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Article

Test and Analysis of Concrete Beams Reinforced by Polyurethane Concrete–Prestressed Steel Wires (PUC–PSWs)

by
Wei Li
1,*,
Jiaqi Qiu
2,
Yi Wang
2,
Xilong Zheng
1 and
Kexin Zhang
2
1
School of Civil and Architectural Engineering, Harbin University, No. 109 Zhongxing Road, Harbin 150086, China
2
School of Transportation and Surveying Engineering, Shenyang Jianzhu University, No. 25 Hunnan Zhong Road, Shenyang 110168, China
*
Author to whom correspondence should be addressed.
Buildings 2024, 14(9), 2746; https://doi.org/10.3390/buildings14092746
Submission received: 14 July 2024 / Revised: 21 August 2024 / Accepted: 24 August 2024 / Published: 2 September 2024
(This article belongs to the Section Building Structures)
Browse Figures
Versions Notes

Abstract

:
In order to solve the problems of low tensile strength of composite mortar prone to cracking when reinforced concrete beams are strengthened by traditional methods, this paper proposes a new polyurethane concrete–prestressing wire (PUC–PSW) reinforcement method using polyurethane concrete (PUC) as the wire embedding material. Twelve reinforced concrete T-beams were tested for PUC–PSW flexural reinforcement. These consisted of one unreinforced beam, four PSW-reinforced beams and seven PUC–PSW-reinforced beams. The wire embedding material, wire anchorage form, PUC material depth, amount of wire and loading type were used as variables. The test results show that PUC–PSW reinforcement can significantly increase the yield load and ultimate load of the reinforced beams by 24.1% and 44.6%, respectively, compared with PSW reinforcement. When the load reached 90 kN, the crack widths of PSW-reinforced beam A2 and PUC–PSW-reinforced beam A8 were 0.17 mm and 0.1 mm, respectively. The ability of PUC–PSW reinforcement to limit crack development is better than that of PSW reinforcement, especially after the main beam steel yield. The strength, stiffness and crack-limiting ability of the reinforced beam increase with the PUC thickness of the reinforced layer.

1. Introduction

Bridges play a critical role in road traffic and are vital to the overall transport system. With socio-economic progress, the transport sector is booming and the volume and load capacity of vehicles have increased remarkably, changing dramatically from what they were in the past. Due to rapidly increasing vehicle loads, acid rain, temperature and other natural factors, a series of damages such as structural cracks, deterioration of concrete materials and corrosion of reinforcing steel are caused in bridges during their normal service life [1,2,3]. Damage of the bridge leads to the safety, applicability and durability of the bridge structure being affected to different degrees, resulting in the reduction of the bridge’s bearing capacity, and even endangering the normal operation of the bridge [4]. Appropriate bridge strengthening methods can improve the bearing capacity of bridges and extend their service life [5,6,7].
The bonding steel plate reinforcement method can limit the development of cracks and improve the flexural performance of bridges [8,9,10]. However, steel plates are difficult to accommodate when faced with irregular concrete surfaces and are prone to rusting problems in natural surroundings. The method of attaching fiber materials is to attach high-strength fiber materials to the surface of the reinforced members with epoxy resin to exert the high strength and durability of the composite materials [11,12,13]. However, carbon fiber cloth/plates are weaker and not as effective in improving structural stiffness, as well as having a greater material cost.
Wire mesh–composite mortar reinforcement of reinforced concrete beams has the advantages of convenient construction, less consumables, greater stiffness after reinforcement, stronger resistance to deformation, ability to withstand higher loads and better ductility in case of damage. However, the reinforcement method has a long construction period for wet work, the high strength of the material cannot be fully exploited and the cost is high [14,15].
The prestressed steel wire (PSW) reinforcement method is to anchor the PSW to the reinforced beam body, and spray composite mortar to bond it to the reinforced beam body as a whole [16,17,18]. Compared with the wire mesh–composite mortar reinforcement method, this reinforcement method can give full play to the tensile strength of the wire, and the utilization of the material is greatly improved. The PSW reinforcement method is an active reinforcement method, which improves the stiffness and bearing capacity of the main beam more obviously [19,20]. However, the low tensile strength of the polymer mortar (PM) sprayed on the outside of the wire produces cracking of the composite mortar layer at the original cracks of the reinforced beam. This reduces the durability of the prestressing strands and affects the durability of the original reinforced concrete girders, especially for bridges across rivers with high humidity.
Polyurethane concrete material has the characteristics of light weight, high strength and high toughness, and the material itself has good bond strength and acid and alkali corrosion resistances, due to which it is commonly used in bridge deck paving layers and in the repair of broken concrete structures [21,22,23,24,25,26]. Therefore, the advantages of polyurethane concrete, being wear-resistant, waterproof and corrosion-resistant, with good bond strength among other qualities, are usually used in bridge deck pavement to provide the solution for low-temperature shrinkage, water damage, aging and other typical problems. The advantages of polyurethane concrete, such as early strength and good bond strength, are utilized for effective rapid repair of concrete structures.
In this study, the polyurethane concrete–prestressed steel wire (PUC–PSW) method is proposed to strengthen reinforced concrete structures. The steel wires are tensional and anchored on the surface of the reinforced beam body, and then the steel wires are embedded in the PUC composite material by pouring said material, which is a composite reinforcement method combining the active force of the steel wires and the passive force of the PUC. Compared with the traditional composite mortar–prestressing strand reinforcement, PUC-PSW reinforcement can make full use of the superior material properties of PUC, and the construction is more convenient and can significantly improve the load-carrying capacity of the reinforced beam. For the experimental study on the flexural reinforcement of PUC–PSWs, the reinforcement effect of PUC material was explored with the embedding material, anchoring form of wires, thickness of PUC material, number of wires and loading method as variables.

2. Materials

2.1. Concrete and Steel Bar

All specimens were made of the same concrete mix, and the compressive strength of the concrete cube was 40 MPa. The reinforcing bar yield strength test process is given as follows: select the appropriate steel bar specimen, use the universal testing machine for loading test, gradually apply the load and record the deformation of the specimen and the load value until the observation of obvious plastic deformation through test data is done to calculate the yield strength. The main beam uses 18 mm-diameter grade III rebars in the longitudinal direction, and a yield strength of 418 MPa was measured. The erection bars are 10 mm-diameter grade II rebars, and the measured yield strength is 250 MPa. The measured yield strength of the first-grade light round steel bars with a diameter of 8 mm is 320 MPa.

