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Polymer-reinforced concrete (FRP) is considered an innovative and economical method of structural repair. In this study, two typical materials [carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP)] were selected to study the reinforcing effect of concrete in harsh environments. The resistance of concrete containing FRP to sulphate attack and related freeze-thaw cycles has been discussed. Electron microscopy to study the surface and internal degradation of concrete during conjugated erosion. The degree and mechanism of sodium sulfate corrosion were analyzed by pH value, SEM electron microscopy, and EMF energy spectrum. Axial compressive strength tests have been used to evaluate the reinforcement of FRP-constrained concrete columns, and stress-strain relationships have been derived for various methods of FRP retention in an erosive coupled environment. Error analysis was performed to calibrate experimental test results using four existing predictive models. All observations indicate that the degradation process of FRP-restricted concrete is complex and dynamic under conjugate stresses. Sodium sulfate initially increases the strength of concrete in its raw form. However, subsequent freeze-thaw cycles can exacerbate concrete cracking, and sodium sulfate further reduces the strength of concrete by promoting cracking. An accurate numerical model is proposed to simulate the stress-strain relationship, which is critical for designing and evaluating the life cycle of FRP-constrained concrete.
As an innovative concrete reinforcement method that has been researched since the 1970s, FRP has the advantages of light weight, high strength, corrosion resistance, fatigue resistance and convenient construction1,2,3. As costs decrease, it is becoming more common in engineering applications such as fiberglass (GFRP), carbon fiber (CFRP), basalt fiber (BFRP), and aramid fiber (AFRP), which are the most commonly used FRP for structural reinforcement4, 5. The proposed FRP retention method can improve concrete performance and avoid premature collapse. However, various external environments in mechanical engineering often affect the durability of FRP-limited concrete, causing its strength to be compromised.
Several researchers have studied stress and strain changes in concrete with different cross-sectional shapes and sizes. Yang et al. 6 found that ultimate stress and strain correlated positively with growth in fibrous tissue thickness. Wu et al.7 obtained stress-strain curves for FRP-constrained concrete using various fiber types to predict ultimate strains and loads. Lin et al.8 found that FRP stress-strain models for round, square, rectangular, and elliptical bars also differ greatly, and developed a new design-oriented stress-strain model using the ratio of width and corner radius as parameters. Lam et al.9 observed that the non-uniform overlap and curvature of the FRP resulted in less fracture strain and stress in the FRP than in slab tensile tests. In addition, scientists have studied partial constraints and new constraint methods according to different real-world design needs. Wang et al. [10] performed axial compression tests on fully, partially and unrestricted concrete in three limited modes. A “stress-strain” model has been developed and the coefficients of the limiting effect for partially closed concrete are given. Wu et al. 11 developed a method for predicting the stress-strain dependence of FRP-constrained concrete that takes into account size effects. Moran et al.12 evaluated the axial monotonic compression properties of constrained concrete with FRP helical strips and derived its stress-strain curves. However, the above study mainly examines the difference between partially enclosed concrete and fully enclosed concrete. The role of FRPs partially limiting concrete sections has not been studied in detail.
