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The paper discusses the effect of different curing methods on the evaporation of water from freshly placed concrete. The curing methods employed within the current experimental programme comprised plastic sheeting and burlap and were compared to the evaporation of water from an uncovered concrete surface. Samples were exposed to an ambient temperature of 40â-¦C and a relative humidity of approximately 30% for a period of 72 h. The effectiveness of each curing method was quantified in terms of both the cumulative water loss and rate of evaporation from the concrete surface. When compared to the uncovered surface, both curing systems resulted in a reduction in both the total amount of water loss and rate of evaporation, particularly over the initial 6 h after mixing. A local maximum in the rate of evaporation coincided with the maximum in internal temperature occurring within the concrete, the latter being due to setting of the cement binder.
Data are presented on the influence of plastic sheeting and burlap on water loss from fresh concrete exposed to a high ambient temperature (40â-¦C) and low relative humidity (30% RH). The efficacy of each curing method was quanti9ed in terms of the cumulative water loss and rate of evaporation from the concrete surface. For the materials and test conditions employed within the experimental programme, the results indicate that both curing systems reduced both the amount and rate of water loss from the concrete surface.
The most efficient curing method was plastic sheeting, particularly over the period prior to setting, although at the later stages of the test, both curing systems gave a similar effect with regard to the total and rate of water loss.
A local maximum in the rate of evaporation for covered and control concrete specimens coincided with the peak in internal temperature occurring within the concrete. Consequently, the classic, two-region response, associated with the rate of drying from a saturated porous material, cannot be invoked to explain the drying behavior of concrete during the initial 24-h after mixing.
(Nasir Shafiq, and J.G. Cabrera, 2003)
Effects of initial curing condition on the fluid transport properties in OPC and fly ash blended cement concrete This paper presents an experimental study of the influence of two initial curing conditions, wet (fog room) and dry (65% RH and 20 _C), on the transport properties of fluid in normal concrete (100% OPC) and blended cement concrete (OPC/FA).
After 28 days initial curing, concrete samples were dried at different relative humidities at 20 _C for about 12 weeks when the equilibrium moisture condition was achieved. Transport properties that include oxygen permeability, water permeability and oxygen diffusion were measured at the equilibrium condition of the samples, and total porosity and degree of saturation were also determined.
The initial curing condition has significant effects on the transport properties; in particular the most prominent effects were observed on fly ash blended cement concrete, which performed extremely well when
Initially cured in wet conditions Based on the results and the discussion of oxygen permeability, water permeability, and oxygen diffusion of the different concrete, the following conclusions are drawn:
1. Age and exposure conditions for initial curing of concrete are very important particularly for high performance blended concrete.
2. Fly ash blended cement concrete requires prolonged wet curing conditions to produce lower porosity and higher resistance against the transport of aggressive fluid into concrete; prolonged wet curing should therefore achieve higher durability concrete.
3. Drying at different equilibrium moisture conditions plays a significant role in the development of porosity and transport properties of fluid in concrete.
4. This study, based on initial curing and then drying to different equilibrium moisture conditions simulates real life exposure conditions for concrete.
(M.N. Haquea, H. Al_Khaiata, O. Kayalib, 2006)
Long-term strength and durability parameters of lightweight concrete in hot regime: importance of initial curing The paper presents results on strength development and durability of 35 and 50MPa total lightweight concretes exposed to hot marine exposure conditions for a period of 7 years. Initial water curing of 7 days and subsequent seaside exposure was found more beneficial for the strength development of lightweight concrete. One day of initial curing and subsequent seaside exposure was not very conducive for the strength development.
A marginal degradation in both the stiffness and the modulus of rupture of the concretes over the exposure period was observed. Likewise, the water penetrability of the two mixtures, for all the three initial curing regimes, increased over a period of 7 years. This establishes that the compressive strength of concrete is not synonymous with its durability. Overall, 3-7 days of initial water curing seems most desirable to enhance the durability of concrete exposed to hot salty marine exposure conditions.
