PSI - Issue 11

Alisson Rodrigues de Oliveira Dias et al. / Procedia Structural Integrity 11 (2018) 84–90 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

85

2

Concrete is a building material widely used as a primary structural material in construction because of its numerous advantages over other materials such as high compressive strength, durability, ease of manufacturing and non combustible properties (Kim et al., 2009; Kodur, 2014). When subjected to high temperatures, as in the case of fires, the concrete undergoes significant physical-chemical changes (Poon et al., 2001, Georgali, Tsakiridis, 2005, Arioz, 2007, Yang et al., 2009, Kodur, 2014).This exposure, among other problems, can cause significant deterioration, such as loss of strength, reduction of modulus of elasticity and degradation of durability in the concrete (Haddad, Shannis, 2004, Xiao, könig, 2004, husem, 2006, yiching et al., 2011), also cracking, spalling of concrete, destruction of the bond between the cement paste and the aggregates and a gradual deterioration of the hardened cement paste (Georgali, Tsakiridis, 2005). The concrete density decreases with the increase of temperature (Pliya, Beaucor, Nounowé, 2011, Kodur, 2014), this is due to the loss of moisture (Kodur, 2014). This deterioration is mainly caused by differences in the thermal expansion coefficients of the constituent materials, such as cement paste, aggregates, as well as it is related to the changes in concrete constituents at very high temperatures (Park et al., 2015). The fire response of structural elements depends on the thermal and mechanical properties of the concrete. These properties vary significantly with temperature and also depend on the composition and characteristics of the materials, as well as the rate of heating and other environmental conditions (Kodur, Raut, 2010). Non-destructive evaluation based on ultrasonic waves is a reliable method to evaluate the degradation of cement based materials (Yim et al., 2012), besides this tests have proved to be very helpful in the inspection of old structures (Bogas, Gomes, Gomes, 2013). Development of the nondestructive technique for assessment of fire-damaged concrete strength is of great importance (Lin et al, 2011). The test performed to determine the ultrasonic pulse velocity (UPV) consists in measuring the transit time of an ultrasonic pulse through the material. In concrete is considered one of the non-destructive procedures with more potential to evaluate the quality and the characteristics on site. According to the theory of the sound propagation in solids, the sound transmission velocity is a function of the density and the elastic modulus of the material, and it is independent of the excitation frequency that causes the perturbation (Carcaño, Moreno, 2008). The transmission of pulse waves through a concrete mass is highly influenced by the microcracking of the concrete. Thus, decreasing the pulse rate with increasing temperature is a sensitive measure of the progress of cracking in the material (Savva, Manita, Sideris, 2005). In this sense, the present work aims analyzing the behavior of the concrete submitted to high temperatures through the UPV.

2. Experimental details

The guiding idea of this research was to evaluate the changes of the UPV in concrete when subjected to high temperatures. For this purpose, was investigated the influence of the type of cooling (in water and in air, for 30 MPa concrete) and fck (20 MPa, 30 MPa, 40 MPa and 50 MPa). The concrete mixes used are shown in Table 1 with the materials and their amounts. A slump of 8 ± 2 mm, obtained according to ABNT NBR NM 67, was adopted for concrete consistency.

Table 1 - Consumption of materials per cubic meter (m³) of concrete, in kilograms.

Coarse aggregate 12,5mm

Coarse aggregate 19mm

Additive (%)

Fine aggregate

a/c

Mix

Fck (MPa)

Cement

T20 T30 T40 T50

20,0 30,0 40,0 50,0

278 332 435 542

860 794 710

277 264 266

694 703 709

0,23 0,72 0,80 1,02

0,65 0,54 0,41 0,33

997,6

-

673,4