2.2. Polyurethane Concrete

PUC is a polymer material, which is mainly composed of polyurethane raw materials, ordinary Portland cement and molecular sieve. The PUC coordination is shown in Table 1. The specific configuration process includes the following: before the start of the test, the appropriate amount of ordinary silicate cement, molecular sieve activation powder, isocyanate and polyether was prepared and poured into a container and weighed according to proportion using an electronic scale. The weighed materials were poured into an empty bucket and mixed quickly. After mixing, the polyurethane concrete was poured into the prepared wooden formwork. After pouring the polyurethane concrete, it takes some time to cure. After curing, the polyurethane concrete specimens were obtained for the experiments.
PUC material density is 1550 kg/m3. The elastic modulus of polyurethane concrete is 6721 MPa. PUC compressive strength measurements are generally conducted by the rebound method. The hardness of the concrete surface is measured using a rebound meter and the compressive strength is presumed from the rebound value. Flexural strength measurements are generally conducted using the four-point bending test, in which a concentrated load is applied to the specimen at two symmetrical loading points until the specimen breaks. Data are collected to obtain the test results. The compressive strength test adopts a cube test mold with dimensions of 70 mm × 70 mm × 70 mm, and the bending strength adopts a cuboid test mold with dimensions of 450 mm × 100 mm × 100 mm. The compact resistance of the PUC material reached 59.3 MPa, while the fracture strength had an average value of 41.5 MPa. These strength properties were verified by tests, and the specific data can be understood by viewing Figure 1 and Figure 2.

2.3. Steel Wires

The high-strength steel wires are composed of multiple steel wires stranded together, their surface galvanized; the sectional structure is in the form of 1 × 7, and its tensile strength is 1650 MPa. The stress–strain curve of the steel wire with a diameter of 4 mm, when performing the stretch test, is detailed in Figure 3. The average ultimate tensile strength of the wire was 1255.6 MPa, and the average elastic modulus was 138.3 GPa. The ultimate tensile strain of each specimen was greater than 2.5%, and the average value was 2.68%. The stress at 0.2% residual strain is defined as the nominal yield strength, which is approximately 85% of the ultimate tensile strength and takes the value of 1067 MPa.

3. Test Beam Design

3.1. Design of Test Beam

In this study, flexural tests on 12 T-section simply supported beams were carried out. The beam specimen had a total length of 3000 mm and a clear span of 2700 mm, and the distance of the pure flexural section was 900 mm. The longitudinal tensile steel bar number was 2, and the longitudinal reinforcement ratio was 0.91%. The diameter of the erection bars was 10 mm. In order to prevent insufficient shear strength, first-order optical round bars with a diameter of 8 mm were configured along the length direction of the specimen. The spacing between the pure bending sections was 150 mm and between the shear bending sections was 80 mm. The specific parameters of the section are shown in Figure 4.
In order to investigate the effect of the PUC–PSW reinforcement technique on the flexural performance of members, the reinforced beams were divided into three groups, namely, control beams, PSW and PUC–PSW, where the control beams group comprised beams CB, while the PSW group included beams A1, A2, A2-1, A3 and the PUC–PSW group beams A4, A5, A6, A7, A8, A9, A9-1. The experimental variables used were: PSW embedment material, PSW anchorage form, PUC material thickness, PSW quantity and whether the beam was pre-cracked. The beam parameters are shown in Table 2, where beam CB is a control beam and is not reinforced. In order to study the effect of PSW embedding material on the performance of the reinforced beams, the reinforced beams are divided into two parts: one is PSW-reinforced beam with a total of four pieces, and the other is PUC–PSW-reinforced beam with a total of seven pieces. The diagram of the reinforced beam is shown in Figure 5. The cross-sectional diagram of reinforced beams I-I is shown in Table 3.
Beam A1, A2, A2-1 and A3 are the four girders that have been strengthened with PSWs, and these wires were all tensioned at a contained stress of 700 MPa. Among them, the steel wires of Beam A1 have no embedding material and use non-bonded PSW reinforcement. Beam A2-1 adopts seven PSWs, and the other two beams are reinforced with five steel wires. In order to study the reinforcement effect of cracked beams, Beam A3 was preloaded to 50 kN, and the cracks in the beam body were ensured to be no more than 0.2 mm wide. Then the load was removed for reinforcement. The thickness of all the reinforced beam mortar is 20 mm. Except for Beam A1, which is anchored by a single anchorage device, the other three beams are anchored by a combination of mechanical anchorage of the anchorage device and bonded anchorage of composite mortar.
Beams A4 to A9-1 include seven PUC–PSW-reinforced beams. Among them, there are no PSWs in beam A4, which are simple PUC-reinforced beams. To investigate the effect of PUC bonded anchorage, Beam A6 was under shear loading at the anchorage at the end of the beam after the PUC was poured, allowing the wires to be bonded and anchored only through the PUC. Beam A7 is preloaded to 50 kN, while ensuring that the beam cracks do not exceed 0.2 mm, and then the load is removed for reinforcement. In order to study the influence of different PUC thicknesses on the performance of reinforced beams, the thickness of PSW embedded PUC material in Beam A8 is set to 30 mm. In order to study the effect of the number of PSWs on the performance of reinforced beams, the number of wires in Beams A9 and A9-1 is set to two, and the thickness of PUC material embedded in the wires of Beam A9-1 is 25 mm.

