Mechanical and Durability Performance Evaluation of Crumb Rubber-Modified Epoxy Polymer Concrete Overlays

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Mechanical and durability performance evaluation of crumb rubber-modified epoxy polymer concrete overlays

Jiaqing Wang, Qingli Dai

^⇑

, Shuaicheng Guo, Ruizhe Si

Department of Civil and Environmental Engineering, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931-1295, United States

h i g h l i g h t s

Recycled crumb rubber particles were applied into polymer concrete for improved performance.

Both mechanical properties and thermal-related durability performance were increased.

Rubber epoxy concrete can better protect concrete bridge decks with increased ductility.

a r t i c l e i n f o

Article history:

Received 27 October 2018

Received in revised form 7 January 2019 Accepted 12 January 2019

Available online 29 January 2019

Keywords:

Polymer concrete overlay Waste tire rubber Mechanical property Thermal property Microstructure

a b s t r a c t

The epoxy-based polymer concrete has been widely used for concrete repairing and overlays due to good mechanical properties and durability. To reduce environmental landfill problems with the accumulation of tire rubbers, scrap tire rubbers were added to epoxy polymer concrete in this investigation. The crumb rubber (with mesh size #50, 0.279 mm) were introduced into epoxy concrete with two different contents of 5% and 10% based on the epoxy monomer weight. The mechanical properties including direct tensile strength, compressive strength, splitting tensile strength and interface bond strength. Thermal- and moisture-related durability performance of rubberized epoxy concrete were measured and compared with the control samples. The compressive strength and splitting tensile strength were improved with the added 5% solid rubbers, and slightly reduced with 10% content. With a specially-designed testing method, the interface tensile bonding strength between the epoxy concrete overlay and concrete slab were measured as higher than 250 psi for both control and rubberized samples. The thermal conductivity of polymer concrete was reduced with the increase of rubber contents. In addition, very low water absorption rates (<0.5%) were measured with all types of epoxy concrete samples. The interface microstructure with SEM also indicated the good bonds between rubber particles and epoxy resins.

The overall test results showed the enhanced performance of rubber-modified epoxy concrete which can facilitate the tire rubber recycling into epoxy polymer concrete for protecting the exist concrete pave- ment structures.

1. Introduction

Recycling of waste tire rubbers into construction materials has been widely investigated in the past decades[1,2]. Vast amounts of research have been conducted to introduce waste tire rubber into conventional Portland cement concrete as partial replacement of coarse/fine aggregates[3]. Similar research were also conducted with fine crumb rubber modified asphalt binder [4–6]. Prior research has concluded that recycled rubber particles can help to modify the performance of Portland cement concrete and asphalt

binder. In the case of rubberized concrete with the replacement of rubber aggregate, the mechanical properties, such as the compressive strength, splitting tensile strength, and flexural strength, could be reduced with high rubber contents [7]; however, the toughness and flexural displacement can be increased when compared with conventional Portland cement concrete[8–10]. The risk of brittle failure could also be minimized by adding soft waste tire rubbers [11]. Although the possibility of recycling rubbers into Portland cement concrete has been verified to show various posi- tive property changes, the application is still limited since the reduction in strength. Therefore, waste tire rubber recycling for civil materials is still limited.

Polymer concrete, which is composed of polymer resin and aggregates, has been extensively used for conventional concrete https://doi.org/10.1016/j.conbuildmat.2019.01.085

⇑ Corresponding author.

E-mail addresses: [email protected] (J. Wang), [email protected] (Q. Dai), [email protected](S. Guo),[email protected](R. Si).

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Construction and Building Materials

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repairs, as well as overlays of concrete bridge decks in the 21st century[12,13]. Conventional Portland cement binder was replaced by polymer resin adhesive to establish aggregate structures[14]. Sev- eral different types of polymer binder were widely used for mixing polymer concrete, such as polyester-styrene, acrylics, and epoxy.

