Experimental laboratory validation of reproducing road viaducts concreting processes

Abstract

In the latest period, hundreds of concrete viaducts were built in Poland within a short time range. Their characteristic feature was the use of high class concrete containing the CEM I 52.5 cement. The concreting works often took place during winter time. Despite successful laboratory tests, the concrete prepared on the concreting site often did not achieve the assumed freeze-thaw resistance. The lack of freeze-thaw resistance in concrete had an unusual course. A higher than normative decrease of strength with simultaneous high tightness was observed. There was no surface spalling, typical for the low freeze resistance concrete, no edge curvature, etc. In addition, on the sample surface, a characteristic mesh of white leakage of unknown chemical compounds around big aggregate grains was identified. Those compounds were thoroughly investigated with the use the SEM analysis. It was found that the direct reason of lowering the concrete strength after 150 cycles of freezing and defrosting was the occurrence of numerous micro-cracks formed during the hydration of high amount of clinker cement in concrete block. Moreover, these phenomena were enhanced by the occurrence of white, needle-shaped, hard-soluble salts localised in the boundary between aggregate and cement paste. Based on the EDS analysis, these salts were identified as nickel and calcium phosphates, ettringite ana thaumasite formations and others, which delayed cement hydration [1,2,7,8,9]. The variety of occurring white salts forced the commencement of model laboratory tests on a specially built station for reproducing the processes taking place during concreting and developing recommendations for the construction workmanship.