Influence of Early Freezing on the Pore Structure Characteristics of Concrete and Its Correlation with Mechanical Properties

Abstract

Early freezing of concrete is common in the construction of water conservancy projects in northern China. Early freezing damage induces the deterioration of the mechanical properties of concrete structures, which seriously affects the safety, stability, and service life of engineering structures. Through a laboratory uniaxial compression test and a computed tomography (CT) test, the influence law of freezing time and freezing temperature on the mechanical properties of concrete is analyzed herein. The three-dimensional pore structure of concrete at different freezing times is reconstructed. The pore distribution and pore structure characteristic parameters of concrete at different freezing times are studied. The correlation between mechanical properties and the pore structure of early frozen concrete is determined. The results show that with the delay of freezing time, the porosity of concrete test blocks increases first and then decreases. The average pore surface area primarily decreases and then increases. The average pore diameter increases with the trend of a quadratic parabola. The average pore form factor primarily decreases, then increases, and then decreases. The average pore surface area has the best correlation with the compressive strength and elastic modulus of early frozen concrete. The outcomes suggest that the average pore surface area should be preferred when constructing the mesoscopic damage index of early frozen concrete. Relevant results provide support for revealing the macro and micro damage mechanisms of early frozen concrete.

1. Introduction

Early freezing is a common phenomenon in the construction of hydraulic concrete structures in northern China [1,2,3,4,5]. After being subjected to frost damage during the early curing of concrete, the free water inside it will freeze to form ice crystals. On the one hand, this generates a frost heave force and induces the generation and expansion of micro-cracks inside the concrete, causing frost heave damage to the concrete. On the other hand, the decrease in free water content inside the concrete slows down the hydration reaction process, which affects the strength and deformation properties of the concrete [6]. Research on the mechanical properties and frost damage characteristics of concrete under the influence of early freezing has become a hot topic and has attracted attention both domestically and internationally, which is an important basis for the prevention and control of early freezing damage to concrete structures.

Many studies have been carried out at both the macro and micro levels [7,8,9,10,11]. Macroscopic properties research mainly focuses on damage morphology, compressive strength, relative dynamic modulus of elasticity, quality loss, and permeability, and the research on microscopic characteristics is mainly based on the analysis of pore structure characteristics [12,13]. Qin et al. [14] used freeze–thaw tests to investigate the quality loss, compressive strength loss, and internal pore distribution changes in early frost-damaged concrete. Xu et al. [15] conducted extensive experimental studies on the damage characteristics and effects of early frozen concrete with the compressive strength, relative dynamic modulus of elasticity, and damage thickness as evaluation indices, and the results indicate that the curing age of concrete during freezing, i.e., the freezing time, has a significant impact on the compressive strength of concrete, while the freezing temperature has a relatively small impact [16,17]. When the starting freezing time is between the initial and final setting periods, the mechanical properties of concrete specimens are most affected by freezing. Hu et al. [18] analyzed the impact of freezing time on the mechanical properties of and damage to concrete, with the damage morphology, strength loss, relative dynamic modulus of elasticity, permeability, and pore structure parameters as evaluation indices [19]. Xu et al. [20] studied the correlation between the mechanical properties of early frozen concrete and the fractal dimension of the pore volume under salt freezing conditions by using nuclear magnetic resonance technology and fractal theory. They built the correlation model between the fractal dimension DMAX and compressive strength. Research by Wen et al. [21] found that the early freezing effect can lead to a decrease in the strength of rubber concrete and an increase in chloride ion permeability. Abundant research results have been achieved on the damage to and mechanical properties of concrete under the influence of early freezing. Many scholars have recognized the importance of early freezing for the pore structure of concrete, but the characterization of pore structure is relatively complex, and it is difficult to fully reflect its characteristic distribution with a single characterization index. There is relatively little research on the impact of early freezing on the pore structure characteristics of concrete under multiple characterization parameters, especially in terms of the correlation between the microscopic pore structure and macroscopic mechanical properties of concrete damaged by early freezing.

In this study, we carried out uniaxial compression tests and CT scanning tests on concrete at different freezing times and temperatures, aiming to analyze the influence of freezing time and freezing temperature on the strength and deformation of concrete and investigate the distribution and variation of the pore structure and pore characteristic parameters of concrete at different freezing times. The macroscopic mechanical properties were characterized by the compressive strength and elastic modulus, and a correlation analysis between the mechanical properties and pore structure of early frozen concrete was conducted. This provided a theoretical basis for revealing the mechanism of early frost damage in concrete curing.