Solid epoxy resin is classified as a non-hazardous material [15], thus it is safe for civil construction applications and long-term service. The curing process of the epoxy resin is also simple; it can activate with hardeners at room temperature and achieve the reasonable strength in a short period of time. Shortened curing times are desirable for rapid repairing of concrete structures as they can help reduce traffic delays, for instance[16]. Once epoxy concrete is used for repairing the concrete structures, it will achieve a high strength overall as well as produce a strong bond in the interlayer with the substrate concrete materials. Epoxy concrete overlays for bridge decks or floors is also popular since its significant durability, chemical attack resistance, water resistance, and fast-curing process[12]. However, some issues still limit the wide application of epoxy resin based polymer concrete.

Since the thermal coefficients between epoxy concrete and traditional concrete materials are different, failure could occur due to rapid temperature changes. These significant differences in thermal coefficients are especially of concern in the northeast area of the United States. In addition, the relative higher cost of epoxy concrete also restricts its advantages when compared with commonly used high-early-strength Portland cement[17]. Since the toughening mechanisms have been found for brittle epoxies, the toughness of ordinary epoxy concrete can be modified with the addition of an elastomeric phase [18]. There are several different liquid elas- tomers that have been widely used for toughening brittle epoxy resins, especially, carboxyl-terminated butadiene-acrylonitrile (CTBN) type rubbers, which can be defined as liquid rubber. Low modulus epoxy concrete could be produced by introducing CTBN, and, consequently, provide a good compatibility with the concrete material and resist thermal cracks[19]. Therefore, extensive application of epoxy concrete could be considered to combine with plastics and waste tire rubber, which may help to furtherly improve the toughness and durability performance. Jo, Park[20]

studied the properties of polymer concrete made with recycled polyethylene terephthalate (PET) plastic waste and recycled concrete aggregates. They found that favorable mechanical properties could be achieved with recycled PET. Shen, Shan[21]investigated the mechanical properties of polymer-modified porous concrete with introduction of rubber aggregates, it was found that the energy absorption capacity, ductility, abrasion resistance, and impact resistance of polymer modified porous concrete were improved with added rubber. Diaconescu, Barbuta[22]predicted the properties of polymer concrete with tire rubber powder using neural network, comparable compressive strength and splitting tensile strength were achieved in polymer-rubber concrete based on that of traditional Portland cement concrete. Thereby, the recycling of solid waste tire rubbers into epoxy resin based polymer concrete may become another environmentally-friendly method for producing polymer concrete with good mechanical properties and durability performance. The successful addition of waste tire

rubbers into epoxy concrete will improve the performance of cur- rent epoxy concrete overlay system.

In this evaluation, the recycled tire rubber particles (mesh size

#50 minus) were used as an additive for the epoxy concrete at two contents: 5% and 10%, based on the monomer weight of epoxy. In total, three different types of epoxy concrete samples were prepared, including control sample, 5% and 10% solid rubber modified sample. The mechanical properties of rubberized epoxy concrete samples were thoroughly evaluated and compared with the original epoxy concrete samples. Besides, the performance of epoxy concrete overlay systems were also evaluated with the interface bond test, thermal compatibility test and water absorption test.

Besides these, the ESEM image analysis to demonstrate the rubber-modified epoxy internal and interface morphology. The mixture design, test methods, and test results are comprehensively discussed in the following sections.

2. Selection of materials and mixture design

In this investigation, the epoxy was obtained from The Euclid Chemical Company. The epoxy material is a two-part based mixture, which consisted of part A: an epoxy resin (composed of Bisphenol A Polyglycidyl ether resin, Epichlorohydrin castor oil based epoxy resin, and Epichlorohydrin), and part B: a hardener (composed of 4-Nonylphenol, Poly diamine, and Bisphenol A).

The epoxy used in this investigation was cured at room temperature. The gelation time is approximately 45 min. The mixing ratio of part A to part B was determined to be 1:1 by weight. The crushed basalt aggregate with a #8 mesh size (2.38–4.76 mm) was used, which has specific gravity of 2.85, absorption rate of 1.55%, and compressive strength of 28,000 psi. In addition, the recycled waste tire rubber particles with the mesh size of #50 (0.297–0.595 mm) were selected. The epoxy concrete was then modified with the introduction of different contents of solid rubbers (5% and 10%), the control samples were also produced without rubbers for comparison. The mixing ratios of different components for distinct samples are presented inTable 1. The materials that used in this investigation are shown inFig. 1.

