DOI: 10.14457/tu.the.2022.134
title: Development of low curing sensitivity and high crack resisting concrete

description: This research aims to study the mechanisms and techniques for developing low curing sensitivity concrete with high crack resisting. This type of concrete can be applied in structures with large exposed surface areas, difficult to cure, such as pavement, slabs, and industrial floors. The compressive strength of concrete at 28 days is designed in the range of 32-45 MPa (cube) and curing sensitivity index at 28 days lower than 5% by strength indicator compared with conventional ready-mixed concrete supplied by a ready-mixed concrete company. The effects of multi-binder systems of cement (OPC), calcined clay (CC), limestone powder (LP), and fly ash (FA) on the curing sensitivity, shrinkage under free and restrained conditions were considered. Binary systems of limestone powder (10%, 20%), calcined clay (10%, 20%, 30%) and fly ash (30%) as well as ternary systems of fly ash with limestone powder (OPC:FA:LP), calcined clay with limestone powder (OPC:CC:LP) or calcined clay with fly ash (OPC:CC:FA), and a quaternary system of calcined clay, limestone powder and fly ash (OPC:CC:LP:FA) were tested in this study. The ratios of ternary systems were 70OPC:20FA:10LP, 70OPC:20CC:10LP, and 70OPC:20CC:10FA. For quaternary binder system, the ratio was 70OPC:13CC: 7LP:10FA. Three series of mortar with different water to binder ratios by weight (w/b) of 0.35 and 0.55 and different paste ratios of 1.2 and 1.4 were produced for testing compressive strength. The specimens of each binder system were cured under two conditions: water curing (water-cured) and no curing (air-cured). The curing sensitivity index was calculated by considering compressive strength as an indicator. The dog-bone specimen of pastes was used for evaluating cracking potential under restrained shrinkage condition. It was found that for the mixes with the same water to binder ratio and paste content, the use of fly ash increased curing sensitivity while limestone powder and calcined clay reduced curing sensitivity. Mixes with a higher water to binder ratio (w/b= 0.55) showed higher curing sensitivity compared to mixes with a lower water to binder ratio (w/b= 0.35). Reducing paste content resulted in reduced curing sensitivity of the mortars. Fly ash and limestone powder reduced shrinkage of the cement pastes. FA mixture (30FA) had the longest cracking age and the lowest propagated crack width, followed by LP mixtures (10LP and 20LP). CC mixes did not help to reduce neither shrinkages nor crack width. Combining calcined clay with fly ash and/or limestone powder helped minimizing the disadvantages compared to the binary system of calcined clay. The effects of bottom ash (BA) as fine aggregate on compressive strength, curing sensitivity, free shrinkage, and restrained shrinkage of mortar were studied. Four mixtures with controlled mortar flow including a control mix with cement only, a mixture with 10% LP replacing cement by weight of binder, a mixture with 10% BA replacing river sand by volume, and a mixture with 10% LP + 10% BA, were tested. The results show that using limestone powder and bottom ash reduces compressive strength, but there is a useful performance to decrease the curing sensitivity index. The autogenous and total shrinkages of LP mixes are lower than those of the mixes without limestone powder. The use of bottom ash helps to reduce autogenous shrinkage of the mortars. For restrained shrinkage, LP demonstrates a good performance of prolonging cracking age and reducing propagated crack width. The BA mixes have shorter cracking age than the mixes without bottom ash, but show lower propagated crack width. The effects of maximum aggregate size on compressive strength and curing sensitivity were observed on concrete specimens. Two types of coarse aggregate with the maximum sizes of 19 mm and 25 mm (Gmax19 and Gmax25) were investigated. The binder systems of 25% FA as a control binder system (similar to the mix proportion, which provided by the ready-mixed concrete company). The w/b of 0.5 was controlled for all tested mixtures. The results indicate that the concrete with Gmax25 has lower curing sensitivity and higher compressive strength than the Gmax19 concrete. Based on previous chapters' results, two mix proportions are designed to create concrete with low curing sensitivity and high crack resistance, according to the targets of the study mentioned above. A mixture with 15% LP to replace cement and a mix combined with 15% LP and 10% BA to replace sand were calculated. The Gmax25 coarse aggregate was used for the proposed mixtures. The Gmax25 concrete mixtures with 15% LP and 15% LP + 10% BA have satisfactory compressive strength at 28 days in the range of 32-45 MPa and curing sensitivity index at 28 days lower than 5% by strength indicator compared with conventional ready-mixed concrete according to the objectives of this study.