2. Methods and Materials

2.1. Mix Proportion and Test Block Preparation

The research object was an aqueduct project located in Handan. According to engineering data, the selected concrete grade was C50. The concrete preparation materials mainly included cement, fly ash, coarse aggregates, fine aggregates, and water-reducing agents. The cement was PC 42.5 from Taihang. The fly ash grade was Grade I, sourced from Handan Thermal Power Plant. The apparent density of the fly ash was 2600 kg/m³, with a fineness of 12%. The coarse aggregate was a continuously graded crushed stone with a particle size distribution of 5–20 mm. The apparent density and bulk density of the coarse aggregate were 2432 kg/m³ and 1712 kg/m³, respectively. The fine aggregate was sieved with medium sand. The apparent density and bulk density of the fine aggregate were 2412 kg/m³ and 1642 kg/m³, respectively. The water-reducing agent was a polycarboxylate superplasticizer. According to the Design Specification for Ordinary Concrete Mix Proportions (JGJ55-2011) [22], the mix ratio of concrete specimens is shown in

Table 1. Mix ratio of concrete specimens.

Water and Binder Ratio	Water (kg/m3)	Cement (kg/m3)	Fly Ash (kg/m3)	Sand (kg/m3)	Gravel (kg/m3)	Water Reducing Agent (kg/m3)
0.35	187	427	107	638	1041	5.34

.

The initial setting time and final setting time of the concrete made with this mix proportion, under standard curing conditions, were 3.5 h and 7 h, respectively. After uniaxial compressive strength testing, it was found that after 28 days of curing under standard curing conditions, the uniaxial compressive strength of concrete was 52.4 MPa. According to the climate temperature in Handan, the freezing temperatures were designed to be −5 °C and −9 °C, respectively. According to the setting time of concrete, the timings at which the concrete began freezing were 1.5 h after standard curing (before initial setting), 4 h after standard curing (after initial setting and before final setting), 8 h after standard curing (after final setting), and 24 h after standard curing (after final setting).

The sample used in the experiment was a cube with a size of 100 × 100 × 100 mm, as shown in Figure 1b. For the same uniaxial compressive strength test conditions, we conducted three parallel tests and then took the average value. The number of cube specimens used for uniaxial compressive strength testing was 24. The number of cube specimens used for CT scanning testing was 4. The total number of cube specimens was 28.

During the experiment, the standard curing of the cube test block was carried out until the moment when it began to freeze. Then, the ambient temperature of the HJX/T150 was set to high, and the low-temperature curing box was set to the design freezing temperature. The test block was placed in both the high- and low-temperature curing boxes, and the low-temperature curing was continued for 8 h. Finally, the test block was placed into a YB-60B standard curing box obtained from the Cangzhou Yixuan Testing Instrument Co., Ltd. (Cangzhou, China), and curing was continued until reaching 28 days. The uniaxial compression tests and CT scanning tests were conducted subsequently. The early freezing test device and specimens are shown in Figure 1a,c.

2.2. Compression Test

With the help of the universal hydraulic testing machine, for the test, we adopted the load control mode and conducted uniaxial compression tests on concrete specimens at different freezing temperatures (−5 °C and −9 °C) and different freezing times (1.5 h, 4 h, 8 h, and 24 h). There were four specimens in each group. Based on the stress–strain curve obtained from the experiments and the Standard Test Methods for Physical and Mechanical Properties of Concrete (GB/T50081-2019) [23], the compressive strength and elastic modulus were obtained as the macroscopic mechanical properties of concrete in order to analyze the influence of the freezing temperature and freezing time on the compressive strength and elastic modulus.

2.3. CT Test

The CT experiment was conducted with the Diondo d5 high-resolution omnipotent small-focus CT detection system, as shown in Figure 2. Taking the freezing temperature of −9 °C as an example, the concrete specimens were layered, scanned, and imaged under the conditions of 1.5 h, 4 h, 8 h, and 24 h of freezing time, respectively. Firstly, the two-dimensional images obtained by CT experiments were subjected to image enhancement and binarization processing. Then, they were imported into VG Studio MAX3.4 for a three-dimensional reconstruction of the pore structure. According to the reconstructed three-dimensional pore structure of concrete, the spatial distribution law and pore structure parameters of concrete pores at different freezing times were obtained.

