Retrofitting of heat-damaged fiber-reinforced concrete cylinders using welded wire mesh configurations

This study used a normal-strength mix with coarse aggregate sizes of 20 and 10 mm, aiming for a cylinder compressive strength of 30 MPa after 28 days. The materials that were utilized in the production of all concrete cylinders consisted of ordinary Portland cement (Type I) possessing a specific gravity of 3.15; natural silica sand with a fineness modulus of 1.65 and a specific gravity of 2.60, while the specific gravities for the coarse aggregates and crushed sand were 2.781% and 2.567%, respectively; tap water; and a superplasticizer (melamine formaldehyde sulfonated) with a specific gravity of 1.21. To ensure consistent workability, the superplasticizer was used in all mixtures. Fiber-reinforced concrete was prepared using hooked-end steel (SF) and polypropylene fibers (PP), which were purchased from local market suppliers. The tensile strengths of SF and PP were 1225 and 550 MPa, respectively, while their elastic modulus was 200 and 4 GPa, respectively. The rectangular PP fibers had dimensions of 0.6 mm in thickness, 1 mm in width, and 50 mm in length. The diameter and length of the circular SF were 0.75 mm and 60 mm, respectively. The SF and PP fibers had specific gravities of 7.85 and 0.9, respectively. Figure 1 and Table 1 show the characteristics of the fibers used in the study.

Fig. 1.

Table 2 shows the component quantities used to prepare normal-strength concrete mixes. Table 3 presents the distribution of SF, PP, or hybrid fibers in terms of volume (or by weight) within the concrete mixes. It is important to mention that in Table 3, the abbreviations “M-C0”, “M-SF”, “M-PPF”, and “M-SF+PPF” are used to denote several types of mixes, namely: mixes without SF and PP, mixes with SF fibers only, and mixes with a combination of SF and PP fibers, respectively. Table 3 displays the test matrix used in this study’s experimental program. All test cylinders were replicated three times for the unheated, heated, unstrengthened, and strengthened specimens to ensure the consistency of the data and enhance confidence in the findings of this study. The specimens in this investigation were 48 concrete cylinders that had a diameter of 150 mm and a height of 300 mm. The primary variables considered in the testing program consisted of (i) the influence of various fiber types (SF, PP, and hybrid fibers), (ii) exposure temperature (26°C and 600°C), and (iii) WWM strengthening.

The cylinder specimens were fabricated by molding. To prepare fiber-reinforced concrete, cement, sand, and gravel were first mixed in the desired mixtures. Water was added to the dry mix, and then stirred to make a uniform concrete slurry. The distribution of the fibers in the concrete mixture is a vital factor that significantly influences the mechanical properties of concrete. Therefore, the fibers were then added gradually to the mixer while mixing continued to ensure that the fibers were evenly distributed without forming clumps. The mixing continued for an additional 5 minutes to achieve a uniform distribution of the fibers. To mitigate segregation, the concrete mix was gradually poured into the designated molds. Additionally, a vibrator was used to carefully shake the concrete, preventing the formation of voids. Figure 2 shows the cylinder specimens on the vibration table during their pouring into the molds. The upper surface was carefully leveled using a steel trowel to guarantee a flat surface, and the stresses were not concentrated during the test. We submerged the specimens in a water tank for 28 days to undergo a curing process.

Fig. 2.

Table 4 presents the features of the WWM used in the specimen strengthening. The WWM had a hole size of 13 mm and a diameter of 0.6 mm. Table 4 also presents the properties of the mortar employed in the WWM jacketing. The sandblasting procedure was applied to the concrete surface to create a roughened surface, hence improving the adhesive strength between the strengthening materials and the concrete surface, as shown in Figure 3(a). Prior to the application of the WWM strengthening, the concrete surface underwent cleaning to eliminate any waste particles from the sandblasting procedure. Prior to applying the WWM jacketing, a 2 mm coating of adhesive mortar was placed on the concrete surface. The first layer of WWM was implemented and carefully pushed into the mortar layer, as shown in Figure 3(b). An additional application of adhesive mortar was carried out to cover the WWM completely, as shown in Figure 3(c). The second layer of WWM was applied using the same processes as the first layer. A 10 mm gap was kept at the ends of the specimens to prevent the direct application of load to the WWM during testing. Figure 3(d) shows the final appearance of the specimens after being strengthened with WWM jackets.

Fig. 3.

