# Investigation of Interfacial Bonding Properties of Polyurethane Concrete and Cement Concrete/Steel Reinforcement

Authors: Yanqi Wang; Quansheng Sun; Hongjian Ding; Songbai Leng; Hongchuan Cui; Bowen Xu; Hewei Cui
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

To solve the problem that the repair material of the bridge's expansion joint anchorage area is prone to debonding from the other bridge components, polyurethane concrete was designed and prepared, and the bonding performance of polyurethane concrete was tested by the bond splitting test, bond direct shear test, bond oblique shear test, and steel reinforcement pull-out test. The results show that the interfacial bond strength of polyurethane concrete and cement concrete can be improved by increasing the interfacial roughness. The bond splitting strength increases by up to 91.4%, the bond direct shear strength increases by up to 55.2%, and the bond oblique shear strength increases by up to 28.9%. The bond strength between polyurethane concrete and cement concrete are minimally influenced by temperature. An increase in the diameter of the reinforcement or the length of the bonded anchorage can increase the ultimate pull-out strength of the polyurethane concrete and the steel reinforcement. Polyurethane concrete is a novel composite material with excellent bonding performance with cement concrete and steel reinforcement, which can be widely used in practical projects, such as repairing bridge expansion joint anchorage areas in the future.


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## Body

## 1. Introduction

The bridge expansion joint anchorage area is one of the most frequently damaged components of the bridge structure. Although the cost of bridge expansion joints accounts for only about 1% of the total cost of the bridge structure, about 16% of the bridge damage occurs at the bridge expansion joints [1]. It is important to emphasize that the repair cost of expansion joint damage accounts for as much as 20% of the total bridge repair cost [2], which leads to an increase in bridge maintenance costs. As bridge expansion devices are mostly in municipal or highway node sections, they cannot be closed to traffic for a long time. The frequent maintenance of expansion joints leads not only to substantial direct economic losses but also to indirect economic losses and adverse social impacts because of traffic disruptions during maintenance. The breakage of the anchorage area of the expansion joint is mostly repaired with fast hardening cementitious material [3]. Fast hardening concrete with early-strength, shrinkage-resistant, and frost-resistant properties can be prepared by adding admixtures to cement. To a certain extent, it achieves fast construction in the anchorage area of expansion joints, and its performance can meet the requirements within a certain period of time, however, this type of concrete often has the technical disadvantage of late strength inversion or poor durability because of the use of a large number of early-strength, fast-setting components [4]. Moreover, the low interfacial bond strength of cement concrete is prone to problems, such as the debonding of the interface between old and new concrete and failure of reinforcement anchorage [4, 5]. Therefore, it is necessary to find a novel material with high early strength, short maintenance period, and excellent bonding performance to replace the traditional anchorage zone repair material for the design, maintenance, and extension of bridge service life of the expansion joint anchorage zone.Polyurethane is a kind of block copolymer, which is formed by the condensation reaction of polyol and isocyanate. Because of its excellent mechanical properties, durability, and performance tunability, polyurethane is widely used in medical [6], aerospace [7], automotive [8], and coating [9] applications. Polyurethane concrete is a novel composite material prepared by polyurethane binder, gravel aggregate, additives, etc. [10]. Nikoukalam et al. [11] tested the basic mechanical properties of polyurethane concrete and analyzed the mechanical response characteristics under various loading conditions. Jiang et al. [12] proposed a novel water-containing unsaturated polyurethane concrete to solve the problems of poor compatibility, fast curing, and difficult molding control of common polyurethane concrete. The 1-d compressive strength of this water-unsaturated polyurethane concrete was 22.70 MPa, no softening and disintegration were found after long-term immersion, and the recommended basic ratios at 15°C were given. Zhang et al. [13] invented a polyurethane Portland cement mixture by mixing polyurethane with cement, its tensile strength and plasticity were much higher than those of Portland cement mortar, and the compatibility was the same as that of Portland cement mortar. The use of polyurethane cement for the strengthening of bridge structures can significantly improve the load-bearing capacity and stiffness of bridge structures, and the feasibility of polyurethane-based composites as structural strengthening materials was verified by the strengthening projects of actual bridges. Yilin et al. [14] investigated the tensile/compression strength, elastic modulus, toughness, strain capacity, and damage mechanism of polyurethane cement matrix composites. It was shown that basalt fibers could significantly improve the stiffness of polyurethane concrete. The addition of fibers basically did not affect the mechanical properties of polyurethane concrete, and the toughness of polyurethane concrete can be improved by rubber powder.Because of the unique microphase separation structure of polyurethane, it has a wide range of functional tunability. Changing the content of hard sections and the length of soft sections is an effective way to adjust the engineering properties of polyurethane concrete [15]. Increasing the content of hard segments or larger molecular weight of soft segments is effective in enhancing hydrogen bonding and microphase separation [16]. Polyurethane concrete also possesses a certain temperature sensitivity and exhibits different mechanical properties at different temperature conditions [17–19]. The specimen undergoes brittle damage at low temperatures and plastic damage at high temperatures [10]. The mechanical strength decreases with increasing temperature, while the maximum strain increases with the temperature [16]. Despite the unfavorable influence of temperature on the mechanical properties of polyurethane concrete, the compressive strength is higher than that of fly ash concrete and normal concrete in a dry and hot environment at 80°C [20]. Adjusting the microphase structure to make the dynamic properties of polyurethane better can provide good cushioning and damping for vehicles passing through the expansion joints, which not only improves the comfort of travelling but also reduces the impact effect of the vehicle loads on the bridge structure [21, 22]. The damping ratio of polyurethane concrete is 8.2%, which is 4.3 times higher than that of ordinary concrete. At 50 Hz, the damping ratio of polyurethane concrete beams is 9.5%, which is about 12 times (0.8%) than that of ordinary concrete beams [23].Because of its chemical stability, polyurethane is often used as an anticorrosion coating [24]. The aggregates are protected from corrosive media by the polyurethane coating, and the long-term performance of the polyurethane concrete can be guaranteed. The polyurethane concrete formed by dolerite aggregates as a filler and polyurethane as a binder not only has excellent mechanical properties but also has good corrosion resistance [25]. Xu et al. [26] conducted laboratory accelerated aging tests on polyurethane concrete based on actual climatic and environmental conditions of bridge structures and evaluated the resistance to particle loosening, permanent deformation, low temperature crack resistance, water stability, and fatigue properties by the Cantabro test, low temperature flexural beam test, rutting test, freeze-thaw splitting test, and four-point bending fatigue test. The study showed that the polyurethane concrete has excellent aging resistance and can be used as a durable bridge deck pavement material. In addition, fatigue performance is also an important guarantee of material durability. Gao et al. [27] conducted four-point bending fatigue tests on polyurethane concrete at different temperatures, and the results showed that the fatigue life of polyurethane concrete was better than that of asphalt mixtures under coupled temperature and external forces. A method for predicting the fatigue life of polyurethane concrete was proposed, which can effectively describe the fatigue life and fatigue limit of polyurethane concrete. Jia et al. [28] prepared fatigue-resistant elastic polyurethane concrete by adding rubber to polyurethane concrete. The incorporation of rubber particles can make polyurethane concrete an internal elastic structure and the interfacial bond between the aggregates and the polyurethane was effectively optimized. Fatigue-resistant elastic polyurethane concrete can achieve a fatigue life of 2 million cycles at a strain level of 600 με.In summary, polyurethane concrete has the characteristics of high strength, corrosion resistance, and excellent fatigue performance [10]. Compared with conventional cement concrete, polyurethane concrete has good dynamical performance [23]. However, in the above-mentioned studies, there are still limitations in the study of the interfacial bonding properties of polyurethane concrete and existing structures. This research aims to evaluate the interfacial bonding characteristics between polyurethane concrete and cement concrete/steel reinforcement with a view to achieve the repair of bridge expansion joint anchorage area by polyurethane concrete and to solve the problem that traditional repair materials are prone to debonding from the other bridge components. Therefore, the interfacial bonding performance between the polyurethane concrete and the existing concrete/steel reinforcement are the key to determine the structural repair effect in the anchorage area. In this paper, the interfacial bonding characteristics between polyurethane concrete and cement concrete under three stress conditions of splitting, direct shear, and oblique shear, and the pull-out characteristics between polyurethane concrete and steel reinforcement were investigated respectively through laboratory tests, and the feasibility of polyurethane concrete as an anchorage zone material for bridge expansion joints was evaluated.


## 2. Materials and Methods

### 2.1. Materials

#### 2.1.1. Polyaryl Polymethylene Isocyanate

The isocyanate used in this paper is polyaryl polymethylene isocyanate (PAPI). The main components of the PAPI are diphenylmethane diisocyanate (MDI) and oligomeric isocyanates with 3–6 functional groups. The technical indicators of PAPI tested according to the specified standard [29–33] are shown in Table 1.Table 1
PAPI technical indicators.
IndicatorsResultsAppearanceViscosity at 25°C (mPa•s)250NCO content of isocyanate (%)32.0Density at 25°C (g/cm3)1.22Acidity (%)0.030Hydrolyzed chlorine (%)0.20


#### 2.1.2. Combined Polyether Polyol

The main components of the combined polyether polyol are polyether polyol, chain extender, and crosslinker. The technical indicators of the combined polyether polyols were tested by the specified standard [29, 34, 35] as shown in Table 2.Table 2
Technical indicators of combined polyether polyol.
IndicatorsResultsAppearanceViscosity at 25°C (mPa•s)200Hydroxyl value (mg KOH/g)370Density at 25°C (g/cm3)1.13pH5.4Unsaturation (mmol/g)0.005


#### 2.1.3. Catalyst

The catalyst used in this paper is an aliphatic amine catalyst among tertiary amine catalysts, which is a solution with triethylenediamine (DABCO) as the main component and has a colorless and transparent liquid appearance. The molecular structure of DABCO is shown in Figure1.Figure 1
Triethylenediamine.


#### 2.1.4. Molecular Sieve

Isocyanate is prone to react with water to form polyurea and carbon dioxide gas [13], which can lead to volume expansion of the polyurethane [36]. Therefore, molecular sieves are added to adsorb water in the mixture and thus ensure the volumetric stability of the cured polyurethane concrete. The physicochemical properties of the 5A molecular sieve tested by the specified standard [37] are listed in Table 3.Table 3
Technical indicators of the 5A molecular sieve.
IndicatorsResultsAppearanceSize (μm)2–4Bulk density (g/mL)0.45Static water adsorption rate (%)25.0pH9


#### 2.1.5. Cement

The performance parameters of PO 42.5 cement tested by the specified standard [38] are shown in Table 4.Table 4
Performance parameters of cement.
IndicatorsResultsInitial setting time72Final setting time366Fineness (%)1.8Specific surface area (m2/kg)37428 d shrinkage rate (%)0.08Wear resistance (kg/m2)2.4Flexural strength (MPa)3 days4.628 days7.2Compressive strength (MPa)3 days7.228 days24.6


#### 2.1.6. Aggregate

The technical indicators of granite aggregates tested by specified standard [39] are shown in Table 5. The aggregate grading curves of the aggregates are shown in Figure 2.Table 5
Technical indicators of granite aggregate.
IndicatorsResultsApparent density (g/m3)2.767Bulk density (g/m3)2.683Bulk porosity (%)38.5Crush value (%)23.1Needle flake content (%)3.4Figure 2
Aggregate grading curves.


#### 2.1.7. Steel Reinforcement

The HRB400 hot rolled ribbed steel bars were used in the pull-out test, and the mechanical properties of the steel reinforcement are shown in Table6.Table 6
Mechanical properties of steel reinforcement.
GradeDiameter (mm)Yield strength (MPa)Tensile strength (MPa)Elongation after break (%)Total extension at maximum force (%)HRB40012450.3657.73217.414460.4635.92717.016455.7624.52516.2


### 2.2. Mixture Design

The effects of the three factors, namely, binder-aggregate ratio, cement dosage ratio, and aggregate gradation, on the compressive strength of polyurethane concrete were investigated by the orthogonal test, and the response analysis of each control factor was conducted separately to determine the optimal mixture of polyurethane concrete:(1)
The L9(33) orthogonal table was used for the orthogonal test, where the control factors were binder-aggregate ratio, cement dosage ratio, and aggregate gradation, with three levels for each factor listed in Table 7. The ratio of the two-component polyurethane total mass to the aggregate mass is used as the binder-aggregate ratio. The cement dosage ratio is the ratio of the cement mass to the polyaryl polymethylene isocyanate (or combined polyether polyol) mass.(2)
The polyurethane concrete orthogonal test mixtures are shown in Table8, and a total of 9 groups of specimens from T1-T9 were set up. To reduce the influence of data variability on the test results, three parallel specimens were prepared for each group under the same preparation and maintenance conditions.Table 7
Factor level values.
Factor IDControl factorsFactor levels123ABinder-aggregate ratio10%15%20%BCement dosage ratio1.01.52.0CAggregate gradationAC-5AC-10AC-13Table 8
Orthogonal experimental design.
NumberOrthogonal combinationsA (%)BCT1A1B1C1101.0AC-5T2A1B2C2101.5AC-10T3A1B3C3102.0AC-13T4A2B1C2151.0AC-10T5A2B2C3151.5AC-13T6A2B3C1152.0AC-5T7A3B1C3201.0AC-13T8A3B2C1201.5AC-5T9A3B3C2202.0AC-10Note: A, B, and C are the factor IDs for each control factor in Table7, respectively. For example, A2B3C1 represents the combination of level 2 of the A factor (binder-aggregate ratio), level 3 of the B factor (cement dosage ratio), and level 1 of the C factor (aggregate gradation).


### 2.3. Preparation of Polyurethane Concrete

Flow diagram for the polyurethane concrete preparation process is shown in Figure3:(1)
Water removal: although molecular sieve as a water remover had been added into the mixture, its dosage should not be too large. Too much molecular sieve would affect the performance of the concrete after curing. To ensure the volumetric stability of the polyurethane concrete, the cement and aggregates should be dried in a 101-2A electric blast dryer at 130°C for 4 hours, during which the cement and aggregates are upturned every 30 minutes. After the drying process, the aggregates and cement were stored in sealed containers and cooled to room temperature before they were used in the preparation of polyurethane concrete.(2)
Premixing: premixtures were prepared by mixing aggregates, cement, molecular sieves, and combined polyether polyols. The premixture in homogeneous state should be dry plastic state, and no cement powder should be found. Premixing should last for more than 10 minutes so that the molecular sieve can take full advantage of water removal while fully mixing the premix.(3)
Mixing: the polyisocyanate was added to the premixture and stirred for 1.5 minutes. Add the catalyst dropwise while the mixture is in the flow plastic state, stirring rapidly for 0.5 minutes.(4)
Pouring and molding: the mixed polyurethane concrete was poured into the mold with preapplied release agent and vibrated on a vibrating table for 20 s to make the air bubbles out. The specimens were demolded after 24 h of pouring.Figure 3
Flow diagram for the polyurethane concrete preparation process.


### 2.4. Methods

#### 2.4.1. Basic Mechanical Strength Test of Polyurethane Concrete

(1) Polyurethane Concrete Compressive Strength. It was carried out on the waw-2000a microcomputer-controlled electro-hydraulic servo universal testing machine. As detailed in Figure 4, the specimen size was 100 mm × 100 mm × 100 mm, and the loading speed was 0.5 mm/min. The test temperature was 20°C. The compressive strength of the polyurethane concrete was calculated by the following:(1)fca=FA.Where fca is the polyurethane concrete compressive strength, MP. F is the ultimate load, N. A is the area of compressed surface, mm2.Figure 4
Compressive strength test.(2) Tensile Strength Test. The tensile test specimen is a dumbbell-type thin section specimen as shown in Figure 5(a). The loading speed was 5.0 mm/min. The tensile strength was calculated by the following:(2)TL=Fbc.where TL is the polyurethane concrete tensile strength, MPa. F is the ultimate load, N. b and c are the width and thickness of the tensioned section, respectively, mm.Figure 5
Tensile strength test. (a) Dumbbell type axial tension specimen dimensions, unit: mm. (b) Tensile test loading.
(a)(b)(3) Flexural Strength Test. The polyurethane concrete flexural strength was tested according to the method specified in the JTG 3420–2020 standard [38] at 20°C, as shown in Figure 6. The loading speed was 0.75 kN/s. The flexural strength was calculated by the following:(3)fL=FLbh2,where fL is the flexural strength, MPa. F is the ultimate load, N. L is the distance between supports, mm. b and h are the specimen width and height, respectively, mm.Figure 6
Flexural strength test.


