Fire Resistance of ConcreteAlessandra Mendes1, Dr Frank Collins1, Professor Jay G Sanjayan21 Civil Engineering Department, Monash University, 2 Swinburne University of Technology

Contraction

Compressive Strength Results

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.

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).

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

OPC paste 1 year after exposure to 800ºC

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

The partial replacement with slag consumes CaOH2 reducing or even eliminating the negative effects observed for OPC pastes.

Slag Replacement

Photo: OPC paste disintegrated after 800ºC. All OPC/slag pastes presented no visible cracks.

Scanning Electron Microscope (SEM)

OPC concrete after 800ºC Calcium - Ca

Aluminum - Al Silicon - Si

Magnesium - Mg Iron - Fe

Cement Chemistry at Elevated Temperatures, as in a Fire Event