3. Results and Discussion

3.1. Strength and Deformation Characteristics of Early Frozen Concrete

The variety rule of the compressive strength and elastic modulus of concrete under different freezing temperatures and freezing time conditions is shown in Figure 3. The freezing temperatures are −5 °C and −9 °C, respectively, as shown by the solid line marked with a solid square and the solid line marked with a solid circle in Figure 3. It can be seen that as the freezing time was delayed, the compressive strength and elastic modulus of concrete at different freezing temperatures showed a trend of first decreasing and then increasing. When the freezing time was between the initial setting and the final setting, the concrete was most affected by frost damage. The experimental results are consistent with existing research results [10,11]. When the freezing time was between the initial setting and the final setting, the compressive strength values of the concrete were 30.35 MPa and 27.05 MPa, and the elastic modulus values were 22.5 GPa and 20.2 GPa, respectively. This is mainly because the hydration reaction of cement mainly occurred after the initial setting, while the degree of cement hydration was relatively low before the initial setting. When the freezing time was between the initial setting and the final setting, the frost heave force generated by the formation of ice crystals during the free water freezing process delayed the hydration reaction and strength growth.

In addition, when the test block was transferred to the standard curing environment, the ice crystals melted and continued to complete the hydration reaction to repair the frost heave damage. The earlier the freezing time, the easier it was for the concrete to self-repair, and the faster the strength and elastic modulus of the concrete recovered. When the freezing time was below the initial setting, the freezing temperature had a significant impact on the strength and elastic modulus of concrete. However, when the freezing time was longer than the initial setting, the effect of the freezing temperature on the strength and elastic modulus of the concrete remained unchanged. It can be seen that prior to the standard curing for 4 h, the freezing temperature and freezing time were the key factors affecting the mechanical properties of concrete, while after the standard curing for 4 h, the freezing time was the main factor affecting the mechanical properties of the concrete.

3.2. The Distribution Law of Pores in Early Frozen Concrete

The distribution pattern of three-dimensional pore space in concrete under different freezing times is shown in Figure 4. From the figure, it can be seen that the pore morphology exhibited an irregular polyhedral structure in the concrete. With the increase in the freezing time, the larger pores inside the concrete specimen gradually increased, and compared with other freezing times, the internal pores of the frozen concrete after 4 h of freezing time were relatively developed.

3.3. Characteristics of Pore Structure in Early Frozen Concrete

The porosity, average pore surface area, average pore diameter, and average pore form factor were used to characterize the pore structure characteristics of concrete, and then the impact of the freezing time on the pore structure characteristics was analyzed based on the indoor CT results. The porosity was determined by the ratio of the pore volume to concrete specimen volume. The average pore surface area refers to the ratio of the total pore surface area within the test block to the number of pores. Pore size refers to the equivalent diameter of concrete pores, and average pore size refers to the ratio of the total pore size within the test block to the number of pores. The pore form factor S_p refers to the ratio of the volume of a pore to the volume of the outer sphere of the pore, reflecting the pore shape. The closer the S_p value is to 1, the closer the pore morphology was to the standard sphere. As shown in Equation (1), the average pore form factor refers to the ratio between the sum of the pore form factor and the number of pores in the test block.

$$S_{\mathrm{p}}=\frac{V_{\mathrm{p}}}{\frac{1}{6}{\pi L}_{\mathrm{p}}^3}$$

(1)

where V_p is the pore volume, and L_p is the pore length.