#### 2.4.2. Test Method for Interfacial Bond Strength between Polyurethane Concrete and Cement Concrete

(1) Pretest Preparation. To investigate the effect of interfacial roughness and temperature gradient on the bonding performance between polyurethane concrete and cement concrete, the bonded specimens were addressed as follows:(1)
Cement concrete bonding interface typeTo adapt the bonding test to the actual situation in the anchorage area of the expansion joint, the cement concrete bonding interface was treated with different roughness, and the roughness of the interface was characterized by the sand patch test [40]. The processing ways are shown in Table 9, and the types of bonding interface roughness are shown in Figure 7.(2)
Temperature gradient settingBecause of the microphase separation effect, polyurethane often exhibits different mechanical properties at different temperatures [10]. To investigate the bonding performance of polyurethane concrete with cement concrete at different temperatures, five temperatures of −40°C, −20°C, 0°C, 20°C, and 40°C were set during the bonding test. The specimens were placed in an environmental chamber at the specified temperature to maintain 4 h before the bonding test.Table 9
Bonding interface preparation.
NumberProcessing waysTectonic depth (mm)R0No processing0.2∼0.4R1Steel wire brush bristles. Remove dirt, floating slurry, loose concrete from the surface0.8∼1.3R2Manual chiseling. Chisel away the surface floating slurry and aggregates2.4∼3.2Figure 7
Bonding interface roughness preparation.(2) Bond Splitting Strength Test. A specimen of size 150 mm × 150 mm × 150 mm was subjected in the bond splitting strength test. The cubic cement concrete specimens with dimensions of 150 mm × 150 mm × 150 mm were cut into specimens with dimensions of 150 mm × 150 mm × 75 mm, and then the bonding interface was addressed to different roughness. The addressed cement concrete specimens were placed in a steel formwork precoated with a release agent, and polyurethane concrete was poured in the direction shown in Figure 8(a). The bond-split specimens after 24 h of casting and demoulding are shown in Figure 8(b).Figure 8
Specimen casting diagram and splitting bond specimens.
(a)(b)The bond-splitting strength test was shown in Figure9. The loading rate was 0.06 MPa/s, and the test temperature was 20°C [38]. The bond splitting strength was calculated by (4), and the results were accurate to 0.01 MPa.(4)fts=2PπA,=0.637PA,where fts is the bond splitting strength (MPa). P is the maximum value of the test load (N). A is the area of bonding interface (mm2).Figure 9
Bond splitting strength test.(3) Direct Shear Bond Strength Test. Shear-slip damage is one of the main forms of damage to concrete bond surfaces. To investigate the interfacial mechanical properties of its shear bond strength, a direct shear bond test was conducted on polyurethane concrete and cement concrete using Z-shaped specimens [41]. The size of the specimen and the preparation process are shown in Figures 10 and 11. The specimen consists of two L-shaped specimens, Part L1 and Part L2, and the size of the bond surface is 100 mm × 100 mm. As shown in Figure 11, the L1 cement concrete block was poured in the formwork first and then demoulded after 48 h under standard conditions. After demoulding, the cement concrete block was placed for a further 26 d under standard conditions. The bonding interface was then treated according to the method in Table 9. The treated cement concrete was placed in a dry and ventilated place for 48 h. The purpose was to dry the concrete, especially at the bonding interface, which must be dry and free of water to achieve a reliable bond with the polyurethane concrete. The dried specimen was filled back into the mould, and the polyurethane concrete was poured. The specimen was cured in a dry place at room temperature for 24 h and then demoulded.Figure 10
Dimensions of direct shear bond strength test.Figure 11
Specimen preparation process.
(a)(b)(c)(d)(e)(f)As shown in Figure12, the direct shear bond test was carried out on the WAW-2000A electrohydraulic servo universal testing machine. The loading speed was 0.5 kN/s [42]. Force and displacement were measured by force and displacement transducers on the universal testing machine. BX120-100 resistance strain gauges were attached to both ends of the bond interface to capture the deformation of polyurethane concrete and cement concrete near the bond interface. The JM3816 data tester was used to collect strain data at a frequency of 1 Hz. For evaluating the shear strength of the bonded interface, there are three general indicators, which are as follows: the average bonded shear strength, the local maximum bond strength, and the residual bond strength of the interface [41]. The objective of this paper is to investigate the required bonding performance of polyurethane concrete as an anchorage zone for bridge expansion joints. The average bond shear strength was used as an evaluation index for the bond direct shear strength test, which was calculated according to the following:(5)τ=PA,where τ is the bond direct shear strength (MPa). P is the maximum value of the test load (N). A is the area of bonding interface (mm2).Figure 12
Direct shear bond strength test.(4) Bond Oblique Shear Strength Test. Slowing down or braking of vehicles can cause the bonding interface between the cement concrete and polyurethane concrete in the anchorage area of the expansion joint to be in a state of compression and shear [43]. The oblique shear test is often used to evaluate the compression-shear stress between the interface of two materials [44]. The principle of the oblique shear test and the interface stress state are shown in Figure 13. According to the Moore-Cullen criterion, when the bonded oblique shear specimen is subjected to axial stress (σ0), a tangential stress (τn) parallel to the interface and a normal stress (σn) perpendicular to the interface are generated at the bonded interface [45]. Under the most unfavorable conditions of emergency braking on a downhill section of the vehicle, the ratio of interlaminar normal stress to shear stress is close to 1 : 1. Therefore, a shear angle of 45° is used to correspond to this most unfavorable braking condition [46], and the bonded oblique shear test loading diagram is shown in Figure 14.Figure 13
Oblique shear interface stress state.Figure 14
Bond oblique shear test loading diagram (unit: mm).The specimen preparation process is shown in Figure15. Cement concrete measuring 100 mm × 100 mm × 50 mm was first poured into the mould and cured under standard conditions for 48 h before being demoulded. After the demoulded cement concrete specimens continued to age for 26 d under standard conditions, the cement concrete specimens were subjected to an interface treatment according to the method in Table 9. The treated specimens were placed in a dry ventilated place for 24 h to make the interface waterless and dry. After the dried specimens were placed back into the mould, the polyurethane concrete was poured into the mould to the specified height. The specimens were cured in a dry place at room temperature for 24 h and then demoulded.Figure 15
Flowchart for bond oblique shear specimen preparation.As shown in Figure16, the bond oblique shear test was carried out on the WAW-2000A electrohydraulic servo universal testing machine. The size of the specimen for the bond oblique shear test was 100 mm × 100 mm × 100 mm, and the loading speed was 10 mm/min [46]. Force and displacement were measured by force and displacement transducers on the universal testing machine. The bond oblique shear strength was calculated according to the following:(6)τ=P×sinαS,where τ is the bonded oblique shear strength (MPa), P is the maximum value of the test load (N), S is the area of shear cross-section (mm2), and α is the shear inclination.Figure 16
Bond oblique shear specimens and test loading.
(a)(b)(5) Pull-Out Test. As shown in Figure 17, the length of the bonded section of the steel reinforcement and the polyurethane concrete is l (mm). The length of the nonbonded section is 150 l (mm). The nonbonded section of the steel reinforcement was first wrapped with tape, then a PVC pipe of the appropriate inner diameter was placed over the taped section, and finally, the gap between the reinforcement and the PVC pipe was filled with sealant. The reinforcement in the nonbonded section can be completely isolated from the polyurethane concrete, which prevents the polyurethane concrete from penetrating into the nonbonded section during casting. As shown in Tables 10 and 12, types of pull-out specimens numbered in the form of reinforcement diameter-bonded anchorage length were carried out. For example, the specimen numbered 14-6 d represents a 14 mm diameter reinforcement with a bonded anchorage length of 6 times the diameter of the reinforcement.Figure 17
Pull-out specimens of steel reinforcement bonded to polyurethane concrete.Table 10
Pull-out test of steel reinforcement bonded to polyurethane concrete.
NumberSpecimen IDDiameter of reinforcement (mm)Bonding and anchoring lengthl (mm)112-4 d12448212-5 d12560312-6 d12672412-7 d12784514-4 d14456614-5 d14570714-6 d14684814-7 d14798916-4 d164641016-5 d165801116-6 d166961216-7 d167112The pull-out test was carried out on the WAW-2000A microcomputer-controlled electro-hydraulic servo universal testing machine with a counterforce frame device as shown in Figure18. During loading, the reinforcement was held in place by the lower fixture, while the polyurethane concrete was driven upwards by a counterforce frame fixed to the loading beam. The loading values were collected by force transducers on the test machine. High precision displacement meters were installed on the upper surface of bearing steel plate on the polyurethane concrete and the free end of the reinforcement. The JM3816 static strain tester was used for the collection of the displacement meter data. The loading was displacement-controlled with a loading speed of 0.3 mm/min. The load and displacement data were collected at a frequency of 1 Hz. The test was terminated when the following three conditions occurred in the specimen: (1) splitting damage to the polyurethane concrete, (2) a fracture occurred in the reinforcement or the reinforcement was pulled out by more than 30 mm, and (3) the slippage of the reinforcement at the loading end was greater than 10 mm and the load value fluctuated very little.Figure 18
Reinforcement pull-out test. (a) Reinforcement pull-out test setup. (b) Schematic diagram of the reinforcement pull-out test setup.
(a)(b)The average bond stress of polyurethane concrete and reinforcement was calculated by (7), where the average bond stress corresponding to the ultimate load is its bond strength.(7)τ=Fπdl,where τ is the average bond stress, MPa. F is the load at the loaded end, N. d is the reinforcement diameter, mm. l is the anchorage length of the reinforcement, mm.


## 2.1. Materials

### 2.1.1. Polyaryl Polymethylene Isocyanate

The isocyanate used in this paper is polyaryl polymethylene isocyanate (PAPI). The main components of the PAPI are diphenylmethane diisocyanate (MDI) and oligomeric isocyanates with 3–6 functional groups. The technical indicators of PAPI tested according to the specified standard [29–33] are shown in Table 1.Table 1
PAPI technical indicators.
IndicatorsResultsAppearanceViscosity at 25°C (mPa•s)250NCO content of isocyanate (%)32.0Density at 25°C (g/cm3)1.22Acidity (%)0.030Hydrolyzed chlorine (%)0.20


### 2.1.2. Combined Polyether Polyol

The main components of the combined polyether polyol are polyether polyol, chain extender, and crosslinker. The technical indicators of the combined polyether polyols were tested by the specified standard [29, 34, 35] as shown in Table 2.Table 2
Technical indicators of combined polyether polyol.
IndicatorsResultsAppearanceViscosity at 25°C (mPa•s)200Hydroxyl value (mg KOH/g)370Density at 25°C (g/cm3)1.13pH5.4Unsaturation (mmol/g)0.005


### 2.1.3. Catalyst

The catalyst used in this paper is an aliphatic amine catalyst among tertiary amine catalysts, which is a solution with triethylenediamine (DABCO) as the main component and has a colorless and transparent liquid appearance. The molecular structure of DABCO is shown in Figure1.Figure 1
Triethylenediamine.


### 2.1.4. Molecular Sieve

Isocyanate is prone to react with water to form polyurea and carbon dioxide gas [13], which can lead to volume expansion of the polyurethane [36]. Therefore, molecular sieves are added to adsorb water in the mixture and thus ensure the volumetric stability of the cured polyurethane concrete. The physicochemical properties of the 5A molecular sieve tested by the specified standard [37] are listed in Table 3.Table 3
Technical indicators of the 5A molecular sieve.
IndicatorsResultsAppearanceSize (μm)2–4Bulk density (g/mL)0.45Static water adsorption rate (%)25.0pH9


### 2.1.5. Cement

The performance parameters of PO 42.5 cement tested by the specified standard [38] are shown in Table 4.Table 4
Performance parameters of cement.
IndicatorsResultsInitial setting time72Final setting time366Fineness (%)1.8Specific surface area (m2/kg)37428 d shrinkage rate (%)0.08Wear resistance (kg/m2)2.4Flexural strength (MPa)3 days4.628 days7.2Compressive strength (MPa)3 days7.228 days24.6


### 2.1.6. Aggregate

The technical indicators of granite aggregates tested by specified standard [39] are shown in Table 5. The aggregate grading curves of the aggregates are shown in Figure 2.Table 5
Technical indicators of granite aggregate.
IndicatorsResultsApparent density (g/m3)2.767Bulk density (g/m3)2.683Bulk porosity (%)38.5Crush value (%)23.1Needle flake content (%)3.4Figure 2
Aggregate grading curves.


### 2.1.7. Steel Reinforcement

The HRB400 hot rolled ribbed steel bars were used in the pull-out test, and the mechanical properties of the steel reinforcement are shown in Table6.Table 6
Mechanical properties of steel reinforcement.
GradeDiameter (mm)Yield strength (MPa)Tensile strength (MPa)Elongation after break (%)Total extension at maximum force (%)HRB40012450.3657.73217.414460.4635.92717.016455.7624.52516.2


## 2.1.1. Polyaryl Polymethylene Isocyanate

The isocyanate used in this paper is polyaryl polymethylene isocyanate (PAPI). The main components of the PAPI are diphenylmethane diisocyanate (MDI) and oligomeric isocyanates with 3–6 functional groups. The technical indicators of PAPI tested according to the specified standard [29–33] are shown in Table 1.Table 1
PAPI technical indicators.
IndicatorsResultsAppearanceViscosity at 25°C (mPa•s)250NCO content of isocyanate (%)32.0Density at 25°C (g/cm3)1.22Acidity (%)0.030Hydrolyzed chlorine (%)0.20


## 2.1.2. Combined Polyether Polyol

The main components of the combined polyether polyol are polyether polyol, chain extender, and crosslinker. The technical indicators of the combined polyether polyols were tested by the specified standard [29, 34, 35] as shown in Table 2.Table 2
Technical indicators of combined polyether polyol.
IndicatorsResultsAppearanceViscosity at 25°C (mPa•s)200Hydroxyl value (mg KOH/g)370Density at 25°C (g/cm3)1.13pH5.4Unsaturation (mmol/g)0.005


## 2.1.3. Catalyst

The catalyst used in this paper is an aliphatic amine catalyst among tertiary amine catalysts, which is a solution with triethylenediamine (DABCO) as the main component and has a colorless and transparent liquid appearance. The molecular structure of DABCO is shown in Figure1.Figure 1
Triethylenediamine.


## 2.1.4. Molecular Sieve

Isocyanate is prone to react with water to form polyurea and carbon dioxide gas [13], which can lead to volume expansion of the polyurethane [36]. Therefore, molecular sieves are added to adsorb water in the mixture and thus ensure the volumetric stability of the cured polyurethane concrete. The physicochemical properties of the 5A molecular sieve tested by the specified standard [37] are listed in Table 3.Table 3
Technical indicators of the 5A molecular sieve.
IndicatorsResultsAppearanceSize (μm)2–4Bulk density (g/mL)0.45Static water adsorption rate (%)25.0pH9


## 2.1.5. Cement

The performance parameters of PO 42.5 cement tested by the specified standard [38] are shown in Table 4.Table 4
Performance parameters of cement.
IndicatorsResultsInitial setting time72Final setting time366Fineness (%)1.8Specific surface area (m2/kg)37428 d shrinkage rate (%)0.08Wear resistance (kg/m2)2.4Flexural strength (MPa)3 days4.628 days7.2Compressive strength (MPa)3 days7.228 days24.6


## 2.1.6. Aggregate

The technical indicators of granite aggregates tested by specified standard [39] are shown in Table 5. The aggregate grading curves of the aggregates are shown in Figure 2.Table 5
Technical indicators of granite aggregate.
IndicatorsResultsApparent density (g/m3)2.767Bulk density (g/m3)2.683Bulk porosity (%)38.5Crush value (%)23.1Needle flake content (%)3.4Figure 2
Aggregate grading curves.


