Effect of Surface Reinforcer on Compressive Strength and Microscopic Mechanism of Freeze–Thaw-Deteriorated Concrete

Abstract

Due to the deterioration caused by repeated freeze–thaw cycles, concrete materials in cold regions often develop cracks, which have serious effects and challenge the durability of concrete-based structures. Therefore, it is worthwhile to repair and strengthen freeze–thaw-damaged concrete to extend the service duration of the structure. In the present study, to investigate the restorative effect of surface reinforcer on freeze–thaw-deteriorated concrete specimens, the effects of the surface reinforcer type, its action duration, and the number of applications on the strength and deformation parameters of repaired specimens were systematically studied. Moreover, pore size distribution, pore structure at different depths, and the micromorphology characteristics were investigated by nuclear magnetic resonance (NMR), pore structure, and scanning electron microscope (SEM) tests to reveal the repair mechanism of different surface reinforcer types. The results indicated that the compressive strength of freeze–thaw-deteriorated concrete could be increased by up to 15.67% after the application of the surface reinforcer. Both the values of compressive strength and deformation modulus E₅₀ increased with the increase in the action duration. In addition, the surface reinforcer could efficiently penetrate the interior of the deteriorated specimen and was able to decrease the total proportion of multi-harmful pores and harmful pores. Furthermore, the pore structure parameters could be significantly improved at a depth of 10 mm; however, the reparative effect of the surface reinforcer gradually decreased with the increase in the action depth. The surface reinforcer could efficiently promote the second hydration of cement and generate more cementitious materials to fill the microvoids, thereby improving the compactness and mechanical properties of the repaired specimens.

1. Introduction

Concrete has been widely used in construction, transport, and other fields by virtue of its low energy consumption, low cost, and high durability, and has become one of the cornerstones for building the edifice of modern civilization [1,2,3]. Undoubtedly, concrete will continue to be the main building material in the present and future [4,5,6].

Owing to the increase in their service life, the durability of concrete-based structures is becoming increasingly important. The deformation of the various constituent materials of prepared concrete is inconsistent, and initial stress generated by mutual constraints results in the appearance of invisible microcracks between the bonding surface of the aggregate and cement stone or in the cement stone itself. Theoretical and engineering practice research has shown that when the crack width is less than 0.05 mm, there is little effect on the use of the structure [7]. However, due to the influence of external factors (e.g., load action, temperature difference, uneven settlement, and non-standard construction operation), cracks in concrete at different positions further expand and gradually penetrate and converge, which leads to serious quality problems or tragic accidents, resulting in economic losses and even casualties. The present analysis shows that the presence of cracks is unavoidable in concrete engineering, and the damage to concrete structures can be remarkably severe. According to relevant statistics [8,9], more than 50% of Europe’s annual construction budget is spent on restoration and renovation works, and the United States is expected to spend up to USD 300 billion a year on repairs or reconstruction. Therefore, it is vital to repair concrete cracks in a timely manner for the improvement of the safety and durability of concrete-based buildings.

Two approaches are generally adopted to effectively repair deteriorated concrete structures with cracks and to extend the service life of said concrete structures [10,11,12]. Specifically, the first common method employs supporting structures (e.g., an anchor bolt, anchor cable, soil nail, or protective net) to strengthen the deteriorated concrete. Another effective approach is the selection of suitable defect-repair materials (e.g. padding [13], grouting [14], structural reinforcement [15], and surface repair [16,17]) to enhance the durability of concrete materials by filling the macroscopic and microscopic cracks which have emerged; this approach has been widely used in recent years [18]. The repair materials method possesses many advantages compared with the supporting structures approach, such as a low comprehensive cost and convenient construction, and it is environmentally friendly. Therefore, it has been highly valued by both researchers and engineers, and thus has informed a series of industry standards and technical regulations to systematically guide the restoration work of concrete-based structures.

