Creep characteristics of concrete made of brick chips in Bangladeshi environment

Abstract

To study the effect of brick chips as coarse aggregate on creep behavior of concrete, a comprehensive testing program is conducted at Bangladesh University of Engineering & Technology, Dhaka, Bangladesh. Concrete cylinder specimens having compressive strength of 17.2, 24.0 and 27.5 MPa are prepared from both natural stone and crushed clay brick aggregate. Mix design ratios are evaluated in a way so that volumetric content of coarse aggregate (both brick and stone) remain approximately same for all concrete samples. Specimens are then subjected to creep testing at 7th and 28th day after casting and creep strain data are recorded up to 300 days. Results show that although strength and other environmental parameters remain same, concrete made from crushed clay brick as coarse aggregate have higher creep strain than that of concrete made from natural stone aggregate. This increase in creep strain ranges from 30% to as high as 45% for the 300 day loading history considered. Additionally, in order to select an appropriate model to predict creep in brick aggregate concrete, effectiveness of five widely used prediction models are examined. For this, predicted creep strain from ACI 209, CEB-FIP 90, B3, GL 2000 and Euro code 2 models are compared with experimental results. Using statistical analysis, it is established that prediction of creep by GL2000 model is closest to the experimental result. A modification factor is then proposed which may be incorporated so that prediction of creep strains by GL 2000 model for brick aggregate concrete becomes more realistic. Furthermore, a simple design oriented empirical model containing only two parameters has been developed to predict creep behavior of concrete made of crushed clay bricks as coarse aggregate for a stress/strength ratio of 0.35. For each concrete strength category, using the available test result a hyperbolic equation is developed from their creep-time behavior. These equations are then combined and modified according to statistical norms to finally obtain a generalized equation. Comparison of creep strain obtained from this equation with that of experimental values show that the proposed model can closely predict creep in brick aggregate made concrete.