## 2.1.7. Steel Reinforcement

The HRB400 hot rolled ribbed steel bars were used in the pull-out test, and the mechanical properties of the steel reinforcement are shown in Table6.Table 6
Mechanical properties of steel reinforcement.
GradeDiameter (mm)Yield strength (MPa)Tensile strength (MPa)Elongation after break (%)Total extension at maximum force (%)HRB40012450.3657.73217.414460.4635.92717.016455.7624.52516.2


## 2.2. Mixture Design

The effects of the three factors, namely, binder-aggregate ratio, cement dosage ratio, and aggregate gradation, on the compressive strength of polyurethane concrete were investigated by the orthogonal test, and the response analysis of each control factor was conducted separately to determine the optimal mixture of polyurethane concrete:(1)
The L9(33) orthogonal table was used for the orthogonal test, where the control factors were binder-aggregate ratio, cement dosage ratio, and aggregate gradation, with three levels for each factor listed in Table 7. The ratio of the two-component polyurethane total mass to the aggregate mass is used as the binder-aggregate ratio. The cement dosage ratio is the ratio of the cement mass to the polyaryl polymethylene isocyanate (or combined polyether polyol) mass.(2)
The polyurethane concrete orthogonal test mixtures are shown in Table8, and a total of 9 groups of specimens from T1-T9 were set up. To reduce the influence of data variability on the test results, three parallel specimens were prepared for each group under the same preparation and maintenance conditions.Table 7
Factor level values.
Factor IDControl factorsFactor levels123ABinder-aggregate ratio10%15%20%BCement dosage ratio1.01.52.0CAggregate gradationAC-5AC-10AC-13Table 8
Orthogonal experimental design.
NumberOrthogonal combinationsA (%)BCT1A1B1C1101.0AC-5T2A1B2C2101.5AC-10T3A1B3C3102.0AC-13T4A2B1C2151.0AC-10T5A2B2C3151.5AC-13T6A2B3C1152.0AC-5T7A3B1C3201.0AC-13T8A3B2C1201.5AC-5T9A3B3C2202.0AC-10Note: A, B, and C are the factor IDs for each control factor in Table7, respectively. For example, A2B3C1 represents the combination of level 2 of the A factor (binder-aggregate ratio), level 3 of the B factor (cement dosage ratio), and level 1 of the C factor (aggregate gradation).


## 2.3. Preparation of Polyurethane Concrete

Flow diagram for the polyurethane concrete preparation process is shown in Figure3:(1)
Water removal: although molecular sieve as a water remover had been added into the mixture, its dosage should not be too large. Too much molecular sieve would affect the performance of the concrete after curing. To ensure the volumetric stability of the polyurethane concrete, the cement and aggregates should be dried in a 101-2A electric blast dryer at 130°C for 4 hours, during which the cement and aggregates are upturned every 30 minutes. After the drying process, the aggregates and cement were stored in sealed containers and cooled to room temperature before they were used in the preparation of polyurethane concrete.(2)
Premixing: premixtures were prepared by mixing aggregates, cement, molecular sieves, and combined polyether polyols. The premixture in homogeneous state should be dry plastic state, and no cement powder should be found. Premixing should last for more than 10 minutes so that the molecular sieve can take full advantage of water removal while fully mixing the premix.(3)
Mixing: the polyisocyanate was added to the premixture and stirred for 1.5 minutes. Add the catalyst dropwise while the mixture is in the flow plastic state, stirring rapidly for 0.5 minutes.(4)
Pouring and molding: the mixed polyurethane concrete was poured into the mold with preapplied release agent and vibrated on a vibrating table for 20 s to make the air bubbles out. The specimens were demolded after 24 h of pouring.Figure 3
Flow diagram for the polyurethane concrete preparation process.


## 2.4. Methods

### 2.4.1. Basic Mechanical Strength Test of Polyurethane Concrete

(1) Polyurethane Concrete Compressive Strength. It was carried out on the waw-2000a microcomputer-controlled electro-hydraulic servo universal testing machine. As detailed in Figure 4, the specimen size was 100 mm × 100 mm × 100 mm, and the loading speed was 0.5 mm/min. The test temperature was 20°C. The compressive strength of the polyurethane concrete was calculated by the following:(1)fca=FA.Where fca is the polyurethane concrete compressive strength, MP. F is the ultimate load, N. A is the area of compressed surface, mm2.Figure 4
Compressive strength test.(2) Tensile Strength Test. The tensile test specimen is a dumbbell-type thin section specimen as shown in Figure 5(a). The loading speed was 5.0 mm/min. The tensile strength was calculated by the following:(2)TL=Fbc.where TL is the polyurethane concrete tensile strength, MPa. F is the ultimate load, N. b and c are the width and thickness of the tensioned section, respectively, mm.Figure 5
Tensile strength test. (a) Dumbbell type axial tension specimen dimensions, unit: mm. (b) Tensile test loading.
(a)(b)(3) Flexural Strength Test. The polyurethane concrete flexural strength was tested according to the method specified in the JTG 3420–2020 standard [38] at 20°C, as shown in Figure 6. The loading speed was 0.75 kN/s. The flexural strength was calculated by the following:(3)fL=FLbh2,where fL is the flexural strength, MPa. F is the ultimate load, N. L is the distance between supports, mm. b and h are the specimen width and height, respectively, mm.Figure 6
Flexural strength test.


### 2.4.2. Test Method for Interfacial Bond Strength between Polyurethane Concrete and Cement Concrete

(1) Pretest Preparation. To investigate the effect of interfacial roughness and temperature gradient on the bonding performance between polyurethane concrete and cement concrete, the bonded specimens were addressed as follows:(1)
Cement concrete bonding interface typeTo adapt the bonding test to the actual situation in the anchorage area of the expansion joint, the cement concrete bonding interface was treated with different roughness, and the roughness of the interface was characterized by the sand patch test [40]. The processing ways are shown in Table 9, and the types of bonding interface roughness are shown in Figure 7.(2)
Temperature gradient settingBecause of the microphase separation effect, polyurethane often exhibits different mechanical properties at different temperatures [10]. To investigate the bonding performance of polyurethane concrete with cement concrete at different temperatures, five temperatures of −40°C, −20°C, 0°C, 20°C, and 40°C were set during the bonding test. The specimens were placed in an environmental chamber at the specified temperature to maintain 4 h before the bonding test.Table 9
Bonding interface preparation.
NumberProcessing waysTectonic depth (mm)R0No processing0.2∼0.4R1Steel wire brush bristles. Remove dirt, floating slurry, loose concrete from the surface0.8∼1.3R2Manual chiseling. Chisel away the surface floating slurry and aggregates2.4∼3.2Figure 7
Bonding interface roughness preparation.(2) Bond Splitting Strength Test. A specimen of size 150 mm × 150 mm × 150 mm was subjected in the bond splitting strength test. The cubic cement concrete specimens with dimensions of 150 mm × 150 mm × 150 mm were cut into specimens with dimensions of 150 mm × 150 mm × 75 mm, and then the bonding interface was addressed to different roughness. The addressed cement concrete specimens were placed in a steel formwork precoated with a release agent, and polyurethane concrete was poured in the direction shown in Figure 8(a). The bond-split specimens after 24 h of casting and demoulding are shown in Figure 8(b).Figure 8
Specimen casting diagram and splitting bond specimens.
(a)(b)The bond-splitting strength test was shown in Figure9. The loading rate was 0.06 MPa/s, and the test temperature was 20°C [38]. The bond splitting strength was calculated by (4), and the results were accurate to 0.01 MPa.(4)fts=2PπA,=0.637PA,where fts is the bond splitting strength (MPa). P is the maximum value of the test load (N). A is the area of bonding interface (mm2).Figure 9
Bond splitting strength test.(3) Direct Shear Bond Strength Test. Shear-slip damage is one of the main forms of damage to concrete bond surfaces. To investigate the interfacial mechanical properties of its shear bond strength, a direct shear bond test was conducted on polyurethane concrete and cement concrete using Z-shaped specimens [41]. The size of the specimen and the preparation process are shown in Figures 10 and 11. The specimen consists of two L-shaped specimens, Part L1 and Part L2, and the size of the bond surface is 100 mm × 100 mm. As shown in Figure 11, the L1 cement concrete block was poured in the formwork first and then demoulded after 48 h under standard conditions. After demoulding, the cement concrete block was placed for a further 26 d under standard conditions. The bonding interface was then treated according to the method in Table 9. The treated cement concrete was placed in a dry and ventilated place for 48 h. The purpose was to dry the concrete, especially at the bonding interface, which must be dry and free of water to achieve a reliable bond with the polyurethane concrete. The dried specimen was filled back into the mould, and the polyurethane concrete was poured. The specimen was cured in a dry place at room temperature for 24 h and then demoulded.Figure 10
Dimensions of direct shear bond strength test.Figure 11
Specimen preparation process.
(a)(b)(c)(d)(e)(f)As shown in Figure12, the direct shear bond test was carried out on the WAW-2000A electrohydraulic servo universal testing machine. The loading speed was 0.5 kN/s [42]. Force and displacement were measured by force and displacement transducers on the universal testing machine. BX120-100 resistance strain gauges were attached to both ends of the bond interface to capture the deformation of polyurethane concrete and cement concrete near the bond interface. The JM3816 data tester was used to collect strain data at a frequency of 1 Hz. For evaluating the shear strength of the bonded interface, there are three general indicators, which are as follows: the average bonded shear strength, the local maximum bond strength, and the residual bond strength of the interface [41]. The objective of this paper is to investigate the required bonding performance of polyurethane concrete as an anchorage zone for bridge expansion joints. The average bond shear strength was used as an evaluation index for the bond direct shear strength test, which was calculated according to the following:(5)τ=PA,where τ is the bond direct shear strength (MPa). P is the maximum value of the test load (N). A is the area of bonding interface (mm2).Figure 12
Direct shear bond strength test.(4) Bond Oblique Shear Strength Test. Slowing down or braking of vehicles can cause the bonding interface between the cement concrete and polyurethane concrete in the anchorage area of the expansion joint to be in a state of compression and shear [43]. The oblique shear test is often used to evaluate the compression-shear stress between the interface of two materials [44]. The principle of the oblique shear test and the interface stress state are shown in Figure 13. According to the Moore-Cullen criterion, when the bonded oblique shear specimen is subjected to axial stress (σ0), a tangential stress (τn) parallel to the interface and a normal stress (σn) perpendicular to the interface are generated at the bonded interface [45]. Under the most unfavorable conditions of emergency braking on a downhill section of the vehicle, the ratio of interlaminar normal stress to shear stress is close to 1 : 1. Therefore, a shear angle of 45° is used to correspond to this most unfavorable braking condition [46], and the bonded oblique shear test loading diagram is shown in Figure 14.Figure 13
Oblique shear interface stress state.Figure 14
Bond oblique shear test loading diagram (unit: mm).The specimen preparation process is shown in Figure15. Cement concrete measuring 100 mm × 100 mm × 50 mm was first poured into the mould and cured under standard conditions for 48 h before being demoulded. After the demoulded cement concrete specimens continued to age for 26 d under standard conditions, the cement concrete specimens were subjected to an interface treatment according to the method in Table 9. The treated specimens were placed in a dry ventilated place for 24 h to make the interface waterless and dry. After the dried specimens were placed back into the mould, the polyurethane concrete was poured into the mould to the specified height. The specimens were cured in a dry place at room temperature for 24 h and then demoulded.Figure 15
Flowchart for bond oblique shear specimen preparation.As shown in Figure16, the bond oblique shear test was carried out on the WAW-2000A electrohydraulic servo universal testing machine. The size of the specimen for the bond oblique shear test was 100 mm × 100 mm × 100 mm, and the loading speed was 10 mm/min [46]. Force and displacement were measured by force and displacement transducers on the universal testing machine. The bond oblique shear strength was calculated according to the following:(6)τ=P×sinαS,where τ is the bonded oblique shear strength (MPa), P is the maximum value of the test load (N), S is the area of shear cross-section (mm2), and α is the shear inclination.Figure 16
Bond oblique shear specimens and test loading.
(a)(b)(5) Pull-Out Test. As shown in Figure 17, the length of the bonded section of the steel reinforcement and the polyurethane concrete is l (mm). The length of the nonbonded section is 150 l (mm). The nonbonded section of the steel reinforcement was first wrapped with tape, then a PVC pipe of the appropriate inner diameter was placed over the taped section, and finally, the gap between the reinforcement and the PVC pipe was filled with sealant. The reinforcement in the nonbonded section can be completely isolated from the polyurethane concrete, which prevents the polyurethane concrete from penetrating into the nonbonded section during casting. As shown in Tables 10 and 12, types of pull-out specimens numbered in the form of reinforcement diameter-bonded anchorage length were carried out. For example, the specimen numbered 14-6 d represents a 14 mm diameter reinforcement with a bonded anchorage length of 6 times the diameter of the reinforcement.Figure 17
Pull-out specimens of steel reinforcement bonded to polyurethane concrete.Table 10
Pull-out test of steel reinforcement bonded to polyurethane concrete.
NumberSpecimen IDDiameter of reinforcement (mm)Bonding and anchoring lengthl (mm)112-4 d12448212-5 d12560312-6 d12672412-7 d12784514-4 d14456614-5 d14570714-6 d14684814-7 d14798916-4 d164641016-5 d165801116-6 d166961216-7 d167112The pull-out test was carried out on the WAW-2000A microcomputer-controlled electro-hydraulic servo universal testing machine with a counterforce frame device as shown in Figure18. During loading, the reinforcement was held in place by the lower fixture, while the polyurethane concrete was driven upwards by a counterforce frame fixed to the loading beam. The loading values were collected by force transducers on the test machine. High precision displacement meters were installed on the upper surface of bearing steel plate on the polyurethane concrete and the free end of the reinforcement. The JM3816 static strain tester was used for the collection of the displacement meter data. The loading was displacement-controlled with a loading speed of 0.3 mm/min. The load and displacement data were collected at a frequency of 1 Hz. The test was terminated when the following three conditions occurred in the specimen: (1) splitting damage to the polyurethane concrete, (2) a fracture occurred in the reinforcement or the reinforcement was pulled out by more than 30 mm, and (3) the slippage of the reinforcement at the loading end was greater than 10 mm and the load value fluctuated very little.Figure 18
Reinforcement pull-out test. (a) Reinforcement pull-out test setup. (b) Schematic diagram of the reinforcement pull-out test setup.
(a)(b)The average bond stress of polyurethane concrete and reinforcement was calculated by (7), where the average bond stress corresponding to the ultimate load is its bond strength.(7)τ=Fπdl,where τ is the average bond stress, MPa. F is the load at the loaded end, N. d is the reinforcement diameter, mm. l is the anchorage length of the reinforcement, mm.


