or collect in grooves. They should form a tough film to withstand early Department of CIVIL Engineering, A.I.T. Chikmagalur
8
construction traffic without damage, be no yellowing, and have good moistureretention properties. Caution is necessary when using curing compounds containing solvents of high volatility in confined spaces or near sensitive occupied spaces such as hospitals because evaporating volatiles may cause respiratory problems. Applicable local environmental laws concerning volatile organic compound (VOC) emissions should be followed. Curing compounds should conform to AASHTO M 148. A method for determining the efficiency of curing compounds, waterproof paper, and plastic sheets is described in AASHTO T 155. ASTM C1151, discontinued in 2000, also evaluates the effectiveness of curing compounds. Curing compounds with sealing properties are specified under ASTM C 1315. As shown in Fig:06
Fig. 06: Liquid membrane-forming curing compounds should be applied with uniform and adequate coverage over the entire surface and edges for effective, extended curing of concrete.
Steam Curing Steam curing is advantageous where early strength gain in concrete is important or where additional heat is required to accomplish hydration, as in cold weather. Two methods of steam curing are used: live steam at atmospheric pressure (for enclosed cast-in-place structures and large precast concrete units) and high-pressure steam in autoclaves (for small manufactured units). Steam curing at atmospheric pressure is generally done in an enclosure to minimize moisture and heat losses. Tarpaulins are frequently used to form the enclosure. Application of steam to the enclosure should be delayed until initial set occurs or delayed at least 3 hours after final placement of concrete to allow for some hardening of the concrete. However, a 3- to 5- hour delay period prior to steaming will achieve maximum early strength. Steam temperature in the enclosure should be kept at about 60°C (140°F) until the desired concrete strength has developed. Strength will not increase significantly if the maximum steam temperature is raised from 60°C to 70°C (140°F to 160°F). Steam-curing temperatures above 70°C (160°F) should be avoided; they are uneconomical and may result in damage. It is recommended that the internal Department of CIVIL Engineering, A.I.T. Chikmagalur
9
temperature of concrete not exceed 70°C (160°F) to avoid heat induced delayed expansion and undue reduction in ultimate strength. Use of concrete temperatures above70°C (160°F) should be demonstrated to be safe by test or historic field data. SELF CURING OF CONCRETE Self curing concrete is the one which can cure itself by retaining its moisture content. A concrete can made to self cure by adding curing ixtures or by the application of curing compounds.
Advantages of self curing 1. Reduces autogenously cracking. 2. Largely eliminates autogenously shrinkage. 3. Reduces permeability. 4. Protects reinforcement steel. 5. Provide greater durability. 6. Increases the early age strength. 7. Improved rheology. 8. Lower maintenance. 9. Higher performance. 10. Doesn’t adversely effects finish ability.
TYPE OF CEMENT Ordinary Portland cement (O.P.C) This is by far the most common cement in use. This is the basic type of cement which is used on large scale in all general types of construction works. The details regarding the composition and properties of this type of cement are given in IS: 269. This cement is irably suitable for use in general concrete constructions where there is no exposure to sulphates in the soil or ground water. These cements are available in different grades viz. 33, 43 and 53 grade. 43 grade O.P.C. In these types of cements, the 28days cement strength is expected to have a minimum value of 43 Mpa. 53 grade O.P.C. In this type of cement, the 28 days cement strength is expected to have a minimum value of 53 Mpa.