1. Initial water curing of 7 days and subsequent seaside exposure is more beneficial for the compressive strength development than it is for the modulus of rupture and the modulus of elasticity of the concretes tested. One day of initial curing and subsequent seaside exposure was not very conducive to the strength and durability characteristics of the concretes. Whereas the compressive strength of the adequately cured concretes increased, there was a marginal degradation in both the stiffness and the modulus of rupture of the concretes over an exposure period of 7 years.
2. The water penetrability, which is indicative of permeability of the concrete, for all the three initial curing regimes, increased over an exposure period of 7 years. This certainly establishes that compressive strength is not synonymous with durability.
3. The depth of water penetration and the depth of carbonation results over a period of 7 years suggest that 35MPa total lightweight concrete in hot marine exposure conditions may not perform adequately.
4. Overall, 3-7 days initial water curing of the 35 and 50MPa LWC's tested is most desirable to enhance their durability in the hot salty marine exposure conditions prevailing in Kuwait like places.
(Metin Husem , Serhat Gozutok, 2004)
The effects of low temperature curing on the compressive strength of ordinary and high performance concreteIn this study, the changing of the compressive strength of ordinary and high performance concrete after having cured at low temperature was investigated experimentally. To accomplish this purpose, concrete specimens of 150 mm diameter and 300 mm high were prepared. After their production the specimens were cured at different conditions for 7 days. Some of them were at 23±2 áµ’C (standard curing); the others were at 10, 5, 0 and - 5 áµ’C, respectively. In the 7th day, some of the specimens cured at different temperature (10, 5, 0 and -5 áµ’C) were broken under uniaxial compression. On the other hand, some of the specimens were applied to standard curing during 28 days. In the end of 28 days, compressive strength of all specimens was obtained. According to the results, compressive strength of the specimens at 10 áµ’C and less than 10 áµ’C during 7 days was lower than that of the specimens at standard curing. In the end of 28th day loss of compressive strength of concrete specimens cured at different temperatures was more than that of specimens cured at standard cure.
The results, can be obtained from this study, are given below.
In ordinary and high performance concrete when cure temperature increases, strength decrease ratio has lessened.
In high performance concrete strength decrease ratio is less than that ordinary concrete because high performance concrete has less pores than ordinary concrete.
Fresh concrete uses water in its capillary pores, completes hydration and increases strength. In high performance concrete because of mineral salts in the water freezing temperature of water is low. So, low temperatures affects high performance concrete less than ordinary concrete. In technique literature, it is said that concrete should not be set under +5 _C without precautions. But according to the test results, even in +10 _C, compressive strength of ordinary and high performance concrete lessen 30% and 19% than specimens have standard cure, respectively. Shortly performed experimental studies show that to reach expected compressive strength of both high performance and ordinary concrete, fresh concrete should be set in its mould and should be compacted, more important than this it should be cured with suitable method and period. It is said in technique literature that concrete should not be set under +5 _C moreover, compressive strength of concrete even 10 _C has un-negligible loses compared with standard cure. That's why if concrete is set in that and lower temperature required precautions should be taken to reach expected strength.
5) Experimental study of early-age behavior of high performance concrete deck slabs under different curing methods
(X. Sharon Huo, and Ling Ung Wong, 2005)
Curing techniques and curing duration have crucial effects to the strength and durability of concrete. Proper curing can protect against moisture loss from fresh concrete. The objective of this experimental study is to examine the early-age behavior of high-performance concrete (HPC) under various curing methods. Laboratory experiments were conducted to investigate the early-age shrinkage development, temperature change, and evaporation rate when different curing methods were used. Four curing techniques and two curing durations were applied to concrete deck slab and cylindrical specimens.