## 2.4.1. Basic Mechanical Strength Test of Polyurethane Concrete

(1) Polyurethane Concrete Compressive Strength. It was carried out on the waw-2000a microcomputer-controlled electro-hydraulic servo universal testing machine. As detailed in Figure 4, the specimen size was 100 mm × 100 mm × 100 mm, and the loading speed was 0.5 mm/min. The test temperature was 20°C. The compressive strength of the polyurethane concrete was calculated by the following:(1)fca=FA.Where fca is the polyurethane concrete compressive strength, MP. F is the ultimate load, N. A is the area of compressed surface, mm2.Figure 4
Compressive strength test.(2) Tensile Strength Test. The tensile test specimen is a dumbbell-type thin section specimen as shown in Figure 5(a). The loading speed was 5.0 mm/min. The tensile strength was calculated by the following:(2)TL=Fbc.where TL is the polyurethane concrete tensile strength, MPa. F is the ultimate load, N. b and c are the width and thickness of the tensioned section, respectively, mm.Figure 5
Tensile strength test. (a) Dumbbell type axial tension specimen dimensions, unit: mm. (b) Tensile test loading.
(a)(b)(3) Flexural Strength Test. The polyurethane concrete flexural strength was tested according to the method specified in the JTG 3420–2020 standard [38] at 20°C, as shown in Figure 6. The loading speed was 0.75 kN/s. The flexural strength was calculated by the following:(3)fL=FLbh2,where fL is the flexural strength, MPa. F is the ultimate load, N. L is the distance between supports, mm. b and h are the specimen width and height, respectively, mm.Figure 6
Flexural strength test.


## 2.4.2. Test Method for Interfacial Bond Strength between Polyurethane Concrete and Cement Concrete

(1) Pretest Preparation. To investigate the effect of interfacial roughness and temperature gradient on the bonding performance between polyurethane concrete and cement concrete, the bonded specimens were addressed as follows:(1)
Cement concrete bonding interface typeTo adapt the bonding test to the actual situation in the anchorage area of the expansion joint, the cement concrete bonding interface was treated with different roughness, and the roughness of the interface was characterized by the sand patch test [40]. The processing ways are shown in Table 9, and the types of bonding interface roughness are shown in Figure 7.(2)
Temperature gradient settingBecause of the microphase separation effect, polyurethane often exhibits different mechanical properties at different temperatures [10]. To investigate the bonding performance of polyurethane concrete with cement concrete at different temperatures, five temperatures of −40°C, −20°C, 0°C, 20°C, and 40°C were set during the bonding test. The specimens were placed in an environmental chamber at the specified temperature to maintain 4 h before the bonding test.Table 9
Bonding interface preparation.
NumberProcessing waysTectonic depth (mm)R0No processing0.2∼0.4R1Steel wire brush bristles. Remove dirt, floating slurry, loose concrete from the surface0.8∼1.3R2Manual chiseling. Chisel away the surface floating slurry and aggregates2.4∼3.2Figure 7
Bonding interface roughness preparation.(2) Bond Splitting Strength Test. A specimen of size 150 mm × 150 mm × 150 mm was subjected in the bond splitting strength test. The cubic cement concrete specimens with dimensions of 150 mm × 150 mm × 150 mm were cut into specimens with dimensions of 150 mm × 150 mm × 75 mm, and then the bonding interface was addressed to different roughness. The addressed cement concrete specimens were placed in a steel formwork precoated with a release agent, and polyurethane concrete was poured in the direction shown in Figure 8(a). The bond-split specimens after 24 h of casting and demoulding are shown in Figure 8(b).Figure 8
Specimen casting diagram and splitting bond specimens.
(a)(b)The bond-splitting strength test was shown in Figure9. The loading rate was 0.06 MPa/s, and the test temperature was 20°C [38]. The bond splitting strength was calculated by (4), and the results were accurate to 0.01 MPa.(4)fts=2PπA,=0.637PA,where fts is the bond splitting strength (MPa). P is the maximum value of the test load (N). A is the area of bonding interface (mm2).Figure 9
Bond splitting strength test.(3) Direct Shear Bond Strength Test. Shear-slip damage is one of the main forms of damage to concrete bond surfaces. To investigate the interfacial mechanical properties of its shear bond strength, a direct shear bond test was conducted on polyurethane concrete and cement concrete using Z-shaped specimens [41]. The size of the specimen and the preparation process are shown in Figures 10 and 11. The specimen consists of two L-shaped specimens, Part L1 and Part L2, and the size of the bond surface is 100 mm × 100 mm. As shown in Figure 11, the L1 cement concrete block was poured in the formwork first and then demoulded after 48 h under standard conditions. After demoulding, the cement concrete block was placed for a further 26 d under standard conditions. The bonding interface was then treated according to the method in Table 9. The treated cement concrete was placed in a dry and ventilated place for 48 h. The purpose was to dry the concrete, especially at the bonding interface, which must be dry and free of water to achieve a reliable bond with the polyurethane concrete. The dried specimen was filled back into the mould, and the polyurethane concrete was poured. The specimen was cured in a dry place at room temperature for 24 h and then demoulded.Figure 10
Dimensions of direct shear bond strength test.Figure 11
Specimen preparation process.
(a)(b)(c)(d)(e)(f)As shown in Figure12, the direct shear bond test was carried out on the WAW-2000A electrohydraulic servo universal testing machine. The loading speed was 0.5 kN/s [42]. Force and displacement were measured by force and displacement transducers on the universal testing machine. BX120-100 resistance strain gauges were attached to both ends of the bond interface to capture the deformation of polyurethane concrete and cement concrete near the bond interface. The JM3816 data tester was used to collect strain data at a frequency of 1 Hz. For evaluating the shear strength of the bonded interface, there are three general indicators, which are as follows: the average bonded shear strength, the local maximum bond strength, and the residual bond strength of the interface [41]. The objective of this paper is to investigate the required bonding performance of polyurethane concrete as an anchorage zone for bridge expansion joints. The average bond shear strength was used as an evaluation index for the bond direct shear strength test, which was calculated according to the following:(5)τ=PA,where τ is the bond direct shear strength (MPa). P is the maximum value of the test load (N). A is the area of bonding interface (mm2).Figure 12
Direct shear bond strength test.(4) Bond Oblique Shear Strength Test. Slowing down or braking of vehicles can cause the bonding interface between the cement concrete and polyurethane concrete in the anchorage area of the expansion joint to be in a state of compression and shear [43]. The oblique shear test is often used to evaluate the compression-shear stress between the interface of two materials [44]. The principle of the oblique shear test and the interface stress state are shown in Figure 13. According to the Moore-Cullen criterion, when the bonded oblique shear specimen is subjected to axial stress (σ0), a tangential stress (τn) parallel to the interface and a normal stress (σn) perpendicular to the interface are generated at the bonded interface [45]. Under the most unfavorable conditions of emergency braking on a downhill section of the vehicle, the ratio of interlaminar normal stress to shear stress is close to 1 : 1. Therefore, a shear angle of 45° is used to correspond to this most unfavorable braking condition [46], and the bonded oblique shear test loading diagram is shown in Figure 14.Figure 13
Oblique shear interface stress state.Figure 14
Bond oblique shear test loading diagram (unit: mm).The specimen preparation process is shown in Figure15. Cement concrete measuring 100 mm × 100 mm × 50 mm was first poured into the mould and cured under standard conditions for 48 h before being demoulded. After the demoulded cement concrete specimens continued to age for 26 d under standard conditions, the cement concrete specimens were subjected to an interface treatment according to the method in Table 9. The treated specimens were placed in a dry ventilated place for 24 h to make the interface waterless and dry. After the dried specimens were placed back into the mould, the polyurethane concrete was poured into the mould to the specified height. The specimens were cured in a dry place at room temperature for 24 h and then demoulded.Figure 15
Flowchart for bond oblique shear specimen preparation.As shown in Figure16, the bond oblique shear test was carried out on the WAW-2000A electrohydraulic servo universal testing machine. The size of the specimen for the bond oblique shear test was 100 mm × 100 mm × 100 mm, and the loading speed was 10 mm/min [46]. Force and displacement were measured by force and displacement transducers on the universal testing machine. The bond oblique shear strength was calculated according to the following:(6)τ=P×sinαS,where τ is the bonded oblique shear strength (MPa), P is the maximum value of the test load (N), S is the area of shear cross-section (mm2), and α is the shear inclination.Figure 16
Bond oblique shear specimens and test loading.
(a)(b)(5) Pull-Out Test. As shown in Figure 17, the length of the bonded section of the steel reinforcement and the polyurethane concrete is l (mm). The length of the nonbonded section is 150 l (mm). The nonbonded section of the steel reinforcement was first wrapped with tape, then a PVC pipe of the appropriate inner diameter was placed over the taped section, and finally, the gap between the reinforcement and the PVC pipe was filled with sealant. The reinforcement in the nonbonded section can be completely isolated from the polyurethane concrete, which prevents the polyurethane concrete from penetrating into the nonbonded section during casting. As shown in Tables 10 and 12, types of pull-out specimens numbered in the form of reinforcement diameter-bonded anchorage length were carried out. For example, the specimen numbered 14-6 d represents a 14 mm diameter reinforcement with a bonded anchorage length of 6 times the diameter of the reinforcement.Figure 17
Pull-out specimens of steel reinforcement bonded to polyurethane concrete.Table 10
Pull-out test of steel reinforcement bonded to polyurethane concrete.
NumberSpecimen IDDiameter of reinforcement (mm)Bonding and anchoring lengthl (mm)112-4 d12448212-5 d12560312-6 d12672412-7 d12784514-4 d14456614-5 d14570714-6 d14684814-7 d14798916-4 d164641016-5 d165801116-6 d166961216-7 d167112The pull-out test was carried out on the WAW-2000A microcomputer-controlled electro-hydraulic servo universal testing machine with a counterforce frame device as shown in Figure18. During loading, the reinforcement was held in place by the lower fixture, while the polyurethane concrete was driven upwards by a counterforce frame fixed to the loading beam. The loading values were collected by force transducers on the test machine. High precision displacement meters were installed on the upper surface of bearing steel plate on the polyurethane concrete and the free end of the reinforcement. The JM3816 static strain tester was used for the collection of the displacement meter data. The loading was displacement-controlled with a loading speed of 0.3 mm/min. The load and displacement data were collected at a frequency of 1 Hz. The test was terminated when the following three conditions occurred in the specimen: (1) splitting damage to the polyurethane concrete, (2) a fracture occurred in the reinforcement or the reinforcement was pulled out by more than 30 mm, and (3) the slippage of the reinforcement at the loading end was greater than 10 mm and the load value fluctuated very little.Figure 18
Reinforcement pull-out test. (a) Reinforcement pull-out test setup. (b) Schematic diagram of the reinforcement pull-out test setup.
(a)(b)The average bond stress of polyurethane concrete and reinforcement was calculated by (7), where the average bond stress corresponding to the ultimate load is its bond strength.(7)τ=Fπdl,where τ is the average bond stress, MPa. F is the load at the loaded end, N. d is the reinforcement diameter, mm. l is the anchorage length of the reinforcement, mm.


## 3. Results and Discussion

### 3.1. Results of Orthogonal Experiments

The results of the orthogonal test were analyzed by the extreme analysis in Table11. The extreme difference of the binder-aggregate ratio is 29.57, which indicates that the binder-aggregate ratio is the control factor with the greatest degree of influence on the compressive strength of polyurethane concrete, followed by the aggregate gradation with an extreme difference value of 3.90, and the cement dosage ratio with an extreme difference value of 0.93, which has the least influence on the compressive strength of polyurethane concrete. The factor level with the maximum average value under each control factor was selected as the polyurethane concrete mixture for the subsequent bond performance tests, i.e., a binder-aggregate ratio of 20%, an aggregate grade of AC-10, and a cement dosage ratio of 2.0. The mass ratio is as follows: polyisocyanate : combined polyether : cement: aggregate (AC-10) = 1 : 1 : 2 : 10, and the percentage of each component is shown in Figure 19.Table 11
Mean principal response.
LevelABC130.3344.0742.60243.2044.3746.50359.9045.0044.33Extreme difference29.570.933.90Rank132Figure 19
The optimal mixture of polyurethane concrete.


### 3.2. Basic Mechanical Strength Test Results of Polyurethane Concrete

#### 3.2.1. Compressive Strength

The results of polyurethane concrete compressive strength test are shown in Table12 and Figure 20. The average ultimate compressive strength of polyurethane concrete is 62.6 MPa, and its compressive process can be divided into linear elastic phase and nonlinear hysteretic elastic phase. Linear elastic stage: the stress-strain curve of the specimen under compression is linearly and positively correlated. Nonlinear hysteretic elastic stage: the stress-strain curve of the specimen is nonlinear, and there is an obvious plateau area. At the early stage of specimen compression, the internal stress caused by external forces is less than the chemical bonding, intermolecular hydrogen bonding, and van der Waals forces within the polyurethane molecule [47, 48]. The polyurethane and gravel aggregate are in a synergistic stress state, and the specimen is in the elastic stress stage. However, the increased stress forces the polyurethane molecular chain segments to deform, move, and misalign, and the force chains inside the specimen matrix undergo redistribution. The disruption of the stress equilibrium throughout the specimen is responsible for the nonlinear trend in the stress-strain curve.Table 12
The polyurethane concrete compressive strength.
NumberC01C02C03C04C05C06MeanCompressive strength (MPa)62.361.164.560.663.463.762.6Figure 20
The compressive strength stress-strain curve of polyurethane concrete.


#### 3.2.2. Tensile Strength

The tensile strength test results of polyurethane concrete are shown in Table13 and Figure 21. The average value of ultimate tensile strength of polyurethane concrete is 12.5 MPa. According to the tensile stress-strain curve in Figure 21, the tensile mechanical behavior of polyurethane concrete can be divided into the linear elastic phase and forced hyperelastic deformation phase. Linear elastic phase: the stress-strain curve of the specimen under tension is linearly and positively correlated. Forced hyperelastic deformation phase: the forced hyperelastic deformation zone includes the strain softening phase and the plateau phase. Although external forces impose the molecular chains to stretch and move, causing large deformation of the specimen, the polyurethane concrete is still able to maintain a high tensile strength.Table 13
The polyurethane concrete tensile strength.
NumberL01L02L03L04L05L06MeanTensile strength (MPa)12.112.711.614.113.611.212.5Figure 21
The tensile strength stress-strain curve of polyurethane concrete.


#### 3.2.3. Flexural Strength

The flexural strength test results of the polyurethane concrete are shown in Table14 and Figure 22. The average value of ultimate flexural strength of polyurethane concrete is 11.4 MPa. According to the flexural stress-strain curve in Figure 22, the flexural mechanical behavior of polyurethane concrete can be divided into linear elastic phase and strain softening phase. Linear elastic phase: the stress-strain curve of the specimen under flexural status is linearly and positively correlated. Strain softening stage: strain softening in this stage is mainly caused by the reduction of the cross-section inertia moment induced by the cracking in the specimen span.Table 14
The flexural strength of polyurethane concrete.
NumberF01F02F03F04F05F06MeanFlexural strength (MPa)10.811.911.210.911.312.211.4Figure 22
The flexural strength stress-strain curve of polyurethane concrete.