The data set corresponding to the four pore structure characteristic parameters of pore volume, pore surface area, pore diameter, and pore form factor in the test block was obtained according to VG Studio software, and the statistical analysis of four parameter data sets was carried out with the help of OriginPro 8.5.0. We aimed to obtain the distribution pattern of pore structure characterization quantities of early frozen concrete at different freezing times, as shown in Figure 5 and Figure 6. It can be seen that the pore volume distribution, which fell mainly between 0.01 and 10 mm³ and accounted for over 90% at different freezing times, obeyed the law of logarithmic normal distribution, and the chi-square values and R² of the distribution fitting were 1.252 and 0.9994, respectively. The distribution of the pore surface area, which fell mainly in the range of 0–5 mm², followed an exponential distribution law, and the chi-square values and R² of the distribution fitting were 0.387 and 0.9999, respectively. The aperture distribution, which was mainly distributed in the 0–2 mm range, followed the law of normal distribution, and the chi-square values and R² of the distribution fitting were 0.152 and 0.999, respectively. The distribution of the pore form factor obeyed the law of normal distribution, and the chi-square value and R² of the distribution fitting were 0.426 and 0.885, respectively. With the increase in the freezing time, the porosity of the concrete test block showed a change law of firstly increasing and then decreasing. The average pore surface area showed a trend of first decreasing and then increasing. The average pore diameter showed a quadratic-parabola increase trend, and the average pore form factor showed a change trend of primarily decreasing and then increasing before decreasing again. Combined with the spatial distribution law of concrete pores, as shown in Figure 4, when the freezing time of concrete was 4 h, the number of irregular small pores increased sharply, which resulted in serious damage to and deterioration of the concrete interior. Then, the inverse correlation law of the average surface area, average pore diameter, and average form factor of pores decreased while the porosity increased.

3.4. Correlation Analysis of Mechanical Properties and Pore Structure Characteristics of Early-Frozen Concrete

According to the processing method proposed by Tian et al. [24], four characteristic parameters of concrete could be obtained, including porosity, average pore surface area, average pore size, and average pore shape factor. Furthermore, we analyzed the variation patterns of the uniaxial compressive strength and elastic modulus with four characteristic parameters. The correlation between the pore structure and mechanical properties of early frozen concrete at different freezing times is shown in Figure 7. It can be seen that with the increase in porosity, the compressive strength and elastic modulus of early frozen concrete showed a trend of first increasing and then decreasing. According to Xu et al. [15], the main reason for this phenomenon is that before the initial setting period, the hydration degree of concrete is relatively low, and the skeleton structure is not formed, resulting in a weak resistance to frost heave damage. After heating up, the hydration reaction can continue to be completed, and the hydration reaction products fill the pores, which to some extent repairs the frost heave damage and reduces the porosity. However, the damage caused by frost heave was still greater than the repaired damage, resulting in a decrease in strength. After the final setting period, a high-strength skeleton structure had been formed inside the concrete, which could effectively resist partial frost heave stress and reduce frost damage. However, due to the relatively high degree of hydration reaction, the production of hydration products was relatively low, the pore filling was limited, and the porosity was relatively high. The porosity of concrete after freezing had a quadratic nonlinear correlation with the compressive strength and elastic modulus. The correlation coefficients were high, reaching 0.84 and 0.97, respectively. The compressive strength and elastic modulus of concrete increased with the increase in the average pore surface area and showed a significant quadratic nonlinear positive correlation with a correlation coefficient higher than 0.9. The average pore size showed a gradually increasing trend along with the compressive strength and elastic modulus of concrete, and the correlation was relatively low, with correlation coefficients of 0.69 and 0.75, respectively. With the increase in the form factor, the compressive strength and elastic modulus of concrete also increased, which showed an obvious quadratic nonlinear positive correlation, and the correlation coefficients reached 0.82 and 0.94, respectively. It can be seen that the correlation between the pore structure characteristics of early frozen concrete and the macro mechanical properties was in order from high to low: average pore surface area, porosity, average pore form factor, and average pore size. Therefore, when establishing the correlation between macroscopic and microscopic damage caused by the early freezing of concrete, it is recommended that one prioritize selecting the average pore surface area to construct a microscopic damage index.

4. Conclusions

• The timing of freezing does not affect the distribution characteristics of the pore structure. At different freezing times, the pore volume, pore surface area, pore size, and pore shape factor follow logarithmic normal, exponential, normal, and normal distribution patterns, respectively.
• With the increase in the freezing time, the porosity of the concrete test block showed a change law of primarily increasing and then decreasing. The average pore surface area showed a change law of primarily decreasing and then increasing. The average pore diameter showed an increasing quadratic parabola trend, and the average pore form factor showed a change trend of first decreasing and then increasing before decreasing again.
• The correlation between the pore structure characteristics and macro mechanical properties from high to low was as follows: average pore surface area, porosity, average pore form factor, and average pore diameter. The average pore surface area should be prioritized when constructing a micro damage index of early frozen concrete.