Department of CIVIL Engineering, A.I.T. Chikmagalur
10
The use of high grade cement should not be taken for granted to yield high grade [strength] concrete as the strength of concrete depends on the mixture of cement, sand, coarse aggregate and water. In fact, the cement’s grade has no relationship to the strength of concrete. It is possible to produce concrete of wideranging strength using a particular grade of cement. Moreover the “grade” has nothing to do with quality; increase in the grade does not increase the quality of the cement. Every structure has to satisfy the requirement of strength and durability. Strength is the ability of the structure to withstand load. Durability refers to the period of trouble free life. A structural cement of concrete may possess high strength, but may deteriorate sooner than expected, making it a material of poor quality. Here the quality is with reference to concrete and not that of the cement. A grade of cement can be said to be of good quality if the concrete made with it satisfies both strength and durability requirements. The strength requirements [i.e. the strength of concrete] is satisfies by choosing the proper amount of cement, limiting the amount of water, consolidating the mixture well and curing the hardened concrete as long as possible. Durability on the other hand depends on the several factors that are attributable both to the material and to the exposed environment. During a recent survey made in Chennai, the only grades of cement freely available was found to be grade 53 and grade 43 was available on special order only and grade 33 was not found available.
Department of CIVIL Engineering, A.I.T. Chikmagalur
11
CHAPTER 2 REVIEW OF LITERATURE Construction industry is growing like day by day even in remote areas and deserted regions. Even India and other countries are facing lot of problems in supplying drinking water to their citizens. Hence construction industries are under pressure in finding out alternative curing methods of curing concrete. The objective of curing is to keep concrete saturated are nearly saturated as possible until the water fill the space in the fresh cement paste have been sequentially reduced by the products of hydration of cement [09]. Curing of concrete is complex phenomenon where the controlling process is hydration of cement [10]. Hydration is an essential process of hardened concrete but some micro cracking can occur as a consequence of hydration especially in HSC. The literature reports that the different compressive strength trends displayed by cement paste and concrete specimen suggest that the presence of aggregate is influencing the behavior of self curing concrete. In the last 100 years there has been placed a certain amount of long lasting good performing and economical concrete. However the deficiencies of much of concrete have been obvious. In the last 75 years there have been great amount of knowledge generated how to make the concrete better by best means of curing ACI committee 308 has studied the subjects since BRYANT MATHER called the industry’s attention to “SELF CURING”. Water retention of concrete containing self curing agents is investigated. Concrete weight loss and internal relative humidity measurements with time were carried out, in order to evaluate the water retention of self curing concrete. Non evaporable water at different ages was measured to evaluate the hydration. Water transport through concrete is evaluated by measuring absorption %, permeable voids %, and water sorptivity and water permeability. The water transport through self curing concrete is evaluated with age. The effect of the concrete mix proportions on the performance of self curing concrete were investigated, such as cement content and w/c ratio. To achieve good cure, excessive evaporation of water from a freshly cast concrete surface should be prevented. Failure to do this will lead to the degree of cement hydration being lowered and the concrete developing unsatisfactory properties. Curing can be performed in a number of ways to ensure that an adequate amount of water is available for cement hydration to occur. However, it is not always possible to cure concrete satisfactorily. This paper is concerned with achieving optimum cure of concrete without the need for applying conventional curing methods. Department of CIVIL Engineering, A.I.T. Chikmagalur
12
The guide to curing concrete is being re written to recognize the value of external curing as an adjunct to internal curing. Curing period is important for concrete in which it attains its maximum strength. Normally all structures made of concrete are cured for a period of 28 days by the application of water. At present, meeting the requirements of drinking water is a global issue. Amidst of this situation. Construction industry is growing rapidly. The scarcity of water is forcing the construction industry to switch over to a curing method which can cure their structures without the use of water. Self curing concrete is the only solution for these problems. Looking at the demand of drinking water in all the metropolitan cities, construction of highways and air fields in remote places where water curing is practically not possible, deserted areas through out the globe, etc. …forced to study on the curing compounds and their performance. Curing compounds are liquids which are usually sprayed directly on to the concrete surface and they are an efficient and cost effective means of curing concrete and may be applied to freshly placed concrete. When used to cure fresh concrete, the timing of the application of the curing compound is critical for maximum effectiveness. They should be applied to the surface of concrete after it has been finished. As soon as free water on the surface has evaporated and there is no water seen visible. They may also be used to reduce the moisture loss from concrete after initial moist curing or removal of form work. In case of columns and beams the application is done after the removal of formwork on the horizontal surface; the curing compound is applied upon the complete disappearance of all bleeding water. After spraying, no further application of water or other material is necessary to ensure continued curing. The concrete surface should not be disturbed until it has sufficient strength to bear surface loads. The applied film should not be walked on before it is fully dry and care should be taken to ensure that the film is not broken. In case the concrete surface has dried, the surface should be sprayed with water and thoroughly wetted and made fully damp before curing compound is applied. The container of curing compound should be well stirred before use. It takes nearly 10 to 15 minutes for its drying and drying and it forms a thin water proofing film on the surface. The fundamental conclusion shows that the efficiency test indeed is significant and worthwhile test, yielding very reasonable test conclusion. Also at an age other than 7 days, a good correlation can be found between the compressive strength and the evaporation.
