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Fire Resistance of Concrete Alessandra Mendes 1 , Dr Frank Collins 1 , Professor Jay G Sanjayan 2

Fire Resistance of Concrete Alessandra Mendes 1 , Dr Frank Collins 1 , Professor Jay G Sanjayan 2 1 Civil Engineering Department, Monash University, 2 Swinburne University of Technology. Cement Chemistry at Elevated Temperatures, as in a Fire Event.

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Fire Resistance of Concrete Alessandra Mendes 1 , Dr Frank Collins 1 , Professor Jay G Sanjayan 2

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  1. Fire Resistance of Concrete Alessandra Mendes1, Dr Frank Collins1, Professor Jay G Sanjayan2 1 Civil Engineering Department, Monash University, 2 Swinburne University of Technology Cement Chemistry at Elevated Temperatures, as in a Fire Event Concrete is made by combining cement , water, fine aggregates (sand) and coarse aggregates. The most common cement used is ordinary Portland cement (OPC). When OPC is mixed with water the general reaction occurs: OPC + H2O → C-S-H + CaOH2 CaOH2 → CaO + H2O This reaction occurs above 400ºC and leads to the contraction and cracking of the OPC paste CaO + H2O → CaOH2 After cooling and in the presence of air moisture, this reaction takes place causing the OPC paste to expand and complete disintegrate Expansion Contraction Slag is a by-product of the steel and iron industry and has cementitious properties. When OPC is partially replaced with slag the following reaction takes place: Slag + CaOH2 → C-S-H Slag Replacement The partial replacement with slag consumes CaOH2 reducing or even eliminating the negative effects observed for OPC pastes. OPC paste 1 year after exposure to 800ºC Photo: OPC paste disintegrated after 800ºC. All OPC/slag pastes presented no visible cracks. Compressive Strength Results Scanning Electron Microscope (SEM) OPC concrete after 800ºC Calcium - Ca Magnesium - Mg Iron - Fe Scanning electron microscope (SEM) enables characterization of the cement paste and concrete microstructure. The first photo on the left relates to a polished specimen of OPC concrete exposed to 800ºC. Magnification of 200x enables visualization of microcraks and dehydrated materials (light grey) formed as a result of the elevated temperatures. Energy–dispersive X-ray (EDX) system colour mapping and phase analysis provide qualitative and semi-quantitative information regarding the chemical elements and phases present in the concrete before/after exposure to elevated temperatures. Examples of the different chemical elements found in OPC concrete after 800ºC are shown (Ca, Al, Si, Mg, Fe). Compressive strength results for OPC and OPC/slag pastes after exposure to elevated temperatures, as in a fire event. OPC pastes presented total strength loss above 400ºC (red dotted-curve). OPC/slag blends (35%, 50% and 65% replacement by weight) with slag presented compressive strength in the range of 15 MPa at temperatures as high as 800ºC. After 1 year, OPC paste reduced to powder (photo top right) while OPC/slag blends presented no visual or strength changes. Silicon - Si Aluminum - Al

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