Analysis of mechanical properties of polyurethane concrete and its bond slip characteristics with rebar

ABSTRACT

In this paper, the bonding properties between rebar and polyurethane concrete (PC) through pull-out tests of PC and rebar is investigated. The effect parameters such as the protective layer thickness of the specimen, the anchorage length of the rebar, the diameter and shape of the rebar on the bonding performance were considered separately. It was shown that the thickness of the protective layer significantly affects the bond strength between the rebar and the PC, and the bond strength increases with the increase of the thickness of the protective layer. The average bond stress is 12.36 MPa for a protective layer thickness of 45 mm, which is an increase of 17.55% compared to 35 mm. The average bond stress is 16.45 MPa at a protective layer thickness of 65, which is a 54.03% increase in stress from 35 mm. The bond strength of rebar to PC decreases with increasing diameter for the same anchorage conditions. The bond stress between the same diameter bars and PC for different anchorage lengths decreases with increasing anchorage length. When the diameter of the rebar is 22 mm, the bond stress between the rebar and the PC is 13.7 MPa, which is a 24.10% reduction in stress compared to 14 mm. The bond strength of rebar to PC decreases with increasing diameter for the same anchorage conditions. And the bond strength of ribbed bars is significantly higher than that of bare round bars. The research results can lay a foundation for the engineering application of polyurethane reinforced concrete.

Keywords:
Polyurethane concrete (PC); Drawing test; Bonding property; Anchor length; Rebar diameter

1. INTRODUCTION

Reinforced concrete structures are one of the most common structural forms used for buildings in China today. Rebar and concrete can work together because the coefficients of temperature linear expansion of rebar and concrete are similar. It is possible to avoid large temperature stresses in both materials when the temperature changes [¹,²,³,⁴,⁵,⁶]. Although concrete and rebar both work in harmony, the mechanical properties of the two materials are very different. Concrete materials have good compressive properties but poor tensile properties. Therefore, when the member is working, it mainly shows the rebar in tension and the concrete in compression. The most important thing is that there is a bond between the concrete and the rebar. The bonding force is mainly divided into three parts: mechanical occlusion force, chemical adhesion force and friction force. The ability of a member to work properly depends on having a good bond between the two. Good bonding allows the two to coordinate deformation to share external loads [⁷,⁸,⁹,¹⁰,¹¹,¹²].

Resin concrete is a composite material formed by polymerization of synthetic resin as a binder mixed with fillers and aggregates. It is superior to ordinary silicate concrete in terms of strength, chemical resistance, water resistance and frost resistance. Its excellent performance reduces the environmental loadability of concrete. It meets the development requirements of environmentally friendly eco-concrete. Resins commonly used in the preparation of resin concrete include polyurethane, epoxy, furan, acrylate, and unsaturated polyvinyl acetate. The mechanical properties of resin concrete are affected by the percentage of aggregate. The tensile strength, flexural strength and ductility of the material increased significantly as the percentage of aggregates in the resin concrete decreased. The tensile, flexural and ductile properties of resin concrete depend on the resin content in the concrete [¹,²,³,⁴.⁵,⁶,⁷,⁸,⁹,¹⁰,¹¹,¹²,¹³,¹⁴,¹⁵,¹⁶]. HALEEM et al. [¹⁷] and HUSSAIN et al. [¹⁸] used polyurethane resin as adhesive and silicate cement or fly ash as filler. Polyurethane cement concrete materials were formulated and applied to flexural rebar of reinforced concrete (RC) scaled beams. The results show that the polyurethane concrete (PC) material has fast early strength development and is characterized by high strength and high toughness. Moreover, the bonding properties are excellent, and the load carrying capacity, stiffness, and ability to limit crack propagation of the scaled-down beams reinforced with PC materials are significantly improved. The PC material considered as light weight material with highly strength due to less density comparing with normal concrete. The increment values of compressive strength was from 5 MPa to 60 MPa of PUC with density 400 kg/m³ density and 1650 kg/m³ respectively, while the flexural tensile strength was increased from 3.3 MPa to 44.3 MPa of PUC with 400 kg/m³ and 1650 kg/m³ density respectively indicating highly improvement in compressive and bending tensile strength compare with conventional concrete. The addition of a small amount of polyurethane improves the plasticity of cement by about 50% without affecting the pumping properties of the material [¹⁹, ²⁰].

