Detection of Microcracking in Concrete Subjected to Elevated Temperature at Very Early Age by Acoustic Emission

Abstract

Microcracks developed considerably in concrete subjected to elevated temperature up to around 60 o C at early ages, especially in low water-to-binder ratio (0.3) concrete. Microcracking was attributed to the stresses induced by the incompatibility in deformation between mortar and aggregate. Differences of coefficients of thermal expansion (CTE) between mortar and coarse aggregate, autogenous shrinkage of mortar and size of coarse aggregate were important factors influencing deterioration. The tensile strength of concrete was severely affected by the extent of microcracks. Concrete using ground granulated blast furnace slag (GGBFS) suffered worse damage than concrete prepared from ordinary Portland cement alone. Attempts were made to apply the acoustic emission (AE) technique to study the process and mechanism of microcracking. The skills required to practice the AE technique at early ages and at high temperature were carefully considered. AE hits agreed with the test results for deformation and tensile strength. Most of the microcracks occurred within the descending period of temperature and were classified into tensile mode. The use of coarse aggregate with larger CTE, saturated fine lightweight aggregate, and the reduction of the maximum size of the aggregate were greatly effective in reducing microcracking and improving the tensile strength of concrete made with GGBFS. Direct tensile strength was more adversely affected by microcracking than splitting tensile strength.

A Numerical Model for Concrete Strength Change under Neutron and Gamma-ray Irradiation

“…Due to its size, the heat of hydration accumulated up to ~60 °C. In case of some aggregates, concrete strength development was stagnated or deteriorated because of increase in number of large pores by formation of densified outer C-S-H due to elevated temperature (Sugiyama and Masuda 1999;Gallucci et al 2013) and the difference between the volume changes in the aggregate and matrix and resultant damage accumulation around aggregates (Son and Hosoda 2010;Maruyama et al 2014b). However, in this calculation, silicate content was high and the thermal expansion of the aggregate was 10 × 10 -6 /K.…”

Review of the Current State of Knowledge on the Effects of Radiation on Concrete

“…The aggregates restrain the shrinkage of cement paste during drying or heating (Carlson 1938;Hansen and Nielsen 1965;Hobbs 1974), and the large difference in volume change, due to environmental conditions, between the aggregates and the cement paste matrix produces cracks in the cement paste that act as pores. These cracks affect the strength and Young's modulus of the concrete (Bažant et al 1982;Son and Hosoda 2010;Maruyama et al 2014b;Lin et al 2015). Therefore, the differential shrinkage is the main factor in the monotonic reduction of the Young's modulus of the concrete (Maruyama et al 2014b).…”

Mechanism for Improving Crack Resistance of Slag Concrete With High Alite Cement

“…The mineral compositions of OPC and HAC are calculated from the chemical composition by Bogue's equations and the main compounds of cements are listed in Table 2. Because the CTE of materials is one of the important parameters affecting microcracking in concrete 9) , two types of coarse aggregates with the same maximum particle size of 19 mm but remarkably different CTEs, namely, limestone and andesite, were used in this research. However, it should be noted here that the chemical reaction between limestone and slag mortar also affects the mechanical properties of concrete 12) .…”

“…2,[239][240][241][242][243][244][245][246][247][248]2014 concrete using ground granulated blast furnace slag (GGBFS) is being widely used all over the world, especially in Japan. However, Son and Hosoda found that slag concrete is also easily subjected to more severe nonstructural microcracks than OPC concrete under temperature variation at early ages 9) . It has also been recognized that slag concrete has lower resistance against carbonation.…”

“…AE measurement at a very early age is difficult because AE sensors cannot be directly attached on the surface of unhardened concrete. Therefore, a waveguide embedded inside concrete had been employed and this worked well in concrete with low water-to-binder ratio (W/B) 5) . Unfortunately, attenuation of acoustic waves in a high moisture content ambient 6) due to energy absorption led to the diminished effectiveness of the waveguide in detecting AE signals in concrete with high W/B.…”