Investigating mix ratios by volume batching for performance specification of in-situ concrete works

Abstract

Volume batching of concrete materials is commonly adopted for in - situ concrete works in most construction sites in Nigeria, most especially where small to medium scale concrete works are involved. Prohibitive cost of procurement and maintenance of standard batching plant may be responsible for the adoption of volume – based batching method. The main objective of this study is to validate the characteristic strength of volume batched nominal mix proportions of 1:2:4, 1:1.5:3 and 1:1:2 for purpose of performance specification for in – situ concrete. The validation process adopted involves direct comparison with weight – based batching of similar nominal mix ratios, considering some important properties as density, slump value and compressive strength. The results indicate strength reduction of 16.7%, 30% and 36% for direct volume batching, while the weight batching recorded strength increase of 47%, 25% and 4% over the expected characteristic strength for the strength classes of C12/15, C16/20 and C20/25 respectively. Further analysis indicated that the conventional volume batch (with coarse/fine aggregate ratio of 2) performed better than the non – conventional volume batch method (with coarse/fine aggregate ratio of 1) despite maintaining the same aggregate/binder ratio. It was concluded that the batch method should be specified alongside the target strength for performance specification of nominal mixes for in – situ concrete work.