Department of CIVIL Engineering, A.I.T. Chikmagalur
13
B.Mangaiarkarasa and S.R.Damodaraswamy conducted compressive test on concrete cubes of 150mm size, the curing compound used is CONCURE WB. Based on the results of the investigation, they concluded the following. Self curing concrete develops higher compressive strength then the water cured concrete in three days. Self curing concrete using wax based curing has an average efficiency of 84.0%. Self cured concrete satisfies serviceability conditions also. Self curing concrete can be practiced in pre fabrication units and in place of water scarcity as well as exposed weather condition. Whenever there is difficulty in water curing, self curing concrete will be very economical in remote areas as well as in water scarcity area. By adapting self curing compound concrete the sustainable development of environment is maintained there is no doubt that self curing concrete play a vital role and dominate the construction industry in future.
Department of CIVIL Engineering, A.I.T. Chikmagalur
14
CHAPTER 3 EXTERNAL SELF CURING Introduction: External self curing concrete is the one which can cure itself by retaining its moisture content. Concrete can made to self cure by the application of curing compounds on the surface of the concrete. The curing compound is applied by means of brushing or spraying. CURING COMPOUND The curing compound used is “CONCURE WB” which is a product of Fosroc chemicals. CONCURE WB is water based concrete curing compound based on a low viscosity wax emulsion. It is supplied as a white emulsion which forms a clear film on drying. When first applied to a fresh cementitious surface the emulsion breaks to form a continuous, non-penetrating white coating. This dries to form a continuous clear film which provides a barrier to moisture loss, ensuring more efficient cement hydration, improved durability and reduced shrinkage. Specifications of CONCURE WB Base –wax Shelf life – 12 months Coverage – 3.5 to 5 m2/litre Cost - Rs.90/litre Specific gravity-0.98 at 25 °C Features:
single application no other curing necessary easy and safe spray application endures hard wearing surface
Application procedure The curing compound is applied by brush or by spraying while the concrete is wet. In case of columns and beams the application is done after the removal of formwork. On the horizontal surface, the curing compound is applied upon the complete disappearance of all bleeding water. After spraying, no further application of water or other material is necessary to ensure continued curing. The concrete surface should not be disturbed until it has sufficient strength to bear surface loads. The applied film should not be walked on before it is fully dry and care should be taken to ensure that the film is not broken.
Department of CIVIL Engineering, A.I.T. Chikmagalur
15
In case the concrete surface has dried, the surface should be sprayed with water and thoroughly wetted and made fully damp before curing compound is applied. The container of curing compound should be well stirred before use. It takes nearly 10 to 15 minutes for its drying and drying and it forms a thin water proofing film on the surface. As shown in Fig: 07.
