Flexural performance evaluation of slab using welded bar mat

ABSTRACT This is an experimental study on flexural performance evaluation when welded bar mats (WBM), which improve the rib shape, strength, and diameter limits of the existing structural welded wire fabric (WWF), are applied to slabs. The purpose of this study was to verify the change in behavior and structural performance when a new material was applied to a structural member. Flexural tests were performed with a total of 10 slab specimens using various variables. The load-deflection relationship and crack patterns of the slab specimens were analyzed to evaluate the applicability of WBM as a structural reinforcement. The results of the structural test were compared with the proposed design formula of the current design standards to check the safety, and the serviceability were verified by comparing the performance with the case of using the conventional rebar. According to the material test results, the yield strength of the steel wires applied to the WBM did not appear clearly, the elastic modulus of the material increased by about 8% compared to that of normal rebar, so that the yield strain and elongation decreased compared to that of normal rebar. When WBM were used for slabs, the nominal strength of the member was found to sufficiently satisfy the design flexural strength. However, as the strength of the material increased, bond failure occurred due to stress concentration between orthogonal steel wires so that deflection and crack concentration phenomena increased.


Introduction
The modern construction market is facing serious problems such as increases in construction costs and relative decreases in productivity in various difficult conditions such as a shortage of professional construction personnel, increases in labor and material costs, and environmental regulations, ( Lee et al. 1994 ) and in addition, many other problems such as the occurrence of safety accidents during field work and poor rebar placing have also been reported. Therefore, instead of field installation of rebar, the use of structural welded wire fabric (WWF) that can be manufactured at a factory in the form of mats to minimize field work, ensure uniform spacing of rebar, and simplify the field rebar placing process, thereby saving labor cost and shortening the construction period, is emerging as an effective alternative. Welded wire fabric is a sort of preassembled rebar manufactured by arranging horizontal and vertical steel wires at right angles and welding the intersections with electrical resistance using cold drawn and surface processed mild steel ( Kim and Kim 2004 ) . It is reported that WWF has high yield strength through this process, but its elongation is lower compared to that of deformed rebar, and it can lead to the degradation of the ductile capacity of the member ( Yoon, Yang, and Yang 1993 ) . In addition, a case was identified indicating that because the yield section is not clear due to the characteristics of steel wires and the bond stress is lowered due to the shape characteristics, the strain localization phenomenon may occur due to the high bearing pressure characteristics of the steel wires welded orthogonally, and these characteristics lead to a brittle failure mode as a result of the breakage of the tensile side steel wires ( Gilbert and Smith 2006 ) . Ferrocement like WWF is widely used to build various elements such as walls, tanks, roofs and bridge decks. Recently, RC structures have collapsed due to large working loads, seismic loads, and durability problems, resulting in billions of dollars in economic loss. Therefore, in the case of new materials such as ferrocement, various experiments and investigations are required to improve the performance until the concrete structure collapses ( Erfan et al. 2021 ) . Studies using many variables have been conducted to confirm the effectiveness of ferrocement, a low-cost building material, and to improve its performance (ACI Committee 549, State of the art report on ferrocement, 1997; ACI Committee 549-1R-88 1993). El-Sayed studied the structural performance and cracking of geopolymer columns using mesh type and number of layers as variables to reduce production cost ( El-Sayed 2021 ) . As a result of the experiment, the number of cracks in the ferrocement column was higher than that of the reinforcement column, and the crack width was also small. In particular, in the case of welded wire mesh columns, it had a significant effect on the enhancement of ultimate load and showed a higher ultimate load than the control group(conventional reinforcing bar reinforced columns). Abdallah et al. analyzed the flexural performance of ferrocement slab panels using welded wire mesh, and as a result of the experiment, they showed excellent ultimate loads under flexural loads ( Abdallah et al. 2019 ) . Basunbul et al. studied the structural performance of a sandwich load-bearing ferrocement wall, and as a result of the experiment, the ferrocement wall panel reinforced with wire mesh showed superior ductility in the transverse and axial directions compared to the same amount of control(conventional reinforced wall panel) ( Basunbul et al. 1991 ) . El-Sayed studied the shear performance and cracking behavior of ferrocement concrete beams using welded wire mesh ( El-Sayed and Erfan 2018 ) . Although there have been previous studies by several researchers on welded wire mesh, it is necessary to review several properties that have not yet been confirmed for this material. In order to identify and solve these problems, this study used welded bar mats (WBM), as shown in Figure 1,to improve the properties of the material and the bond capacity between the structural steel wires and concrete, and to identify the structural performance when WBM were applied to structural members, the flexural performance of the slab members was evaluated. Similar to the existing structural welded wire fabric, WBM are produced by cold working using steel wires, but were improved by refining the shape of the ribs, enhancing the strength of the steel, and enabling the production of larger diameters.