All of the above studies have been conducted on polyurethane concrete (PC), a new type of material. In the actual project, the combination of PC and rebar, can give full play to the superior mechanical properties of both, to obtain more excellent crack prevention, seismic and crack control capabilities. However, there is a lack of systematic experimental research on the bonding and anchoring properties of rebar with polyurethane hundi. Since the mechanical properties of PC materials are significantly different from those of ordinary concrete. The results of existing research on the bonding of ordinary concrete to rebar are no longer applicable to the bonding of PC materials to rebar. In this paper, an experimental study on the pullout performance of PC with rebar is carried out. Considerations are the type of rebar, anchorage length, protective layer thickness, and other test parameters. Exploring the bonding properties between PC and rebar.

2. PULL-OUT TEST BETWEEN REBAR AND PC

2.1. Mechanical properties of materials

2.1.1. PC

PC is polyurethane as binder with cement and fly ash as fillers for PC. The main raw material components are shown in Table 1.

Table 1
Main chemical composition of polyurethane.

CHEMICAL COMPOSITION		PERCENT(%)
Polyol	Polyether	49
	Fluids	1
	Water	0–1
Isocyanate		50–51

PC materials are proportioned according to quality. PC material pouring diagram is shown in Figure 1. The main raw materials for polyurethane are polyurethane polyols and isocyanates. The fillers were cement and fly ash, and molecular sieves were utilized to absorb water. The mix ratio adopts the mass ratio. The curing time for PC is about 7d. Formulated high density PC materials as shown in Table 2. PC has a density of 1560 kg/m³, a compressive strength of 62.1 MPa, an axial tensile strength of 32.2 MPa, and a flexural strength of 42.3 MPa. PC material flexural test chart is shown in Figure 2.

Figure 1
PC material pouring diagram.

Table 2
PC composition.

PC	PERCENT(%)
Polyether polyol	24
Isocyanate	24
Cement	30
Coal ash	20
Molecular sieve	2

Figure 2
PC material flexural test chart.

2.1.2. Rebar

The rebar of this test is made of threaded and light round rebars produced by Benxi Steel Works, and the rebar is HRB400 grade rebar. During the fabrication of the drawing specimen, the drawing bars are the same batch of steel bars produced. The rebar diameters were 14 mm, 18 mm and 22 mm. The measured tensile yield strength of the rebars was 425 MPa. The ultimate tensile strength was 643 MPa. The modulus of elasticity was 2.0 × 10⁵ MPa. The elongation is 33%.

3. SPECIMEN DESIGN AND FABRICATION

The cement and fly ash were baked at high temperature for 2 h and the temperature was reduced to room temperature to remove the free water from the materials therein. Mix cement, fly ash, isocyanate, polyurethane polyol and molecular sieve. Stirring with a stirring rod is used, and the mixing process should ensure that the material is evenly mixed. Tensile specimens were made by pouring the mixed PC material into the tensile member forms. The rebar was centered in the middle of the cube specimen with dimensions shown in Figures 3 and 4. The PC was individually fabricated into dumbbell-shaped specimens with a thickness of 25 mm, a width of 65 mm in the center, and a width of 190 mm on both sides. The dumbbell-type specimen is shown in Figure 5. The drawing test consisted of 32 groups with 3 specimens in each group and 96 specimens in total.

Figure 3
Drawing of specimen for pull-out test (unit: mm).

Figure 4
Drawing specimen PC diagram.

Figure 5
PC axial tensile specimen.