Fig: 07: Application of external self curing compound. Uses of external self curing concrete As a spray applied membrane to retain moisture in concrete for effective curing. Suitable for all general concreting applications and of particular benefit for large area concrete surfaces, such as airport runways, roads and bridgeworks. It is also suitable for piece works. Where, it is difficult to curing. Advantages of external self curing concrete Improved curing of concrete enhances cement hydration and provides a more durable concrete. Control of moisture loss improves surface quality, reducing permeability, producing a hard wearing; dust free Surface and minimizing potential for surface cracking and shrinkage. Fugitive colour provides visual guide during application. Water based, therefore, non-flammable.
Department of CIVIL Engineering, A.I.T. Chikmagalur
16
Spray application reduces labour costs and eliminates the need for alternative curing systems. CHAPTER 4 TEST ON INGREDIENT MATERIALS Introduction: The present investigation is carried out to study the behavior of normal and self curing cured concrete and ingredient material by using O.P.C. 53 grade cement. The tests were carried out at the Civil Engineering Laboratory of Civil Engineering Department, AIT Chikmagalur. Materials used and their properties: It is well known that strength of the concrete is dependent on the properties of its ingredients. The materials used in the present investigations are as follows. • • •
53 grade O.P.C. River sand as fine aggregate Quarried and crushed stone as coarse aggregate
Test on cement: The specific gravity, normal consistency, initial setting time, final setting time and compressive strength of cement were found as per B.I.S specifications. The results are tabulated in tables. Test on fine aggregate: 53 grade O.P.C. was used throughout the investigations. The cement was tested according to B.I.S. specifications to determine its various properties. The overall quantity of cement required for the investigation was procured in a single lot and stored in the appropriate manner. The specific gravity of natural sand was found according to the norms of the Indian Standards and was used throughout in preparing the required mix of concrete. Results are tabulated in table. Sieve analysis of the fine aggregate was also carried out as per the B.I.S. specifications to determine the grading zone. The results of sieve analysis are tabulated in table.
Department of CIVIL Engineering, A.I.T. Chikmagalur
17
Tests on coarse aggregate: The specific gravity of crushed stone aggregates of 20mm and downsize was found according to the norms of Indian standards and were used for all concrete mixes. The results are tabulated in table. Sieve analysis of the coarse aggregate was also carried out as per B.I.S. specifications. The results are tabulated in table. Specimen details: Two types of specimens namely cubes and cylinders were cast. Cubes were used for compressive strength test and Cylinders for split tensile strength test. Curing of the test specimens: The specimens are stored in the laboratory atmosphere for 24 hours from the time of adding water to the ingredients. Temperature was maintained at 270 ± 20C. The specimens were removed from the moulds after 24 hrs. And then kept immersed in clean water for the required age. The water in the tank was changed every week and the temperature was maintained constant. Testing of the specimens: The testing was carried out according to I.S. specifications. Table 1 Results of test on cement Particulars 1 2 3
Type of cement Normal Consistency Specific Gravity Setting time (in min) (a) Initial setting time (b) Final setting time
references 53 grade O.P.C. 34.00 % 2.85 80 min. 460 min.
Department of CIVIL Engineering, A.I.T. Chikmagalur
------------IS:269-1958 IS:269-1976 IS: 269-1976 Should more Than 30 min. Should not be More than 600min.
18
Table 2 Sl No.
Particulars
Compressive Strength in N/mm2(MPa)
--
Type of cement
3 days
1
53 grade O.P.C.
