Residual properties after high temperature exposure for normal weight concrete made with cement blends that include calcined clay and limestone

Abstract

Tests were conducted before and after high temperature exposure on normal weight concrete cylinders with mix designs that were identical except for their cementitious materials: 100 % ordinary Portland cement (OPC-100); 70 % OPC with 30 % metakaolin (i.e. calcined clay) (MK-30); 70 % OPC with 20 % metakaolin and 10 % limestone (LC2-30); and 70 % OPC with 18 % metakaolin, 9 % limestone, and 3 % gypsum (LC3-30). All concretes had comparable workability and compressive strengths at 1–28 days from casting. The inclusion of metakaolin reduced the ambient water permeability (up to 60 % less for MK-30), reflecting the observed change in microstructure from C-S-H gel in the OPC-100 to a denser C-A-S-H gel in mixes with metakaolin. Specimens were heated slowly (at a rate of 1–5 °C/min) to the first onset of spalling (at peak exposure temperatures of 307–350 °C) and then cooled to measure residual strength and water permeability. LC3-30 specimens exhibited the most spalling, and those that survived had the largest drop in residual strength (retaining ∼55 % of 28-day ambient strength, below current code-based values at these concrete temperatures). The other concretes suffered strength losses of 13–38 %, more consistent with code-based values. Residual permeability increased by an order of magnitude for all mixes after heating.