Experimental design
In order to evaluate the performance of flexural members using WBM, a flexural test was conducted with slab specimens along with a material experiment for WBM.
A total of 10 slab specimens were planned as shown in Table 1, and the variables of the test were the shapes of specimens (simple supported slab and continuous slab), the types and strength of reinforcements used, and the compressive strength of concrete.
The first six specimens are slabs simply supported at both ends, and changes in flexural performance according to the strength of reinforcement and concrete were observed for specimens made using conventional rebar and specimens made using WBM.
Specimens 7 to 10 are two-span continuous slabs, and as with the simple supported slabs, those made using the conventional rebar and those made using WBM were compared by observing the changes in the negative moment area above the continuous support point.
All specimens were designed as a rectangular prismatic section, and the cover thickness was set to 20 mm according to the Korean structural design standards. The simple supported slabs were manufactured with length L = 4950 mm, width b = 1200 mm, and thickness H = 150 mm, and the continuous slabs were manufactured with length L = 7100 mm, interior span l -= 3350 mm, width b = 1200 mm, and thickness H = 150 mm. A 400 mm wide beam was installed in the middle of each of the continuous slabs to form an internal support point supporting the slab on both sides. The WBM were manufactured at 200 mm intervals  in both the orthogonal and longitudinal directions using ready-made products, and in the case of the deformed bars, although the same quantity was maintained in the longitudinal direction by placing them at 200 mm intervals as with the WBM, the interval was designed as 450 mm in the orthogonal direction since the quantity corresponding to the shrinkage temperature control rebar required by the design standard was placed. Figures 2, 3 show the details of the rebar placing and cross sections of the simple supported slabs and the continuous slabs.

Simple supported slab
The specimens of simple supported slabs were manufactured in a precast concrete manufacturing factory and initially cured by steam curing, and after 28 days of aging, were transported to the laboratory of Hoseo University and experimented.
The specimen fabrication process is as shown in Figure 4, and the experimental settings are as shown in Figure 5 ( Mansour et al. 2015)., ( Hong 2014 ) Strain gauges were attached to the longitudinal reinforcing bar on the lower side to measure the strain of the  reinforcing bar inside the specimens and the strain at a total of four points, which are two points at the center of the slab and two points at the 3/4 point of the slab, was measured as shown in Figure 6. The deflection of the specimens was measured by installing three LVDTs at intervals of 600 mm including the bottom of the loading point.

Continuous slab
To prevent damage due to movement, the continuous slab specimens were fabricated and cured at the laboratory building of Hoseo University, which is the experimental site, as seen in Figure 7.
As for the experimental setting, a support beam was placed at the center of the specimen as shown in Figure 8, and the LVDTs for measuring the deflection were installed at three points at intervals of 300 mm centering on the bottom of the center of the spans on both sides. As seen in Figure 9, strain gauges for measuring the strain of the reinforcement were installed at four points on the longitudinal rebar at the bottom of the center of each span to measure the strain of the steel at the positive moment point and at four points on the longitudinal rebar at the top of the beam to measure the strain of the steel at the negative moment point.

Concrete
The mechanical properties of concrete were tested for each type of specimen, and compressive strength, modulus of elasticity, and flexural strength were measured.
To measure the compressive strength of concrete, six-cylinder type specimens (Ø100 mm) were tested according to KS F 2405 ( Korean Standard, 2405, 2017),   and the modulus of elasticity was measured using a compressor-meter during the compressive strength test. The flexural strength of concrete was measured using specimens for the flexural strength test according to KS F 2408 ( Korean Standard, 2408, and three specimens per type were tested and the average values were used.
The mechanical properties of concrete such as average compressive strength f cm , elastic modulus E c , and flexural tensile strength f r are shown in detail in Table 2.

Reinforcement
The reinforcements used for the tests were deformed rebar and WBM, and their diameter was D13. As for the test method, KS D 3504 ( Korean Standard, 3504, 2021 ) for deformed rebar and KS D 7017(Korean Standard, 7017, 2021) for WBM were followed. The modulus of elasticity, yield strength f y , tensile strength f t , elongation, and tensile strength/yield strength ratio f t =f y were determined with at least three specimens, and the resultant geometrical and mechanical properties of the steel used are as shown in Table 3. In addition, the representative stress-strain relationship curve of the specimens is as shown in Figure 10.
In Figure 10, the yield point of conventional rebar is clearly shown, while the yield point of WBM is not clearly distinguished. Accordingly, in the case of WBM, the modulus of elasticity was calculated using the 0.2% offset method according to the design standards. In the case of WBM, the elastic modulus of the material increased by 6 ~ 8% compared to normal rebar due to the influence of the cold drawing operation. In addition, the tensile strength/yield strength ratio also showed values in a range of 1.23 ~ 1.30 for normal rebar, but the values were in a range of 1.17 ~ 1.21, which were 93 ~ 95% of those of normal rebar, for WBM. Elongation, which is the strain of the reinforcement measured until break, also showed a decrease to about 2/3 of that of normal rebar in the results of material tests.