3.2. Hardening Procedure

First, the beam end is slotted and installed with the PSW anchorage, as shown in Figure 6a,b. A notch about 100 mm wide is made along the width direction at the end of the beam to expose the main steel bar inside the main beam. The prepared anchor is welded to the exposed steel bar of the main beam, and rust is removed on the surface of the steel bar in order to ensure the welding quality. When welding, attention is paid to adjusting the relative position of the main bar, anchor and beam bottom.
Secondly, the concrete surface of the beam bottom is treated, as shown in Figure 6c,d. In order to strengthen the material and concrete with a better bonding effect, the concrete hammer is used to chisel the concrete surface paste and loose concrete parts, so that the coarse aggregate is exposed, and then an air compressor is used to blow away the floating ash on the concrete surface.
Thirdly, the extruded anchor head fabrication, steel wire feeding and tension anchorage are shown in Figure 6e,f. Since the anchorage used is similar to pier anchorage, the cutting length of the wire should be controlled according to the designed tension control stress. The aluminum alloy sleeve is extruded with a hydraulic tool, and the steel wires and aluminum alloy sleeve become a whole after extruding. Aluminum alloy casing is selected as the extrusion anchor head, the steel wire is stacked into two wires into the casing and then the press is extruded to form the extrusion anchor head, so that the steel wires and aluminum sleeve form a whole. One side of the steel wires is directly inserted into the anchor, and the other end is tensioned with a hand hoist or a wire tensioner. The steel wire is extended and the extruded anchor head exceeds the anchor.
Finally, the PUC material is poured, as shown in Figure 6g,h. The pre-made U-shaped wooden formwork is fixed at the bottom of the beam, and the two end parts of the wooden formwork are closed at the contact position with the concrete, so as to prevent the PUC material from flowing out during the casting procedure. In the early stages of PUC material pouring, the material has similar characteristics to self-compacting concrete and has good fluidity. The formwork will be removed 24 h after pouring. PUC material has the function of protecting and co-anchoring the steel wires.

4. Loads and Measurements

A mechanical jack was used for test loading, and the maximum pressure of the jack was 500 kN. A resistive pressure sensor and data acquisition system were used for pressure measurement during loading. Forward loading was used for testing, and the component was damaged by 5 kN/ grade loading, as shown in Figure 7.
The specific modifications are shown as follows. The arrangement of strain measurement points in the span-center section is shown in Figure 8. During the test, strain gauges were set up on concrete, steel reinforcement, steel strand, polyurethane concrete and composite mortar for measuring the material strains in each part. Concrete strain gauges were arranged at the mid-span of the specimen, and six 100 mm × 3 mm special strain gauges were uniformly pasted on the concrete surface along the height of the cross-section. Four 5 mm × 3 mm strain gauges were pasted on the surface of the two tensile main bars, which were arranged over the loading point section and the mid-span section, respectively.

5. Test Results and Analysis

5.1. Load–Displacement Curves

The damage modes of the beams shown in Figure 9 and Figure 10 illustrate the load–contraction curves of Beams CB, A1, A2 and A5. Beam A1 is an unbonded PSW-reinforced beam, and the stiffness of the reinforced beam is obviously higher than that of the CB beam. When the load attains 135 kN, the reinforced beam yields and its rigidity decreases rapidly until the load attains the ultimate load of 144 kN. Because the steel wires are not bonded to the concrete, the stress on the steel wires is evenly distributed with the increase of load, so the reinforced beam has good ductility on the load–deflection curve. Failure modes of Beams CB and A1 are shown in Figure 9a,b. Beam A2 was reinforced with bonded PSWs, and these wires were inserted into the composite mortar material. At the beginning of the loading, the stiffness of Beam A2 increased slightly compared to Beam A1, but as the load increased, the composite mortar cracked, leading the difference in the stiffnesses between the two beams to become inconspicuous. When the load reaches 141.3 kN, the steel wires break, the deflection reaches 24.6 mm and the load decreases rapidly. The remaining steel wires also break one after another with the increase of the load, and finally a relatively stable load of 105 kN remains. The damage pattern of the beam is shown in Figure 9c. The load–deflection curves of Beams A2, A2-1 and CB are shown in Figure 11. Beams A2-1 and A2 have the same wire embedment material, but Beam A2-1 has more wires embedded than Beam A2. According to the information shown in the figure, Beam A2-1 has a slightly higher stiffness compared to Beam A2 and therefore exhibits a smaller deflection value for the same load. When the steel wire reaches 160.4 kN, the steel wires in the mortar emit a sound of “touching” and break, and the deflection is 23.0 mm. The failure mode of Beam A2-1 is shown in Figure 9d.
The load–contortion curves of Beams CB, A2, A4 and A5 are shown in Figure 12. Beam A5 is a PUC–PSW-reinforced beam with a PUC thickness of 20 mm. When the load achieves 150 kN, the curve’s slope starts to become smaller, which indicates that the stiffness of the primary girder starts to degrade. The strain in the PUC in the reinforced beam rises as the load progresses. When the test load attained the ultimate load of 204.3 kN, the PUC and the wires fractured simultaneously with a “bang” sound. This phenomenon indicates that the consolidated beam has been damaged, and the greatest deflection of Beam A5 at this time is 21.8 mm. The load–contortion curve at this stage is almost horizontal, and the reinforced beam has been undergoing damage, the pattern of which is shown in Figure 9f.
The load–deflection curves of Beams CB, A2, A3, A5 and A7 are shown in Figure 13. Beams A3 and A7 are preloaded to 50 kN and then reinforced after unloading. Beam A3 is a PSW-reinforced beam, and Beam A7 is a PUC–PSW-reinforced beam. The failure mode of the pre-cracked reinforced Beam A3 is similar to that of the directly reinforced Beam A2, and the failure mode of the pre-cracked reinforced Beam A7 is similar to that of the directly reinforced Beam A5. At the initial stage of loading, the rigidity of Beam A2 is slightly less than that of Beam A7, and the discrepancy between the rigidities of these two beams gradually decreases with the growth of loading. However, at the very initial loading stage, the difference in rigidities between Beam A5 and Beam A7 is not apparent. Because PM cracks at the cracks formed during the concrete pre-cracking, the limiting cracks are completely borne by the steel wire, and the preformed cracks are rapidly developed, resulting in the stiffness of the preloaded reinforced Beam A3 being significantly lower than that of the directly reinforced Beam A2. In the case of PUC–PSW-reinforced beams, the PUC and PSWs jointly bear the burden of limiting crack development. The prestress force effect of the wires closes the cracks in the preloaded beams and generates a certain prestress force. Subsequently, the PUC material has a good ability to limit crack development, so that the crack of the pre-cracked reinforced Beam A7 does not develop significantly in the initial stage of loading. Therefore, the stiffness of Beam A5 does not change significantly compared with that of Beam A7 in the initial stage of loading.
Beam A8 is a PUC–PSW-reinforced beam with a PUC thickness of 30 mm. Compared to Beam A5 with 20 mm-thick polyurethane concrete, Beam A8 has higher structural stiffness. On the one hand, the 30 mm-thick PUC layer in the reinforced layer has a larger converted section moment of inertia, so it has a high section bending stiffness. On the other hand, the 30 mm-thick PUC material effectively restricts crack formation and extension during loading, thus improving the overall rigidity of the beam. Similar to Beam A5, Beam A8 behaved similarly in terms of cracking and disruption modes, as demonstrated in Figure 9i.
The load–contortion curves for Beams A5, A8, A9 and A9-1 are shown in Figure 14. Both Beams A9 and A9-1 have two PSWs, but the thicknesses of the embedded PUC material are not the same. Similar to Beams A5 and A8, the stiffness of Beam A9-1 is greater than that of Beam A9. The PUC layer of Beam A9-1 is 25 mm thick and has a larger section area, so it has a larger sectional bending stiffness. Moreover, the PUC material is able to limit the crack unfolding well during the loading process, which in turn increases the rigidity of the beam. The load reaches 179 kN. With the sound of “collision”, the PUC of Beam A9 and the steel wires break simultaneously, and the beam body is damaged. At this time, the deflection is 22.2 mm, and the failure mode is shown in Figure 9i. Beams A5, A8 and A9-1 have deflections of 21.4 mm, 20.4 mm and 21.3 mm, respectively, when they fail. According to the four curves of different numbers of steel wires and various thicknesses of different PUC layers, the deflections of Beams A9-1 and A8 decrease slightly compared to those of Beams A9 and A5, respectively, when the beam body is damaged. Therefore, with the increase of the thickness of the PUC layer, the deflection of the beam when the failure occurs is slightly reduced.