### 3.3. Results of Polyurethane Concrete and Cement Concrete Bond Tests

#### 3.3.1. Bond Splitting Strength

(1) Damage Pattern of Bond Splitting Test. Typical bond splitting damage patterns under different bonded interfaces are shown in Figure 23. It can be observed from Figure 23 that the damage surface of R0 type is relatively straight and basically develops along the bonding surface. Part of the polyurethane binder can be observed on the cement concrete side, and there is also a small amount of cement floating slurry on the polyurethane concrete side. It is mainly because of the presence of floating slurry or small amounts of loose concrete on untreated bond surfaces. When the specimen is subjected to load, the floating slurry or loose concrete has a weaker ability to resist the tensile stress, causing it to be damaged by tensile splitting from the interface bond weakness.Figure 23
Damage patterns of bond splitting test.The damage surface of type R1 occurred on the cement concrete side, and the cement concrete slurry and aggregate can be seen in the central area of the polyurethane concrete side. It is mainly attributed to the fact that when the bond splitting specimens undergo external loads, the middle position of the bonded interface is subjected to the main tensile stress, and the edge position of the bonded interface is subjected to compressive stress. However, the tensile strength of cement concrete is small, and when the tensile stress reaches the tensile limit of cement concrete, the weak middle position of cement concrete is the first to be destroyed.The damage surface of type R2 also occurs on the cement concrete side, however, unlike the interface of type R1, more tensile fracture of the aggregate can be observed on the cement concrete side. It is because more aggregates are exposed at the cement concrete interface after chisel treatment. The bond strength of the polyurethane binder to the aggregate is greater than the tensile strength of the aggregate. The aggregates are prone to be pulled off or out by the polyurethane binder under splitting.(2) Bond-Splitting Strength under Different Interface Roughness and Different Temperatures. The bond-splitting strength is an important indicator to evaluate the ability of the bonded interface to resist tensile stress. The results of the polyurethane concrete and cement concrete bond-splitting strength test are shown in Figure 24. The horizontal coordinate represents the test conditions for different combinations of temperature and interface types, for example, 40-R1 indicates the test condition at the combination of 40°C and R1 interface type. The bond splitting strengths of the specimens with different roughness interfaces are R2 > R1 > R0 in descending order. The type R2 interface is chiseled so that the aggregate can be exposed, and the polyurethane binder can form a reliable bond with the aggregate, resulting in a higher bond splitting strength than type R0 and R1.Figure 24
Bond splitting strength under different interface types and different temperatures.Under five temperature conditions, the bond splitting strength of Type R2 increases by 91.4%, 90.9%, 84.8%, 82.6%, and 83.1% compared to Type R0, and by 17.0%, 18.0%, 15.0%, 17.1%, and 16.3% compared to Type R1, respectively. The greater the roughness of the interface, the greater the resistance to tensile stress of the bond between polyurethane concrete and cement concrete. Compared to the untreated specimens, the chiseled specimens showed an increase in the bond-splitting strength of 82.6% to 91.3%, which means that the chiseled interface can effectively improve the bond-splitting strength. In addition, there are minor fluctuations in the bond-splitting strength of R0, R1, and R2 at different temperatures, and a certain regular trend is not found. The bond splitting strength of polyurethane concrete and cement concrete is less affected by temperature in the range of −40°C to 40°C. It indicates that polyurethane concrete is temperature stable as a repair material in the anchorage zone of expansion joints and provides reliable interfacial bond strength in both high and low temperature conditions.(3) Bond-Splitting Strength under Different Interface Roughness and Cement Concrete of Different Strength Grades. The interfacial bond splitting strengths of polyurethane concrete and different strength grade cement concrete are shown in Figure 25. The horizontal coordinate of Figure 25 indicates the test condition for the combination of different cement concrete and different interface types, for example, C40-R1 indicates the test condition of polyurethane concrete bonded to C40 cement concrete at the R1 interface. As can be seen from Figure 25, the higher the strength grade of cement concrete for the same type of interfacial treatment, the greater the interfacial bond-splitting strength. For the three interface types R0, R1, and R2, compared with the bond-splitting strength of polyurethane concrete and C40 concrete, the bond-splitting strength of polyurethane concrete and C50 concrete increased by 14.3%, 36.2%, and 28.2%, and the bond-splitting strength of polyurethane concrete and C60 concrete increased by 36.0%, 60.9%, and 66.0%. The results show that the bond- splitting strength between polyurethane concrete and cement concrete is considerably influenced by the strength grade of the cement concrete. In addition to the characteristics of the repair material itself, the mechanical properties of the existing concrete in the anchorage area are also critical factors affecting the repair effect of the bridge expansion joint anchorage area.Figure 25
Bond splitting strength with cement concrete of different strength grades.


#### 3.3.2. Bond Direct Shear Strength

(1) Bond Direct Shear Damage Patter. The typical damage patterns of direct shear specimens with different bonding interface forms are shown in Figure 26. The damage surface of R0-type specimen is located on the bonding interface, with a small amount of polyurethane attached to the cement concrete side and a small amount of cement floating slurry attached to the polyurethane concrete side. The low shear strength of the R0-type bonded interface is mainly because of the untreated bonded interface. The weak points of the interface, such as cement floating slurry, loose concrete, etc., have low shear strength, and the local shear damage of the interface decreases the overall shear resistance of the bonded interface.Figure 26
Bond direct shear damage patterns.The damage surface of the R1 specimen occurs on the cement concrete side, and it can be observed that the polyurethane concrete side has some shear fracture of the aggregate. The sheared fine aggregate indicates that the damage of the R1 bond direct shear specimen is caused by the shear damage of the cement concrete near the bonding surface. The R1 type interface is wire brushed, and the weak cement paste on the bonding interface is removed, which allows the polyurethane concrete to bond with a small amount of exposed aggregate, and the bonding interface can withstand greater shear stress.The damage surface of the R2-type specimen also occurs on the cement concrete side, where more coarse aggregate could be observed on the fracture section. The chiseled cement concrete bonding interface has more aggregate exposed, and the coarse aggregate can be tightly bonded to the polyurethane. The shear force at the bonding interface is mainly carried by two components, one being the bonding force between the polyurethane and the cement concrete and the other being the mechanical bite between the coarse aggregate. As the load increases, the shear stress at the bonded interface reaches its limit, and the tiny cracks near the aggregate expand rapidly to the cement concrete side. From the damage morphology of different specimens, it can be observed that the greater the interfacial roughness, the better the shear resistance of the bond interface.(2) Bond Direct Shear Strength under Different Interface Roughness and Different Temperatures. As shown in Figure 27, the horizontal coordinate represents the test conditions of different temperatures and interface types, for example, 40-R1 indicates the test condition at the combination of 40°C and R1 interface type. It can be seen that the shear strength of the bonded interface is R2 > R1 > R0. The R2 type interface has the highest shear strength. Polyurethane can be tightly bonded to the bare aggregate at the cement concrete interface, and the bonding of the polyurethane and the mechanical occlusion between the aggregates withstand the shear stress together. Type R0 has the lowest bond shear strength. Cement float and loose concrete on the bonding interface affect the bonding interface and reduce the bond strength of the bonding interface. The shear strength of the chiseled specimens increased from 50.1% to 55.2% compared to the specimens without any treatment. The interface roughness has an effect on the bond shear strength of polyurethane concrete and cement concrete. The greater the interface roughness, the higher the shear strength of the bond interface. The rate of change in shear strength for each bonding interface was only between 1.5% and 4.4% at the five temperatures of −40°C, −20°C, 0°C, 20°C, and 40°C. Temperature variations have little effect on the interfacial shear strength, which is certainly advantageous for polyurethane concrete when used as a bridge anchorage zone repair material. This property allows the polyurethane concrete to provide reliable shear strength of the bonding interface over a wide temperature range for the expansion joint anchorage zone, which avoids the debonding of the repair material.Figure 27
Bonding direct shear strength at different interface types and different temperatures.(3) Bond Direct Shear Strength under Different Interface Roughness and Cement Concrete of Different Strength Grades. The variation of shear strength under the cement concrete of different strength grades is shown in Figure 28. The horizontal coordinate in Figure 28 represents the combination of different strength grade cement concrete and interface types, for example, C50-R1 represents polyurethane concrete bonded to C50 cement concrete under the R1 interface type. The shear strength of the bonded interface is improved with the increase of concrete strength grade under each bonded interface type, and the shear strength of the three bonded interface types is increased by 25.4%, 15.6%, and 18.6%, respectively, when the concrete strength grade is increased from C40 to C60. The bond shear strength is significantly influenced by the strength grade of cement concrete. According to the shear damage patterns, it is known that the damage interface occurs on the cement concrete side. Therefore, the shear strength of the cement concrete determines the bond shear strength of polyurethane concrete and cement concrete.Figure 28
Bond direct shear strength under different interface roughness and cement concrete of different strength grades.


#### 3.3.3. Bond Oblique Shear Strength

(1) Bond Oblique Shear Damage Patterns. Typical damage patterns of oblique shear bonded specimens with different interface treatments are shown in Figure 29. R0 and R1 bond oblique shear specimens damage interface is relatively straight, and the rupture surface is basically carried out along the bonding surface. R2 oblique shear bond damage interface is an oblique crack that gradually expands to the cement concrete side along the bonding surface. R2 interface has been chiseled and treated, and more aggregates are exposed on the cement concrete side, so not only the coarse aggregate itself can withstand higher shear stress but also can be closely bonded with the polyurethane to form a reliable bonding. As the load increases, the shear stress at the bond interface causes tiny cracks near the aggregate of the cement concrete and rapidly expands into the cement concrete, showing the damage pattern of diagonal cracks.Figure 29
Bond oblique shear damage patterns.(2) Bond Oblique Shear Strength under Different Interface Roughness and Different Temperatures. The bond oblique shear strength of the specimens at each temperature is shown in Figure 30. At the same temperature, the bond oblique shear strength is R2 > R1 > R0. The bond oblique shear strength of R2 type is increased by 2.34 MPa∼2.42 MPa compared with R0 type, with an increase of 27.8%∼28.9%. The interface of R2 type has a larger roughness after artificial chiseling, so that the specimen can withstand greater oblique shear stress when it is subjected to the compound action of compression-shear. In addition, the differences between the maximum and minimum values of bond oblique shear strength of R0, R1, and R2 specimens are 0.08 MPa, 0.07 MPa, and 0.08 MPa, respectively, at different temperatures, and the bond oblique shear strengths of the three interfaces tend to be stable. The interfacial bonding performance of polyurethane concrete and cement concrete has excellent temperature stability in the range of −40°C to 40°C. The bond oblique shear strength is hardly affected by the temperature under the compound action of compression-shear.Figure 30
Bond oblique shear strength under different interfaces and different temperatures.(3) Bond Oblique Shear Strength of Polyurethane Concrete under Cement Concrete of Different Strength Grades. The oblique shear strength under different interface roughness and cement concrete of different strength grades is shown in Figure 31. The oblique shear strength of the three bonded interfaces increases by 22.8%, 14.7%, and 15.7%, with the increase of concrete strength grade, respectively. As shown in Figure 31, the occurrence of oblique shear damage on the cement concrete side indicates that the polyurethane concrete has excellent bonding performance with the cement concrete. The oblique shear strength increases with the concrete strength grade, and higher oblique shear strength can be provided by a rougher bond interface, which is the same as the results of the bond splitting test and the direct shear test.Figure 31
Bond oblique shear strength under different interface roughness and different cement concrete.


### 3.4. Results of Polyurethane Concrete and Steel Reinforcement Pull-Out Test

#### 3.4.1. Pull-Out Strength and Damage Patterns

The typical damage patterns of pull-out test are shown in Figure32. There are two main forms of damage to polyurethane concrete and reinforcement bonded specimens: one is that the reinforcement is pulled out as shown in Figure 32(a), and the other is that the reinforcement is broken as shown in Figure 32(b). The pull-out damage of reinforcement from concrete is caused by the insufficient grip of the concrete on the reinforcement [49]. As shown in Figure 32(a), scraping marks could be seen from the reinforcement projections, and some polyurethane concrete is also observed to have been brought out. In contrast, the damage to the reinforcement fracture is because of the reliable grip of the polyurethane concrete on the reinforcement, which exceeds the ultimate tensile strength of the reinforcement. To further investigate the effect of the anchorage length and diameter of the reinforcement on the pull-out strength, the pull-out specimens of different reinforcement diameters and anchorage lengths were carried to test the pull-out strength. The ultimate pull-out strength and damage patterns are shown in Table 15.Figure 32
Typical damage patterns of reinforcement pull-out test.
(a)(b)Table 15
Results of pull-out test of steel reinforcement and polyurethane concrete.
NumberDiameter (mm)Bonding and anchoring lengthl (mm)Ultimate load (kN)Bonding strength (MPa)Failure mode12-4 d12437.3720.65Pull-out12-5 d549.3921.84Pull-out12-6 d662.3722.98Pull-out12-7 d7(77.38)(24.44)Fracture14-4 d14460.5624.59Pull-out14-5 d580.6926.21Pull-out14-6 d6(94.26)(25.51)Fracture14-7 d7(96.82)(22.46)Fracture16-4 d16482.3725.60Pull-out16-5 d5110.3928.68Pull-out16-6 d6(122.76)(25.44)Fracture16-7 d7(121.49)(21.58)FractureAs shown in Table15, 12-6d, 14-5d, and 16-5d are all reinforcement pull-out damage patterns. The longer the anchorage length, the greater the pull-out strength for the same diameter of steel. The splitting damage pattern of pull-out test, which is often found in the pull-out test of reinforcement and cement concrete [50], does not occur in the pull-out test of reinforcement and polyurethane concrete. As can be seen from Section 3.2, polyurethane concrete has good mechanical properties, especially axial tensile and flexural strengths, which are much higher than those of cement concrete. This property limits the expansion of small and diagonal cracks because of the shear of the concrete and reinforcement. As a result, no form of splitting damage occurs in the tensile tests of polyurethane concrete with reinforcement.The damage patterns of 12-7d, 14-6d, 14-7d, 16-6d, and 16-7d specimens are all steel reinforcement fracture (values in parentheses in Table15). The common point of the five groups of specimens is the long anchorage length of the steel reinforcement, which provides a high bonding strength for the bonding interface for the following three cases: (1) the anchorage length of 12 mm diameter reinforcement is greater than 72 mm, (2) the anchorage length of 14 mm diameter reinforcement is greater than 84 mm, (3) the anchorage length of 16 mm diameter reinforcement is greater than 96 mm, the bond strength of polyurethane concrete and reinforcement is greater than the tensile ultimate strength of the reinforcement, and the pull-out test presents a damage pattern of reinforcement fracture. With the increase of load, the reinforcement steel undergoes the yielding stage, strengthening stage, necking down, and finally fracture damage. Although the peak load in this case does not represent the pull-out strength of the polyurethane and the reinforcement, it can indicate that the bond strength of the polyurethane and the reinforcement is greater than the tensile ultimate load of the reinforcement.


#### 3.4.2. Bond Stress-Slip Curves of Reinforcement Pull-Out Specimens

The bond stress-slip curves of reinforcement pull-out specimens are shown in Figure33. The bond-slip curves can be divided into the following five stages:(1)
Microslip phase: in the initial phase of the test loading, the load rises rapidly within a small amount of slip. The slip at the upper free end of the reinforcement is zero, and a small relative slip occurs at the interface between the reinforcement and the polyurethane concrete at the lower loading end, with no cracks appearing within the polyurethane concrete. The bond at this point is mainly provided by the chemical bond, and the slip at the free end significantly lags behind the loaded end.(2)
Slip stage: the load had increased to the point that the chemical bonding stress at the interface between the reinforcement and the polyurethane concrete was destroyed. The bonding stress was provided by the microfrictional resistance of the ribbed reinforcement and the mechanical bite strength. Mechanical bite forces are developed between the ribs of the reinforcement and the polyurethane concrete. As the external load increases, the shear stress on the polyurethane concrete between the ribs gradually increases. When the shear stress caused by the external load exceeds the shear strength of the polyurethane concrete, slippage occurs between the reinforcement and the polyurethane concrete.(3)
Extreme stage: with the increase of load, the bond stress inside the polyurethane concrete gradually reaches the peak, and the growth rate of slip becomes faster. Since the polyurethane concrete has high mechanical strength, it can effectively prevent the expansion of microcracks and the generation of macro cracks, so there is almost no form of splitting damage.(4)
Declining stage: as the increase of slippage between the ribs of the reinforcement on the polyurethane concrete constantly appears as new extrusion, the new slippage surface at the polyurethane concrete is sheared. At this time, the bond stress rapidly decreases, and the slippage is increasing. Eventually, the specimen losses its load-bearing capacity. The bonding force at this stage is mainly frictional resistance.(5)
Residual stage: when the polyurethane concrete between the ribs of the reinforcement is squeezed to total destruction, the friction between the polyurethane concrete and the reinforcement still provides a residual value for part of the bond stress because of its viscoelasticity. The size of the residual value is related to the contact area of the bond surface. The larger the diameter of the reinforcement, the larger the corresponding residual value.Figure 33
Bond stress-slip curves of polyurethane concrete reinforcement pull-out specimens.
(a)(b)(c)(d)The bond-slip curves for Φ12 reinforcement (bonded anchorage lengths of 4 d, 5 d, and 6 d, respectively) are shown in Figure33(a). The ultimate bond stress at 12-6 d reaches 22.98 MPa, an increase of 5.2% and 11.3% compared to 12-5 d and 12-4 d, respectively. The longer the length of the bonded anchorage, the greater the slip when the peak stress is reached, and the longer the slip phase lasts, as well as the greater the residual bond stress.The bond-slip curves for different bond-anchorage lengths of 14 mm and 16 mm diameter reinforcement are shown in Figures33(b) and 33(c), respectively. The 14-5 d ultimate bond stress is 26.21 MPa, which is 6.6% higher than that of 14-4 d, and the ultimate bond stress corresponds to a slip of 1.56 mm, which is 0.13 mm higher than that of 14-4 d. The ultimate bond stress of 16-5 d is 28.68 MPa, which is 12.0% higher than that of 16-4 d. The slip corresponding to the ultimate bond stress is 2.1 mm, which is 0.47 mm higher than that of 14-4 d.The bond-slip curves for specimens with different reinforcement diameters (all with a bond-anchorage length of 5 d) are shown in Figure33(d). The larger the diameter of the reinforcement, the longer the duration of the microslip phase, and the greater the load required to enter the slip phase. The greater contact area between the larger diameter reinforcement and the polyurethane concrete results in greater frictional resistance and greater residual stresses. Therefore, the larger the diameter of the reinforcement, the slower the tendency for the bond stress to drop.