Prior to heating, the specimens were allowed to dry naturally for 120 days following the end of the 28-day curing phase. This procedure is used to guarantee that the specimens are completely dry and to manage any potential shrinkage strain or explosion brought on by the abrupt temperature increase. As shown in Figure 4, the specimens were heated using an electrical oven with internal dimensions of 1.0 × 1.0 × 1.0 m. The cylinders were subjected to a controlled heating process, increasing their temperature from the ambient temperature (i.e., 26°C) to the predetermined target temperature (i.e., 600°C) at an average rate of 8°C per minute. Figure 5 displays the time-temperature curve used for heating the cylinders. Figure 5 illustrates the typical temperature-time curve employed for conducting structure fire resistance tests according to ISO 834 [49]. The figure clearly demonstrates that the oven utilized in the current investigation was incapable of attaining fast heating rates that simulate those of a typical fire. Nevertheless, it is important to note that the typical fire test may not accurately simulate the true heating of concrete in an actual building during a genuine fire [3]. The factory-supplied Type-K thermocouples were used to measure the internal temperature of the oven. The cylinders were exposed to a high temperature of 600°C for a duration of 3 hours to guarantee that the temperature was evenly distributed inside the cylinder. Following the heating cycle, the cylinders were left to cool before testing and strengthening.

Fig. 4.

Fig. 5.

The cylinders tested in this study were subjected to an axial compression load to determine the stress-strain relationship. The cylinders were topped with sulfur on their top surface to ensure a perfectly flat surface for conducting the test. An axial strain measurement was conducted using a compressometer equipped with two Linear Variable Differential Transformer (LVDTs ) positioned around the specimen, as illustrated in Figure 6. The cylinders were subjected to compressive pressure at a rate of 0.2 MPa/s until failure, following the procedure suggested by ASTM C39 [50].

Fig. 6.

Figure 7 shows typical examples of failure modes observed in control and strengthened specimens following compression tests and exposure to heating. The control samples (without fibers) had brittle failure due to concrete crushing, as shown in Figure 7. The control samples (without fibers) subjected to high temperatures exhibited a reduced tendency for brittle failure. At first, the concrete showed premature cover spalling. Subsequently, there was a failure when the axial load was decreased in comparison with samples of the same kind that were subjected to room temperature. The concrete exhibited a modest apparent visual discoloration under exposure to a temperature of 600°C. Thermal cracking was noted on the concrete’s surface of specimens without fibers at 600°C. However, the failure mode of the heated specimens was remarkably similar to that of the unheated specimens. There are fewer instances of violent failures when compared to the unheated specimens. The C0-RT specimens exhibited significant cracks and crushing of the concrete, first in the upper portion (i.e., the loading point), and then at the middle points of the specimen. Initially, the concrete cover exhibited premature spalling, which was subsequently followed by failure at its strength capability. The same trend was noticed in the PPF-RT, where the cracks started from the loading point to the middle points of the specimen, in comparison with the SF-RT and SF+PPF-RT specimens. Nevertheless, the presence of fibers in mixes prevented thermal cracks on the surface. The crack widths of the specimens with steel fibers (i.e., SF-RT and SF+PPF-RT) were smaller than those of the specimens without fibers (i.e., C0-RT and PPF-RT). In addition, the C0-RT and PPF-RT specimens exhibited narrower cracks compared to the SF-RT and SF+PPF-RT specimens. This result may be attributed to the incorporation of SF, PP, and SF+PP fibers in concrete specimens creating a bridging influence between cracks, similar to the outcomes mentioned in previous studies (e.g., [51, 52]). The inclusion of fiber can change the failure mode from brittle to ductile by enhancing the concrete’s toughness, aligning with findings from previous studies [53, 54]. In general, the specimens without fibers failed in explosive/brittle mode. Conversely, this did not occur in the specimens that had fibers (i.e., became less explosive/brittle) due to the random distribution of the fibers within the concrete matrix. When specimens containing PP fibers were preheated to temperatures of 600°C, the failure of the specimens became more ductile. This can be attributed to the fibers melting, leading to the development of pores within the concrete core. In addition, when SF specimens were subjected to temperatures of 600°C, the failure exhibited increased ductility compared to the specimens without fibers. However, it still displayed greater brittleness than the specimens containing PP fibers. The failure mode of heated specimens containing hybrid fibers exhibited characteristics that fell between those of specimens containing only SF and those containing only PP fibers. Figure 7 is a typical photograph of the failure mode of all the cylinders strengthened with WWM jackets in comparison to the control specimens. Generally, as loading increased, small cracks on the mortar surface that enveloped the WWM were observed. The cracks progressively expanded and grew wider until reaching their peak strength. At this point, the cracks reached their maximum length at the ends of the cylinders’ cross-sections, coinciding with a rapid decrease in the resisting load. The WWM failed across the specimen’s height, which led to the failure of the specimens. Close to the maximum load, the WWM layers were seen to expand outward before experiencing a rapid failure. In some of the specimens, vertical cracks were observed across the specimen’s height, as shown in Figure 7. The cylinders strengthened with WWM jackets experienced failure on the ends (i.e., loading point), followed by further failure at the midpoint. The strengthened specimens subjected to higher temperatures showed reduced susceptibility to brittle failure in comparison to the specimens subjected to room temperature (26°C). In general, the heated specimens produced a quieter sound, while the unheated specimens failed suddenly with a loud explosive sound. The observed test behavior and failure manner indicated that the transverse wires in unheated specimens (i.e., C0-RT-S, SF-RT-S, PPF-RT-S, and SF+PPF-RT-S) experienced hoop tension, resulting in the generation of passive confinement pressure. As the load increased, more prominent and broader cracks became visible, eventually resulting in the separation and protrusion of the mortar, especially in the C0-600-S, SF-600-S, and PPF-600-S specimens, as shown in Figure 7. Several meshes obtained from the failed specimens exhibited fractured horizontal wires, indicating that these wires yielded as a result of hoop tension.