3. Mixing procedures

In this investigation, simple electrical mixing tools were used for the mixing processes. All components were accurately weighed before mixing. As introduced in the mixture design, the binder system was composed of two parts (part A and part B). In regards to the epoxy concrete, before introducing aggregates, part A and part B were separately mixed for 2 min before being combined together, which was followed by sufficient mixing for another 3 min. Then, the aggregate was added to the mixed epoxy com- pound and mixed for an additional 2 min. In samples where rubber was mixed in, rubber particles were mixed with part A of the epoxy for 2 min; part B was mixed separately at the same time. After the rubber was mixed well with part A, part B was added and then mixed together for 2 min. Lastly, 3 min of mixing were performed

Table 1

The mixing ratio for different epoxy concrete samples.

Sample types Mixing ratio

(Part A: Part B)

#50 Rubber content #8 Aggregate content

Control 1:1 by weight 0% 2 times weight of epoxy part A + B

With 5% rubber 1:1 by weight 5% based on the weight of epoxy part A + B 2 times weight of epoxy part A + B With 10% rubber 1:1 by weight 10% based on the weight of epoxy part A + B 2 times weight of epoxy part A + B

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with the addition of basalt aggregates. Immediately after mixing, the different samples were prepared and stored in the curing room until the corresponding test time.

4. Interface microstructure of hardened rubber-modified epoxy concrete

The SEM image of hardened epoxy concrete that produced with solid rubber particles was demonstrated as shown inFig. 2a). To identify the phases of rubber and epoxy, the energy dispersive X- ray (EDX) analysis was conducted to determine the element compounds of rubber and hardened epoxy resin, which were demonstrated in Fig. 3. In the case of the rubber, the main elements were carbon, oxygen, aluminum, silicon, and sulfur, which are typ- ical compounds of tire rubber[6]. Regards to the hardened epoxy resin, it is obviously that the chloride was found, and the main elements were carbon and oxygen, and no other elements were found.

Therefore, the chemical compositions of solid tire rubber and epoxy resin were distinguished. Afterwards, with identified phase compositions, the bond behaviors between rubber particles and epoxy resins were investigated.

In order to capture the interface morphology in rubber- modified epoxy, the sawed surface section of epoxy concrete samples (with 5% rubber content) were polished by sandpaper and then imaged with SEM. As shown inFig. 2b) and c), the good bonding behavior was found between the small rubber particle and hardened epoxy resins. In the vicinity area of rubber particle, the hardened epoxy resin presented dense and consistent structure.

Meanwhile, the rubber particle was coated with epoxy resin and most of interface have intact contact. However, on some locations, some gaps existed due to entrapped air during the mixing process as shown inFig. 2d).

5. Mechanical property and thermal property tests with prepared samples

5.1. Direct tension test and observations of fracture surfaces The direct tension test was performed according to the procedure outlined in ASTM D638 standard [23]. The test specimens

were prepared with the original epoxy binder and rubber particles modified epoxy binder without addition of aggregate, by using the mixing ratios as shown inTable 1. The dimensions of the sample are shown inFig. 4, which followed the requirement of the Type

Ⅰsample in ASTM D638. The dog-bone shaped samples were properly produced and cured for 24 h before testing. The test was conducted by a MTS loading frame with a controlled displacement loading rate of 5 mm/min. The test results were calculated by the average of four specimens for each type of samples.

5.2. Compressive strength test

The compressive strength test was performed by following ASTM C579 standard, method B[24]. The mixing ratios inTable 1 were used. The epoxy concrete samples were produced with a 2⁰⁰2⁰⁰cubic mold and cured for 24 h before performing the test.

For each type of epoxy concrete, three samples were produced.

The compressive loading frame was used. A controlled loading speed of the crosshead was set to 50 psi/s. The compressive strengths of the samples were obtained by the average of the three samples.

5.3. Splitting tensile strength test

The splitting tensile test was conducted following the procedure in ASTM C496 standard[25]. The test specimens were prepared with a cross section dimension of 2 in diameter and a height about 3 in. The test was performed after a 24 h curing period. Three corresponding specimens were tested for each type of sample. The splitting tensile strength was calculated by following Eq.(1).

fsp¼ 2P

p

^DL ^ð1Þ

5.4. Interface bonding strength test

The internal bonding strength test was designed based on ASTM C1583 standard[26], and is centered around the pull-off method [27]. The epoxy concrete overlay was applied to the substrate con- Fig. 1.Ingredient materials for control and rubber-modified samples.