## 3.1. Results of Orthogonal Experiments

The results of the orthogonal test were analyzed by the extreme analysis in Table11. The extreme difference of the binder-aggregate ratio is 29.57, which indicates that the binder-aggregate ratio is the control factor with the greatest degree of influence on the compressive strength of polyurethane concrete, followed by the aggregate gradation with an extreme difference value of 3.90, and the cement dosage ratio with an extreme difference value of 0.93, which has the least influence on the compressive strength of polyurethane concrete. The factor level with the maximum average value under each control factor was selected as the polyurethane concrete mixture for the subsequent bond performance tests, i.e., a binder-aggregate ratio of 20%, an aggregate grade of AC-10, and a cement dosage ratio of 2.0. The mass ratio is as follows: polyisocyanate : combined polyether : cement: aggregate (AC-10) = 1 : 1 : 2 : 10, and the percentage of each component is shown in Figure 19.Table 11
Mean principal response.
LevelABC130.3344.0742.60243.2044.3746.50359.9045.0044.33Extreme difference29.570.933.90Rank132Figure 19
The optimal mixture of polyurethane concrete.


## 3.2. Basic Mechanical Strength Test Results of Polyurethane Concrete

### 3.2.1. Compressive Strength

The results of polyurethane concrete compressive strength test are shown in Table12 and Figure 20. The average ultimate compressive strength of polyurethane concrete is 62.6 MPa, and its compressive process can be divided into linear elastic phase and nonlinear hysteretic elastic phase. Linear elastic stage: the stress-strain curve of the specimen under compression is linearly and positively correlated. Nonlinear hysteretic elastic stage: the stress-strain curve of the specimen is nonlinear, and there is an obvious plateau area. At the early stage of specimen compression, the internal stress caused by external forces is less than the chemical bonding, intermolecular hydrogen bonding, and van der Waals forces within the polyurethane molecule [47, 48]. The polyurethane and gravel aggregate are in a synergistic stress state, and the specimen is in the elastic stress stage. However, the increased stress forces the polyurethane molecular chain segments to deform, move, and misalign, and the force chains inside the specimen matrix undergo redistribution. The disruption of the stress equilibrium throughout the specimen is responsible for the nonlinear trend in the stress-strain curve.Table 12
The polyurethane concrete compressive strength.
NumberC01C02C03C04C05C06MeanCompressive strength (MPa)62.361.164.560.663.463.762.6Figure 20
The compressive strength stress-strain curve of polyurethane concrete.


### 3.2.2. Tensile Strength

The tensile strength test results of polyurethane concrete are shown in Table13 and Figure 21. The average value of ultimate tensile strength of polyurethane concrete is 12.5 MPa. According to the tensile stress-strain curve in Figure 21, the tensile mechanical behavior of polyurethane concrete can be divided into the linear elastic phase and forced hyperelastic deformation phase. Linear elastic phase: the stress-strain curve of the specimen under tension is linearly and positively correlated. Forced hyperelastic deformation phase: the forced hyperelastic deformation zone includes the strain softening phase and the plateau phase. Although external forces impose the molecular chains to stretch and move, causing large deformation of the specimen, the polyurethane concrete is still able to maintain a high tensile strength.Table 13
The polyurethane concrete tensile strength.
NumberL01L02L03L04L05L06MeanTensile strength (MPa)12.112.711.614.113.611.212.5Figure 21
The tensile strength stress-strain curve of polyurethane concrete.


### 3.2.3. Flexural Strength

The flexural strength test results of the polyurethane concrete are shown in Table14 and Figure 22. The average value of ultimate flexural strength of polyurethane concrete is 11.4 MPa. According to the flexural stress-strain curve in Figure 22, the flexural mechanical behavior of polyurethane concrete can be divided into linear elastic phase and strain softening phase. Linear elastic phase: the stress-strain curve of the specimen under flexural status is linearly and positively correlated. Strain softening stage: strain softening in this stage is mainly caused by the reduction of the cross-section inertia moment induced by the cracking in the specimen span.Table 14
The flexural strength of polyurethane concrete.
NumberF01F02F03F04F05F06MeanFlexural strength (MPa)10.811.911.210.911.312.211.4Figure 22
The flexural strength stress-strain curve of polyurethane concrete.


## 3.2.1. Compressive Strength

The results of polyurethane concrete compressive strength test are shown in Table12 and Figure 20. The average ultimate compressive strength of polyurethane concrete is 62.6 MPa, and its compressive process can be divided into linear elastic phase and nonlinear hysteretic elastic phase. Linear elastic stage: the stress-strain curve of the specimen under compression is linearly and positively correlated. Nonlinear hysteretic elastic stage: the stress-strain curve of the specimen is nonlinear, and there is an obvious plateau area. At the early stage of specimen compression, the internal stress caused by external forces is less than the chemical bonding, intermolecular hydrogen bonding, and van der Waals forces within the polyurethane molecule [47, 48]. The polyurethane and gravel aggregate are in a synergistic stress state, and the specimen is in the elastic stress stage. However, the increased stress forces the polyurethane molecular chain segments to deform, move, and misalign, and the force chains inside the specimen matrix undergo redistribution. The disruption of the stress equilibrium throughout the specimen is responsible for the nonlinear trend in the stress-strain curve.Table 12
The polyurethane concrete compressive strength.
NumberC01C02C03C04C05C06MeanCompressive strength (MPa)62.361.164.560.663.463.762.6Figure 20
The compressive strength stress-strain curve of polyurethane concrete.


## 3.2.2. Tensile Strength

The tensile strength test results of polyurethane concrete are shown in Table13 and Figure 21. The average value of ultimate tensile strength of polyurethane concrete is 12.5 MPa. According to the tensile stress-strain curve in Figure 21, the tensile mechanical behavior of polyurethane concrete can be divided into the linear elastic phase and forced hyperelastic deformation phase. Linear elastic phase: the stress-strain curve of the specimen under tension is linearly and positively correlated. Forced hyperelastic deformation phase: the forced hyperelastic deformation zone includes the strain softening phase and the plateau phase. Although external forces impose the molecular chains to stretch and move, causing large deformation of the specimen, the polyurethane concrete is still able to maintain a high tensile strength.Table 13
The polyurethane concrete tensile strength.
NumberL01L02L03L04L05L06MeanTensile strength (MPa)12.112.711.614.113.611.212.5Figure 21
The tensile strength stress-strain curve of polyurethane concrete.


## 3.2.3. Flexural Strength

The flexural strength test results of the polyurethane concrete are shown in Table14 and Figure 22. The average value of ultimate flexural strength of polyurethane concrete is 11.4 MPa. According to the flexural stress-strain curve in Figure 22, the flexural mechanical behavior of polyurethane concrete can be divided into linear elastic phase and strain softening phase. Linear elastic phase: the stress-strain curve of the specimen under flexural status is linearly and positively correlated. Strain softening stage: strain softening in this stage is mainly caused by the reduction of the cross-section inertia moment induced by the cracking in the specimen span.Table 14
The flexural strength of polyurethane concrete.
NumberF01F02F03F04F05F06MeanFlexural strength (MPa)10.811.911.210.911.312.211.4Figure 22
The flexural strength stress-strain curve of polyurethane concrete.


## 3.3. Results of Polyurethane Concrete and Cement Concrete Bond Tests

### 3.3.1. Bond Splitting Strength

(1) Damage Pattern of Bond Splitting Test. Typical bond splitting damage patterns under different bonded interfaces are shown in Figure 23. It can be observed from Figure 23 that the damage surface of R0 type is relatively straight and basically develops along the bonding surface. Part of the polyurethane binder can be observed on the cement concrete side, and there is also a small amount of cement floating slurry on the polyurethane concrete side. It is mainly because of the presence of floating slurry or small amounts of loose concrete on untreated bond surfaces. When the specimen is subjected to load, the floating slurry or loose concrete has a weaker ability to resist the tensile stress, causing it to be damaged by tensile splitting from the interface bond weakness.Figure 23
Damage patterns of bond splitting test.The damage surface of type R1 occurred on the cement concrete side, and the cement concrete slurry and aggregate can be seen in the central area of the polyurethane concrete side. It is mainly attributed to the fact that when the bond splitting specimens undergo external loads, the middle position of the bonded interface is subjected to the main tensile stress, and the edge position of the bonded interface is subjected to compressive stress. However, the tensile strength of cement concrete is small, and when the tensile stress reaches the tensile limit of cement concrete, the weak middle position of cement concrete is the first to be destroyed.The damage surface of type R2 also occurs on the cement concrete side, however, unlike the interface of type R1, more tensile fracture of the aggregate can be observed on the cement concrete side. It is because more aggregates are exposed at the cement concrete interface after chisel treatment. The bond strength of the polyurethane binder to the aggregate is greater than the tensile strength of the aggregate. The aggregates are prone to be pulled off or out by the polyurethane binder under splitting.(2) Bond-Splitting Strength under Different Interface Roughness and Different Temperatures. The bond-splitting strength is an important indicator to evaluate the ability of the bonded interface to resist tensile stress. The results of the polyurethane concrete and cement concrete bond-splitting strength test are shown in Figure 24. The horizontal coordinate represents the test conditions for different combinations of temperature and interface types, for example, 40-R1 indicates the test condition at the combination of 40°C and R1 interface type. The bond splitting strengths of the specimens with different roughness interfaces are R2 > R1 > R0 in descending order. The type R2 interface is chiseled so that the aggregate can be exposed, and the polyurethane binder can form a reliable bond with the aggregate, resulting in a higher bond splitting strength than type R0 and R1.Figure 24
Bond splitting strength under different interface types and different temperatures.Under five temperature conditions, the bond splitting strength of Type R2 increases by 91.4%, 90.9%, 84.8%, 82.6%, and 83.1% compared to Type R0, and by 17.0%, 18.0%, 15.0%, 17.1%, and 16.3% compared to Type R1, respectively. The greater the roughness of the interface, the greater the resistance to tensile stress of the bond between polyurethane concrete and cement concrete. Compared to the untreated specimens, the chiseled specimens showed an increase in the bond-splitting strength of 82.6% to 91.3%, which means that the chiseled interface can effectively improve the bond-splitting strength. In addition, there are minor fluctuations in the bond-splitting strength of R0, R1, and R2 at different temperatures, and a certain regular trend is not found. The bond splitting strength of polyurethane concrete and cement concrete is less affected by temperature in the range of −40°C to 40°C. It indicates that polyurethane concrete is temperature stable as a repair material in the anchorage zone of expansion joints and provides reliable interfacial bond strength in both high and low temperature conditions.(3) Bond-Splitting Strength under Different Interface Roughness and Cement Concrete of Different Strength Grades. The interfacial bond splitting strengths of polyurethane concrete and different strength grade cement concrete are shown in Figure 25. The horizontal coordinate of Figure 25 indicates the test condition for the combination of different cement concrete and different interface types, for example, C40-R1 indicates the test condition of polyurethane concrete bonded to C40 cement concrete at the R1 interface. As can be seen from Figure 25, the higher the strength grade of cement concrete for the same type of interfacial treatment, the greater the interfacial bond-splitting strength. For the three interface types R0, R1, and R2, compared with the bond-splitting strength of polyurethane concrete and C40 concrete, the bond-splitting strength of polyurethane concrete and C50 concrete increased by 14.3%, 36.2%, and 28.2%, and the bond-splitting strength of polyurethane concrete and C60 concrete increased by 36.0%, 60.9%, and 66.0%. The results show that the bond- splitting strength between polyurethane concrete and cement concrete is considerably influenced by the strength grade of the cement concrete. In addition to the characteristics of the repair material itself, the mechanical properties of the existing concrete in the anchorage area are also critical factors affecting the repair effect of the bridge expansion joint anchorage area.Figure 25
Bond splitting strength with cement concrete of different strength grades.


### 3.3.2. Bond Direct Shear Strength

(1) Bond Direct Shear Damage Patter. The typical damage patterns of direct shear specimens with different bonding interface forms are shown in Figure 26. The damage surface of R0-type specimen is located on the bonding interface, with a small amount of polyurethane attached to the cement concrete side and a small amount of cement floating slurry attached to the polyurethane concrete side. The low shear strength of the R0-type bonded interface is mainly because of the untreated bonded interface. The weak points of the interface, such as cement floating slurry, loose concrete, etc., have low shear strength, and the local shear damage of the interface decreases the overall shear resistance of the bonded interface.Figure 26
Bond direct shear damage patterns.The damage surface of the R1 specimen occurs on the cement concrete side, and it can be observed that the polyurethane concrete side has some shear fracture of the aggregate. The sheared fine aggregate indicates that the damage of the R1 bond direct shear specimen is caused by the shear damage of the cement concrete near the bonding surface. The R1 type interface is wire brushed, and the weak cement paste on the bonding interface is removed, which allows the polyurethane concrete to bond with a small amount of exposed aggregate, and the bonding interface can withstand greater shear stress.The damage surface of the R2-type specimen also occurs on the cement concrete side, where more coarse aggregate could be observed on the fracture section. The chiseled cement concrete bonding interface has more aggregate exposed, and the coarse aggregate can be tightly bonded to the polyurethane. The shear force at the bonding interface is mainly carried by two components, one being the bonding force between the polyurethane and the cement concrete and the other being the mechanical bite between the coarse aggregate. As the load increases, the shear stress at the bonded interface reaches its limit, and the tiny cracks near the aggregate expand rapidly to the cement concrete side. From the damage morphology of different specimens, it can be observed that the greater the interfacial roughness, the better the shear resistance of the bond interface.(2) Bond Direct Shear Strength under Different Interface Roughness and Different Temperatures. As shown in Figure 27, the horizontal coordinate represents the test conditions of different temperatures and interface types, for example, 40-R1 indicates the test condition at the combination of 40°C and R1 interface type. It can be seen that the shear strength of the bonded interface is R2 > R1 > R0. The R2 type interface has the highest shear strength. Polyurethane can be tightly bonded to the bare aggregate at the cement concrete interface, and the bonding of the polyurethane and the mechanical occlusion between the aggregates withstand the shear stress together. Type R0 has the lowest bond shear strength. Cement float and loose concrete on the bonding interface affect the bonding interface and reduce the bond strength of the bonding interface. The shear strength of the chiseled specimens increased from 50.1% to 55.2% compared to the specimens without any treatment. The interface roughness has an effect on the bond shear strength of polyurethane concrete and cement concrete. The greater the interface roughness, the higher the shear strength of the bond interface. The rate of change in shear strength for each bonding interface was only between 1.5% and 4.4% at the five temperatures of −40°C, −20°C, 0°C, 20°C, and 40°C. Temperature variations have little effect on the interfacial shear strength, which is certainly advantageous for polyurethane concrete when used as a bridge anchorage zone repair material. This property allows the polyurethane concrete to provide reliable shear strength of the bonding interface over a wide temperature range for the expansion joint anchorage zone, which avoids the debonding of the repair material.Figure 27
Bonding direct shear strength at different interface types and different temperatures.(3) Bond Direct Shear Strength under Different Interface Roughness and Cement Concrete of Different Strength Grades. The variation of shear strength under the cement concrete of different strength grades is shown in Figure 28. The horizontal coordinate in Figure 28 represents the combination of different strength grade cement concrete and interface types, for example, C50-R1 represents polyurethane concrete bonded to C50 cement concrete under the R1 interface type. The shear strength of the bonded interface is improved with the increase of concrete strength grade under each bonded interface type, and the shear strength of the three bonded interface types is increased by 25.4%, 15.6%, and 18.6%, respectively, when the concrete strength grade is increased from C40 to C60. The bond shear strength is significantly influenced by the strength grade of cement concrete. According to the shear damage patterns, it is known that the damage interface occurs on the cement concrete side. Therefore, the shear strength of the cement concrete determines the bond shear strength of polyurethane concrete and cement concrete.Figure 28
Bond direct shear strength under different interface roughness and cement concrete of different strength grades.