The measured experiment data were also compared with several shrinkage prediction models. The results show that proper moisture-curing methods can effectively reduce concrete temperature due to hydration heat and limit the development of early-age shrinkage strains. The concrete of a longer curing duration would yield lower shrinkage deformation and lower evaporation rate
Conclusions and recommendations
Based on the literature information and the experimental results, the following conclusions are obtained:
1. Moisture-curing methods could effectively reduce concrete temperature due to hydration heat during early-age. Cotton mats and burlaps retained more moisture on the concrete surface and reduced the
temperature in concrete during the very early-age. Re-wetting the burlaps and cotton mats is necessary to cool the concrete.
2. Choosing proper curing materials is very important to have lower shrinkage strains in HPC. The wet burlaps, cotton mat, the polyethylene blankets are good curing methods to limit the shrinkage strain development. The curing duration of concrete also affects the shrinkage development. The concrete of a longer curing duration would yield lower shrinkage deformation and lower evaporation rate.
3. Cost analysis shows that burlap and polyethylene blankets are economic and practical curing materials because of being able to provide good curing and their reusability.
4. The comparison study shows that the CEB-FIP model can predict the studied HPC shrinkage very well. In addition, the ACI 209R-92 model can provide good prediction for very early-age shrinkage and GL 2000 model can provide reasonable prediction for the later-age shrinkage. This study is based on HPC with local materials and specified mixture proportions.
(Maher A. Bader, 2003)
This paper reports the results of a study conducted to evaluate the performance of concrete in a coastal environment exposed to below ground conditions in a coastal area. The concrete specimens were prepared with varying water/cement ratio, cement content, and polymer/epoxy additions and varying consolidation efforts prior to exposure to below ground conditions in a coastal area for more than four years.
The performance of the concrete specimens exposed to the highly concentrated chloride and sulfate environment was evaluated by measuring the chloride diffusion and reduction in compressive strength due to sulfate attack. Results indicated that the mix design parameters, such as water-cement ratio and cement content, significantly affected both the chloride diffusion and the sulfate resistance of concrete.
Similarly, the level of consolidation and the period of curing influenced the performance of concrete in the aggressive environment. Further, the performance of latex and epoxy modified concrete was better than that of polymer concrete.
( Kefeng Tan, Odd E. Gjorv, 1995)
performance of concrete under different curing conditions the effect of curing conditions on strength and permeability of concrete was studied. Test results showed that after 3 and 7 days moist curing only the concretes with w/c ratios equal to or less than 0.4 were accepted, while after 28 days of moist curing however, even the concrete with w/c of 0.6 could be accepted. Silica fume has a significant effect on the resistance to water penetration.
For the concretes both with and without silica fume and with w/c + s of 0.5, the 28 day compressive strengths of 3 and 7 days moist curing were higher than those of 28 days moist curing, and the silica fume concrete seemed to be less sensitive to early drying. The curing temperatures did not affect the water penetration of concrete, but affected the chloride penetration and compressive strength of concrete significantly.
State of the Art
In hot dry climates, such as exist in the Arabian Gulf region, plastic shrinkage cracking can develop when the rate of evaporation exceeds the rate at which bleed water rises to the concrete surface. Furthermore, with or without plastic shrinkage cracking, surface drying can still lead to a reduction in abrasion resistance and an increase in porosity and permeability at the critical near surface, or 'cure affected zone'.
When placing concrete in a dry, hot weather climate, precautions are needed to prevent rapid, early drying of the concrete surface. Given that drying begins when the evaporation rate exceeds the bleeding rate, risk of drying depends on the environment and on the bleeding rate of the concrete.
The most severe evaporation rates in the Gulf region occur in August. Furthermore, the most hostile 6-h period of the day is roughly between 13.00 and 19.00 h, whereas the most benign 6-h period is between 02.00 and 08.00h
In hot weather conditions, it is advisable to place concrete in the later hours of the day when the ambient temperature decreases, in order to make the setting and hardening of the concrete coincide with this decrease.
