Internal Curing for Concrete Pavment

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 r ate and eventually r esults in limite d strength and h igher p ermeability. Generally, curing is done either by supplying additional water from external source or by peventing m oisture l oss t hrough pl astic s heeting. H owever, i n a developing country l ike Bangladesh, curing i s c onsidered a s a n a dditional s tep a nd of ten overlooked. There is a lso s carcity of water i n m any regions of t he 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 W eight Aggregate (LWA) t o produce internal cu ring concrete c an be considered a s a n effective s olution i n B angladesh since bur nt c lay i s cheap and locally available. U nit weight of burnt c lay is nearest 1000 k g/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 w ith d esirable me chanical p roperties. A t th e s ame time , th is process of IC can s ave extra water required for pondi ng ( natural c uring pr ocess). Therefore, internal cu ring al so has significant environmental impact. However, very limited s tudy i s available o n internal cu ring o f concrete in co ntext o f Bangladesh. Therefore, i n t he p resent study, a co mprehensive ex perimental program has b een designed and implemented to i nvestigate t he effect o f l ocally available material in internal curing of co ncrete mixes. A total o f eighteen mixes were designed among which ni ne mixes ha d three d ifferent p ercent replacement o f coarse aggregate w ith locally available li ghtweight aggregate ( 10%, 2 0% a nd 30% r eplacement of S tone Chips with burnt c lay chips) and t hree di fferent water c ement ratios ( 0.4, 0.45 and 0.5). Six mixes w ere d one with no r eplacements f or comparison. Three c oncrete mixes were also prepared using SAP (from readily available baby di apers) us ing admixture having three different water cement ratios ( 0.4, 0.45 and 0.5). Admixture was us ed t o improve w orkability of c oncrete u sing SAP. For IC, different c uring conditions were simulated. In one method, samples were air cured by placing them in a dry p lace inside t he laboratory w ith and without polythene c over. Samples ( both covered and uncovered) were also naturally cured. Such condition was simulated by placing t hem i n a nope n dry space out side t he laboratory. These curing c onditions 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 a t pa rticular temperature and r elative humidity condition. So, both L WA a nd S AP a re suitable c andidates f or internal c uring. Compressive strength te sts of d ifferent co ncrete specimens under v arious curing conditions were done on 3, 7 a nd 28 d ay. 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 t o e valuate t he effect of di fferent mixpr oportions and curing conditions on internal curing capability of proposed utilization of burnt clay and SAP application. It i s found t hat i n all cases of proposed m ethods c oncrete m ixes experience internal curing. Comparison of test results reveals that mixes covered with polythene sheets a nd ha ving 20% replacement with bur nt c lay produced hi ghest compressive strength and lowest permeability (both water and chloride permeability) as compared to mixes with no replacement.