Engineering properties of binary cement concretes and their effect on punching shear of flat slabs

• Main Author: Samad, Shahab
• Other Authors: Kew, Hessian ; Georgopoulos, Costas ; Limbachiya, Mukesh C.
• Published: Kingston University 2013
• Subjects: 624; Civil engineering
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.587390

Concrete is the most important building material in the world due to the fact that it is versatile and gives architectural freedom. For sustainable construction solutions concrete is the material of choice if the embodied CO2 content is considered. In concrete, cement is the main constituent and due to the limit on the availability of natural minerals used, the energy released and the CO2 emissions produced during its manufacture, it can be partially replaced using industrial by-products e.g. Pulverised Fuel Ash (PFA), Ground Granulated Blast furnace Slag (GGBS) and silica fume. The effects of the partial replacement of cement with these industrial by products on fresh and hardened properties of concrete cured under summer and winter environments are established and compared with the Portland cement (PC) concrete. Early age strength of concrete containing GGBS and PFA is less than the PC concrete, which would prevent its use in the in the post tensioned concrete and in fast-track construction, where early removal of formwork, or early application of load to the structure are the main requirements. For this reason, and due to the demand of high strength concrete in construction, for its improved durability properties, concrete containing GGBS and PFA was produced by keeping the water/cement ratio low and a superplasticiser was used to achieve the required workability. At low water/cement ratio, concrete containing GGBS up to 50 % and PFA Lip to 30 % can achieve the required early age strength for the removal of formwork if cured properly. At the age of 28 days, the flexural strength and modulus of elasticity of concrete in which PC is partially replaced with GGBS and PFA are increased in comparison to where this is not carried out. For practical applications of the sustainable concrete mixes in structural concrete production and due to the limited data availability on punching shear strength, concrete containing GGBS and PFA in flat slab specimens were tested for this property. In flat slabs without beams, the design criteria is often the resistance against punching shear failure at the column/slab connection. Punching failure is the separation of the portion of the slab surrounding the column from the rest and is a brittle failure. Experimental punching shear results are compared to the estimates of BS 8110, BS EN1992-1-1 and ACI 318 and it was found that the estimates of ACI318 and BS 8110, ignoring the partial safety factors, are close to the experimental results and the estimates of BS EN 1992-1-1 are over conservative. Based on the test results of punching shear resistance of flat slabs, and the materials used, it is concluded that concrete containing GGBS up to 50 % and PFA up to 30 % can be used in flat slabs without any special design requirements and the design rules given in different codes of practice can be used without modifications. It is evaluated that for a concrete, designed for characteristic strength of 30 MPa, a reduction of 152 kg/m3 of CO2 and 0.65 GJ/m30f energy consumption can be achieved by replacing PC with 50 % GGBS and a reduction of 62.5 kg/m3 of C02 and 0.27 GJ/m3 of energy can be achieved by using 30 % PFA concrete.