5.2. Cracks

The cracking loads of the test beams are shown in Table 4. Unreinforced CB beams developed cracking at loads up to 20 kN, while Beams A1 and A2 had cracking loads of 40 kN each, which is a 100% increase over the unreinforced beams. The prestressing effect makes the PSW reinforcement more effective after the initial stresses are obtained, resulting in higher opening loads for the reinforced beams. For example, the opening load of Beam A4 was 25 kN, which was only 25% higher than that of the control beam. In contrast, the PUC–PSW-strengthened Beam A5 had a casing load of 45 kN, which was 80% higher than that of A4. The ability of Beam A5 to withstand higher casing loads is mainly attributed to the prestressing effect.
Under the action of cracking load, the concrete at the bottom of the beam produces a small tensile strain, and the limiting force of PUC material on the crack is not stronger than that of PSWs. Due to the good bonding properties of the PUC and the prestressing force effect of the wires, even though the A6 girder lacked the anchorage effect of the girder-end anchorage, the cracking load was not reduced in the case of the A5 girder in the early phase of loading compared to the A5 girder. When the wires at the end of the A6 girder were sheared, the prestressing effect still caused compressive stresses in the PUC, and these compressive stresses effectively acted on the main girder. The compressive stresses acting on the main girder were not reduced due to the good bonding properties of the PUC material. Therefore, the increase of cracking load of PUC–PSW-reinforced beams is mainly due to the prestressing effect of PSWs.
The load–crack curves of Beams CB, A2, A4, A5 and A8 are shown in Figure 15. These crack values present the mean of the three dominant crack measurements in the span. When the load reaches 60 kN, the crack widths of Beams CB, A2, A4, A5 and A8 are 0.21 mm, 0.14 mm, 0.11 mm, 0.07 mm and 0.05 mm, respectively. When the load reaches 90 kN, the crack widths of Beams CB, A2, A4, A5 and A8 are 0.27 mm, 0.17 mm, 0.17 mm, 0.12 mm and 0.1 mm, respectively. The crack widths of Beams A2, A4, A5 and A8 are smaller than those of the control beams under the same loading. Therefore, both PSW reinforcement and PUC–PSW reinforcement methods can effectively limit crack development, but the latter method is more effective.
In the initial stage of loading, PSW-reinforced Beam A2 shows better crack restraint ability than Beam A4, and the prestressing effect in this stage is stronger than that of PUC. When the load exceeds 100 kN, the ability of PUC to restrain the crack is very obvious. When the load exceeds 100 kN, the reinforcement begins to yield and the stretching force at the crack aperture where the PUC material restricts the crack becomes greater than that of the wire.
Compared to PSW reinforcement, PUC–PSW reinforcement is considerably more successful in confining crack propagation throughout the loading procedure. The concrete cracks reinforced by PSWs and the corresponding composite mortar cracks are shown in Figure 16. The composite mortar cracks and concrete cracks appear at the same time, so that the limiting cracks in the reinforcement layer are completely borne by the steel wires. However, due to the fact that PUC has a high tensile capacity, the PUC–PSW-reinforced beams did not exhibit surface cracking when the concrete cracked, a condition shown in Figure 17. For PUC–PSW-reinforced Beam A5, the PUC and the wires in the reinforced layer work together to limit crack development. When the load exceeds 120 kN, Beam A5 exhibits better crack limiting ability than Beam A2. When the load reaches 120 kN, five Beam A steel bars begin to yield, the tensile force of the PUC material at the crack opening increases rapidly and PUC plays a dominant role in the PUC–PSW reinforcement layer after the steel bars yield. However, the ability of Beam A2 to limit crack development decreases after the steel bars yield, because only the wires provide a small tensile force at the crack opening. When the load exceeds 150 kN, the ability of Beam A5 to limit crack development begins to decrease, probably because the stress of the wires in the PUC–PSW layer hardly increases at this time, and the limiting crack development in the reinforced layer is borne by the PUC material until the structure fails. Although the crack limiting ability is reduced after 150 kN, the bearing capacity of the structure still maintains a high level.
The load–crack curves of Beams A2, A2-1, A9 and A9-1 are shown in Figure 18. The steel wires of Beams A2, A2-1 and A9 are embedded in a material with a thickness of 20 mm. Beams A2 and A2-1 are composite mortar, and A9 beams are PUC material. When the load is less than 70 kN, although Beams A2-1 and A2 are arranged with five and three PSWs more than Beam A9, respectively, the crack widths of the three beams under the same load are not much different in the early stage of crack formation. With the increase of load, after the load exceeds 70 kN, the split widths of Beams A9 and A2-1 remain approximately the same, but the split width of Beam A2 quickly increases with the growth of load, which is noticeably larger than that of the other two beams. Because Beam A2 is less than Beam A2-1 with two PSWs, with the increases of load and steel wire tensile stress, and a larger number of steel wires under the same conditions, the total tensile stress is larger, so the ability to restrict crack development is strong. Compared with Beam A2-1, although Beam A9 has five fewer steel wires than Beam A2-1, PUC material and PSWs jointly participate in the force, and the PUC material will not crack with the cracking of concrete and can produce tensile force at the crack to limit the cracking, so even if the PUC lacks five steel wires, it still has a good ability to limit the crack development. When the load exceeds 110 kN, the crack of Beam A2-1 develops faster than that of Beam A9. With the continuous increase of load, the gap between the crack widths of two beams gradually increases. Since both Beams A2-1 and A9 began to enter the yield stage at 110 kN, for Beam A2-1 reinforced with PSWs, the limiting crack development work in the reinforced layer is fully undertaken by the steel wires, while for A9 beams reinforced with PUC–PSW, the limiting crack development work in the reinforced layer is jointly undertaken by the steel wires and PUC. With the increase of the strain of the reinforced layer in the reinforced beam, PSWs begin to enter the nominal yield stage, and the stress increases slowly, while the stress of PUC material continues to increase with the increase of strain until the reinforced beam fails. Beam A9-1 has the same wire arrangement as Beam A9, but is 5 mm thicker than the PUC material of Beam A9. Since the magnitude of the tensile stress in the PUC material continues to escalate with rising strain, an initial gap between the split widths of these two beams occurs during the inception of the initial phase of loading. This gap in crack width gradually widened as the loading value increased until it eventually led to the damage of both the beams.