In order to facilitate the measurement of the deformation of the rebars during the loading test, the specimens are placed on a specific test loading device for the pull-out test. The test device mainly consists of loading device, LVDT displacement gauge, drawing specimen, PTFE plate and penetrating type pressure transducer. The top and bottom plates of the fixture in the loading unit are connected by four 25 mm diameter high tensile steel rebars. And the top and bottom plates of the fixture are fixed in both directions by using high-strength screws at both ends of the rebar. The test bars are threaded through the pre-set center hole in the fixture base plate and connected to the lower collet of the tester. A penetrating pressure transducer is placed between the bottom surface of the specimen and the bottom plate of the fixture in order to actually measure the pullout load during loading. The top plate of the fixture is connected to the upper collet of the tester by a 25 mm diameter high strength, precision-rolled rebar. A steel plate and a PTFE plate are placed between the bottom surface of the specimen and the bottom plate of the fixture in order to improve the boundary conditions and to facilitate the centering. The specimen number and parameters are shown in Table 3, mainly considering 3 parameters, protective layer thickness, diameter and shape of rebar, and anchorage length.

Table 3
Drawing specimen parameter table.

SPECIMEN NUMBER	THICKNESS OF PROTECTIVE LAYER (MM)	ANCHOR LENGTH (MM)	REBAR DIAMETER (MM)
1	65	35	18 (Ribbed)
2	65	55	14 (Ribbed)
3	65	55	18 (Ribbed)
4	65	55	22 (Ribbed)
5	65	55	22 (Round)
6	65	85	14 (Ribbed)
7	65	85	18 (Ribbed)
8	65	85	22 (Ribbed)
9	65	85	22 (Round)
10	65	105	18 (Ribbed)
11	35	85	18 (Ribbed)
12	45	85	18 (Ribbed)
13	55	85	18 (Ribbed)

4. LOAD AND MEASUREMENT SOLUTIONS

According to the Standard for Test Methods of Physical and Mechanical Properties of Concrete (GB/T50081-2019), the pull-out test adopts a microcomputer-controlled electro-hydraulic servo universal testing machine. Specimen arrangement detail is shown in Figure 6. To facilitate the installation of the specimen on the testing machine, an auxiliary reaction frame for placing the specimen was fabricated. The test-measured bar slip is the relative slip between the stressed end of the bar and its corresponding concrete end section. The test loads and rebar slips were collected by computer. The loading was carried out by force control, and the loading rate was controlled to be 0.1 kN/s during the test loading. The pull-out load of the plate specimen is measured by means of a penetrating pressure transducer in the loading device. At the same time, the static stress-strain testing system was used to automatically collect the load and displacement data with an acquisition frequency of 0.33 Hz. The collected data are automatically recorded by a computer for subsequent analysis of the results. Stop the test when the free end of the specimen with slip damage reaches 15 mm.

Figure 6
Specimen arrangement detail.

According to the tensile value measured in the pull-out test, the bond stress at the pull-out interface can be found according to the following formula:

(1)$$\tau =\frac{P}{\pi d{l}_a}$$

Where: P is the pulling force; τ is bonding stress; d is rebar diameter; la is anchoring length.

5. INFLUENCE OF ANCHORAGE PARAMETERS ON BOND STRENGTH BETWEEN REBAR AND PC

5.1. Axial tensile strength of PC

The average value of axial tensile strength of PC materials was 32.2 MPa as shown in Table 4.

Table 4
Axial tensile strength table for PC materials.

NUMBER	PC1	PC 2	PC 3	PC 4	PC 5	PC 6	AVERAGE VALUE
DISSOCIATION (MPA)	31.7	33.0	32.2	33.3	32.8	30.1	32.2

The intrinsic relationship curves for each specimen based on axial tension of PC material collected during the test are shown in Figure 7. Direct stretching of the intrinsic relationship curves, as shown in Figure 8.

Figure 7
Axial tensile intrinsic relationship curve.

Figure 8
Axial tensile intrinsic relationship graph (average value).

