Influence of elevated temperatures on the mechanical properties of blended cement concretes prepared with limestone and siliceous aggregates

Abstract

The investigation performed was aimed at showing the influence of high temperatures on the mechanical properties and properties that affect the measurement by non-destructive methods (rebound hammer and pulse velocity) of concrete containing various levels (10% and 30%) of pozzolanic materials. Three types of Pozzolans, one natural pozzolan and two lignite fly ashes (one of low and the other of high lime content) were used for cement replacement. Two series of mixtures were prepared using limestone and siliceous aggregates. The W/b and the cementitius material content were maintained constant for all the mixtures. Concrete specimens were tested at 100, 300, 600 and 750 °C for 2 h without any imposed load, and under the same heating regime. At the age of 3 years, tests of compressive strength, modulus of elasticity, rebound hummer and pulse velocity were come out. Results indicate that the residual properties of concrete strongly depend on the aggregates' and the binder type. Relationships between strength of concrete as well as rebound and pulse velocity versus heating temperatures are established. The above results are evaluated to establish a direct relationship between non-destructive measurements and compressive strength of concrete exposed to fire.

Introduction

Concretes containing mineral admixtures are used extensively throughout the world for their good performance and for ecological and economic reason. The effect of high temperature on concrete containing fly ash or natural pozzolans has not been investigated in detail unlike fly ash concrete that has been under investigation since the 1960s. Researchers and investigators differ in their opinion regarding the changes in the properties of concretes, particularly in the range of 100–300 °C. Above 300 °C, there is a uniformity of opinion concerning a decrease in mechanical characteristics [1], [2], [3], [4], [5]. However, decreases in strength, reported in the literature reveal significant quantitative differences due to the variety of high temperature condition tested, and the variety of constituent materials of concrete used. It is recognized that the behavior of concrete subjected to high temperatures is a result of many factors [6], [7]; such as heating rate, peak temperatures, dehydration of C–S–H gel, phase transformations, and thermal incompatibility between aggregates and cement paste. On the other hand, quality control of concrete, by means of non-destructive methods, in structures subjected to fire or not so high temperature exposure conditions, is not particularly easy to be carried out. The correlation already exists usually refers to the hydration age of 28 days.

The scope of this work is to provide experimental data on the residual mechanical properties of concretes subject to heat, containing pozzolanic materials, and to the properties that affect measurements by non-destructive methods (rebound hammer and pulse velocity). These properties are very important for the safe design of concrete and in the repair of concrete structures.