5.3. Yield Load and Ultimate Load

The bearing capacities of the test beams are shown in Table 3. The load comparison of Beams CB, A1, A2 and A2-1 is shown in Figure 19. The buckling load of Beam A1 is 94 kN, which is 23% greater than that of the control beam. The yield loads of Beams A2 and A3 are 96.7 kN and 94.1 kN, respectively, which are 30% and 27% greater compared to the control beam. Numerically, the buckling loads of Beams A2 and A3 are not considerably elevated compared to Beam A1. Due to the premature cracking of the composite mortar, the mortar does not increase the yield load of beams. As there are two more steel wires in Beam A2-1 than Beams A1 and A2, the yield load of Beam A2-1 has been significantly increased. Under normal circumstances, directly increasing the amount of prestressed tendons can effectively increase the yield load. Beam A4 is a single polyurethane-concrete-reinforced beam with a yield load of 98.8 kN, a slight increase over the PSW-reinforced beam. Although there are no PSWs in the PUC material, a single PUC material can still greatly increase the yield load of the beam.
The ultimate strengths of Beams CB, A1, A2 and A2-1 are shown in Figure 19. The ultimate loads of PSW-reinforced Beams A2 and A3 were 141.3 kN and 144.5 kN, respectively, which were 40% and 43% greater than that of the control beams, and both beams suffered wire fracture damage. Beam A2-1 has two more steel wires than Beams A1 and A2, resulting in 13.1% and 14.5% increases in the ultimate load compared with Beams A1 and A2, respectively. By increasing the amount of wire configuration, the ultimate load-carrying capacity of the concrete beams can be effectively enhanced, and this enhancement is greater than the corresponding increment in yield load. Beam A4, with an ultimate load of 168.7 kN, shows a significant ultimate strength enhancement compared to Beams A2 and A3 despite the absence of PSWs in its PUC. In the PSW-reinforced beams, the wires yielded before the beams were damaged, whereas the tentative strengths of the PUC-reinforced beams continued to increase until the final destruction of the structure.
The load comparison of Beams CB, A2, A3, A5 and A7 is shown in Figure 20. The yield loads of Beams A5 and A7 are 120 kN and 118.6 kN, respectively, which are significant improvements over the PSW-strengthened Beams A2 and A3. Due to the good mechanical properties of PUC material, the embedded PUC material has a great influence on the yield strength of the structure. Compared with Beam A2-1, with the increase of the number of steel wires, the amplitude of the yield load increased by embedding PUC material is more obvious.
The ultimate load comparison of Beams CB, A2, A3, A5 and A7 is shown in Figure 20. The load carrying capacities of Beams A5 and A7 were 204.3 kN and 192.2 kN, respectively, increases of 44.6% and 36.0% compared to Beam A2. With the same wire configuration, embedding the wires in the PUC can significantly increase the load-carrying capacity of the reinforced beams, and the improvement is considerable. The failure modes of both Beams A5 and A7 are that the wires and PUC break at the same time. Therefore, the thicker PUC layer can further improve the load-carrying capacity of the beam.
The load comparison of Beams A5, A6 and A8 is shown in Figure 21. The yield load of Beam A8 increased by 137% over the control beam and 14.2% over Beam A5, indicating that the yield load of the reinforced beams increases with the increase in the depth of the PUC. Beam A6’s yield load is only 51% greater than control’s, while that of Beam A5 with beam-end anchorage is 61.5% higher. Compared with Beam A5, the yield load of Beam A6 decreased by 6.7%. Therefore, when only relying on PUC anchoring steel wires, when the yield load is reached, the concrete at both ends of the small oblique cracks and the PUC material has a small slip.
As shown in Figure 21, the ultimate carrying capacity of Beam A8 is 228 kN, which is 11.8% higher than that of Beam A5 due to the addition of a 10 mm PUC layer. Because the concrete at the end of the PUC–PSW-reinforced layer of Beam A6 has a small oblique crack at yield, the deflection increases rapidly with the continuous increase of load, but the load increases slowly. As the end of the reinforced layer suddenly peeled off from the concrete, the ultimate strength was 133.1 kN, 53.5% lower than that of Beam A5.
The yield load comparison of Beams A2-1, A9 and A9-1 is shown in Figure 22. Although Beam A9 has five fewer steel wires than Beam A2-1 and they are all embedded in the material with a thickness of 20 mm, the yield load of Beam A9 is 6.9% higher than that of Beam A2-1 because the two beams are embedded in different materials. Beam A9-1 is embedded in a 25 mm-thick PUC material and also has five fewer steel wires than Beam A2-1, and the yield load is 130.2 kN, 26.2% higher than that of Beam A2-1. It is shown that if the number of steel wires is reduced under necessary conditions, the requirement of yield strength can be satisfied by embedding steel wires into PUC materials.
The ultimate load comparison of Beams A2-1, A9 and A9-1 is shown in Figure 22. Although Beam A9 has five fewer steel wires than Beam A2-1, the ultimate strength of Beam A9 is 10.6% higher than Beam A2-1 because of the different embedding materials. Beam A9-1 has five fewer steel wires than Beam A2-1. Because the thickness of PUC material embedded in the wires of Beam A9-1 is 25 mm, the ultimate load capacity of Beam A9-1 reaches 207.8 kN, which is 28.4% higher than that of Beam A2-1. Therefore, in the project, if it is necessary to reduce the number of steel wires and still ensure that the bearing capacity meets the requirements, the steel wires embedded in the PUC material can achieve the purpose.