### 3.3.3. Bond Oblique Shear Strength

(1) Bond Oblique Shear Damage Patterns. Typical damage patterns of oblique shear bonded specimens with different interface treatments are shown in Figure 29. R0 and R1 bond oblique shear specimens damage interface is relatively straight, and the rupture surface is basically carried out along the bonding surface. R2 oblique shear bond damage interface is an oblique crack that gradually expands to the cement concrete side along the bonding surface. R2 interface has been chiseled and treated, and more aggregates are exposed on the cement concrete side, so not only the coarse aggregate itself can withstand higher shear stress but also can be closely bonded with the polyurethane to form a reliable bonding. As the load increases, the shear stress at the bond interface causes tiny cracks near the aggregate of the cement concrete and rapidly expands into the cement concrete, showing the damage pattern of diagonal cracks.Figure 29
Bond oblique shear damage patterns.(2) Bond Oblique Shear Strength under Different Interface Roughness and Different Temperatures. The bond oblique shear strength of the specimens at each temperature is shown in Figure 30. At the same temperature, the bond oblique shear strength is R2 > R1 > R0. The bond oblique shear strength of R2 type is increased by 2.34 MPa∼2.42 MPa compared with R0 type, with an increase of 27.8%∼28.9%. The interface of R2 type has a larger roughness after artificial chiseling, so that the specimen can withstand greater oblique shear stress when it is subjected to the compound action of compression-shear. In addition, the differences between the maximum and minimum values of bond oblique shear strength of R0, R1, and R2 specimens are 0.08 MPa, 0.07 MPa, and 0.08 MPa, respectively, at different temperatures, and the bond oblique shear strengths of the three interfaces tend to be stable. The interfacial bonding performance of polyurethane concrete and cement concrete has excellent temperature stability in the range of −40°C to 40°C. The bond oblique shear strength is hardly affected by the temperature under the compound action of compression-shear.Figure 30
Bond oblique shear strength under different interfaces and different temperatures.(3) Bond Oblique Shear Strength of Polyurethane Concrete under Cement Concrete of Different Strength Grades. The oblique shear strength under different interface roughness and cement concrete of different strength grades is shown in Figure 31. The oblique shear strength of the three bonded interfaces increases by 22.8%, 14.7%, and 15.7%, with the increase of concrete strength grade, respectively. As shown in Figure 31, the occurrence of oblique shear damage on the cement concrete side indicates that the polyurethane concrete has excellent bonding performance with the cement concrete. The oblique shear strength increases with the concrete strength grade, and higher oblique shear strength can be provided by a rougher bond interface, which is the same as the results of the bond splitting test and the direct shear test.Figure 31
Bond oblique shear strength under different interface roughness and different cement concrete.


## 3.3.1. Bond Splitting Strength

(1) Damage Pattern of Bond Splitting Test. Typical bond splitting damage patterns under different bonded interfaces are shown in Figure 23. It can be observed from Figure 23 that the damage surface of R0 type is relatively straight and basically develops along the bonding surface. Part of the polyurethane binder can be observed on the cement concrete side, and there is also a small amount of cement floating slurry on the polyurethane concrete side. It is mainly because of the presence of floating slurry or small amounts of loose concrete on untreated bond surfaces. When the specimen is subjected to load, the floating slurry or loose concrete has a weaker ability to resist the tensile stress, causing it to be damaged by tensile splitting from the interface bond weakness.Figure 23
Damage patterns of bond splitting test.The damage surface of type R1 occurred on the cement concrete side, and the cement concrete slurry and aggregate can be seen in the central area of the polyurethane concrete side. It is mainly attributed to the fact that when the bond splitting specimens undergo external loads, the middle position of the bonded interface is subjected to the main tensile stress, and the edge position of the bonded interface is subjected to compressive stress. However, the tensile strength of cement concrete is small, and when the tensile stress reaches the tensile limit of cement concrete, the weak middle position of cement concrete is the first to be destroyed.The damage surface of type R2 also occurs on the cement concrete side, however, unlike the interface of type R1, more tensile fracture of the aggregate can be observed on the cement concrete side. It is because more aggregates are exposed at the cement concrete interface after chisel treatment. The bond strength of the polyurethane binder to the aggregate is greater than the tensile strength of the aggregate. The aggregates are prone to be pulled off or out by the polyurethane binder under splitting.(2) Bond-Splitting Strength under Different Interface Roughness and Different Temperatures. The bond-splitting strength is an important indicator to evaluate the ability of the bonded interface to resist tensile stress. The results of the polyurethane concrete and cement concrete bond-splitting strength test are shown in Figure 24. The horizontal coordinate represents the test conditions for different combinations of temperature and interface types, for example, 40-R1 indicates the test condition at the combination of 40°C and R1 interface type. The bond splitting strengths of the specimens with different roughness interfaces are R2 > R1 > R0 in descending order. The type R2 interface is chiseled so that the aggregate can be exposed, and the polyurethane binder can form a reliable bond with the aggregate, resulting in a higher bond splitting strength than type R0 and R1.Figure 24
Bond splitting strength under different interface types and different temperatures.Under five temperature conditions, the bond splitting strength of Type R2 increases by 91.4%, 90.9%, 84.8%, 82.6%, and 83.1% compared to Type R0, and by 17.0%, 18.0%, 15.0%, 17.1%, and 16.3% compared to Type R1, respectively. The greater the roughness of the interface, the greater the resistance to tensile stress of the bond between polyurethane concrete and cement concrete. Compared to the untreated specimens, the chiseled specimens showed an increase in the bond-splitting strength of 82.6% to 91.3%, which means that the chiseled interface can effectively improve the bond-splitting strength. In addition, there are minor fluctuations in the bond-splitting strength of R0, R1, and R2 at different temperatures, and a certain regular trend is not found. The bond splitting strength of polyurethane concrete and cement concrete is less affected by temperature in the range of −40°C to 40°C. It indicates that polyurethane concrete is temperature stable as a repair material in the anchorage zone of expansion joints and provides reliable interfacial bond strength in both high and low temperature conditions.(3) Bond-Splitting Strength under Different Interface Roughness and Cement Concrete of Different Strength Grades. The interfacial bond splitting strengths of polyurethane concrete and different strength grade cement concrete are shown in Figure 25. The horizontal coordinate of Figure 25 indicates the test condition for the combination of different cement concrete and different interface types, for example, C40-R1 indicates the test condition of polyurethane concrete bonded to C40 cement concrete at the R1 interface. As can be seen from Figure 25, the higher the strength grade of cement concrete for the same type of interfacial treatment, the greater the interfacial bond-splitting strength. For the three interface types R0, R1, and R2, compared with the bond-splitting strength of polyurethane concrete and C40 concrete, the bond-splitting strength of polyurethane concrete and C50 concrete increased by 14.3%, 36.2%, and 28.2%, and the bond-splitting strength of polyurethane concrete and C60 concrete increased by 36.0%, 60.9%, and 66.0%. The results show that the bond- splitting strength between polyurethane concrete and cement concrete is considerably influenced by the strength grade of the cement concrete. In addition to the characteristics of the repair material itself, the mechanical properties of the existing concrete in the anchorage area are also critical factors affecting the repair effect of the bridge expansion joint anchorage area.Figure 25
Bond splitting strength with cement concrete of different strength grades.


## 3.3.2. Bond Direct Shear Strength

(1) Bond Direct Shear Damage Patter. The typical damage patterns of direct shear specimens with different bonding interface forms are shown in Figure 26. The damage surface of R0-type specimen is located on the bonding interface, with a small amount of polyurethane attached to the cement concrete side and a small amount of cement floating slurry attached to the polyurethane concrete side. The low shear strength of the R0-type bonded interface is mainly because of the untreated bonded interface. The weak points of the interface, such as cement floating slurry, loose concrete, etc., have low shear strength, and the local shear damage of the interface decreases the overall shear resistance of the bonded interface.Figure 26
Bond direct shear damage patterns.The damage surface of the R1 specimen occurs on the cement concrete side, and it can be observed that the polyurethane concrete side has some shear fracture of the aggregate. The sheared fine aggregate indicates that the damage of the R1 bond direct shear specimen is caused by the shear damage of the cement concrete near the bonding surface. The R1 type interface is wire brushed, and the weak cement paste on the bonding interface is removed, which allows the polyurethane concrete to bond with a small amount of exposed aggregate, and the bonding interface can withstand greater shear stress.The damage surface of the R2-type specimen also occurs on the cement concrete side, where more coarse aggregate could be observed on the fracture section. The chiseled cement concrete bonding interface has more aggregate exposed, and the coarse aggregate can be tightly bonded to the polyurethane. The shear force at the bonding interface is mainly carried by two components, one being the bonding force between the polyurethane and the cement concrete and the other being the mechanical bite between the coarse aggregate. As the load increases, the shear stress at the bonded interface reaches its limit, and the tiny cracks near the aggregate expand rapidly to the cement concrete side. From the damage morphology of different specimens, it can be observed that the greater the interfacial roughness, the better the shear resistance of the bond interface.(2) Bond Direct Shear Strength under Different Interface Roughness and Different Temperatures. As shown in Figure 27, the horizontal coordinate represents the test conditions of different temperatures and interface types, for example, 40-R1 indicates the test condition at the combination of 40°C and R1 interface type. It can be seen that the shear strength of the bonded interface is R2 > R1 > R0. The R2 type interface has the highest shear strength. Polyurethane can be tightly bonded to the bare aggregate at the cement concrete interface, and the bonding of the polyurethane and the mechanical occlusion between the aggregates withstand the shear stress together. Type R0 has the lowest bond shear strength. Cement float and loose concrete on the bonding interface affect the bonding interface and reduce the bond strength of the bonding interface. The shear strength of the chiseled specimens increased from 50.1% to 55.2% compared to the specimens without any treatment. The interface roughness has an effect on the bond shear strength of polyurethane concrete and cement concrete. The greater the interface roughness, the higher the shear strength of the bond interface. The rate of change in shear strength for each bonding interface was only between 1.5% and 4.4% at the five temperatures of −40°C, −20°C, 0°C, 20°C, and 40°C. Temperature variations have little effect on the interfacial shear strength, which is certainly advantageous for polyurethane concrete when used as a bridge anchorage zone repair material. This property allows the polyurethane concrete to provide reliable shear strength of the bonding interface over a wide temperature range for the expansion joint anchorage zone, which avoids the debonding of the repair material.Figure 27
Bonding direct shear strength at different interface types and different temperatures.(3) Bond Direct Shear Strength under Different Interface Roughness and Cement Concrete of Different Strength Grades. The variation of shear strength under the cement concrete of different strength grades is shown in Figure 28. The horizontal coordinate in Figure 28 represents the combination of different strength grade cement concrete and interface types, for example, C50-R1 represents polyurethane concrete bonded to C50 cement concrete under the R1 interface type. The shear strength of the bonded interface is improved with the increase of concrete strength grade under each bonded interface type, and the shear strength of the three bonded interface types is increased by 25.4%, 15.6%, and 18.6%, respectively, when the concrete strength grade is increased from C40 to C60. The bond shear strength is significantly influenced by the strength grade of cement concrete. According to the shear damage patterns, it is known that the damage interface occurs on the cement concrete side. Therefore, the shear strength of the cement concrete determines the bond shear strength of polyurethane concrete and cement concrete.Figure 28
Bond direct shear strength under different interface roughness and cement concrete of different strength grades.


## 3.3.3. Bond Oblique Shear Strength

(1) Bond Oblique Shear Damage Patterns. Typical damage patterns of oblique shear bonded specimens with different interface treatments are shown in Figure 29. R0 and R1 bond oblique shear specimens damage interface is relatively straight, and the rupture surface is basically carried out along the bonding surface. R2 oblique shear bond damage interface is an oblique crack that gradually expands to the cement concrete side along the bonding surface. R2 interface has been chiseled and treated, and more aggregates are exposed on the cement concrete side, so not only the coarse aggregate itself can withstand higher shear stress but also can be closely bonded with the polyurethane to form a reliable bonding. As the load increases, the shear stress at the bond interface causes tiny cracks near the aggregate of the cement concrete and rapidly expands into the cement concrete, showing the damage pattern of diagonal cracks.Figure 29
Bond oblique shear damage patterns.(2) Bond Oblique Shear Strength under Different Interface Roughness and Different Temperatures. The bond oblique shear strength of the specimens at each temperature is shown in Figure 30. At the same temperature, the bond oblique shear strength is R2 > R1 > R0. The bond oblique shear strength of R2 type is increased by 2.34 MPa∼2.42 MPa compared with R0 type, with an increase of 27.8%∼28.9%. The interface of R2 type has a larger roughness after artificial chiseling, so that the specimen can withstand greater oblique shear stress when it is subjected to the compound action of compression-shear. In addition, the differences between the maximum and minimum values of bond oblique shear strength of R0, R1, and R2 specimens are 0.08 MPa, 0.07 MPa, and 0.08 MPa, respectively, at different temperatures, and the bond oblique shear strengths of the three interfaces tend to be stable. The interfacial bonding performance of polyurethane concrete and cement concrete has excellent temperature stability in the range of −40°C to 40°C. The bond oblique shear strength is hardly affected by the temperature under the compound action of compression-shear.Figure 30
Bond oblique shear strength under different interfaces and different temperatures.(3) Bond Oblique Shear Strength of Polyurethane Concrete under Cement Concrete of Different Strength Grades. The oblique shear strength under different interface roughness and cement concrete of different strength grades is shown in Figure 31. The oblique shear strength of the three bonded interfaces increases by 22.8%, 14.7%, and 15.7%, with the increase of concrete strength grade, respectively. As shown in Figure 31, the occurrence of oblique shear damage on the cement concrete side indicates that the polyurethane concrete has excellent bonding performance with the cement concrete. The oblique shear strength increases with the concrete strength grade, and higher oblique shear strength can be provided by a rougher bond interface, which is the same as the results of the bond splitting test and the direct shear test.Figure 31
Bond oblique shear strength under different interface roughness and different cement concrete.