30.2
7 days 45.7
28days 59.8
Compressive Strength of Cement 70.6mm X 70.6mm X 70.6mm= Size of the specimen
Table 3 Properties of the aggregate used Sl Particulars No. 1 Specific gravity 2 Fineness modulus 3
Fine Coarse Aggregate Aggregate 2.50 2.60 2.85
Grading zone Zone II
7.13
20 mm and Downsize
Department of CIVIL Engineering, A.I.T. Chikmagalur
Reference IS:2386(partIII)1963 Is;2836(partIII)1963 IS:383-1970 IS:460-1962 IS 383-1963
19
Table 4 Sieve analysis of fine aggregate Weight of sample taken =1000gm Sl.No 1 2 3 4 5 6 7
IS sieve size 10mm 4.75mm 2.36mm 1.18mm 600μ 300 μ 150 μ
Weight retained(gm) 0 24 32 180 394 282 84
Cumulative Wt. retained 0 24 56 236 630 912 996
Cumulative % Wt. retained 0 2.40 5.60 23.60 63.00 91.20 99.60 ∑=285.40
% finer 100.00 97.60 94.40 76.40 37.00 8.80 0.40
Calculations: Fineness modulus = 285.40/100 =2.85
Table 5 Sieve analysis of coarse aggregate Weight of sample taken =2000gms IS sieve Sl.No size
Wt. retained
Cumulative Wt.retained
Cumulative % wt.retained
%finer
1 2 3 4 5 6 7 8 9 10
0 0 560 1152 276 09 03 0 0 0
0 0 560 1712 1988 1997 2000 2000 2000 2000
0 0 28.00 85.60 99.40 99.85 100.00 100.00 100.00 100.00 ∑ =712.85
100 100 72.00 14.40 0.60 0.15 0.00 0.00 0.00 0.00
80mm 40mm 20mm 10mm 4.75mm 2.36mm 1.18mm 600 300 150
Fineness modulus =712.85/100 =7.13
Department of CIVIL Engineering, A.I.T. Chikmagalur
20
CHAPTER 5 TESTS ON FRESH CONCRETE Measurement of workability of concrete: Workability is defined as the amount of useful internal work necessary to achieve full compaction. It is also defined as the ease with which concrete can be placed and degree to which it resists segregation. It is also given a new definition which includes all the essential properties of the concrete in plastic condition i.e. mixing ability, transportability, modulability and ease compaction. Some of the important factors that affect the workability of concrete are, • • • •
Relative quantities of paste and aggregate. Plasticity of the paste itself. Maximum size and grading of aggregate. Shape and surface characteristics of aggregate particle.
Consistency of the concrete is an important component of workability and refers in a way to the wetness of concrete. However it must be assumed that the wetter the mix more workable it is. If a mix is too wet, segregation may occur resulting in honey combing, excessive bleeding and sand streaking on the formed surfaces. On the other hand if the mix is too dry it may be difficult to place and compact and segregation may occur because of lack of cohesiveness and plasticity of the plastic. In the present investigations workability connected with physical quantity is correlated by slump test, compaction factor test and Vee-Bee consistometer test. The merit of Vee-Bee test is that it simulates atleast in some respects, the compaction of concrete by vibration in practice. Slump test: Slump test gives an idea about consistency of concrete mix and indirectly measures workability of the concrete. Depending on the slump values of concrete can be classified into different categories as per IS: 1199-1959. Classification of concrete Stiff Poorly mobile Mobile Cast mix
Slump value (mm) 0 10-30 40-150 >150
In this investigation work, slump test was conducted for all types of cement chosen for grades M25 and the results are tabulated in the below table
Department of CIVIL Engineering, A.I.T. Chikmagalur
21
Compaction factor test: The compaction factor is defined as the ratio of weight of partially compacted concrete to weight of fully compacted concrete. This test mainly deals with the amount of energy required to compact a particular mix of concrete, which is a measure of workability. The test gives a rough idea of workability of the given concrete mix. Depending upon the compaction factor test values the concrete mix can be classified into different categories as per IS: 1199-1959.
Compaction factor
Quantity of mix
0.95
Good
0.92
Medium
0.85
Bad(poor)
The results of compaction factor test are tabulated in below table
Table 6 Slump test values of concrete mixes
Sl.No.
Grade of concrete
w/c ratio
Slump (mm)
1
M25 O.P.C.