Fig. 7.

The loss in compressive strength exhibited variability across different mixes based upon the specific type of fibers employed, as displayed in Figure 8. The investigation demonstrated that the residual compressive strength of all the concretes, regardless of the presence of fibers, declined as the exposure temperature rose. This phenomenon can be attributed to the decrease in moisture content and the breaking down of the hydrated cement paste [48]. The findings indicate that the inclusion of steel fibers alone resulted in a 34.7% increase in compressive strength at room temperature. Nevertheless, the addition of PP fibers alone led to a slight increase of 8.8% in compressive strength at room temperature. The incorporation of hybrid fibers (SF and PP) resulted in a significant enhancement in compressive strength at normal room temperature conditions. Specifically, the compressive strength increased by approximately 34.0%. The notable enhancement in compressive strength observed in the concrete with steel fibers can be attributed to the utilization of steel fibers with a larger aspect ratio of (L/d = 80) in this study, which effectively harnesses the strength of the fibers. The observed range of enhancement in compressive strength aligns with previous research findings [3, 35, 36].

Fig. 8.

After exposing the specimens to a temperature of 600°C, the degree of decrease in compressive strength ranged from 23.7% to 53.3%. The percentage loss in strength for plain concrete was 53.3%, whereas the samples with SF alone, PP alone, and hybrid of steel and PP showed a reduction of 29.9%, 45.1%, and 23.7%, respectively. The range of reduction in strength is within the range of the previous studies [15]. This clearly shows that the presence of PP fiber can contribute to enhancing the overall performance of concrete under high-temperature conditions due to an increase in pore volume after the melting of the fibers, potentially allowing gases to escape and thereby contributing to enhancing the compressive strength of the material [1, 7, 55].

Figure 8 presents the average compressive strength of all the specimens. Based on the results, it is evident that the strengthened concrete specimens exhibited greater compressive strength in comparison to the unstrengthened specimens. This can be attributed to increased confinement due to the presence of WWM jacketing. When exposed to room temperature and enclosed in WWM jacketing, the C0-RT-S, SF-RT-S, PPF-RT-S, and SF+PPF-RT-S specimens showed an increase in compressive strength of 68.5%, 74.4%, 82.7%, and 63.2%, in comparison to the C0-RT, SF-RT, PPF-RT, and SF+PPF-RT control specimens that were exposed to room temperature, respectively. Furthermore, when exposed to a temperature of 600°C, the compressive strength of the C0-600-S, SF-600-S, PPF-600-S, and SF+PPF-600-S specimens increased by 112.1%, 98.2%, 91.7%, and 43.4% in comparison to the C0-600, SF-600, PPF-600, and SF+PPF-600 control specimens that were exposed to room temperature, respectively. Application of WWM jacketing to strengthen specimens that had been damaged by being subjected to high temperatures resulted in a 38.8%, 4.9%, and 9.4% increase in compressive strength for SF-600-S, PPF-600-S, and SF+PPF-600-S specimens, respectively, in comparison to the initial compressive strength of the undamaged specimens C0-RT, SF-RT, PPF-RT, and SF+PPF-RT. However, the C0-600-S specimen showed a slight decrease of 1% compared with C0-RT, as shown in Figure 8.