Much valuable research has been carried out to determine the effect of surface reinforcers on the mechanical properties and strengthening mechanism of repaired concrete materials. For instance, Li et al. [19] studied the variation in the compressive strength of concrete according to its inorganic coating surface reinforcer content after high-temperature treatment. The results showed that the inorganic coating surface reinforcer could significantly improve the compressive strength of deteriorated concrete derived at high temperatures. Liu et al. [20] invented a new kind of surface reinforcer and applied it to repair concrete in pavements, and engineering application results showed that the surface reinforcer could not only enhance the mechanical properties of concrete but could also greatly improve its durability. Meanwhile, other surface reinforcers were also developed for the improvement of the strength grade [21], anti-permeability [22,23], and carbonization resistance [24] of concrete materials. It is worth noting that, in addition to the expiration of the service period of concrete structures, concrete will also be subjected to many devastating unnatural factors, such as fire and seawater erosion. Therefore, Wang et al. [25] conducted deterioration tests of concrete induced by high temperatures and sulfate erosion, respectively, and the effects of the content and action duration of surface reinforcer on the compressive strength, elastic modulus, carbonization resistance, and permeability resistance of repaired concrete were systematically studied. The test results showed that the performance parameters of repaired concrete at both an early stage and at 28 d were higher than those of the unrepaired sample, and the improvement rate increased with the increase in the amount of surface reinforcer. Moreover, the lower the strength grade of concrete, the greater the promoting effect of surface reinforcer. In addition to the mechanical properties of repaired concrete materials, Shi et al. [26] analyzed the effects of surface reinforcer on the degree of hydration, surface microcracks, and capillary structure of concrete, and observed that polymer-based surface reinforcers could inhibit the occurrence of surface cracks in concrete specimens and enhance the carbonization resistance of concrete. To date, previous investigations have mainly concentrated on the improvement of the physical and mechanical parameters of repaired concrete following the effects of a surface reinforcer, while few studies are available in the existing literature that address the variation in the microstructure of deteriorated concrete following the effects of a surface reinforcer. To study the effects of surface reinforcers on the strength and microscopic mechanism of damaged concrete, a series of freeze–thaw cycles were induced to test the deterioration of concrete specimens, and then different types of surface reinforcer were employed to repair the deteriorated concrete specimens. Accordingly, the sensitivities of surface reinforcer type, number of applications, and action duration on the strength and deformation parameters of repaired concrete were systematically investigated. Moreover, nuclear magnetic resonance (NMR), pore structure, and scanning electron microscope (SEM) tests were carried out to study the pore size distribution, pore structure characteristics, and micromorphology of repaired concrete.

2. Experimental Materials and Methods

2.1. Materials and Mix Design

In this test, ordinary P.O 42.5 Portland cement was selected from the Huainan Bagongshan factory. The specific surface area and density were 332 m²/kg and 3130 kg/m³, respectively. The chemical composition of cement obtained by tests is shown in

Table 1. Chemical composition of cement (%).

SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	Na2O	K2O
21.80	3.31	2.78	64.12	0.75	1.03	0.04	0.68

.

The concrete surface reinforcer was a water-soluble liquid chemical hardener of a colorless and transparent appearance. The main ingredients of the concrete surface reinforcer were reactive alkali metal silicates or modified alkali metal silicates, catalysts, and additives. In the present experiment, three types of surface reinforcer with good performance were selected, which were marked as type A, type B, and type C according to the differences in their chemical composition and proportion. The promotional effect of surface reinforcers was explored through three variables: surface reinforcer type, number of applications, and action duration. The numbers of applications were 2, 4 and 8, respectively. The action duration was 3 d, 7 d, and 14 d, respectively. During the experiment, a brush was used to apply the surface reinforcer to the specimens.

C40 ordinary concrete was produced and 1 m³ of concrete comprised cement, sand, coarse aggregate, and water in amounts of 380 kg/m³, 630 kg/m³, 1120 kg/m³ and 190 kg/m³, respectively. The concrete test specimen mixture proportions are shown in

Table 2. Mixture proportions (kg/m³).

Cement	Fine Aggregate	Coarse Aggregate	Water
380	630	1120	190

. The cube compressive strength of the specimen was measured as 48.61 MPa after 28 days of standard curing.

For subsequent analyses, the specimens under different test conditions were numbered; for instance, B-4-7 represented the type of surface reinforcer (type B), which was applied 4 times and had an action duration of 7 d after application; A-2-14 represented the type of surface reinforcer (type A), which was applied 2 times and had an action duration of 14 d after application. In addition, S-1 represented specimens cured with saturated calcium hydroxide and D-1 represented the deterioration of a specimen subjected to 80 freeze–thaw cycles.

2.2. Test Design

2.2.1. Fast Freezing Test

In this study, the freeze—thaw test was carried out according to the requirements of the Standard for Long-term Performance and Durability Test Methods for Ordinary Concrete (GB/T 50082-2009) [27]. The freeze—thaw test adopted a TEST-1000 High and Low Temperature Humidity and Heat Test Chamber. The freezing temperature was set to −20 °C, the melting temperature to 20 °C, and the freeze—thaw cycle to 8 h (i.e., the freezing time was 4 h and the thawing time was 4 h). Freeze—thaw cycles were performed 0, 20, 40, 60, and 80 times, respectively. The specimen was immersed in water at 20 °C for 4 days after 28 days of maintenance, and then removed and placed in a freeze—thaw test chamber for freeze–thaw cycle testing. The specimen was then allowed to dry and the compressive strength was measured after reaching the set number of freezing and thawing times. The compressive strength of the specimen subjected to 80 freeze—thaw cycles was recorded as the compressive strength of the deteriorated specimen. The freeze—thaw cycle test parameters are shown in

Table 3. Freeze–thaw cycle test parameters.

