Efficiency of internal curing in concrete using local materials with different curing conditions

Abstract

In concrete production, curing is of utmost importance to ensure desirable properties like strength, durability, shrinkage etc. Loss of water through evaporation reduces the hydration rate and eventually results in limited strength and higher permeability. Generally, curing is done either by supplying additional water from external source or by preventing moisture loss through plastic sheeting. However, in a developing country like Bangladesh, curing is considered as an additional step and often overlooked. There is also scarcity of water in many regions of the country. In addition, many local contractors do not have the knowledge and skill to ensure proper curing. As a result, durability of general concreting work has become a concern in the country. Under such scenario, internal curing (IC) could be adopted to improve the overall quality of concrete. IC refers to the process by which the hydration of cement occurs because of the availability of additional internal water that is not part of the mixing water as per mix design. But generally production of this type of concrete is difficult and costly. Lightweight aggregates absorb considerable water during mixing which apparently can transfer to the paste during hydration. Utilization of over burnt clay as Light Weight Aggregate (LWA) to produce internal curing concrete can be considered as an effective solution in Bangladesh since burnt clay is cheap and locally available. Unit weight of burnt clay is nearest 1000 kg/m3 and water absorption capacity of more than 10%. So it can be recommended as LWA with high absorption capacity. However, people striving for high strengths are not eager to use lightweight aggregates. A promising solution might be a partial replacement of coarse aggregate with lightweight aggregates. Super absorbent polymers (SAP) can also be used as a means of internal curing since they absorb large amount of water when get saturated. If SAP is mixed with ingredients and segregation can be avoided then it can be a solution to ensure internal curing. IC can assure proper hydration and eventually results in concrete with desirable mechanical properties. At the same time, this process of IC can save extra water required for ponding (natural curing process). Therefore, internal curing also has significant environmental impact. However, very limited study is available on internal curing of concrete in context of Bangladesh. Therefore, in the present study, a comprehensive experimental program has been designed and implemented to investigate the effect of locally available material in internal curing of concrete mixes. A total of eighteen mixes were designed among which nine mixes had three different percent replacement of coarse aggregate with locally available lightweight aggregate (10%, 20% and 30% replacement of Stone Chips with burnt clay chips) and three different water cement ratios (0.4, 0.45 and 0.5). Six mixes were done with no replacements for comparison. Three concrete mixes were also prepared using SAP (from readily available baby diapers) using admixture having three different water cement ratios (0.4, 0.45 and 0.5). Admixture was used to improve workability of concrete using SAP. For IC, different curing conditions were simulated. In one method, samples were air cured by placing them in a dry place inside the laboratory with and without polythene cover. Samples (both covered and uncovered) were also naturally cured. Such condition was simulated by placing them in an open dry space outside the laboratory. These curing conditions were selected to replicate the ambient conditions. In order to evaluate internal curing capacity of LWA and SAP, desorption tests were conducted and found that both of them desorbs huge amount of water at particular temperature and relative humidity condition. So, both LWA and SAP are suitable candidates for internal curing. Compressive strength tests of different concrete specimens under various curing conditions were done on 3, 7 and 28 day. Modulus of elasticity was also determined at the age of 28 day. Durability tests (water permeability and chloride permeability) were conducted on 28 day cured specimens. A comprehensive comparative study was then carried out to evaluate the effect of different mix proportions and curing conditions on internal curing capability of proposed utilization of burnt clay and SAP application. It is found that in all cases of proposed methods concrete mixes experience internal curing. Comparison of test results reveals that mixes covered with polythene sheets and having 20% replacement with burnt clay produced highest compressive strength and lowest permeability (both water and chloride permeability) as compared to mixes with no replacement. Though, SAP desorbs more amount of water than LWA and can be considered as better applicant for internal curing, but better compressive strength and better durability results are obtained from concrete having burnt clay with 20% replacement. Therefore burnt clay can be selected as an effective internal curing material in Bangladesh.