In addition, the study evaluated the performance of FRP-restricted concrete in terms of compressive strength, strain change, initial modulus of elasticity, and strain-hardening modulus under various conditions. Tijani et al. 13,14 found that the repairability of FRP-limited concrete decreases with increasing damage in FRP repair experiments on initially damaged concrete. Ma et al. [15] studied the effect of initial damage on FRP-constrained concrete columns and considered that the effect of damage degree on tensile strength was negligible, but had a significant effect on lateral and longitudinal deformations. However, Cao et al. 16 observed stress-strain curves and stress-strain envelope curves of FRP-constrained concrete affected by initial damage. In addition to studies on initial concrete failure, some studies have also been conducted on the durability of FRP-limited concrete under harsh environmental conditions. These scientists studied the degradation of FRP-restricted concrete under harsh conditions and used damage assessment techniques to create degradation models to predict service life. Xie et al. 17 placed FRP-constrained concrete in a hydrothermal environment and found that hydrothermal conditions significantly affected the mechanical properties of FRP, resulting in a gradual decrease in its compressive strength. In an acid-base environment, the interface between CFRP and concrete deteriorates. As the immersion time increases, the rate of release of the energy of destruction of the CFRP layer decreases significantly, which ultimately leads to the destruction of interfacial samples18,19,20. In addition, some scientists have also studied the effects of freezing and thawing on FRP-limited concrete. Liu et al.21 noted that CFRP rebar has good durability under freeze-thaw cycles based on relative dynamic modulus, compressive strength, and stress-strain ratio. In addition, a model is proposed that is associated with the deterioration of the mechanical properties of concrete. However, Peng et al.22 calculated the lifetime of CFRP and concrete adhesives using temperature and freeze-thaw cycle data. Guang et al. 23 conducted rapid freeze-thaw tests of concrete and proposed a method for assessing frost resistance based on the thickness of the damaged layer under freeze-thaw exposure. Yazdani et al. 24 studied the effect of FRP layers on the penetration of chloride ions into concrete. The results show that the FRP layer is chemically resistant and insulates the inner concrete from the outer chloride ions. Liu et al.25 simulated peel test conditions for sulfate-corroded FRP concrete, created a slip model, and predicted degradation of the FRP-concrete interface. Wang et al. 26 established a stress-strain model for FRP-constrained sulphate-eroded concrete through uniaxial compression tests. Zhou et al. [27] studied damage to unconfined concrete caused by combined freeze-thaw cycles of salt and for the first time used a logistic function to describe the failure mechanism. These studies have made significant progress in evaluating the durability of FRP-limited concrete. However, most researchers have focused on modeling erosive media under one unfavorable condition. Concrete is often damaged due to associated erosion caused by various environmental conditions. These combined environmental conditions severely degrade the performance of FRP-restricted concrete.
Sulfation and freeze-thaw cycles are two typical important parameters affecting the durability of concrete. FRP localization technology can improve the properties of concrete. It is widely used in engineering and research, but currently has its limitations. Several studies have focused on the resistance of FRP-restricted concrete to sulfate corrosion in cold regions. The process of erosion of fully enclosed, semi-enclosed and open concrete by sodium sulfate and freeze-thaw deserves more detailed study, especially the new semi-enclosed method described in this article. The reinforcement effect on concrete columns was also studied by exchanging the order of FRP retention and erosion. Microcosmic and macroscopic changes in the sample caused by bond erosion were characterized by electron microscope, pH test, SEM electron microscope, EMF energy spectrum analysis and uniaxial mechanical test. In addition, this study discusses the laws governing the stress-strain relationship that occurs in uniaxial mechanical testing. The experimentally verified limit stress and strain values ​​were validated by error analysis using four existing limit stress-strain models. The proposed model can fully predict the ultimate strain and strength of the material, which is useful for future FRP reinforcement practice. Finally, it serves as the conceptual basis for the FRP concrete salt frost resistance concept.
This study evaluates the deterioration of FRP-limited concrete using sulfate solution corrosion in combination with freeze-thaw cycles. Microscopic and macroscopic changes caused by concrete erosion have been demonstrated using scanning electron microscopy, pH testing, EDS energy spectroscopy, and uniaxial mechanical testing. In addition, the mechanical properties and stress-strain changes of FRP-constrained concrete subjected to bonded erosion were investigated using axial compression experiments.
FRP Confined Concrete consists of raw concrete, FRP outer wrap material and epoxy adhesive. Two external insulation materials were selected: CFRP and GRP, the properties of the materials are shown in Table 1. Epoxy resins A and B were used as adhesives (mixing ratio 2:1 by volume). Rice. 1 illustrates the details of the construction of concrete mix materials. In Figure 1a, Swan PO 42.5 Portland cement was used. Coarse aggregates are crushed basalt stone with a diameter of 5-10 and 10-19 mm, respectively, as shown in fig. 1b and c. As a fine filler in Fig. 1g used natural river sand with a fineness modulus of 2.3. Prepare a solution of sodium sulfate from the granules of anhydrous sodium sulfate and a certain amount of water.
The composition of the concrete mixture: a – cement, b – aggregate 5–10 mm, c – aggregate 10–19 mm, d – river sand.
The design strength of concrete is 30 MPa, which results in a fresh cement concrete settlement of 40 to 100 mm. The concrete mix ratio is shown in Table 2, and the ratio of coarse aggregate 5-10 mm and 10-20 mm is 3:7. The effect of interaction with the environment was modeled by first preparing a 10% NaSO4 solution and then pouring the solution into a freeze-thaw cycle chamber.