5.4. Strain Analysis

The stress–strain curves of Beams A2 and A5 are shown in Figure 23. When Beam A2 is loaded to a yield load of 96.7 kN, the strain in the wire is 7092 με, resulting in a relative stiffness of 1016 MPa. After subtracting the initial strain, the net increase of the wire strain is 2132 με. Under the load of 96.7 kN, the strain of Beam A5 is 6594 με and the corresponding stress is 961 MPa. The net increase in wire strain after subtracting the initial strain from the wire of Beam A5 is 1580 με, which is much more modest than the net increase in wire string strain of Beam A2. In addition, the stretch in the reinforcement of Beam A5 is much lighter than that of Beam A2 under the same test load. When the tensile strain at the bottom of the beam is close to 600 με, the composite mortar of Beam A2 cracks, so that the composite mortar cracks before the load reaches 96.7 kN. However, when the load reaches 96.7 kN, the strain value of the polyurethane concrete material at the bottom of the beam is 1873 με, and the corresponding stress is about 11.3 MPa. Therefore, the PSW embedded PUC material was able to delay the yielding of the main girder reinforcement. When the yield load of 120 kN was achieved in Beam A5, the net strain of the wire increased by 2155 με minus the initial opening strain. At this load, the net gain in the wire of Beam A2 was 2804 με, which was much greater than the net gain in strain in the wire of Beam A5. At this point, the strain in the Beam A5 PUC material is 2494 με, and the resulting strain is approximately 14.9 MPa. Therefore, the PUC material can effectively increase the bearing capacity of the main beam. Under the ultimate load of 228.8 kN, the maximum strain of beam A5 PUC material is 7369 με and the corresponding stress is 37.0 MPa.
The load–strain curves of rebars, wires and PUC of Beams CB, A5 and A6 are shown in Figure 24. The load–strain curve of Beam A6 was similar to that of Beam A5 under below-yield loading. The results indicate that it is feasible for the wires to be anchored by PUC only during the initial stages of loading. With the increase of load, the strain of the reinforced layer increases, but it does not cause the slip of the wire. At 133 kN, the reinforced beam fails due to anchoring failure. At this time, the strain of the steel wire is 6900 με, and the strain of the PUC material is 1950 με, which corresponds to the strength of about 11.9 MPa under the ultimate load, which is only 30% of the ultimate strength of the material. The strength of the material is not utilized to its optimum in this case, and the extreme strength improvement is not as pronounced as in Beam A5. In order to make full use of PUC material without causing premature stripping damage, it is necessary to anchor the steel wire with beam-end anchorage.
According to Figure 25, the strain variation rules of steel bars, wires and PUC in Beams A5 and A8 are similar. Since the stringers embedded in the PUC in Beam A8 are 10 mm thicker than those in Beam A5, the strains of the reinforcement, wires and PUC soil in Beam A8 will be smaller than those in Beam A5 under the same loading.
The strains of Beam A2-1 and A9 rebars, steel wires, composite mortar and PUC material are shown in Figure 26. When Beam A2-1 is loaded to the yield load of 103.2 kN, the strain of the wire is 6905 με and the corresponding stress is 987 MPa. After the initial strain is subtracted from Beam A2-1 wires, the net strain increase of the wires is 2007 με. Under the load of 103.2 kN, the strain of Beam A9 is 6977 με and the corresponding stress is 992 MPa. After the initial strain is subtracted from the wires of Beam A9, the net strain increase of wire is 2086 με, which is close to the net strain increase of Beam A2-1 wires. Although Beam A9 is equipped with five fewer PSWs than Beam A2-1, under the same test load, the strain value of Beam A9 reinforcement is close to that of Beam A2. When the tensile strain of the beam bottom is close to 700 με, the composite mortar of the Beam A2-1 cracks, so that the composite mortar cracks before the load reaches 103.2 kN. However, when the load reaches 103.2 kN, the strain value of the PUC material at the bottom of the beam is 1956 με, and the corresponding stress is about 12.1 MPa. Therefore, although the number of PSWs used is reduced, the PUC material embedded with the PSWs can guarantee the yield strength of the reinforced beam. When the load reaches 161.8 kN, Beam A2-1 reaches the ultimate strength and the steel wires have nominal yield, and the steel bars have already entered the yield stage. However, when the load reaches 161.8 kN, the steel wires of Beam A9 have not reached the nominal yield strength, and the PUC material has not broken. At this time, the PUC strain is 4806 με and the corresponding stress is 27.8 MPa, which has not reached the PUC tensile limit strength.
The strains of Beams A5 and A9 rebars, wires and PUC material are shown in Figure 27. Beams A5 and A9 are arranged with five and two steel wires, respectively, which are embedded in the PUC material of 20 mm thickness. As can be seen from the figures, the trends of the load–strain curves of the reinforcement, wires and PUC materials in the two beams are quite comparable. Because there are three more steel wires in Beam A5 than in Beam A9, the strains of steel bars, steel wires and PUC in Beam A5 are smaller than those of Beam A9 under the same load. After the steel bars of Beam A5 yielding, the tensile strain difference of the wires of two beams under the same load gradually increases. As the steel bars enter the yield state, the further increase of the bearing capacity of the reinforced beam is mainly borne by the steel wires and PUC in the reinforced layer. As the number of steel wires in Beam A5 is more than that in Beam A9, the bearing capacity of the steel wires in Beam A5 increases greatly with the increase of load, resulting in the tensile strain difference of the steel wires of the two beams gradually increasing.
The strains of rebars, wires and PUC materials of Beams A9 and A9-1 are shown in Figure 28. Two PSWs are arranged in each of the two reinforced beams, and Beams A9 and A9-1 are embedded in 20 mm and 25 mm PUC material, respectively. According to the figures, the load–strain curves of reinforcement, wires and PUC in Beams A9-1 and A9 have similar variation trends. Since the thickness of the wire inserted into the PUC in Beam A9-1 is 5 mm larger than that in Beam A9, the strains of the bars, wires and PUC in beam A9-1 will be smaller than those in Beam A9 under the same load. As the wire enters the nominal yield state, the difference in the tensile strains of the PUC materials between the two beams under the same load gradually grows, due to the fact that the reinforcement has entered the yield state. As the load grows, the load-carrying capacity of the beams increases mainly by the PUC soil material. Therefore, the difference in tensile strains of PUC materials between the two beams gradually increases.

