Enhancement in the quality of near surface concrete using some formwork liners

Abstract

Abstract The durability of concrete cast near the conventional formwork (steel or wood) is reduced because of higher water-cement (w/c) ratio as compared with the concrete present in the core of the member. Various controlled permeability formwork (CPF) liners absorb or drain out excess water near formwork, can reduce the increase in w/c ratio of concrete near the formwork. This paper presents a quantitative study showing improvement in the properties of near surface concrete (NSC) by the use of a commercially available CPF liner, and comparing the performance of more easily available and cheaper alternatives to CPF liner used. Two types of concrete mixes with identical w/c ratio but different water contents were used. Three specimens were made for each of the two types of concrete mix and one surface for each of the specimen was cast using one of the CPF liners among a commercially manufactured CPF, particle board, and gunny bag. Four tests: Rapid chloride permeability test, AgNO3 colorimetry test, initial surface absorption test and pull off test were performed to study durability properties of NSC. Study of physical appearance in terms of blowholes and grey area on the concrete surface using image processing is also presented in the paper. Though commercially manufactured CPF produces better surface than the particle board and gunny bag, the improvement in NSC properties by use of gunny bag is also noteworthy.