Freezing Time/h	Melting Time/h	Freeze–Thaw Cycle/h	Freeze–Thaw Cycles Number
4	4	8	0, 20, 40, 60, and 80

.

2.2.2. Surface Reinforcer Test

To allow the specimen to fully absorb the surface reinforcer, the specimen’s four surfaces, aside from the casting surface and the bottom surface, were sanded. The surface reinforcer was then fully and evenly applied to the four sanded surfaces using a brush. In order to enable the surface reinforcer to fully penetrate, each treatment was left for thirty minutes. A woven bag was used to cover the specimen’s surface after application to keep it from coming into excessive contact with air. The compressive strength test was then performed following the action period.

2.2.3. Uniaxial Compression Test

The press adopted the fully automatic DYE-2000S type computerized pressure-tester. The loading rate of concrete specimens was set at 1.0 MPa/s, and each group prepared 4 parallel concrete specimens.

2.2.4. NMR Test

NMR was a phenomenon that occurred in a system of nuclei with magnetic moments and angular momentum [28,29], where the excited magnetic nuclei interacted with other nuclei in thermal motion, absorbing and releasing energy. The transverse relaxation time (T₂) of pore water in concrete could be expressed as follows:

$$\frac{1}{T_2}=\rho \left(\frac{S}{V}\right)=F_s\frac{\rho }{R}$$

(1)

where $\rho$ is the surface relaxation strength; R, S, and V are the radius, surface area, and volume of the pores, respectively; F_s is the geometric shape factor; and its values are 1, 2, and 3 for fractures, cylindrical pores, and spherical pores [30,31,32,33], respectively.

2.2.5. SEM Test

SEM is an analytical instrument that utilized secondary electrons and back-scattered electron signals to acquire information on the physical and chemical properties of the sample under examination. With the improvement of science and technology, its properties could be magnified hundreds of thousands of times, and its resolution could be up to the nanometer level, which makes it an extremely important tool for morphology and composition analysis [34,35,36,37].

2.3. Pore Structure Test

Four pore parameters were measured from the pore structure test, namely the air content, spacing factor, average chord length, and specific surface area of the pores [38,39,40]. The air content was the percentage of the specimen pore volume; the spacing factor was the maximum distance between any point in the cement stone and the spherical surface of any neighboring pore; the average chord length was the ratio of the sum of the chord lengths of the pores to the total number of pores; and the specific surface area of the pore was the surface area of the concrete per unit volume of pore after hardening.

The American Society for Testing and Materials (ASTM C457-2016 [41]) straight-line conductor method was used to conduct the pore structure test utilizing the Model TR-AHS Hardened Concrete Pore Structure Tester. The procedure for preparing a pore structure specimen was as follows: Firstly, a cutting machine was used to cut 100 mm × 100 mm × 10 mm slices; secondly, the slices were roughly ground, finely ground, roughly polished, and finely polished with five different mesh sizes of sandpaper, and the test pieces were cleaned and dried after the above steps were completed; then, the polished surfaces of the specimens were blackened with a black water-based marker and put in a drying box at 105 °C for 30 min. Then, the pores were filled with barium sulfate powder to melt into the pores, the excess powder was removed, and the rest of the surface was left black. Finally, the specimen was placed on the test bench for measurement. The observed area of the specimen was 80 mm × 80 mm. The experimental procedure is shown in Figure 1.

3. Experimental Results and Discussion

3.1. Stress–Strain Curves

Figure 2 presents the axial stress–strain curves of repaired concrete specimens under different test conditions.

