PSI - Issue 64

Sothyrak Rath et al. / Procedia Structural Integrity 64 (2024) 122–129 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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several theories have been developed to explain it (Collins (1944); Litvan (1972); Powers (1975, 1945)). Among them, the osmotic pressure theory (Powers (1975)) is considered the most comprehensive. This theory states that as the temperature decreases, water in the capillary pores freezes, forcing water in smaller pores to travel to larger pores to re-establish equilibrium. This process generates internal pressure that can damage the pores. Given the importance of entrained air voids for frost resistance, the theory suggests that air voids compete with larger pores to attract water. Thus, the addition of air voids can have both positive and negative effects on the frost resistance of concrete structures. Consequently, these theories suggest that pore structure is crucial for frost resistance, as it directly relates to the amount of freezable water in the concrete system, which can be overstressed by frozen water. To evaluate frost damage in concrete, certain standards (ASTM C666 / C666M-15 (2015); JIS A1148 (2015); Setzer et al. (2004)) have been proposed for laboratory-scale evaluation, using ultrasonic pulse transmission time and fundamental transverse frequency measurements. To determine the degree of damage from these measurements, the initial (pre-damage) pulse transmission time and fundamental transverse frequency are necessary. This implies that using these techniques directly for in-situ assessment cannot determine the degree of frost damage based on the measured data. Additionally, as these methods are applied on the concrete surface for the measurement, factors such as moisture content, carbonation, presence of reinforcement, and age of concrete, which usually affect in-situ measurement can influence the results (Naik et al. (2003); Yiching et al. (2003)). Therefore, an indirect estimation method that addresses these influences should be investigated for evaluating frost damage in concrete. Given that the pore structure of concrete is strongly correlated with frost damage phenomena, an estimation method based on the concrete pore structure should be considered. The drilling powder method, causing 1-mm damage, has been proposed to minimally evaluate the pore structure of cement paste and concrete (Rath et al. (2023); Tanaka and Sakai (2021)). This method uses the drilled powder of concrete, at any desired depth, for porosity evaluation, thereby minimizing the influence of parameters from in-situ assessment. This study examines the feasibility of using this method to evaluate the degree of frost damage in concrete. Additionally, to assess the proposed method's feasibility, a 5-mm concrete core was prepared for comparison in estimating the degree of frost damage. 2. Materials and methods 2.1. Sample preparation Concrete samples were prepared using ordinary Portland cement (density: 3.15 g/cm 3 , Blaine fineness: 3,480 cm 2 /g), sand (crushed hard sandstone, maximum particle size: 5 mm, saturated surface dry density: 2.65 g/cm 3 , water absorption rate: 1.32%, fineness modulus: 2.4), and gravel (crushed hard sandstone, maximum particle size: 20 mm, saturated surface dry density: 2.67 g/cm 3 , water absorption rate: 0.84%). Additionally, chemical admixtures were used, including a superplasticiser (SP) (Master Builder), thickener (TH) (KAO Global Chemicals Japan), antifoaming agent (AF) (Dow & Toray Industries), and air-entraining admixture (AEA) (Master Builder). We prepared samples using two types of concrete mixtures, without and with AEA, which are denoted as S1 and S2, respectively, as listed in Table 1. Additionally, the curing conditions, duration, and air content of the fresh concrete (measured following JIS A1128 standard (JIS A 1128, 2019)) are listed in Table 1. Water curing was performed by submerging the samples in water at 20 °C, while air curing was conducted in a room at 20 °C with 60 ± 10% relative humidity. Prism specimens measuring 10 × 10 × 40 cm and cylindrical specimens measuring 10 × 20 cm (diameter × height) were prepared. The prism specimens were used for exposure to F – T cycles, while the cylindrical specimens were used for extracting powder and core samples. The specimens were demolded 24 h after casting and cured for 28 d. After curing, the specimens were kept in a room for 28 d under the same environmental conditions as air curing. Subsequently, the powder and core samples were extracted, and cyclical F – T exposure was initiated. 2.2. Drilling powder preparation and pore structure evaluation Powder samples were obtained using a 4-mm drilling bit to extract 2.0 ± 0.5 g of powder from the concrete samples. Although a 1-mm drilling bit could be used, we chose a larger bit to reduce the working time for this study. Drilling was perpendicular to the specimen height, avoiding regions within 25 mm of the top and bottom surfaces. The powder