Concrete mixtures were prepared in a 0.5 m3 forced mixer and the entire batch of concrete was used to lay the required samples. First of all, the concrete ingredients are prepared according to Table 2, and the cement, sand and coarse aggregate are premixed for three minutes. Then evenly distribute the water and stir for 5 minutes. Next, concrete samples were cast into cylindrical molds and compacted on a vibrating table (mould diameter 10 cm, height 20 cm).
After curing for 28 days, the samples were wrapped with FRP material. This study discusses three methods for reinforced concrete columns, including fully enclosed, semi-constrained, and unrestricted. Two types, CFRP and GFRP, are used for limited materials. FRP Fully enclosed FRP concrete shell, 20 cm high and 39 cm long. The top and bottom of the FRP-bound concrete were not sealed with epoxy. The semi-hermetic testing process as a recently proposed airtight technology is described as follows.
(2) Using a ruler, draw a line on the concrete cylindrical surface to determine the position of the FRP strips, the distance between the strips is 2.5 cm. Then wrap the tape around the concrete areas where FRP is not needed.
(3) The concrete surface is polished smooth with sandpaper, wiped with alcohol wool, and coated with epoxy. Then manually stick the fiberglass strips onto the concrete surface and press out the gaps so that the fiberglass is fully adherent to the concrete surface and avoids air bubbles. Finally, glue the FRP strips onto the concrete surface from top to bottom, according to the marks made with a ruler.
(4) After half an hour, check whether the concrete has separated from the FRP. If the FRP is slipping or sticking out, it should be fixed immediately. Molded specimens must be cured for 7 days to ensure cured strength.
(5) After curing, use a utility knife to remove the tape from the concrete surface, and finally get a semi-hermetic FRP concrete column.
The results under various constraints are shown in fig. 2. Figure 2a shows a fully enclosed CFRP concrete, Figure 2b shows a semi-generalized CFRP concrete, Figure 2c shows a fully enclosed GFRP concrete, and Figure 2d shows a semi-constrained CFRP concrete.
Enclosed styles: (a) fully enclosed CFRP; (b) semi-closed carbon fiber; (c) completely enclosed in fiberglass; (d) semi-enclosed fiberglass.
There are four main parameters that are designed to investigate the effect of FRP constraints and erosion sequences on the erosion control performance of cylinders. Table 3 shows the number of concrete column samples. The samples for each category consisted of three identical status samples to keep the data consistent. The mean of three samples was analyzed for all experimental results in this article.
(1) Airtight material is classified as carbon fiber or fiberglass. A comparison was made of the effect of two types of fibers on the reinforcement of concrete.
(2) Concrete column containment methods are divided into three types: fully limited, semi-limited and unlimited. The erosion resistance of semi-enclosed concrete columns was compared with two other varieties.
(3) The erosion conditions are freeze-thaw cycles plus sulfate solution, and the number of freeze-thaw cycles is 0, 50 and 100 times, respectively. The effect of coupled erosion on FRP-constrained concrete columns has been studied.
(4) The test pieces are divided into three groups. The first group is FRP wrapping and then corrosion, the second group is corrosion first and then wrapping, and the third group is corrosion first and then wrapping and then corrosion.
The experimental procedure uses a universal testing machine, a tensile testing machine, a freeze-thaw cycle unit (CDR-Z type), an electron microscope, a pH meter, a strain gauge, a displacement device, an SEM electron microscope, and an EDS energy spectrum analyzer in this study. The sample is a concrete column 10 cm high and 20 cm in diameter. The concrete was cured within 28 days after pouring and compaction, as shown in Figure 3a. All samples were demoulded after casting and kept for 28 days at 18-22°C and 95% relative humidity, and then some samples were wrapped with fiberglass.
Test methods: (a) equipment for maintaining constant temperature and humidity; (b) a freeze-thaw cycle machine; (c) universal testing machine; (d) pH tester; (e) microscopic observation.