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crete block, which could pseudo-simulate an application for concrete pavements. The substrate concrete block was designed with dimensions of 10⁰⁰10⁰⁰3⁰⁰. The concrete slab was produced with the rubberized concrete material, which contained 10% rubber particles as partial replacement of conventional aggregate [28]. The

slab was cured for 28 days before applying the epoxy concrete overlay. Before constructing the epoxy overlay, the concrete slab was oven dried to eliminate moisture and the surface was cleaned and polished. The surface of the slab was evenly divided into two areas as shown inFig. 5a). Plastic dividers were then fixed on the Fig. 3.The EDX analysis results: a) On rubber particle; b) On hardened epoxy.

Fig. 2.SEM imaging of the sawed surface section of solid rubber modified epoxy concrete: a) Good bond between rubber particle and hardened epoxy; b) rubber particles was wrapped by hardened epoxy; c) good bond between rubber and hardened epoxy resin; d) Gap between rubber particle and hardened epoxy;

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surface of the substrate slab to retain the epoxy overlay. The edge of the mold was properly sealed with water resistant tape in order to eliminate leaking. The control epoxy concrete overlay and rubber modified (5% rubber content) epoxy concrete overlay were mixed and poured on the surface at the same time as shown in Fig. 5b). This procedure can provide consistent substrate material

for different types of overlay polymer concrete. The overlay was poured such that it would have thickness of 0.5 in for both types of epoxy concrete. After a 24 h curing period for the epoxy overlay, the concrete block with the coated epoxy concrete overlay was then removed from the molds for the cutting process. The block was separated into two slabs from the boundary line established by the middle plastic divider between the control side and the 5% rubber modified side, which is shown inFig. 6a). The specimens were drilled out from the slabs as shown inFig. 6b), the diameter of the cylinder samples was measured as 50 mm. Then, the samples were glued to aluminum plate fixtures on both sides by an epoxy adhesive. The finished specimens were then cured for 24 h before testing as shown inFig. 7.

The test was then operated on a material testing system (MTS), which has a maximum loading capacity of 50,000 lbs. Before applying the load, aluminum plates were attached to the loading fixture as demonstrated in Fig. 8, and the position of the sample was aligned in the vertical direction. The displacement controlled loading rate of 50 mm/min was utilized.

Fig. 5.The sample preparation of epoxy concrete overlay: a) Established mold for applying epoxy concrete overlay; b) Applied two types of epoxy overlay on a concrete substrate.

Fig. 6.The drilled samples for bonding test: a) Cured epoxy concrete overlay with substrate concrete slab after sawing; b) Drilled samples from the sawed slab.

Fig. 4.Test specimen dimensions.

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5.5. Thermal conductivity test

The thermal conductivities of hardened epoxy resin and epoxy concrete were measured by a KD2 pro thermal properties analyzer at room temperature, which is based on the line heat-source tech- nique [29]. The 100 mm length heated single-needle probe was

inserted into the sample material, which can measure the thermal conductivity between 0.10 and 4.00 W/mK. The cylinder samples with dimensions of 2⁰⁰diameter and 6⁰⁰height were prepared with a 1/8⁰⁰driller in order to insert the needle sensor. The samples were stored at room temperature for 24 h after drilling process. The thermal conductivity of each sample type was obtained by calcu- lating the average of the three measurements. Each measurement was followed by 15 min recover time to eliminate the temperature vibration for every sample.