5.2. Thickness of protective layer

The parameters for selection of protective layer thickness of rebar were 35 mm, 45 mm, 55 mm and 65 mm. Comparison of bond stresses for each group of specimens with different protective layer thicknesses is shown in Figure 9. The relationship between the bond stress between the rebar and PC and the thickness of the protective layer is plotted in Figure 10. From the figure, the bond strength between the rebar and PC increases with the increase in the thickness of the protective layer. The average bond stress is 10.68 MPa for a protective layer thickness of 35 mm. The average bond stress is 12.36 MPa for a protective layer thickness of 45 mm. Stress increased by 32.3% from 35 mm. The average bond stress is 16.45 MPa at a protective layer thickness of 65 mm. Stress increased by 54.03% from 35 mm. The thickness of the protective layer significantly affects the bond strength between the rebar and the PC. Since the rebar anchorage damage mode is mostly splitting damage, the larger the protective layer, the greater the splitting strength of the PC cube specimens.

Figure 9
Bond stress diagram for each group of specimens at each thickness of protective layer.

Figure 10
Bond stress-protection layer thickness relationship diagram.

5.3. Rebar anchoring length

The rebar anchorage length was selected with four parameters of 35 mm, 55 mm, 85 mm and 105 mm as shown in Figure 11. The relationship between the bond stress between the rebar and the PC and the anchorage depth is shown in Figure 12. The bond strength between rebar and PC decreases with the increase of anchorage length. The bond stress between the rebar and PC was 19.64 MPa at an anchorage length of 35 mm. The bond stress between the rebar and the PC was 16.47 MPa for an anchorage length of 55 mm. Stress was reduced by 16.14% compared to 35 mm. The bond stress between the rebar and PC was 13.28 MPa at an anchorage length of 85 mm. The stress increased by 30.86% over the anchorage length of 35 mm. The average bond stress between the rebar and PC was 10.65 MPa for an anchorage length of 105 mm. The stress increased by 45.77% over the anchorage length of 45 mm. The bond strength between the rebar and the PC decreases with increasing rebar anchorage length. The rebar pullout force is closely related to the bond area of the rebar. Anchorage depth and adhesive field are positively correlated.

Figure 11
Bond stress diagrams for each group of specimens at each anchorage depth.

Figure 12
Bond stress - anchorage depth relationships.

5.4. Diameter and shape of rebars

The parameters for selection of rebar diameters were 14 mm, 18 mm and 22 mm. The rebar type selection parameters are taken as 22 mm diameter rebar and light round rebar as shown in Figure 13. The relationship between the bond stresses between the reinforcement and the PC with respect to the diameter of the rebar and the type of rebar is shown in Figure 14. The anchorage lengths are all 55 mm and negative correlation between adhesive intensity and diameter. The bond stress between the rebar and the PC is 18.05 MPa when the diameter of the rebar is 14 mm. With a rebar diameter of 18 mm, the bond stress is 16.47 MPa. The stress was reduced by 8.35% compared to the diameter of 14 mm. With a rebar diameter of 22 mm, the bond stress between the rebar and the PC is 13.7 MPa, which is a 24.10% reduction in stress compared to 14 mm. The bond stress of 22 mm diameter bare round rebar is only 6.4 MPa, which is only 47.4% of that of ribbed rebar under the same conditions. This is because the mechanical bite force of ribbed deformed rebar is greater than that of bare round rebar. The bond strength of the rebar to the urethane concrete decreases with increasing diameter for the same anchorage conditions. The bond stress between the same diameter rebar and PC at different anchorage lengths decreases with increasing anchorage length.

Figure 13
Bond stress diagrams for each group of specimens for each rebar diameter.

Figure 14
Plot of bond stress-bar diameter and type relationship.

The comparison of bond stresses between different diameter rebars and PC at an anchorage length of 85 mm is shown in Figure 15. Average bond stress is shown in Table 5. The average bond stress is 13.54 MPa for a diameter of 14 mm. With a rebar diameter of 18 mm, the average bond stress is 12.88 MPa. The stress was reduced by 4.87% from d of 14 mm. With a rebar diameter of 22 mm, the average bond stress is 11.35 MPa. The stress was reduced by 11.88% from d of 18 mm. The binder stress of 22 mm diameter bare round bar is only 4.0 MPa, with 35.24% of that of ribbon bar in the similar condition.

Figure 15
Bond stress diagrams - each rebar diameter (l_a = 85 mm).

Table 5
Average bond stress.