0.55
90
Department of CIVIL Engineering, A.I.T. Chikmagalur
22
CHAPTER 6 TESTS ON HARDENED CONCRETE Details of the standard specimens: Two types of specimens namely cubes and cylinders were cast. Cubes were used for compression strength test and cylinders for split tensile strength test.The details of the standard specimens used in the investigation are shown in table. Table 7 Details of the standard specimens: Type of test
Type of specimen
Dimensions (mm)
Compression test
Cubes
150 x 150 x 150
Split tension test
Cylinder
100 ᴓ x 200 depth
Test for compressive strength: The specimens were removed from the curing tank and its surfaces are cleaned with cotton waste. They were tested in wet condition in a Compression Testing Machine. The rate of loading was maintained at 140 kg/cm2/minute as per the requirements given in the code of practice (IS: 516-1969). Three specimens of 150mm cubes were tested for required age and the average value of compressive strength was calculated. The results of compressive strength test were tabulated in table. Table 7
Average Compressive strength of conventionally cured concrete: Compressive strength, MPa (N/mm2)
Type of cement OPC 53 grade
3 days 14.07
7 days 25.77
Department of CIVIL Engineering, A.I.T. Chikmagalur
28 days 33.48
23
Table 8
Average Compressive strength of external self cured concrete: Compressive strength, MPa (N/mm2)
Type of cement
OPC 53 grade
3 days
7 days
28 days
17.18
27.12
34.59
Test for Split Tensile Strength: The tests were performed while they were in wet condition in a Compression Testing Machine. Three specimens were tested and the mean value was computed and the results were tabulated in table. Table 9
Average Split tensile strength of conventionally cured concrete: Split tensile strength, MPa (N/mm2)
Type of cement
OPC 53 grade
3 days 1.01
7 days 1.17
28 days 1.91
Table 10
Average Split tensile strength of external self cured concrete: Split tensile strength, MPa (N/mm2)
Type of cement
OPC 53 grade
3 days 1.06
7 days 1.30
Department of CIVIL Engineering, A.I.T. Chikmagalur
28 days 2.31
24
CHAPTER 7 Interpretation of test results
Setting time of cement: The initial and final setting time of the cement (53 grade O.P.C) are shown in the table (1). Workability of concrete: The workability test results with respect to slump test are tabulated in table(6) Compressive strength of concrete: Normally compressive strength of the concrete is a measure of quality of concrete for a particular mix. The results of the compressive strength are tabulated in table 7 & 8 and variations are shown in fig.
Comparison between the compressive strength of conventional cured concrete And external self cured concrete.
1) Ordinary Portland cement: From the test results it is observed that the 3, 7 and 28 day’s compressive strength of external self cured concrete is more than that for conventional cured concrete. It is observed that the increase in compressive strength of external self cured concrete is about 22.10% at the age of 3 days, 5.24% at the age of 7 days and 3.32% at the age of 28 days as compare to conventional cured concrete.
Department of CIVIL Engineering, A.I.T. Chikmagalur
25
Splitting tensile strength of concrete: Split tensile strength is the method of determining tensile strength of concrete. The results and variations of split tensile strength are tabulated in table 9 & 10. And variations are shown in fig.
Comparison between the split tensile strength of conventional cured concrete And external self cured concrete.
1) Ordinary Portland cement: From the test results it is observed that the 3, 7 and 28 days split tensile strength of external self cured concrete is greater than that for conventional cured concrete. It is observed that the increase in split tensile strength of external self cured concrete is about 4.95% at the age of 3 days, 11.11% at the age of 7 days and 20.94% at the age of 28 days as compare to conventional cured concrete.
Department of CIVIL Engineering, A.I.T. Chikmagalur
26
TABLES
Department of CIVIL Engineering, A.I.T. Chikmagalur
27
COMPRESSIVE STRENGTH OF SELF CURED CONCRETE: TYPE OF CEMENT
3 DAYS
7DAYS
28 DAYS
O.P.C.