The stress-strain response for concrete is a significant property for evaluating the overall behavior of RC structures under high temperatures (i.e., 600°C). Figures 9(a) and 9(b) show the stress-axial strain curves for unheated and heated specimens, respectively. To determine the average stress, the compressive load was divided by the cylinder’s cross-sectional area. The axial strains were calculated as the ratio of the average axial displacement (as measured by the LVDTs) and the length of the gage. The elastic modulus decreases as the temperature rises from ambient (i.e., 26°C) to 600°C, which can be clearly noted in the stressstrain curves. As shown in Figure 9(a), the stressstrain curves for the unheated specimens demonstrated a generally linear trend until it reached peak strengths of 30.2, 40.7, 32.9, 40.5, 50.6, 71.0, 60.1, and 66.1 MPa for C0-RT, SF-RT, PPF-RT, SF+PPF-RT, C0-RT-S, SF-RT-S, PPF-RT-S, and SF+PPF-RT-S, respectively. The stress-strain curves demonstrated a decrease in slope, leading to a nonlinear behavior. After reaching maximum strength, the concrete had cracks that progressively widened, leading to complete failure due to the concrete splitting. The same trend was noticed in the heated specimens; the stress-strain curves for the unheated specimens demonstrated a generally linear trend until they reached peak strengths, as shown in Figure 9(b). The specimens with fibers (SF and a combination of SF+PPF) showed a gradual decline in the curve due to the ability of the fibers to bridge the cracks, which resulted in enhanced post-peak ductility, as shown in Figure 9. Although the heated specimens exhibited a decrease in compressive strength, the stress-strain curves did not have a sudden drop but rather were nearly flat curves, as shown in Figure 9(b). Analysis of those curves indicates that when subjected to a temperature of 600°C, concrete with and without fibers experiences a greater reduction in stiffness compared to a reduction in compressive strength, which will be discussed in the following section. The decrease in strength due to temperature increases can be attributed to the hydrated cement paste disintegrating and losing moisture content. Other researchers [3] have also reported similar findings, showing that the residual strength of a concrete mix with fibers significantly decreases when subjected to temperatures over 600°C. Furthermore, it is seen that the concrete with SF exhibited a superior capacity for energy absorption (the area under the stress-strain curve) and elastic modulus following exposure to high temperatures in comparison to the concretes with no fibers, PP, and hybrid fibers. The reason for this is that SFs possess superior mechanical characteristics (i.e., elastic modulus and strength) compared to PP fibers, and SFs are also capable of withstanding higher temperatures. PP fibers, however, could melt when exposed to high temperatures. Furthermore, the decrease in modulus of elasticity and energy absorption capacity caused by heating was comparatively smaller in concretes that contained hybrid fibers, in contrast to concretes that just had PP fibers. The addition of SF addressed the deficiencies of the PP fibers, namely in terms of tensile strength and elastic modulus. However, the PP fibers contributed to the reduction of porosity in the concrete mix [3]. Figure 9 presents comparisons of the stress-strain curves for the strengthened and unstrengthened specimens. The figure demonstrates that the use of WWM jacketing greatly enhanced the specimen’s strength, strain at ultimate strength, and the post-peak response of all concrete mixes, regardless of whether heated or unheated. SF concrete specimens and hybrid fiber concrete specimens (i.e., SF+PPF) have shown superior performance compared to the other types of mixes (C0 and PPF specimens) in both unstrengthened and strengthened conditions. The failure of the strengthened specimens was primarily attributed to vertical crack widening caused by the yielding of the WWM with an inability to maintain the specimen’s strength. The observed specimen behavior and failure pattern indicate that the WWM experienced hoop tension, resulting in passive confinement pressure. As the axial load increased, the cracks became more prominent and wider, occasionally resulting in mortar separation and bulging, as shown in Figure 7. Several WWMs in the crushed specimens exhibited fractured horizontal wires, indicating that these wires yielded as a result of hoop tension.

Fig. 9.