BAR DIAMETER (MM)	SURFACE (MM)	STRENGTH (MPA)
14	Ribbed	13.54
18	Ribbed	12.88
22	Ribbed	11.35
22	rib	4.00

6. BOND STRESS-SLIP CURVE

The more exemplary adhesive slip curves among the groups of samples were chosen for contrasting center of analysis in this experiment, and the adhesive slip curves are shown in Figure 16.

Figure 16
Bond-slip curves for deformed rebars and bright-round rebars.

From the adhesive slip curve, similar to the adhesive slip curve of rebar concrete, the curve is divided into three parts: the rising section, the falling section and the residual section. The stages are as follows:

1. (1) Rising segment: As the tension increases, free slip begins to appear and grows nonlinearly. There is a complete loss of chemical adhesion, and interior fractures progress gradually from the holding layer to the skin. When the interior fractures developed along the weakest point of the protective layer to the concrete surface, the specimens showed splitting cracks along the longitudinal direction of the rebar and slowly extended from the loaded end to the free end, when the bond was at its maximum. In the comparative tests, the ultimate bond stress of the deformed rebar was significantly higher than that of the bare round rebar.

2. (2) Descending Segment: After the concrete splits, the loads begin to drop, for members without lateral restraint. Due to the disappearance of the mechanical occlusion at the concrete grip level leading to a sharp decrease in the bond stress. Only a little friction remains between the rebar and the PC remains. At the same time slip has increased dramatically.

3. (3) Residual segments: A As the slippage increases to one horizontal rib distance, the bond stress begins to decrease gradually and the free end slippage increases substantially. In the case of bare round rebars, the load remains essentially constant and the bond is provided mainly by the friction between the reinforcing steel and the PC material. In the case of ribbed rebar, the load is gradually decreased until the reinforcing steel is completely pulled out. Mechanical occlusion forces remain between the rebar and the urethane concrete during the residual phase.

7. CONCLUSION

In this paper, the bonding properties between rebar and polyurethane concrete (PC) through pull-out tests of PC and rebar is investigated. The effect of parameters such as the protective layer thickness of the specimen, the anchorage length of the rebar, the diameter and shape of the rebar on the bonding performance were considered separately. The conclusions are as follows:

1. The average bond stress is 12.36 MPa for a protective layer thickness of 50 mm, which is an increase of 17.55% compared to 40 mm. The average bond stress is 16.45 MPa at a protective layer thickness of 70 mm, Which is a 54.03% increase in stress from 40 mm. The thickness of the protective layer significantly affects the bond strength between the rebar and the PC. Appropriate thickness of protective layer should be adopted in practical engineering.

2. With a rebar diameter of 20 mm, the bond stress between the rebar and the PC is 13.7 MPa, which is a 24.10% reduction in stress compared to 12 mm. The bond stress of 20 mm diameter bare round rebar is only 6.4 MPa, which is only 47.4% of that of ribbed rebar under the same conditions. The bond strength of the rebar to the urethane concrete decreases with increasing diameter for the same anchorage conditions. The bond stress between the same diameter rebar and PC at different anchorage lengths decreases with increasing anchorage length.

3. The anchorage lengths were all 50 mm, and with a rebar diameter of 20 mm, the bond stress between the rebar and the PC was 13.7 MPa, which is a 24.10% reduction in stress compared to 12 mm. The bond stress of 20 mm diameter bare round rebar is 6.4 MPa, which is only 47.4% of that of ribbed rebar under the same conditions. The bond strength of the rebar to the urethane concrete decreases with increasing diameter for the same anchorage conditions. The bond stress decreases with increasing anchorage length between the reinforcement bars of the same diameter and the PC at different anchorage lengths.

4. Through the drawing test of polyurethane concrete and steel bar, we can understand the working mechanism of polyurethane concsssrete and steel bar, which can lay a foundation for the application of polyurethane reinforced concrete materials in engineering.

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Publication Dates

• Publication in this collection
  14 Feb 2025
• Date of issue
  2025

History

• Received
  18 Oct 2024
• Accepted
  23 Dec 2024