26.57
41.1
47.46
BLENDED CEMENT
22.62
39.55
51.67
Department of CIVIL Engineering, A.I.T. Chikmagalur
28
SPLITING TENSILE STRENGTH OF SELF CURED CONCRETE: TYPES OF CEMENT
3 DAYS
7 DAYS
28 DAYS
O.P.C.
2.32
3.74
3.43
BLENDED CEMENT
2.3
3.76
3.36
Department of CIVIL Engineering, A.I.T. Chikmagalur
29
CHARTS
Department of CIVIL Engineering, A.I.T. Chikmagalur
30
Compressive strength in Mpa
FIG 1:COMPRESSIVE STRENGTH OF CONVENTIONAL CURED CONCRETE 120 100 80
P.P.C O.P.C
60 40 20 0 3
7
28
AGE IN DAYS
FIG 2: SPLITTING TENSILE STRENGTH OF CONVENTIONAL CURED CONCRETE
Split tensile strength inMpa
12 10 8 P.P.C. O.P.C.
6 4 2 0 3
7
28
Age in days Department of CIVIL Engineering, A.I.T. Chikmagalur
31
FIG 3:FLEXURAL STRENGTH OF CONVENTIONAL CURED CONCRETE
flexural strength in Mpa
14 12 10 8
P.P.C
6
O.P.C
4 2 0 3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
32
Department of CIVIL Engineering, A.I.T. Chikmagalur
33
FIG 4:COMPRESSIVE STRENGTH OF SELF CURED CONCRETE
Compressive strength in Mpa
120 100 80 P.P.C
60
O.P.C
40 20 0 3
7
28
Age in days
Split tensile strength in Mpa
FIG 5:SPLITTING TENSILE STRENGTH OF SELF CURED CONCRETE 8 7 6 5
P.P.C
4
O.P.C
3 2 1 0 3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
34
FIG 6:FLEXURAL STRENGTH OF SELF CURED CONCRETE
Flexural strength in Mpa
10 9 8 7 6
P.P.C
5 4
O.P.C
3 2 1 0 3
7
28
Age in days
Compressive strength in Mpa
FIG 7:COMPRESSIVE STRENGTH OF CONVENTIONAL AND SELF CURED CONCRETE 120 100 80
SELF CURED
60
CONVENTION AL CURED
40 20 0 3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
35
Split tensile strength in Mpa
FIG 8:SPLITTING TENSILE STRENGTH OF CONVENTIONAL AND SELF CURED CONCRETE OF O.P.C 10 9 8 7 6 5 4 3 2 1 0
self cured conventional cured
3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
36
FIG 9:FLEXURAL STRENGTH OF CONVENTIONAL AND SELF CURED CONCRETE OF O.P.C
Flexural strength in Mpa
12 10 8
self cured
6
conventional cured
4 2 0 3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
37
FIG 10:COMPRESSIVE STRENGTH OF CONVENTIONAL AND SELF CURED CONCRETE OF BLENDED CEMENT
Compressive strength in Mpa
120 100 80
self cured
60
conventional cured
40 20 0 3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
38
Split tensile strength in Mpa
FIG 11:SPLITTING TENSILE STRENGTH OF CONVENTIONAL AND SELF CURED CONCRETE OF BLENDED CEMENT 10 9 8 7 6 5 4 3 2 1 0
self cured conventional cured
3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
39
FIG 12:FLEXURAL STRENGTH OF CONVENTIONAL AND SELF CURED CONCRETE OF BLENDED CEMENT
Flexural strength in Mpa
12 10 self cured
8 6
conventional cured
4 2 0 3
7
28
Age in days
Department of CIVIL Engineering, A.I.T. Chikmagalur
40
DESIGN CONCRETE MIX (As per IS:10262-2009)
DESIGN BY IS 10262-2009 METHOD Design stipulations: • • • •
Characteristic compressive strength required in the field at 28 days Maximum size of aggregates Degree of quality control Type of exposure
25N/mm2 20mm Good Mild
Test data for materials Cement used Grade of cement Specific gravity of cement Specific gravity of fine aggregate Specific gravity of coarse aggregate Slump value Maximum size of aggregate Fine aggregate falls into