The freeze-thaw experiment uses the flash freeze method as shown in Figure 3b. According to GB/T 50082-2009 “Durability Standards for Conventional Concrete”, concrete samples were completely immersed in 10% sodium sulfate solution at 15-20°C for 4 days before freezing and thawing. After that, the sulfate attack begins and ends simultaneously with the freeze-thaw cycle. The freeze-thaw cycle time is 2 to 4 hours, and the defrosting time should not be less than 1/4 of the cycle time. The sample core temperature should be maintained within the range from (-18±2) to (5±2) °С. The transition from frozen to defrosting should take no more than ten minutes. Three cylindrical identical samples of each category were used to study the weight loss and pH change of the solution over 25 freeze-thaw cycles, as shown in Fig. 3d. After every 25 freeze-thaw cycles, the samples were removed and the surfaces cleaned before determining their fresh weight (Wd). All experiments were carried out in triplicate of the samples, and the average values ​​were used to discuss the test results. The formulas for the loss of mass and strength of the sample are determined as follows:
In the formula, ΔWd is the weight loss (%) of the sample after every 25 freeze-thaw cycles, W0 is the average weight of the concrete sample before the freeze-thaw cycle (kg), Wd is the average concrete weight. weight of sample after 25 freeze-thaw cycles (kg).
The strength degradation coefficient of the sample is characterized by Kd, and the calculation formula is as follows:
In the formula, ΔKd is the rate of strength loss (%) of the sample after every 50 freeze-thaw cycles, f0 is the average strength of the concrete sample before the freeze-thaw cycle (MPa), fd is the average strength of the concrete sample for 50 freeze-thaw cycles (MPa).
On fig. 3c shows a compressive testing machine for concrete specimens. In accordance with the “Standard for Test Methods for the Physical and Mechanical Properties of Concrete” (GBT50081-2019), a method for testing concrete columns for compressive strength is defined. The loading rate in the compression test is 0.5 MPa/s, and continuous and sequential loading is used throughout the test. The load-displacement relationship for each specimen was recorded during mechanical testing. Strain gauges were attached to the outer surfaces of the concrete and FRP layers of the specimens to measure axial and horizontal strains. The strain cell is used in mechanical testing to record the change in specimen strain during a compression test.
Every 25 freeze-thaw cycles, a sample of the freeze-thaw solution was removed and placed in a container. On fig. 3d shows a pH test of a sample solution in a container. Microscopic examination of the surface and cross section of the sample under freeze-thaw conditions is shown in Fig. 3d. The state of the surface of various samples after 50 and 100 freeze-thaw cycles in sulfate solution was observed under a microscope. The microscope uses 400x magnification. When observing the surface of the sample, the erosion of the FRP layer and the outer layer of concrete is mainly observed. Observation of the cross section of the sample basically selects the erosion conditions at a distance of 5, 10 and 15 mm from the outer layer. The formation of sulfate products and freeze-thaw cycles requires further testing. Therefore, the modified surface of the selected samples was examined using a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS).
Visually inspect the sample surface with an electron microscope and select 400X magnification. The degree of surface damage in semi-enclosed and jointless GRP concrete under freeze-thaw cycles and exposure to sulfates is quite high, while in fully enclosed concrete it is negligible. The first category refers to the occurrence of erosion of free-flowing concrete by sodium sulfate and from 0 to 100 freeze-thaw cycles, as shown in Fig. 4a. Concrete samples without frost exposure have a smooth surface without visible features. After 50 erosions, the pulp block on the surface partially peeled off, exposing the white shell of the pulp. After 100 erosions, the shells of the solutions completely fell off during a visual inspection of the concrete surface. Microscopic observation showed that the surface of the 0 freeze-thaw eroded concrete was smooth and the surface aggregate and mortar were in the same plane. An uneven, rough surface was observed on a concrete surface eroded by 50 freeze-thaw cycles. This can be explained by the fact that some of the mortar is destroyed and a small amount of white granular crystals adhere to the surface, which is mainly composed of aggregate, mortar and white crystals. After 100 freeze-thaw cycles, a large area of ​​white crystals appeared on the surface of the concrete, while the dark coarse aggregate was exposed to the external environment. Currently, the concrete surface is mostly exposed aggregate and white crystals.
Morphology of an erosive freeze-thaw concrete column: (a) unrestricted concrete column; (b) semi-enclosed carbon fiber reinforced concrete; (c) GRP semi-enclosed concrete; (d) fully enclosed CFRP concrete; (e) GRP concrete semi-enclosed concrete.