Load-deflection relationship
Since the effect of self-load on the flexural strength is large due to the property of the slab specimens, the experimental results were analyzed considering the effect of self-load on the external force data measured during the experiment. That is, as seen in Figure 11, since the load effect due to the slab specimen's self-load corresponds to about 20 kN when converted into the concentrated load of the experimental apparatus, the origin of the data measured in the experiment was matched with the corresponding load in the graph calculated by section analysis and the measurements were compared with the analysis values. The line indicating the analysis results in Figure 11 is the result of a cross-sectional analysis using the characteristic values measured in material tests for all of concrete, WBM, and deformed rebar, and this graph was compared with the experimental results considering the effect of self-load.
As shown in Figure 11(a), in the case of the f ck =21MPa specimens (Nos. 1-3), only the self-load exceeded the crack moment M cr of the slab so that a large initial deflection occurred, and accordingly, the experimental results were corrected assuming the initial deflection due to self-load as 10.4 mm, as seen in the figure.
Meanwhile, for the f ck =35MPa specimens (Nos. 4-6), the crack moment increased due to the increase in the The load was applied in one direction at a constant speed using a 30ton hydraulic jack (stroke 300 mm), and the loading was stopped temporarily at each step of loading to measure cracks. Table 3. Geometric and mechanical properties of reinforcement.
Reinf.  compressive strength of concrete so that the deflection due to self-load was analyzed to be about 4.87 mm, corresponding to the elastic deflection. From the analysis of the experimental results in Figure 11 and Table 4, all specimens are seen to satisfy the design strength of the section analysis. That is, the specimens of all variables were analyzed to have flexural resistance strength exceeding the nominal strength for design. In particular, the specimen with f ck =35MPa did not crack due to its self-load, indicating that the experimental results were very similar to the results of section analysis.
In the case of the WBM specimens, the yield point of the reinforcement was not clearly observed in the experimental results, but showed a pattern of continuously resisting the load even after the design strength. As seen in Figures 12, 13, WBM specimens were analyzed to have the ability to resist the flexural strength of the slab even after the yield strain of the reinforcement. Therefore, it was judged that the structural design is possible using the existing design formula even when WBM are used as reinforcements of flexural members.
However, for f ck =21MPa specimens, the WBM specimens satisfied the flexural strength according to the results of the entire section analysis, but the deflection was shown to increase a little compared to the normal reinforced concrete specimens of the same strength. This phenomenon is considered to be attributable to the small number of cracks and increases in crack width as shown in Table 5, which was judged to be due to the bond capacity between the WBM and concrete. On the other hand, for f ck =35MPa, the stiffness of almost all specimens was found to be similar. Stiffness of specimen was calculated by the slope of loaddeflection curve. As a result, it was analyzed that the effect on stiffness according to the use of WBM is not large. With regard to the strain of reinforcements, as seen in Figures 12, 13, all specimens stably manifested strength up to or over yield strain, and in the case of W400F21 (No. 5), the strain excessively increased in the end part and the center of the span due to the problem of strain gauges, but as for the rest of the specimens, the strain was analyzed to be almost the same.

Crack and failure modes
During the flexural tests, the number of cracks on the side of the specimens and changes in the maximum crack width were observed at each stage of certain loads, and the results are as shown in Table 5. According to the results of analyzing the crack pattern at the time of failure, the number of cracks occurring in the WBM specimens was less than that of the general reinforced concrete specimens. Maximum crack width of the WBM specimens was larger and height of the cracks was higher than the general reinforced concrete specimens.
This phenomenon of concentration of cracks was thought to happen because the rib effect of WBM is more disadvantageous than that of conventional rebar. However, in the case of WBM, the effect on the maximum bond strength is considered insignificant due to the effect of orthogonal welded rebar.
Height of the crack according to the concrete compressive strength was shown to be lower in f ck =21MPa specimens than in f ck =35MPa specimens, and this is  *P cr d : Design cracking load, **P y d : Design yield load, ***P self : Equivalent line load for self-weight, ****P max : Maximum load considered to be attributable to the changes in the neutral axis following the increase in the compressive strength of concrete. Figure 14 shows the final crack pattern in the event of the failure of simple supported slab specimens.