Figure 10 shows the compressive strength of unheated and heated specimens to evaluate the fiber type effects on the compressive strength following exposure to high temperatures. This describes the residual strength of the concrete after being subjected to high temperature (i.e., 600°C) compared to its strength at room temperature (i.e., 26°C). The results presented clearly demonstrate that the inclusion of fibers has a substantial effect on the compressive strength of concrete, regardless of fiber type. Incorporating fibers significantly mitigated the decline in compressive strength caused by exposure to high temperatures compared to concrete mixes without fiber. Therefore, the decrease in the compressive strength of heated fiber-reinforced concrete cylinders was smaller compared to the control concrete specimens (i.e., plain mix). The SF-RT, PPF-RT, and SF+PPF-RT showed an increased ratio of 34.7%, 8.9%, and 34.1%, respectively, compared to control concrete specimens (i.e., C0-RT). However, the reduction in the compressive strength of heated specimens of C0-600, SF-600, PPF-600, and SF+PPF-600 was 53.3%, 29.9%, 45.3%, and 23.7% compared to unheated specimens of C0-RT, SF-RT, PPF-RT, and SF+PPF-RT, respectively. Based on the findings provided in Figure 10, it can be generally concluded that the fibers can be ranked in terms of increasing efficiency as follows: SF alone, hybrid fibers (SF+PPF), and finally PPF alone. The steel fibers are superior because of their bridging effect. The rationale for this is that SFs have better mechanical properties (e.g., elastic modulus and strength) than PP fibers, and SFs can also endure greater temperatures. The enhanced compressive strength of the fiber-reinforced mixes can be attributed to the strong bonding effect between the concrete matrix and the fibers. This bonding effect reduces the formation and propagation of cracks, allowing the specimens to continue resisting axial loads even in the presence of cracks [53]. In addition to being shorter, PP fibers have a lower elasticity modulus and tensile strength compared to SFs. Thus, PP fibers have the ability to bridge microcracks but do not have a substantial influence on the compressive strength, which is comparable to the outcome mentioned in previous studies (e.g., [7, 56, 57]).

Fig. 10.

The RC members’ stiffness significantly influences the RC structures’ load distribution. Therefore, the strengthening method for damaged RC members needs to be able to restore both stiffness and compressive strength. Enhanced understanding of the effects of being subjected to high temperatures and the WWM jacketing on the elastic behavior of the specimens was achieved by evaluating the initial stiffness of the tested cylinders. The initial stiffness was determined as a ratio of the stress to the strain at 40% of the ultimate axial load. At this stage, no cracks were detected in the cylinder specimens. Table 5 illustrates the compressive strength and initial stiffness test results for each concrete cylinder. Table 5 and Figure 9 demonstrate that subjecting the cylinder specimens to a temperature of 600°C for a duration of 3 hours resulted in a notable reduction in the initial stiffness. Regarding the exposure to a temperature of 600°C, the use of WWM jacketing as well as the presence of fibers resulted in decreased initial stiffness. The reduction in the initial stiffness of heated specimens of C0-600, SF-600, PPF-600, and SF+PPF-600 showed an increased ratio of 89.2%, 82.9%, 85.0%, and 83.7% compared to unheated specimens of C0-RT, SF-RT, PPF-RT, and SF+PPF-RT, respectively. It can be noticed that the heated specimens that had PP fiber showed the lowest initial stiffness compared to all specimens, which can be attributed to the fact that PP melts when exposed to high temperatures. However, strengthening the heated specimens led to a reduction in the loss of initial stiffness compared to the heated specimens before strengthening. For example, the initial stiffness loss reached 27.6% in the presence of SF, compared to 82.9% for the same specimen before strengthening. Nevertheless, the presence of fibers in mixes resulted in increased initial stiffness at room temperature. The SF-RT, PPF-RT, and SF+PPF-RT showed an increased ratio of 32.6%, 18.8%, and 34.1%, respectively, compared to control concrete specimens (i.e., C0-RT). In addition, the same trend was noticed when using WWM jacketing to strengthen the specimens. The C0-RT-S, SF-RT-S, PPF-RT-S, and SF+PPF-RT-S showed an increased ratio of 28.9%, 77.4%, 8.0%, and 71.8%, respectively, compared to control concrete specimens (i.e., C0-RT). The stiffness of the specimens in the elastic zone was dependent on the confinement strength and the elastic modulus of the WWM. The result showed that the PP fibers make a small contribution to the specimen’s initial stiffness, which can be attributed to the PP fibers having a lower elastic modulus and tensile strength compared to SFs. The decreased initial stiffness of heated specimens is primarily attributed to the occurrence of micro-cracks caused by heat and the subsequent loss of water, which leads to the softening and creation of pores in the concrete. Consequently, heated concrete specimens are anticipated to display greater lateral expansion when compressed axially compared to unheated concrete specimens [13].