3.2. Compressive Strength

The compressive strength values of concrete specimens are shown in Figure 3. The data revealed that the freeze–thaw cycles reduced the compressive strength of standard concrete specimens, and concrete surface reinforcers were able to improve the compressive strength of freeze–thaw-deteriorated concrete specimens. The compressive strength of concrete could be increased by up to 15.67% after the application of the surface reinforcer. The surface reinforcer type, the action duration, and the number of applications had a significantly positive effect on the promotion of compressive strength. To be specific, the compressive strength decreased from 48.61 MPa to 46.39 MPa after 80 freeze–thaw cycles, with a reduction rate of 4.57%. Surface reinforcer type A exhibited a better strength-promoting effect compared to surface reinforcer types B and C. For instance, for the repaired concrete specimen, after experiencing four applications and a 7 d action duration, the compressive strength of concrete repaired by surface reinforcer type A was 52.47 MPa, which was higher than that of 51.67 MPa and 51.03 MPa for surface reinforcer types B and C. In addition, with the same number of applications, the compressive strength increased with the increase in the action duration. Moreover, for the repaired concrete specimen, after experiencing four applications, the compressive strengths of the repaired concrete specimens under the action duration of 3 d, 7 d, and 14 d were 50.33 MPa, 52.47 MPa, and 53.66 MPa, respectively, which improved by 8.5%, 13.1%, and 15.7%, compared to the deterioration specimen of 46.39 MPa. In addition to the type and action duration of surface reinforcer, the number of applications affected the compressive strength of concrete specimens. For example, the compressive strengths of B-2-7, B-4-7, and B-8-7 were 49.61 MPa, 51.67 MPa, and 50.28 MPa, respectively, which increased by 6.9%, 11.3%, and 8.4% compared to the deterioration specimen. Therefore, excessive applications of surface reinforcer could not produce any obvious enhancement of compressive strength, and the effect of action duration on the degree of increase in amplitude of repaired concrete was higher than that of number of applications in this study.

3.3. E₅₀

The deformation modulus E₅₀ was an important indicator for evaluating the deformation properties of concrete materials. In this study, the ratio of 50% peak stress in the stress–strain curve to its corresponding strain was selected as the deformation modulus [42]:

$$E_{50}=\frac{{\sigma }_{0.5}}{{\epsilon }_{0.5}}$$

(2)

According to the above calculation method, the values of E₅₀ with various conditions were obtained, as shown in Figure 4. It can be observed that the deformation modulus E₅₀ declined from 1567 MPa to 1285 MPa after undergoing 80 freeze–thaw cycles, with a reduction rate of 18%, while the E₅₀ of the repaired specimens significantly increased after the application of the surface reinforcer. Specifically, after undergoing four applications and a 7 d action duration, the E₅₀ of the repaired specimens coated with surface reinforcer types A, B, and C were 1707 MPa, 1615 MPa, and 1657 MPa, respectively, which was 33%, 26%, and 29% higher than that of the deteriorated specimens. Moreover, after four applications, when the action duration increased from 3 d to 14 d, the E₅₀ of the specimens repaired by reinforcer type A increased from 1595 MPa to 1881 MPa, with a larger-scale increase of 17.93%. After applying surface reinforcer type A and with 7 d action duration, the deformation modulus of repaired specimens subjected to two, four, and eight applications could be improved 1.25, 1.33, and 1.26 times compared with the deteriorated one. This phenomenon illustrates that there exists a threshold for the increase in the E₅₀ with the number of applications.

3.4. NMR Results

Based on Equation (1), the pore proportion with different sizes could be calculated for concrete specimens. However, there is no unified standard to accurately distinguish the pore size scale of cement-based materials (i.e., concrete, mortar, and cement soil). In the present study, the pore size division method of concrete proposed by Wu et al. [43] was adopted to quantitatively analyze the degree of pore structure-repair of deteriorated concrete specimens by different surface reinforcer types. Wu et al. [43] divided the pore size into four intervals according to the pore radius R: Harmless pores (R ≤ 10 nm), less-harmful pores (10 nm < R ≤ 25 nm), harmful pores (25 nm < R ≤ 100 nm), and multi-harmful-pores (R > 100 nm), as shown in Figure 5. Table 3 illustrates the percentages of four kinds of pores under different conditions.

Figure 5 and

Table 4. Pore proportions with different sizes (%).

Specimen Number	Harmless Pores	Less-Harmful Pores	Harmful Pores	Multi-Harmful Pores
S-1	70	16	2	12
D-1	67	17	5	11
A-4-7	70	17	5	8
B-4-7	70	17	3	10
C-4-7	69	17	4	10
A-4-3	69	17	5	9
A-4-14	77	14	2	7
A-2-7	71	16	4	9

clearly demonstrate that, for the standard concrete specimen, the proportions of harmless pores, less-harmful pores, harmful pores, and multi-harmful pores were 70%, 16%, 2%, and 12%, respectively, while the values changed to 67%, 17%, 5%, and 11% for freeze–thaw-treated specimens, respectively. The overall proportion of multi-harmful pores and harmful pores was 14% for S-1, and this increased to 16% after 80 freeze–thaw cycles. In addition, the ratio of different pore sizes changed after the application of the surface reinforcer, with a decrease in the total proportion of multi-harmful pores and harmful pores, and an increase in the total proportion of harmless pores and less-harmful pores. The above phenomenon revealed that the surface reinforcer penetrated the interior of the deteriorated concrete specimen and was able to fill the harmful pores, thus decreasing the porousness and enhancing the compactness of the concrete specimens. Moreover, the total proportion of multi-harmful pores and harmful pores decreased with the increase in the action duration and number of applications, as shown in Figure 5b,c. Under the same action duration and number of applications, the repairing effect of surface reinforcer type C on the pore structure of deteriorated concrete was weaker than that of surface reinforcer types A and B.

