1 Introduction
2 Experimental Investigation
2.1 Material
Component | Mass (%) |
---|---|
CaO | 56.74 |
SiO2
| 22.83 |
Al2O3
| 8.66 |
Fe2O3
| 2.89 |
MgO | 4.40 |
SO3
| 2.80 |
Na2O | 0.10 |
K2O | 0.73 |
TiO2
| 0.58 |
P2O5
| 0.13 |
MnO | 0.05 |
Cr2O3
| 0.017 |
SrO | 0.045 |
ZnO | 0.008 |
Cl | 0.000 |
2.2 Desorption of MCA
2.3 Concrete Mixtures
2.4 Experimental Setup
Symbol | Experimental curing conditions | Simulated curing conditions |
---|---|---|
NC | Normal curing under water | Proper external curing |
IC1 | Inside with cover | Curing under shade with covering |
IC2 | Inside without cover | Curing under shade without covering |
IC3 | Outside with cover | Exposed field condition with covering |
IC4 | Outside without cover | Exposed field condition without covering |
IC3a | 3 days under normal curing and then outside with cover | Exposed field condition under covering after 3 days of NC |
IC4a | 3 days under normal curing and then outside without cover | Exposed field condition without covering after 3 days of NC |
IC3b | 7 days under normal curing and then outside with cover | Exposed field condition under covering after 7 days of NC |
IC4b | 7 days under normal curing and then outside without cover | Exposed field condition without covering after 7 days of NC |
3 Results and Discussion, Phase 1
3.1 Compressive Strengthand Modulus of Elasticity
Percent replacement (%) | Curing conditions | |||
---|---|---|---|---|
IC1 | IC2 | IC3 | IC4 | |
10 | 31.5 | 30 | 35 | 38 |
20 | 43 | 41 | 42 | 46 |
30 | 25 | 26 | 26 | 36 |
Percent replacement | Curing conditions | |||
---|---|---|---|---|
IC1 | IC2 | IC3 | IC4 | |
10 | 18 | 7.5 | 15 | 11 |
20 | 19.5 | 18.5 | 19.5 | 17 |
30 | 20 | 10 | 12.5 | 11 |
3.2 Chloride Permeability Test
4 Results and Discussion, Phase 2
4.1 Internal Relative Humidity (RH)
4.2 Compressive Strength
4.3 Splitting Tensile Strength Test Results
5 Economics of Internal Curing
6 Conclusions
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MCA has required desorption capacity to be considered as an effective internal curing medium. It is observed that desorption capacity of MCA depends on temperature and relative humidity. Higher temperature (in the range of 30–34 °C) and lower relative humidity (in the range of 60–73%) conditions are favorable to desorption by MCA. However, it is also found that considerable amount of water can be desorbed by MCA even at higher relative humidity of 85% or more.
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It is apparent from sample internal RH that internal curing ensures additional water within concrete. At all ages, internally cured samples experienced relatively high internal RH than control samples.
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Strength and durability of internally cured samples having MCA as internal curing medium and polythene sheet covering are significantly higher than those of control samples with conventional stone chips as coarse aggregate in the absence of proper external curing. The best performance was observed when 20% stone chips was replaced by MCA, which produced comparable compressive strength, elastic modulus and tensile strength as obtained by normally cured control specimens.
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It is evident from past experience that proper external curing is not practiced in number of general construction sites in Bangladesh which may result in poor performance. Field conditions in several construction sites, particularly in outskirts of major cities, are similar to IC4 condition considered in the study. In such cases, internal curing can produce about 30% or more increase in compressive strength at 28 days. Moreover, about 30% and 10% increase in modulus of elasticity and tensile strength, respectively, can be achieved through internal curing, as found from this study.
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Durability performance of MCA concrete is a concern because MCA as coarse aggregate increases the permeability due to its porous structure. Consequently, most of the internally cured samples showed higher permeability. However, moderate chloride permeability was achieved from 20% replacement of stone chips with MCA. This means that such proportion of MCA within concrete desorbs sufficient water to ensure proper hydration which eventually produces better performing concrete without requiring any external water. Proper covering should be employed for ensuring better performance through internal curing mechanism with MCA.
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Internal curing can be recommended in adverse curing conditions, because internally cured samples showed significantly better performance than that of control samples under similar curing conditions without supply of external water.
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Twenty percent partial replacement of stone chips with MCA, 0.4 w/c ratio and utilization of proper covering (preferably polythene) are recommended as optimum combination for producing internally cured concrete.
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Internal curing technique, considered in this study, can be very effective where curing water and skilled labor are not easily available, which is very common in many parts of the world.
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Internal curing using MCA is also a simple and inexpensive method to execute. Therefore, internally cured concrete can be a considered as a promising solution for improving the overall quality of general concreting work of Bangladesh.