The second category is the corrosion of semi-hermetic CFRP and GRP concrete columns under freeze-thaw cycles and exposure to sulfates, as shown in Fig. 4b, c. Visual inspection (1x magnification) showed that a white powder gradually formed on the surface of the fibrous layer, which quickly fell off with an increase in the number of freeze-thaw cycles. The unrestricted surface erosion of semi-hermetic FRP concrete became more pronounced as the number of freeze-thaw cycles increased. The visible phenomenon of “bloating” (the open surface of the solution of the concrete column is on the verge of collapse). However, the peeling phenomenon is partially hampered by the adjacent carbon fiber coating). Under the microscope, synthetic carbon fibers appear as white threads on a black background at 400x magnification. Due to the round shape of the fibers and exposure to uneven light, they appear white, but the carbon fiber bundles themselves are black. Fiberglass is initially white thread-like, but upon contact with the adhesive it becomes transparent and the state of the concrete inside the fiberglass is clearly visible. The fiberglass is bright white and the binder is yellowish. Both are very light in color, so the color of the glue will hide the fiberglass strands, giving the overall look a yellowish tint. The carbon and glass fibers are protected from damage by an external epoxy resin. As the number of freeze-thaw attacks increased, more voids and a few white crystals became visible on the surface. As the sulfate freezing cycle increases, the binder gradually becomes thinner, the yellowish color disappears and the fibers become visible.
The third category is the corrosion of fully enclosed CFRP and GRP concrete under freeze-thaw cycles and exposure to sulfates, as shown in Fig. 4d, e. Again, the observed results are similar to those for the second type of constrained section of the concrete column.
Compare the phenomena observed after applying the three containment methods described above. The fibrous tissues in fully insulated FRP concrete remain stable as the number of freeze-thaw cycles increases. On the other hand, the adhesive ring layer is thinner on the surface. Epoxy resins mostly react with active hydrogen ions in open-ring sulfuric acid and hardly react with sulfates28. Thus, it can be considered that erosion mainly changes the properties of the adhesive layer as a result of freeze-thaw cycles, thereby changing the reinforcing effect of FRP. The concrete surface of FRP semi-hermetic concrete has the same erosion phenomenon as unrestricted concrete surface. Its FRP layer corresponds to the FRP layer of fully enclosed concrete, and the damage is not obvious. However, in semi-sealed GRP concrete, extensive erosional cracks occur where the fiber strips intersect with the exposed concrete. Erosion of exposed concrete surfaces becomes more severe as the number of freeze-thaw cycles increases.
The interiors of fully enclosed, semi-enclosed, and unrestricted FRP concrete showed significant differences when subjected to freeze-thaw cycles and exposure to sulfate solutions. The sample was cut transversely and the cross section was observed using an electron microscope at 400x magnification. On fig. 5 shows microscopic images at a distance of 5 mm, 10 mm and 15 mm from the boundary between concrete and mortar, respectively. It has been observed that when sodium sulfate solution is combined with freeze-thaw, concrete damage is progressively broken down from the surface to the interior. Because the internal erosion conditions of CFRP and GFRP-constrained concrete are the same, this section does not compare the two containment materials.
Microscopic observation of the inside of the concrete section of the column: (a) completely limited by fiberglass; (b) semi-enclosed with fiberglass; (c) unlimited.
Internal erosion of FRP fully enclosed concrete is shown in fig. 5a. Cracks are visible at 5 mm, the surface is relatively smooth, there is no crystallization. The surface is smooth, without crystals, 10 to 15 mm thick. Internal erosion of FRP semi-hermetic concrete is shown in fig. 5 B. Cracks and white crystals are visible at 5mm and 10mm, and the surface is smooth at 15mm. Figure 5c shows sections of concrete FRP columns where cracks were found at 5, 10 and 15 mm. A few white crystals in the cracks became progressively rarer as the cracks moved from the outside of the concrete to the inside. Endless concrete columns showed the most erosion, followed by semi-constrained FRP concrete columns. Sodium sulfate had little effect on the interior of fully enclosed FRP concrete samples over 100 freeze-thaw cycles. This indicates that the main cause of erosion of fully constrained FRP concrete is associated freeze-thaw erosion over a period of time. Observation of the cross section showed that the section immediately prior to freezing and thawing was smooth and free of aggregates. As the concrete freezes and thaws, cracks are visible, the same is true for aggregate, and the white granular crystals are densely covered with cracks. Studies27 have shown that when concrete is placed in a sodium sulfate solution, sodium sulfate will penetrate into the concrete, some of which will precipitate as sodium sulfate crystals, and some will react with cement. Sodium sulfate crystals and reaction products look like white granules.