3.5. Pore Structure

Due to the fact that NMR could only measure the pore size distribution of the repaired concrete specimens, the remediation effect of surface reinforcer under different layer depths could not reveal the effect that four applications and 7 d action duration had, and the pore structure of the specimen repaired by surface reinforcer type A was estimated by measuring the pore structure on the surface (0 mm in depth), at a depth of 10 mm and at a depth of 20 mm, respectively. Specimen cutting positions were shown in Figure 6.

Figure 7 presented the pore morphology and pore parameters of the repaired specimens at different depths. It could be observed that air content, spacing, and average chord length on the surface of repaired specimens increased gradually, and the specific surface area decreased gradually with increased depth. Specifically, the air content was 12.3% on the surface of repaired concrete specimens, and the values were 13.95%, and 15.77% at depths of 10 mm and 20 mm, respectively, with increases of 13.4% and 28.2%, respectively. The spacing factor and average chord length of the repaired specimens at 20 mm depth were 1.33 times and 1.43 times higher than those at the surface position. Additionally, the specific surface area decreased from 6.15 mm⁻¹ to 4.3 mm⁻¹ when the depth increased from 0 mm to 20 mm, which reduced by a larger scale of 30%.

The above data clearly illustrate that the repair effect of surface reinforcers gradually decreased with an increase in the action depth. In addition, in this research the pore structure parameters could be significantly improved within 10 mm depth.

3.6. SEM Results and Analysis

Images magnified from 100× to 20,000× were utilized to analyze the microstructure characteristics of concrete under different repair conditions, as shown in Figure 8. From Figure 8a, it can be observed that freeze–thaw cycles induced an increase in the number of pores and the roughness of the surface. However, the flatness of the specimens increased and the number of pores markedly decreased after the application of the surface reinforcer, as shown in Figure 8b. Figure 8c reveals that, for the standard concrete specimens, a large amount of C-S-H gel could be found on the surface of the matrix.

When the surface reinforcer was applied, compared with the standard concrete, under the same magnification condition (10,000×), the surface reinforcer had an obvious promoting effect on the numerous cementitious products generated by the secondary reaction between the surface reinforcer and the hydration product. Therefore, the surface reinforcer was able to fill the microvoids, as shown in Figure 8d. The above phenomenon indicated that the surface reinforcer could effectively help to increase the compactness and reduce the void ratio of the concrete specimen, resulting in an increase in the specimens’ macroscopic mechanical properties. In addition, the microscopic morphology and elemental composition of specimens coated with different types of surface reinforcer were also obtained, as shown in Figure 8e–g. By comparing the above images, it could be observed that two microscopic morphology shapes (i.e., spherical and columnar) of surface reinforcer were observed for types A, B, and C in the present research. The primary constituents of surface reinforcer A are Si, O, and Ca. Regarding surface reinforcer B, the biggest percentage is accounted for by Ca, Si, and K. Ca and Si are the primary components of surface reinforcer C. In summary, the main components of the above three types of surface reinforcer were Si, Al, O, and K.

4. Conclusions

(1)

Three types of surface reinforcer were able to efficiently improve compressive strength and deformation modulus E₅₀, and the action duration and number of applications had a positive effect on the repair effect of the surface reinforcer.

(2)

As the surface reinforcer penetrated the specimens, the proportions of harmful pores and multi-harmful pores decreased and transformed into less-harmful pores and harmless pores. This contributed to the compressive strength of the concrete.

(3)

With the increase in the depth of repaired concrete, the air content, spacing factor, and average chord length decreased, while the specific surface area of the specimens gradually increased. In addition, in this research the pore structure parameters were significantly improved at 10 mm depth.

(4)

Surface reinforcer was able to generate more cementitious materials and fill the microvoids after penetrating the specimen. Two microscopic morphology shapes (i.e., spherical and columnar) of surface reinforcer were observed for types A, B, and C in the present research. The main components of the above three types of surface reinforcer were Si, Al, O, and K.