: Ordinary Portland cement : 53 grade : 2.85 : 2.50 : 2.60 : 90mm : 20mm : Zone-II
Department of CIVIL Engineering, A.I.T. Chikmagalur
41
Target mean strength of concrete: fck1 = fck + (1.65 x S) = 25 + (1.65 x 4.0) = 31.60 N/mm2. fck1 = Target average compressive strength at 28 days, fck= Characteristic compressive strength at 28 days, s = Standard deviation From table no.01 of IS10262-2009, S=4 N/mm2 Selection of water cement ratio: From table no. of IS456-2000 maximum, W/C ratio =0.55 (For mild exposure condition) Selection of water content: From table 2 of IS10262-2009 for 20mm nominal maximum size aggregate and fine aggregate conforming to grading zone 2 and for 25-50mm slump range maximum water content per cubic meter of concrete =186 kg. Estimated water content for 90mm slump = 186 + (6/100) x 186 = 197 Kg/m3 Calculation of cement content: W/C ratio = 0.55 Cement content = 197/0.55 . = 358.18 Kg/m3 From table no.05 of IS456-2000 minimum cement for mild exposure condition = 300Kg/m3 Therefore 358.183 Kg/m3 >300Kg/m3 Hence ok Proportions of volume of coarse aggregate and fine aggregate content Department of CIVIL Engineering, A.I.T. Chikmagalur
42
From table 3, volume of coarse aggregate corresponding to 20 mm size aggregate and fine aggregate zone2, for W/C ratio of 0.50=0.62 Here, W/C ratio = 0.55, therefore W/C ratio is increased by 0.05 correspondingly the volume of coarse aggregate is lower by 0.01 Therefore, the volume of coarse aggregate = 0.61 Volume of fine aggregate = 1-0.61=0.39
Mix calculation The mix calculation per unit volume of concrete shall be as follows. a) Volume of concrete = 1 m3 b) Volume of cement= mass / (specific gravity x 1000) = 358.18 / (2.85 x 1000) = 0.126 m3 c) Volume of water = mass / (specific gravity x 1000) = 197 / (1 x 1000) = 0.197 m3 d) Volume of aggregate = 1 - 0.126 - 0.197 = 0.677 m3 e) Mass of coarse aggregate = volume of aggregate x proportion of aggregate x specific gravity of aggregate x 1000 = 0.677 x 0.61 x 2.60 x 1000 = 1073.72 Kg f) Mass of fine aggregate = volume of aggregate x proportion of aggregate x specific gravity of aggregate x 1000 = 0.677 x 0.39 x 2.50 x 1000 = 660.08 Kg Mix proportion for 1 m3 concrete: W/C ratio
Water In litre
Cement In Kg
Fine aggregate In Kg
Department of CIVIL Engineering, A.I.T. Chikmagalur
Coarse aggregate in Kg 43
0.55
197
358.18
660.08
1073.72
1
1.84
3.00
Department of CIVIL Engineering, A.I.T. Chikmagalur
44
Related Documents 171j1w
Self Curing Concrete 6cc38
December 2019 34
Self Curing Of Concrete. l3x5x
December 2019 37
Concrete Curing 316d70
April 2020 25
Vertex Self Curing Liquid 3dm5s
May 2023 0
Self Compacting Concrete w1r5f
December 2020 0
Self-cleaning Concrete 20u64
October 2019 30More Documents from "Sujay Raghavendra N" 6ko3o
Underwater Construction 6o5u3g
November 2019 42
Tmt Bars. 1b1r3o
December 2019 57
Maintenance Of Bridges 71101w
April 2022 0
Microsilica As ixture In High Strength Concrete 1d4jb
December 2019 57
Self Compacting Concrete Report 5g561i
November 2021 0