6. Conclusions

In this study, PUC material is used as the embedded material of steel wires to form a new method of PUC–PSW reinforcement. Compared with other traditional reinforcement methods, the lightweight and high-strength PUC material not only bonds and anchors PSWs, but also passively participates in the structural forces, reducing the amount of PSWs. The high bonding and toughness of PUC increase the durability of prestressing strands. The early strength characteristics of PUC improve the efficiency of reinforcement. In this study, the static tests on the PUC–PSW-reinforced beam are conducted and analyzed. The test results show the following.
(1) Compared to PSW strengthening, PUC–PSW strengthening can improve the yield and ultimate loads of the strengthened beams considerably. The yield load and ultimate load of the PUC–PSW-reinforced beams (20 mm-thick PUC) increase by 24.1% and 44.6%, respectively, compared with the corresponding PSW-reinforced beam. Even if the steel wires in the reinforced layer are removed, the yield load and ultimate load increase by 2.2% and 19.4%, respectively. PUC can additionally increase the yield and ultimate loads of reinforced beams, reduce the amount of strand and reduce the tension or stress in the anchorage zone at the end of the beam, which is conducive to structural safety.
(2) The ability of PUC–PSW reinforcement to limit crack development is better than that of PSW reinforcement, especially after the main beam steel bars yield. When the load reaches 90 kN, the crack widths of Beams A2, A4, A5 and A8 were 0.17 mm, 0.17 mm, 0.12 mm and 0.10 mm, respectively. The PUC material did not crack throughout the loading process until the final reinforcement layer fractured; combined with the material’s good chemical resistance, this phenomenon can theoretically increase the durability of the strand’s use. PUC has a good resistance to cracking and corrosion, which enhances the durability of the reinforced beams and prolongs the service life of the reinforced beams.
(3) Strength, stiffness and crack-limiting capacity of reinforced beams improve with the growing thickness of the PUC of the reinforcement layer and decrease with the decreasing prestressing of the wires.
(4) In this study, an experimental investigation on PUC–PSW-reinforced RC beams was carried out, and it was verified that the PUC–PSW reinforcement method offers greater strength, stiffness and durability than the composite mortar–prestressing strand reinforcement method. The next step is to carry out a theoretical study on the flexural calculation of PUC–PSW-reinforced RC beams to lay the foundation for its application in engineering design.

