Effect of mixture proportions on the drying shrinkage and permeation properties of high strength concrete containing class F fly ash

Sustainability of concrete can be improved by using large volume of fly ash as a replacement of cement and by ensuring improved durability of concrete. Durability of concrete containing large volume of class F fly ash is dependent on the design of mixture proportions. This paper presents an experimental study on the effect of mixture proportions on the drying shrinkage and permeation properties of high strength concrete containing large volume local class F fly ash. Concrete mixtures were designed with and without adjustments in the water to binder ratio (w/b) and the total binder content to take into account the incorporation of fly ash up to 40% of total binder. Concretes, in which the mixture proportions were adjusted for fly ash inclusion achieved equivalent strength of the control concrete and showed enhanced properties of drying shrinkage, sorptivity, water permeability and chloride penetration as compared to the control concrete. The improvement of durability properties was less significant when no adjustments were made to the w/b ratio and total binder content. The results show the necessity of the adjustments in mixture proportions of concrete to achieve improved durability properties when using class F fly ash as a cement replacement

Strain localization and its impact on the ductility of reinforced concrete slabs containing welded wire reinforcement

Welded wire fabric (WWF) is commonly used in reinforced concrete (r.c.) slabs. WWF is classified in Australia as Class L or low ductility reinforcement and, as such, the characteristic strain at peak stress (termed the uniform elongation) is not less than 0.015 and the ratio of tensile strength to yield stress (0.2% proof stress) is not less than 1.03. A r.c. slab containing low ductility steel usually fails in bending by fracture of the tensile reinforcement at the critical section, well before the concrete in the compression zone becomes overstressed, and the conventional understanding of ductile under-reinforced flexural failure is not valid. The failure is brittle and results in complete collapse of the span, often with little or no warning. This paper explores the collapse load behaviour of slabs containing WWF, highlighting the great significance of strain localization in lightly reinforced slabs and its adverse impact on ductility. The results of tests on several simply-supported and continuous one-way slabs reinforced with WWF are used to illustrate the discussion