Load-deflection relationship
In the load-deflection relationship of two-span continuous slabs, all specimens with f ck =21MPa (Nos. 7,8) were shown to sufficiently satisfy the flexural resistance strength based on section analysis, as seen in Table 6 and Figure 15, when the effect of self-load was considered as with simple supported slabs. As for the load-deflection relationship measured at the center of the left and right slabs, whereas the deflection on the left and right sides was shown to be similar for normal reinforced concrete slabs, there were slight deviations between the left and right sides in the case of the WBM slabs, but the flexural resistance strength of all specimens was shown to exceed the design resistance strength for section analysis, indicating that there is no problem in the expression of flexural performance in terms of strength. Since excessive  Figure 14. Crack pattern at failure (simple supported). deflection occurring due to an experimental problem may cause serious damage in the negative moment section, the experiment was stopped when the maximum crack width in the negative moment section exceeded 1.8 mm. Therefore, the yield point of the reinforcement could not be found in the load-deflection curve, but stable flexural behaviors were shown even at 134% of the design strength or higher strength.
Therefore, it was judged that both normal rebar and WBM have sufficient flexural performance. Meanwhile, the occurrence of larger deflection in the WBM specimen (No. 8) than in conventional reinforced concrete was analyzed to be attributable to the fact that the local bond of the WBM wires between the orthogonal welding rebar is poorer than that of conventional rebar, as with simple supported slabs.
In specimens (No. 9, 10) with a compressive strength of concrete of f ck =35MPa, the maximum crack width in the negative moment area was shown to be large when the maximum resistance strength of the specimens was 102 ~ 107% of the design flexural strength, and this was judged to be the result of the effect of bond strength between orthogonal welded steel wires that increased as the effect of the load increased. Therefore, in the case of WBM, it can be analyzed that flexural performance similar to that of conventional rebar can be expected at the level of ordinary strength, but the improvement of adhesion performance is required as the strength increases. Also, the strain in the negative moment area at the top of the beam was larger than the strain that occurred in the central section of both spans, and this is considered a valid result given the effect of the load. In addition, it was judged that there was no significant difference between normal rebar and WBM in the distribution of strain as shown in Figure 16.

Cracks and failure mode
In the case of continuous slabs, micro cracks with a width of about 0.1 mm that crossed the slab occurred on the top of the beam during lifting for the removal of the lower form in all specimens except for C-WBM400F21 (No. 8), but the experiment was carried out ignoring the influence because the cracks were closed in the process of specimen setting.
Then, the cracks were measured by stage of certain loads as shown in Table 7. When the f ck =21MPa specimens failed, the crack pattern was opposite to the results of experiments with simple supported slabs as the crack dispersion on the moment section on the top of the beam was shown to be more excellent than that in the case or ordinary reinforced concrete slabs, as seen in Figure 17(a,b). On the other hand, for WBM slabs with f ck =35MPa, many grid pattern cracks occurred on the top of the slabs, as shown in Figure 17(c,d) and this was analyzed to be a phenomenon that happens when the failure of the bond between orthogonal rebar occurs as the level of load increases.

Conclusion
The results of experiments on the flexural performance of slab specimens made using WBM with conventional rebar and welding wires are summarized as follows.
(1) As a result of testing the material properties of the WBM(modified welded wire fabric), the yield strength of WBM did not appear clearly, but the modulus of elasticity increased by 6-8% and the elongation decreased to about 2/3 compared with conventional re-bar.
(2) In terms of flexural strength, all specimens were evaluated to have resistance strength exceeding the nominal strength required by the design standards. It means that strength design using the current design standards is possible when WBM are used as reinforcements of flexural members. However, the safety factor of the maximum resistance strength was evaluated to be lower when high-strength concrete over 35 MPa level and steel wires over 500 MPa level were used than when normal strength concrete and  steel wires were used. Therefore, it is thought necessary to consider the foregoing when highstrength materials are used. (3) As for the safety factor in terms of flexural strength of WBM specimens, the safety factor of the two-span continuous slab decreased significantly more than that of the one-span simple supported slab. This phenomenon is due to the increase in the load concentration in the negative moment area. (4) As for the deflection and crack patterns of members, the deflection of WBM specimens was shown to be larger and the cracks were concentrated. This was analyzed to be attributable to the fact that in the case of WBM, although the overall bond and splice performances could be satisfied due to the effect of the orthogonal welded steel wires, the bond performance of the WBM itself by the rib shape was worse to that of the reinforcing bars, and it was evaluated that the slip of the WBM between the orthogonal welded wires occurs significantly. It could be led to the occurrence of local bond failure. This phenomenon appeared larger as the stress level of the members (5) increased, and accordingly, it is considered that additional studies on the improvement of the rib shape of the steel wires are necessary to apply the WBM up to the high strength level.

Data availability statement
The data that support the findings of this study are available from the corresponding author, [GH Hong(honggh@hoseo. edu)], upon reasonable request.

Disclosure statement
No potential conflict of interest was reported by the author(s).