In cold weather conditions, it is advisable to place concrete in the early hours of the day when the ambient temperature rises, in order to make the setting and hardening of concrete coincide with this increase.
Finally, on the subject of compressive strength, the main conclusion, from an industrial point of view, is that it is advisable to place concrete in hot weather in the afternoon and in cold weather in the morning. This statement has been proved in realistic conditions at a ready-mix concrete plant.
The five curing methods are considered to encompass the conventional methods commonly encountered on site:
Using air drying and is designated as the no curing.
Sprinkling water over the concrete twice a day for seven days.
Sprinkling water over the concrete twice a day for seven days using a plastic cover, the Plastic film used for curing concrete shall be tough, pliable, moisture proof, and sufficiently durable to retain its moisture proof properties while it is in place on the surface of the concrete. The plastic film shall be white pigmented material. The film shall be not less than 0.85 ml (21 µm) thick shall have not less than 70% daylight reflectance relative to magnesium oxide when tested in accordance with ASTM E 1347, and shall be opaque. If the thickness of plastic film is less than 3.4 ml (85 µm), it shall not be used more than once for curing concrete.
Sprinkling water over the concrete twice a day for seven days using a burlap cover, Burlap shall be clean, evenly woven, free of encrusted concrete or other contaminating materials, and shall be reasonably free from cuts, tears, broken or missing yarns, and thin, open, or weak places.
Using SINAK S-102 curing which is designed as a replacement for all forms of traditional wet cure methods such as blankets, ponding, plastic sheeting, etc. and is as convenient to apply as the less effective membrane forming curing compounds conforming to ASTM C-309. SINAK S-102 works by penetrating concrete and reacting with the soluble calcium compounds within to form additional calcium silicate, a major factor in sound, healthy concrete. These reactions slow the escaping water, aiding the curing process. This significantly reduces hairline cracking and micro-cracking. The appearance and surface of the treated concrete is unaffected, and there is no surface coating or film that can be worn or weathered away. Concrete cured with SINAK S-102â„¢ is protected against chloride penetration on traffic-bearing and nontraffic surfaces, surface scaling, and freeze-thaw damage. The treated surface is more resistant to abrasion and dusting. Treated surfaces are compatible with bond-breakers, paints, flooring materials, staining processes, resinous coatings, patching, crack and joint materials.
Cold weather curing of concrete is best done with thermal blankets. Available with reinforced polyethylene, curing blankets are filled with layers of polypropylene foam for un-paralleled "R" value. Closed-cell polypropylene foam won't absorb water. Since the core stays dry, the blanket maintains its thermal conductivity so concrete can achieve the greatest possible strength. A dry core also makes for a lighter blanket, and that in turn speeds placement, reducing costs. Curing blankets resist tears and punctures so they can be reused again and again. Between uses the blankets are easily folded for storage and transport.
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Programme to Completion
Appendix a. Initial Project Proposal
Concrete is a composite material, consisting of coarse aggregate, sand, cement, and water. Curing is a process in which the cement and water in the concrete bond and harden throughout a chemical reaction which is called hydration. As they bond, the cement and water connect the aggregate and sand in order to form a strong foundation structure.
Curing have a major impact on the quality of the finished work, horizontal surfaces are used as detectors for problems that arise from not curing in an uncured slab, whether decorative used its likely to develop a pattern of fine cracks (called crazing) and the surface will have lower strength. Curing can guarantee that concrete will present a good service over the life of the structure and even when a good quality concrete is used it can be ruined by not having the proper curing practices. (Neville 1996)
Curing can have a marked effect on the hydration of cements. And this will be reflected on the performance of concrete. The particular performance requirements to resist different aggressive situations need to be considered carefully in the light of the potential benefits of curing. It is clear from the available evidence that compressive strength development in structures is one of the properties least sensitive to curing (Bob Cather 1994).The curing process can play an important role in evaluating of the ultimate strength, durability, volume stability, permeability to water, and resistance to freezing, thawing, as a matter of fact good curing can delay shrinkage and in the case of wet curing can delay shrinkage until after curing is complete. (Nasvik, Joe 2002)
2- Research Aims and Objectives
The primary aim of this project is to study the concrete curing process and practices and its effect in improving the concrete strength, durability and decreasing the chance of cracking .Also it aims to compare concrete curing between Britain and Kuwait in order to evaluate the possibility of introducing advanced practices.