FRP completely limits concrete cracks in conjugated erosion, but the section is smooth without crystallization. On the other hand, FRP semi-closed and unrestricted concrete sections have developed internal cracks and crystallization under conjugated erosion. According to the description of the image and previous studies29, the joint erosion process of unrestricted and semi-restricted FRP concrete is divided into two stages. The first stage of concrete cracking is associated with expansion and contraction during freeze-thaw. When sulphate penetrates the concrete and becomes visible, the corresponding sulphate fills cracks created by shrinkage from freeze-thaw and hydration reactions. Therefore, sulfate has a special protective effect on concrete at an early stage and can improve the mechanical properties of concrete to a certain extent. The second stage of sulfate attack continues, penetrating cracks or voids and reacting with the cement to form alum. As a result, the crack grows in size and causes damage. During this time, the expansion and contraction reactions associated with freezing and thawing will exacerbate internal damage to the concrete, resulting in a reduction in bearing capacity.
On fig. 6 shows the pH changes of concrete impregnation solutions for three limited methods monitored after 0, 25, 50, 75, and 100 freeze-thaw cycles. Unrestricted and semi-closed FRP concrete mortars showed the fastest pH rise from 0 to 25 freeze-thaw cycles. Their pH values ​​increased from 7.5 to 11.5 and 11.4, respectively. As the number of freeze-thaw cycles increased, the pH rise gradually slowed down after 25-100 freeze-thaw cycles. Their pH values ​​increased from 11.5 and 11.4 to 12.4 and 11.84, respectively. Because the fully bonded FRP concrete covers the FRP layer, it is difficult for sodium sulfate solution to penetrate. At the same time, it is difficult for the cement composition to penetrate into external solutions. Thus, the pH gradually increased from 7.5 to 8.0 between 0 and 100 freeze-thaw cycles. The reason for the change in pH is analyzed as follows. The silicate in concrete combines with hydrogen ions in water to form silicic acid, and the remaining OH- raises the pH of the saturated solution. The change in pH was more pronounced between 0-25 freeze-thaw cycles and less pronounced between 25-100 freeze-thaw cycles30. However, it was found here that the pH continued to increase after 25-100 freeze-thaw cycles. This can be explained by the fact that sodium sulfate reacts chemically with the interior of the concrete, changing the pH of the solution. Analysis of the chemical composition shows that concrete reacts with sodium sulfate in the following way.
Formulas (3) and (4) show that sodium sulfate and calcium hydroxide in cement form gypsum (calcium sulfate), and calcium sulfate further reacts with calcium metaaluminate in cement to form alum crystals. Reaction (4) is accompanied by the formation of basic OH-, which leads to an increase in pH. Also, since this reaction is reversible, the pH rises at a certain time and changes slowly.
On fig. 7a shows the weight loss of fully enclosed, semi-enclosed, and interlocked GRP concrete during freeze-thaw cycles in sulfate solution. The most obvious change in mass loss is unrestricted concrete. Unrestricted concrete lost about 3.2% of its mass after 50 freeze-thaw attacks and about 3.85% after 100 freeze-thaw attacks. The results show that the effect of conjugated erosion on the quality of free-flow concrete decreases as the number of freeze-thaw cycles increases. However, when observing the surface of the sample, it was found that the loss of mortar after 100 freeze-thaw cycles was greater than after 50 freeze-thaw cycles. Combined with the studies in the previous section, it can be hypothesized that the penetration of sulfates into concrete leads to a slowdown in mass loss. Meanwhile, internally generated alum and gypsum also result in slower weight loss, as predicted by chemical equations (3) and (4).
Weight change: (a) relationship between weight change and number of freeze-thaw cycles; (b) relationship between mass change and pH value.
The change in weight loss of FRP semi-hermetic concrete first decreases and then increases. After 50 freeze-thaw cycles, the mass loss of semi-hermetic fiberglass concrete is about 1.3%. Weight loss after 100 cycles was 0.8%. Therefore, it can be concluded that sodium sulfate penetrates into free-flowing concrete. In addition, observation of the surface of the test piece also showed that the fiber strips could resist mortar peeling in an open area, thereby reducing weight loss.
The change in mass loss of fully enclosed FRP concrete is different from the first two. Mass does not lose, but adds. After 50 frost-thaw erosions, the mass increased by about 0.08%. After 100 times, its mass increased by about 0.428%. Since the concrete is completely poured, the mortar on the surface of the concrete will not come off and is unlikely to result in loss of quality. On the other hand, the penetration of water and sulfates from the high content surface into the interior of the low content concrete also improves the quality of the concrete.