## 3.4. Results of Polyurethane Concrete and Steel Reinforcement Pull-Out Test

### 3.4.1. Pull-Out Strength and Damage Patterns

The typical damage patterns of pull-out test are shown in Figure32. There are two main forms of damage to polyurethane concrete and reinforcement bonded specimens: one is that the reinforcement is pulled out as shown in Figure 32(a), and the other is that the reinforcement is broken as shown in Figure 32(b). The pull-out damage of reinforcement from concrete is caused by the insufficient grip of the concrete on the reinforcement [49]. As shown in Figure 32(a), scraping marks could be seen from the reinforcement projections, and some polyurethane concrete is also observed to have been brought out. In contrast, the damage to the reinforcement fracture is because of the reliable grip of the polyurethane concrete on the reinforcement, which exceeds the ultimate tensile strength of the reinforcement. To further investigate the effect of the anchorage length and diameter of the reinforcement on the pull-out strength, the pull-out specimens of different reinforcement diameters and anchorage lengths were carried to test the pull-out strength. The ultimate pull-out strength and damage patterns are shown in Table 15.Figure 32
Typical damage patterns of reinforcement pull-out test.
(a)(b)Table 15
Results of pull-out test of steel reinforcement and polyurethane concrete.
NumberDiameter (mm)Bonding and anchoring lengthl (mm)Ultimate load (kN)Bonding strength (MPa)Failure mode12-4 d12437.3720.65Pull-out12-5 d549.3921.84Pull-out12-6 d662.3722.98Pull-out12-7 d7(77.38)(24.44)Fracture14-4 d14460.5624.59Pull-out14-5 d580.6926.21Pull-out14-6 d6(94.26)(25.51)Fracture14-7 d7(96.82)(22.46)Fracture16-4 d16482.3725.60Pull-out16-5 d5110.3928.68Pull-out16-6 d6(122.76)(25.44)Fracture16-7 d7(121.49)(21.58)FractureAs shown in Table15, 12-6d, 14-5d, and 16-5d are all reinforcement pull-out damage patterns. The longer the anchorage length, the greater the pull-out strength for the same diameter of steel. The splitting damage pattern of pull-out test, which is often found in the pull-out test of reinforcement and cement concrete [50], does not occur in the pull-out test of reinforcement and polyurethane concrete. As can be seen from Section 3.2, polyurethane concrete has good mechanical properties, especially axial tensile and flexural strengths, which are much higher than those of cement concrete. This property limits the expansion of small and diagonal cracks because of the shear of the concrete and reinforcement. As a result, no form of splitting damage occurs in the tensile tests of polyurethane concrete with reinforcement.The damage patterns of 12-7d, 14-6d, 14-7d, 16-6d, and 16-7d specimens are all steel reinforcement fracture (values in parentheses in Table15). The common point of the five groups of specimens is the long anchorage length of the steel reinforcement, which provides a high bonding strength for the bonding interface for the following three cases: (1) the anchorage length of 12 mm diameter reinforcement is greater than 72 mm, (2) the anchorage length of 14 mm diameter reinforcement is greater than 84 mm, (3) the anchorage length of 16 mm diameter reinforcement is greater than 96 mm, the bond strength of polyurethane concrete and reinforcement is greater than the tensile ultimate strength of the reinforcement, and the pull-out test presents a damage pattern of reinforcement fracture. With the increase of load, the reinforcement steel undergoes the yielding stage, strengthening stage, necking down, and finally fracture damage. Although the peak load in this case does not represent the pull-out strength of the polyurethane and the reinforcement, it can indicate that the bond strength of the polyurethane and the reinforcement is greater than the tensile ultimate load of the reinforcement.


### 3.4.2. Bond Stress-Slip Curves of Reinforcement Pull-Out Specimens

The bond stress-slip curves of reinforcement pull-out specimens are shown in Figure33. The bond-slip curves can be divided into the following five stages:(1)
Microslip phase: in the initial phase of the test loading, the load rises rapidly within a small amount of slip. The slip at the upper free end of the reinforcement is zero, and a small relative slip occurs at the interface between the reinforcement and the polyurethane concrete at the lower loading end, with no cracks appearing within the polyurethane concrete. The bond at this point is mainly provided by the chemical bond, and the slip at the free end significantly lags behind the loaded end.(2)
Slip stage: the load had increased to the point that the chemical bonding stress at the interface between the reinforcement and the polyurethane concrete was destroyed. The bonding stress was provided by the microfrictional resistance of the ribbed reinforcement and the mechanical bite strength. Mechanical bite forces are developed between the ribs of the reinforcement and the polyurethane concrete. As the external load increases, the shear stress on the polyurethane concrete between the ribs gradually increases. When the shear stress caused by the external load exceeds the shear strength of the polyurethane concrete, slippage occurs between the reinforcement and the polyurethane concrete.(3)
Extreme stage: with the increase of load, the bond stress inside the polyurethane concrete gradually reaches the peak, and the growth rate of slip becomes faster. Since the polyurethane concrete has high mechanical strength, it can effectively prevent the expansion of microcracks and the generation of macro cracks, so there is almost no form of splitting damage.(4)
Declining stage: as the increase of slippage between the ribs of the reinforcement on the polyurethane concrete constantly appears as new extrusion, the new slippage surface at the polyurethane concrete is sheared. At this time, the bond stress rapidly decreases, and the slippage is increasing. Eventually, the specimen losses its load-bearing capacity. The bonding force at this stage is mainly frictional resistance.(5)
Residual stage: when the polyurethane concrete between the ribs of the reinforcement is squeezed to total destruction, the friction between the polyurethane concrete and the reinforcement still provides a residual value for part of the bond stress because of its viscoelasticity. The size of the residual value is related to the contact area of the bond surface. The larger the diameter of the reinforcement, the larger the corresponding residual value.Figure 33
Bond stress-slip curves of polyurethane concrete reinforcement pull-out specimens.
(a)(b)(c)(d)The bond-slip curves for Φ12 reinforcement (bonded anchorage lengths of 4 d, 5 d, and 6 d, respectively) are shown in Figure33(a). The ultimate bond stress at 12-6 d reaches 22.98 MPa, an increase of 5.2% and 11.3% compared to 12-5 d and 12-4 d, respectively. The longer the length of the bonded anchorage, the greater the slip when the peak stress is reached, and the longer the slip phase lasts, as well as the greater the residual bond stress.The bond-slip curves for different bond-anchorage lengths of 14 mm and 16 mm diameter reinforcement are shown in Figures33(b) and 33(c), respectively. The 14-5 d ultimate bond stress is 26.21 MPa, which is 6.6% higher than that of 14-4 d, and the ultimate bond stress corresponds to a slip of 1.56 mm, which is 0.13 mm higher than that of 14-4 d. The ultimate bond stress of 16-5 d is 28.68 MPa, which is 12.0% higher than that of 16-4 d. The slip corresponding to the ultimate bond stress is 2.1 mm, which is 0.47 mm higher than that of 14-4 d.The bond-slip curves for specimens with different reinforcement diameters (all with a bond-anchorage length of 5 d) are shown in Figure33(d). The larger the diameter of the reinforcement, the longer the duration of the microslip phase, and the greater the load required to enter the slip phase. The greater contact area between the larger diameter reinforcement and the polyurethane concrete results in greater frictional resistance and greater residual stresses. Therefore, the larger the diameter of the reinforcement, the slower the tendency for the bond stress to drop.


## 3.4.1. Pull-Out Strength and Damage Patterns

The typical damage patterns of pull-out test are shown in Figure32. There are two main forms of damage to polyurethane concrete and reinforcement bonded specimens: one is that the reinforcement is pulled out as shown in Figure 32(a), and the other is that the reinforcement is broken as shown in Figure 32(b). The pull-out damage of reinforcement from concrete is caused by the insufficient grip of the concrete on the reinforcement [49]. As shown in Figure 32(a), scraping marks could be seen from the reinforcement projections, and some polyurethane concrete is also observed to have been brought out. In contrast, the damage to the reinforcement fracture is because of the reliable grip of the polyurethane concrete on the reinforcement, which exceeds the ultimate tensile strength of the reinforcement. To further investigate the effect of the anchorage length and diameter of the reinforcement on the pull-out strength, the pull-out specimens of different reinforcement diameters and anchorage lengths were carried to test the pull-out strength. The ultimate pull-out strength and damage patterns are shown in Table 15.Figure 32
Typical damage patterns of reinforcement pull-out test.
(a)(b)Table 15
Results of pull-out test of steel reinforcement and polyurethane concrete.
NumberDiameter (mm)Bonding and anchoring lengthl (mm)Ultimate load (kN)Bonding strength (MPa)Failure mode12-4 d12437.3720.65Pull-out12-5 d549.3921.84Pull-out12-6 d662.3722.98Pull-out12-7 d7(77.38)(24.44)Fracture14-4 d14460.5624.59Pull-out14-5 d580.6926.21Pull-out14-6 d6(94.26)(25.51)Fracture14-7 d7(96.82)(22.46)Fracture16-4 d16482.3725.60Pull-out16-5 d5110.3928.68Pull-out16-6 d6(122.76)(25.44)Fracture16-7 d7(121.49)(21.58)FractureAs shown in Table15, 12-6d, 14-5d, and 16-5d are all reinforcement pull-out damage patterns. The longer the anchorage length, the greater the pull-out strength for the same diameter of steel. The splitting damage pattern of pull-out test, which is often found in the pull-out test of reinforcement and cement concrete [50], does not occur in the pull-out test of reinforcement and polyurethane concrete. As can be seen from Section 3.2, polyurethane concrete has good mechanical properties, especially axial tensile and flexural strengths, which are much higher than those of cement concrete. This property limits the expansion of small and diagonal cracks because of the shear of the concrete and reinforcement. As a result, no form of splitting damage occurs in the tensile tests of polyurethane concrete with reinforcement.The damage patterns of 12-7d, 14-6d, 14-7d, 16-6d, and 16-7d specimens are all steel reinforcement fracture (values in parentheses in Table15). The common point of the five groups of specimens is the long anchorage length of the steel reinforcement, which provides a high bonding strength for the bonding interface for the following three cases: (1) the anchorage length of 12 mm diameter reinforcement is greater than 72 mm, (2) the anchorage length of 14 mm diameter reinforcement is greater than 84 mm, (3) the anchorage length of 16 mm diameter reinforcement is greater than 96 mm, the bond strength of polyurethane concrete and reinforcement is greater than the tensile ultimate strength of the reinforcement, and the pull-out test presents a damage pattern of reinforcement fracture. With the increase of load, the reinforcement steel undergoes the yielding stage, strengthening stage, necking down, and finally fracture damage. Although the peak load in this case does not represent the pull-out strength of the polyurethane and the reinforcement, it can indicate that the bond strength of the polyurethane and the reinforcement is greater than the tensile ultimate load of the reinforcement.


## 3.4.2. Bond Stress-Slip Curves of Reinforcement Pull-Out Specimens

The bond stress-slip curves of reinforcement pull-out specimens are shown in Figure33. The bond-slip curves can be divided into the following five stages:(1)
Microslip phase: in the initial phase of the test loading, the load rises rapidly within a small amount of slip. The slip at the upper free end of the reinforcement is zero, and a small relative slip occurs at the interface between the reinforcement and the polyurethane concrete at the lower loading end, with no cracks appearing within the polyurethane concrete. The bond at this point is mainly provided by the chemical bond, and the slip at the free end significantly lags behind the loaded end.(2)
Slip stage: the load had increased to the point that the chemical bonding stress at the interface between the reinforcement and the polyurethane concrete was destroyed. The bonding stress was provided by the microfrictional resistance of the ribbed reinforcement and the mechanical bite strength. Mechanical bite forces are developed between the ribs of the reinforcement and the polyurethane concrete. As the external load increases, the shear stress on the polyurethane concrete between the ribs gradually increases. When the shear stress caused by the external load exceeds the shear strength of the polyurethane concrete, slippage occurs between the reinforcement and the polyurethane concrete.(3)
Extreme stage: with the increase of load, the bond stress inside the polyurethane concrete gradually reaches the peak, and the growth rate of slip becomes faster. Since the polyurethane concrete has high mechanical strength, it can effectively prevent the expansion of microcracks and the generation of macro cracks, so there is almost no form of splitting damage.(4)
Declining stage: as the increase of slippage between the ribs of the reinforcement on the polyurethane concrete constantly appears as new extrusion, the new slippage surface at the polyurethane concrete is sheared. At this time, the bond stress rapidly decreases, and the slippage is increasing. Eventually, the specimen losses its load-bearing capacity. The bonding force at this stage is mainly frictional resistance.(5)
Residual stage: when the polyurethane concrete between the ribs of the reinforcement is squeezed to total destruction, the friction between the polyurethane concrete and the reinforcement still provides a residual value for part of the bond stress because of its viscoelasticity. The size of the residual value is related to the contact area of the bond surface. The larger the diameter of the reinforcement, the larger the corresponding residual value.Figure 33
Bond stress-slip curves of polyurethane concrete reinforcement pull-out specimens.
(a)(b)(c)(d)The bond-slip curves for Φ12 reinforcement (bonded anchorage lengths of 4 d, 5 d, and 6 d, respectively) are shown in Figure33(a). The ultimate bond stress at 12-6 d reaches 22.98 MPa, an increase of 5.2% and 11.3% compared to 12-5 d and 12-4 d, respectively. The longer the length of the bonded anchorage, the greater the slip when the peak stress is reached, and the longer the slip phase lasts, as well as the greater the residual bond stress.The bond-slip curves for different bond-anchorage lengths of 14 mm and 16 mm diameter reinforcement are shown in Figures33(b) and 33(c), respectively. The 14-5 d ultimate bond stress is 26.21 MPa, which is 6.6% higher than that of 14-4 d, and the ultimate bond stress corresponds to a slip of 1.56 mm, which is 0.13 mm higher than that of 14-4 d. The ultimate bond stress of 16-5 d is 28.68 MPa, which is 12.0% higher than that of 16-4 d. The slip corresponding to the ultimate bond stress is 2.1 mm, which is 0.47 mm higher than that of 14-4 d.The bond-slip curves for specimens with different reinforcement diameters (all with a bond-anchorage length of 5 d) are shown in Figure33(d). The larger the diameter of the reinforcement, the longer the duration of the microslip phase, and the greater the load required to enter the slip phase. The greater contact area between the larger diameter reinforcement and the polyurethane concrete results in greater frictional resistance and greater residual stresses. Therefore, the larger the diameter of the reinforcement, the slower the tendency for the bond stress to drop.


## 4. Conclusions

The bond strength properties of polyurethane concrete and cement concrete/steel reinforcement, respectively, have been investigated through laboratory tests, and the following conclusions are drawn:(1)
The optimum mixture for polyurethane concrete is the combination polyether : polyisocyanate : cement : aggregate = 1 : 1 : 2 : 10. The compressive strength of polyurethane concrete at the optimum mixture is 62.1 MPa.(2)
Polyurethane concrete and cement concrete interface bonding performance has excellent temperature stability in the −40°C ∼ 40°C temperature range. Its interface bonding strength is almost unaffected by the change of temperature.(3)
Increasing the roughness of the interface can significantly enhance the interfacial bonding performance of polyurethane concrete and cement concrete. Compared with the untreated specimens, the bond-splitting strength of the chiseled specimens increased by 82.6%∼91.4%, up to 2.94 MPa. The bond shear strength increased by 51.0%∼55.2%, up to 5.42 MPa. The bond oblique shear strength increased by 27.8%∼28.9%, up to 10.82 MPa.(4)
The strength of the existing cement concrete has a significant influence on the bond strength of the interface. The higher the strength of the existing concrete, the better the mechanical properties of the bonded interface.(5)
The ultimate bond strength of polyurethane concrete and reinforcement can be raised by increasing the diameter of the reinforcement or the bonding and anchoring length. In the bridge expansion joint anchorage zone, the reinforcement is completely encapsulated by the anchoring concrete. The polyurethane concrete thus provides a reliable grip for the reinforcement.Polyurethane concrete has excellent interfacial bonding properties with cement concrete and reinforcement, as well as temperature stability, which is a potential new repair material that can be widely used in the future in the repair of expansion joint anchorage areas.



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*Source: 5644468-2022-10-06.xml*