Author Contributions

Writing—original draft, W.L.; Data curation, J.Q.; Writing—review & editing, Y.W.; Data curation, X.Z.; Formal analysis, K.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Compressive test photo of polyurethane concrete material.
Figure 1. Compressive test photo of polyurethane concrete material.
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Figure 2. Flexural test photo of polyurethane concrete material.
Figure 2. Flexural test photo of polyurethane concrete material.
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Figure 3. Tensile stress–strain curve of steel wires.
Figure 3. Tensile stress–strain curve of steel wires.
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Figure 4. Specimen dimensions and reinforcement: (a) longitudinal section; (b) horizontal section.
Figure 4. Specimen dimensions and reinforcement: (a) longitudinal section; (b) horizontal section.
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Figure 5. Profile of the reinforced concrete beam.
Figure 5. Profile of the reinforced concrete beam.
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Figure 6. Reinforcement steps: (a) anchorage; (b) beam-end anchorage; (c) beam surface chisel; (d) main beam erection; (e) anchor head extrusion; (f) beam-end anchorage; (g) polyurethane concrete mixing; (h) polyurethane concrete pouring.
Figure 6. Reinforcement steps: (a) anchorage; (b) beam-end anchorage; (c) beam surface chisel; (d) main beam erection; (e) anchor head extrusion; (f) beam-end anchorage; (g) polyurethane concrete mixing; (h) polyurethane concrete pouring.
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Figure 7. Static load test: (a) Front view, (b) Side view.
Figure 7. Static load test: (a) Front view, (b) Side view.
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Figure 8. Cross-section layout of strain gauges.
Figure 8. Cross-section layout of strain gauges.
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Figure 9. Failure modes of the beams: (a) Beam CB; (b) Beam A1; (c) Beam A2; (d) Beam A2-1; (e) Beam A4; (f) Beam A5; (g) Beam A6; (h) Beam A8; (i) Beam A9; (j) Beam A9-1.
Figure 9. Failure modes of the beams: (a) Beam CB; (b) Beam A1; (c) Beam A2; (d) Beam A2-1; (e) Beam A4; (f) Beam A5; (g) Beam A6; (h) Beam A8; (i) Beam A9; (j) Beam A9-1.
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Figure 10. Load–deflection curves of Beams CB, A1, A2 and A5.
Figure 10. Load–deflection curves of Beams CB, A1, A2 and A5.
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Figure 11. Load–deflection curves of Beams CB, A2 and A2-1.
Figure 11. Load–deflection curves of Beams CB, A2 and A2-1.
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Figure 12. Load–deflection curves of Beams CB, A2, A4 and A5.
Figure 12. Load–deflection curves of Beams CB, A2, A4 and A5.
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Figure 13. Load–deflection curves of Beams CB, A2, A3 and A7.
Figure 13. Load–deflection curves of Beams CB, A2, A3 and A7.
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Figure 14. Load–deflection curves of Beams A5, A8, A9 and A9-1.
Figure 14. Load–deflection curves of Beams A5, A8, A9 and A9-1.
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Figure 15. Load–crack width curves of Beams CB, A2, A4, A5 and A8.
Figure 15. Load–crack width curves of Beams CB, A2, A4, A5 and A8.
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Figure 16. Cracks of concrete and the corresponding composite mortar.
Figure 16. Cracks of concrete and the corresponding composite mortar.
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Figure 17. Concrete cracks and intact polyurethane concrete.
Figure 17. Concrete cracks and intact polyurethane concrete.
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Figure 18. Load–crack width curves of Beams A2, A2-1, A9 and A9-1.
Figure 18. Load–crack width curves of Beams A2, A2-1, A9 and A9-1.
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Figure 19. Load comparison of Beams CB, A1, A2 and A2-1.
Figure 19. Load comparison of Beams CB, A1, A2 and A2-1.
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Figure 20. Load comparison of Beams CB, A2, A3, A5 and A7.
Figure 20. Load comparison of Beams CB, A2, A3, A5 and A7.
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Figure 21. Load comparison of Beams A5, A6 and A8.
Figure 21. Load comparison of Beams A5, A6 and A8.
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Figure 22. Load comparison of Beams A2-1, A9 and A9-1.
Figure 22. Load comparison of Beams A2-1, A9 and A9-1.
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Figure 23. Load–strain curves of Beam A2 and A5 rebar, wires and insert materials.
Figure 23. Load–strain curves of Beam A2 and A5 rebar, wires and insert materials.
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Figure 24. Load–strain curves of Beams CB, A5 and A6 rebars, steel wires and polyurethane concrete materials.
Figure 24. Load–strain curves of Beams CB, A5 and A6 rebars, steel wires and polyurethane concrete materials.
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Figure 25. Load–strain curves of Beams A5 and A8 rebars, steel wires and polyurethane concrete materials.
Figure 25. Load–strain curves of Beams A5 and A8 rebars, steel wires and polyurethane concrete materials.
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Figure 26. Load–strain curves of Beams A2-1 and A9 rebars, steel wires, composite mortar and polyurethane concrete materials.
Figure 26. Load–strain curves of Beams A2-1 and A9 rebars, steel wires, composite mortar and polyurethane concrete materials.
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Figure 27. Load–strain curves of Beams A5 and A9 steel bars, steel wires and polyurethane concrete materials.
Figure 27. Load–strain curves of Beams A5 and A9 steel bars, steel wires and polyurethane concrete materials.
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Figure 28. Load–strain curves of Beams A9 and A9-1 steel bars, steel wires and polyurethane concrete materials.
Figure 28. Load–strain curves of Beams A9 and A9-1 steel bars, steel wires and polyurethane concrete materials.
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Table 1. Composition of polyurethane concrete.
Table 1. Composition of polyurethane concrete.
Chemical CompositionPercentage (%)
Polyether25
Isocyanate25
Portland cement45
Molecular sieve5
Table 2. Parameters of beam reinforcements.
Table 2. Parameters of beam reinforcements.
GroupBeam NumberNumber of Steel WirePrestress (MPa)Embedded MaterialMaterial Thickness (mm)Anchorage FormPreloadReinforcement under Load
Control beamCB-------
PSWA15700--Anchor gear--
A25700Mortar20Anchor gear + Mortar--
A2-17700Mortar20Anchor gear + Mortar--
A35700Mortar20Anchor gear + MortarPreload-
PUC–PSWA40700PUC20---
A55700PUC20Anchor gear + PUC--
A65700PUC20PUC--
A75700PUC20Anchor gear + PUCPreload-
A85700PUC30Anchor gear + PUC--
A92700PUC20Anchor gear + PUC--
A9-12700PUC25Anchor gear + PUC--
Table 3. Cross-sectional diagrams of reinforced beams I-I.
Table 3. Cross-sectional diagrams of reinforced beams I-I.
Beam NumberCross SectionBeam NumberCross Section
A1Buildings 14 02746 i001A5, A6, A7Buildings 14 02746 i002
A2, A3Buildings 14 02746 i003A8Buildings 14 02746 i004
A2-1Buildings 14 02746 i005A9Buildings 14 02746 i006
A4Buildings 14 02746 i007A9-1Buildings 14 02746 i008
Table 4. Cracking loads, yield loads and ultimate loads of the beams.
Table 4. Cracking loads, yield loads and ultimate loads of the beams.
Group NumberBeam NumberCracking Load
(kN)		Increase Ratio
(%)		Yield Load
(kN)		Increase Ratio
(%)		Ultimate Load
(kN)		Increase Ratio
(%)
Control beamCB20-74.3-101.0-
Steel wireA14010091.423.0143.141.7
A24010096.730.1141.339.9
A2-150150103.238.9161.860.2
A3--94.126.6144.543.1
PUC–PSWA4252598.833.0168.767.0
A545125120.061.5204.3102.3
A650150112.050.7133.131.8
A7--118.659.6192.290.3
A855175137.084.4228.5126.2
A93050110.348.5179.077.2
A9-13575130.275.2207.8105.7
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MDPI and ACS Style

Li, W.; Qiu, J.; Wang, Y.; Zheng, X.; Zhang, K. Test and Analysis of Concrete Beams Reinforced by Polyurethane Concrete–Prestressed Steel Wires (PUC–PSWs). Buildings 2024, 14, 2746. https://doi.org/10.3390/buildings14092746

AMA Style

Li W, Qiu J, Wang Y, Zheng X, Zhang K. Test and Analysis of Concrete Beams Reinforced by Polyurethane Concrete–Prestressed Steel Wires (PUC–PSWs). Buildings. 2024; 14(9):2746. https://doi.org/10.3390/buildings14092746

Chicago/Turabian Style

Li, Wei, Jiaqi Qiu, Yi Wang, Xilong Zheng, and Kexin Zhang. 2024. "Test and Analysis of Concrete Beams Reinforced by Polyurethane Concrete–Prestressed Steel Wires (PUC–PSWs)" Buildings 14, no. 9: 2746. https://doi.org/10.3390/buildings14092746

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