Several studies have previously been conducted on the relationship between pH and mass loss in FRP-restricted concrete under erosive conditions. Most of the research mainly discusses the relationship between mass loss, elastic modulus and strength loss. On fig. 7b shows the relationship between concrete pH and mass loss under three constraints. A predictive model is proposed to predict concrete mass loss using three retention methods at different pH values. As can be seen in Figure 7b, Pearson’s coefficient is high, indicating that there is indeed a correlation between pH and mass loss. The r-squared values ​​for unrestricted, semi-restricted, and fully restricted concrete were 0.86, 0.75, and 0.96, respectively. This indicates that the pH change and weight loss of fully insulated concrete is relatively linear under both sulfate and freeze-thaw conditions. In unrestricted concrete and semi-hermetic FRP concrete, the pH gradually increases as the cement reacts with the aqueous solution. As a result, the concrete surface is gradually destroyed, which leads to weightlessness. On the other hand, the pH of fully enclosed concrete changes little because the FRP layer slows down the chemical reaction of the cement with the water solution. Thus, for a fully enclosed concrete, there is no visible surface erosion, but it will gain weight due to saturation due to the absorption of sulfate solutions.
On fig. 8 shows the results of an SEM scan of samples etched with sodium sulfate freeze-thaw. Electron microscopy examined samples collected from blocks taken from the outer layer of concrete columns. Figure 8a is a scanning electron microscope image of unenclosed concrete before erosion. It is noted that there are many holes on the surface of the sample, which affect the strength of the concrete column itself before frost-thawing. On fig. 8b shows an electron microscope image of a fully insulated FRP concrete sample after 100 freeze-thaw cycles. Cracks in the sample due to freezing and thawing may be detected. However, the surface is relatively smooth and there are no crystals on it. Therefore, unfilled cracks are more visible. On fig. 8c shows a sample of semi-hermetic GRP concrete after 100 frost erosion cycles. It is clear that the cracks widened and grains formed between the cracks. Some of these particles attach themselves to cracks. An SEM scan of a sample of an unrestricted concrete column is shown in Figure 8d, a phenomenon consistent with semi-restriction. To further elucidate the composition of the particles, the particles in the cracks were further magnified and analyzed using EDS spectroscopy. Particles basically come in three different shapes. According to the energy spectrum analysis, the first type, as shown in Figure 9a, is a regular block crystal, mainly composed of O, S, Ca and other elements. By combining the previous formulas (3) and (4), it can be determined that the main component of the material is gypsum (calcium sulfate). The second one is shown in Figure 9b; according to the energy spectrum analysis, it is an acicular non-directional object, and its main components are O, Al, S and Ca. Combination recipes show that the material consists mainly of alum. The third block shown in Fig. 9c, is an irregular block, determined by energy spectrum analysis, mainly consisting of components O, Na and S. It turned out that these are mainly sodium sulfate crystals. Scanning electron microscopy showed that most of the voids were filled with sodium sulfate crystals, as shown in Figure 9c, along with small amounts of gypsum and alum.
Electron microscopic images of samples before and after corrosion: (a) open concrete before corrosion; (b) after corrosion, the fiberglass is completely sealed; (c) after corrosion of GRP semi-enclosed concrete; (d) after corrosion of open concrete.
The analysis allows us to draw the following conclusions. The electron microscope images of the three samples were all 1k× and cracks and erosion products were found and observed in the images. Unrestricted concrete has the widest cracks and contains many grains. FRP semi-pressure concrete is inferior to non-pressure concrete in terms of crack width and particle count. Fully enclosed FRP concrete has the smallest crack width and no particles after freeze-thaw erosion. All of this indicates that fully enclosed FRP concrete is the least susceptible to erosion from freeze and thaw. Chemical processes inside semi-enclosed and open FRP concrete columns lead to the formation of alum and gypsum, and sulfate penetration affects porosity. While freeze-thaw cycles are the main cause of concrete cracking, sulfates and their products fill some of the cracks and pores in the first place. However, as the amount and time of erosion increases, the cracks continue to expand and the volume of alum formed increases, resulting in extrusion cracks. Ultimately, freeze-thaw and sulfate exposure will reduce the strength of the column.