Experimental investigation on structural concrete masonry in fire: emphasis on the thermal behavior and residual strength

Abstract

This paper aims to analyze the thermal behavior and residual mechanical properties of concrete hollow-blocks structural masonry and its component materials in fire situation using experimental investigation. Compression tests were carried out on blocks, prisms and small walls at room temperature and after being exposed for 70 minutes to the ISO 834 Standard Fire. The test at high temperatures was run using a furnace powered by natural gas and instrumented with thermocouples to measure temperatures in the specimens. The influence of the initial concrete strength on masonry behavior was evaluated considering the use of blocks with different strengths at room temperature. In addition, exposure to fire was also investigated considering masonry elements with no coverings and submitted to two different fire exposure conditions: one or both sides. The results indicate a substantial loss in the masonry load capacity at high temperatures, especially in cases of fire exposure on both sides, where the residual compressive strength resulted, on average, between 20% and 27% for the blocks and approximately 14% for prisms and small walls. Its performance with fire heating up on only one face is much higher, with an average residual masonry strength equal to 46% compared to its strength at room temperature. The obtained results are also useful for evaluating masonry regarding the integrity and thermal insulation criteria, the latter achieved with little over 60 minutes of testing.

Residual mechanical resistance of concrete blocks and laying mortars after high temperatures

“…In the absence of specific normative codes for the hollow concrete block analysed, for the purpose of comparison, Figure 10 presents a suggestion of the evolution of residual compressive strength, after heating, for mixtures of concrete with silica aggregate given by international standards 14,15 . The figure also shows the results for the residual compressive strength of the concrete blocks evaluated in this work and those found in the literature 3,6 …”

“…14,15 The figure also shows the results for the residual compressive strength of the concrete blocks evaluated in this work and those found in the literature. 3,6 Figure 10 shows a sharp drop in residual compressive strength for temperatures above 500 C in all results presented that used siliceous aggregate. Blocks with siliceous aggregate present greater resistance loss between 500 C and 700 C, the temperature range in which quartz expansion occurs due to a polymorphic transformation (573 C), contributing to the increase of internal stresses and cracking of the material.…”

“…The methodology for heating and cooling mortar samples and blocks was adapted from the standard stipulated in RILEM TC 200, 7 adapted to the geometry of the evaluated samples, according to the procedure defined in Medeiros et al 2 and Leal et al 6 Concrete blocks and mortar specimens were heated and cooled at a rate of 1°C/minute, remaining at the maximum temperature of interest for 1 h followed by a slow rate cooling inside the oven (Figure 5) and after‐cooling evaluation of the mechanical strength of the blocks.…”

“…Leal et al 6 evaluated the residual mechanical strength of blocks, prisms, small walls and laying mortar after heating. In the research, hollow concrete blocks of aggregate of normal weight and with nominal compressive strength of 4 and 10 MPa (gross area).…”

“…The aggregates and cementitious materials chosen for the block can help to reduce these negative effects, in addition, the geometry of the blocks and the porosity of the dry concrete used in the units can be taken as parameters of relative influence on the evaluated residual mechanical properties. 4 Leal et al 6 evaluated the residual mechanical strength of blocks, prisms, small walls and laying mortar after heating. In the research, hollow concrete blocks of aggregate of normal weight and with nominal compressive strength of 4 and 10 MPa (gross area).…”