1. Evaluate the effect of different curing materials and techniques on concrete properties
2. Identify the selection of the curing methods and materials
3. Verify effectiveness of several curing methods
5. Highlighting a variety of of the international curing specifications
6. Define the relation between various concrete test measurements and concrete properties affected by curing
7. Design a product and conduct experiments
8. Discuss the results
9. Write down conclusions and recommendation
10. Report results and research.
3- Literature review
Different resources are available to build the literature review of this topic and will be used in the next phase as listed below:
1) Nasvik, Joe (2002) "Curing concrete slabs "
2) Haque, M.N. (February 1990), "Some Concretes Need 7 Days Initial Curing".
3) Jackson, F.H. and Kellerman, W.F., "Tests of Concrete Curing Materials," Journal of the American Concrete Institute,
4) Kropp, J. and Hilsdorf, H.K. (1995), Performance Criteria for Concrete Durability
5) Senbetta E, et al., (1994). "Curing and Curing Materials," Significance of Tests and Properties of Concrete and Concrete-Making Materials, P. Klieger and J. Lamond.
6) Austin, S.A., and Robins, P.J. (1997) "Influence of early curing on the sub-surface permeability and strength of silica fume concrete," Magazine of Concrete Research,
7) Suprenant, B.A., and Malisch, W.R.,(1998). "Are your slabs dry enough for floor coverings?"
8) Swazey, M.A., "Early Concrete Volume Changes and their Control," Journal of the American Concrete Institute,
After finalizing the aims and objectives of the study the literature review became as follows:
1) W.J. McCartera, and A.M. Ben-Salehb, (2000), "Influence of practical curing methods on evaporation of water from freshly placed concrete in hot climates".
2) Nasir Shafiq, and J.G. Cabrera, (2003), "Effects of initial curing condition on the fluid transport properties in OPC and fly ash blended cement concrete".
3) M.N. Haquea, et al., (2006), "Long-term strength and durability parameters of lightweight concrete in hot regime: importance of initial curing, H.Al_Khaiata, and O. Kayalib".
4) Metin Husem, and Serhat Gozutok, (2004), "The effects of low temperature curing on the compressive strength of ordinary and high performance concrete".
5) X. Sharon Huo, and Ling Ung Wong, (2005), "Experimental study of early-age behavior of high performance concrete deck slabs under different curing methods".
6) Maher A. Bader, (2003), "Performance of concrete in a coastal environment".
7) Kefeng Tan, and Odd E. Gjorv, (1995), "Performance of Concrete under Different Curing Conditions".
4- Research Methodology
The main aim of the paper is to compare between curing in Britain and Kuwait, by presenting the application and selection of the curing methods and materials in these two systems and compare them. The Comparison will include several elements that indicate the curing process and practices in each country such as: climatic conditions, duration procedures for verification, verification of effectiveness and accounting for concrete materials and mixture proportions
Standard testing procedures are needed in order to assess the efficiency of the different curing methods and techniques. The focus will be on temperature and climate variation during the year (winter/summer wise) and during the same day (morning/night wise).
After defining the relation between various concrete test measurements and concrete properties affected by curing, and conducting various experiments results will be in form of measured strength of concrete treated under different conditions in order to verify of effectiveness of curing practices used in both countries UK and Kuwait, then evaluate the possibility of introducing advanced practices.
Taking into consideration several factors concern the variety between the two countries such as construction methods itself and types of projects constructed and the climate variation since it is a chemical reaction between cement minerals and water and climate dependent.