Abstract
The freeze-thaw durability of shotcrete can be improved by adding an air-entraining agent in cold areas. The main focus of this paper is to investigate the changes in the internal pore structure of C25 ordinary shotcrete and shotcrete mixed with a RM-YQ air-entraining agent using computed tomography (CT) scanning technique during freeze-thaw cycles. The macroscopic tests were conducted, including mass loss, dynamic modulus of elasticity and ultrasonic wave velocity tests. Results were compared, and the freeze-thaw durability characteristics of shotcrete mixed with the air-entraining agent were revealed. Adding an air-entraining agent could reduce the number of pores largely that ranged mainly from 0.01 mm2 to 1.00 mm2 (excluding the pores or bubbles < 0.01 mm2 because of the precision of the CT scanning system), and could therefore improve the initial pore structure of the formed shotcrete. During first few freeze-thaw cycles, just few small pores formed. After cement mortar fragmentations appeared, the number of small pores (0.01 mm2 to 0.50 mm2) in ordinary shotcrete increased significantly. The pore structure deteriorated largely. However, this could be prevented effectively by adding an air-entraining agent. Therefore, the freeze–thaw durability of shotcrete was improved.
Highlights
In cold regions, shotcrete undergoes freeze-thaw cycles because of the alternating changes in negative and positive temperatures
The results were recorded, and the figures were provided to analyse and compare the different effects of the freeze-thaw cycles on the damage to the internal structure of the ordinary shotcrete and the shotcrete mixed with the air-entraining agent
The test results were analysed and compared to reveal the different effects of freeze-thaw cycles on the internal pore structure of the ordinary shotcrete and the shotcrete mixed with the air-entraining agent
Summary
Shotcrete undergoes freeze-thaw cycles because of the alternating changes in negative and positive temperatures. Current studies have rarely investigated the mechanism of freeze-thaw damage to shotcrete mixed with an air-entraining agent. Nondestructive detecting technologies, including the acoustic emission (AE) method (Suzuki et al 2007), the ultrasonic imaging method (Molero et al 2012; Ranz et al 2014) and the computed tomography (CT) scanning, have been applied to investigate the internal microstructure of concrete Among these methods, CT scanning technique is widely used, because it can obtain visualised and accurate results. On the basis of the results of previous studies, the CT scanning technique was applied to investigate the changes in the internal pore structure of shotcrete mixed with an air-entraining agent and ordinary shotcrete during freeze-thaw cycles. The test results were analysed and compared to determine the freeze-thaw durability characteristics of shotcrete mixed with an air-entraining agent
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