5.6. Thermal compatibility test

This test was conducted by following ASTM C884 standard[30], which requires a freeze-thaw cycle circumstance. The temperature of the freezer was controlled as22°C, at the same time, the room temperature for the thaw procedure was retained at 24°C. The test specimens were prepared with the substrate concrete slab and epoxy concrete coating layer. Those concrete slabs were prepared with dimensions of 12in by 12in by 3in, and the thickness of the overlay was operated as 0.5 in. After 28-day curing of the concrete slabs, the concrete samples were oven-dried to remove moisture, the surface was then finished with proper polishing. The prepared wood mold was then attached to the top surface of the concrete blocks to retain the epoxy overlay. Afterwards, the epoxy overlays were poured on the surface. The epoxy overlays were finished by following the mixing ratios inTable 1, the control samples and 5% rubber modified samples were prepared. For each cycle, the samples were stored in the freezer for 24 h, and then removed them to room temperature for another 24 h. The behavior of the samples was detected after total 5 cycles. To evaluate the thermal compatibility between the epoxy concrete coating layer and the substrate concrete block, the delamination of the epoxy layer from the concrete block or the presence of horizontal cracks in the concrete near the interface can be determined as failure.Fig. 9shows the coated concrete slabs placed in the freezer.

5.7. Water absorption rate

The water resistance ability of the rubberized epoxy concrete was compared with control specimens. The test was conducted Fig. 7.Prepared samples for interface bond test.

Fig. 8.Test setup of internal bonding strength.

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by following ASTM D570 standard[31], except the thickness of the specimens was 1 in. The specimens were sliced from the cylinder samples that with the diameter of 2 in. Four specimens were prepared for each group (control samples, 5% rubber modified samples, 10% rubber modified samples). The specimens were oven dried at 80°C for 3 h to eliminate all moisture before measuring the initial weight. All samples were then immediately emerged into tap water at room temperature of 23 ± 1°C. After 24 h, the samples were moved out and the water on the surface of each sample was wiped away. The measurements were conducted with the surface saturated dry (SSD) specimens.

6. Mechanical and thermal property tests results and discussions

6.1. Direct tensile strength of hardened epoxy resin without aggregate The direct tensile test results are shown inFig. 10. The direct tensile strength of the hardened original epoxy resin was approximately 920 psi. After introduction of rubber particles, the direct tension strength was reduced with the increase of rubber content.

However, with the 5% rubber particles, it was reduced by about 4.7% based on the strength of control specimens. In the 10% rubber samples, the direct tensile strength had a significant decrease at

about 23.4%. The epoxide groups of the epoxy resin can react with the hardener, which will form a cross-linked structure[16]. Since the addition of solid rubber particles, some of the cross-link structure at the cross-section could be cut off since the solid rubbers did not have chemical reactions with the epoxide group or the hardener. The effective cross-linked volume in the cross-section area of the specimens could be decreased, which led to the reduction of direct tensile strength. However, the direct tensile strength of 5% rubber modified samples still maintained a tensile strength at 874 psi. These results indicated the possibility of recycling scrap tire rubbers into epoxy resin. The properties of epoxy concrete and tire rubber modified epoxy concrete were thus evaluated in the following sections.

6.2. Compressive strength of epoxy concrete specimens

The results of the compressive strength test are summarized in Fig. 11. The control epoxy concrete that was produced in this investigation showed an average compressive strength of 3987 psi. After introducing 5% rubber particles into the epoxy concrete, the compressive strength was increased to 4350 psi, an increase of 9.1%

compared to the control specimens. With the increase of rubber content to 10%, the compressive strength was less than both the control samples and 5% rubber content samples. When compared to the control samples, the 10% rubber content samples had a compressive strength that decreased by 2%. The structure of epoxy concrete is simply made of epoxy resin binder and aggregates, with the introduction of low-content rubber particles, the hardened epoxy binder might be toughened by well-dispersed fine rubber particles similar to the mechanism of rubberized concrete[8]. This added low-content rubber particles can also reduce the brittleness of the control epoxy resin, and thus contribute to resist cracking when the structure deforms due to external forces[32]. The failure of the brittle sample (such as epoxy) was mainly affected by the development of the main cracks. The main crack will suddenly grow and the specimen will lose its carrying capacity. With the added 5% rubber particles, the epoxy resin could be properly toughened and thus slow the growth of the cracks. In addition, when the crack developed, the small rubber particles could also release the stress once the crack tips go through them[33]. How- ever, after applying more rubber as in the 10% rubber samples, the integrity of the modified concrete samples was reduced. Also the added rubber particle is a relatively soft material when compared with the epoxy matrix, thus the specimen’s loading carrying Fig. 9.The test specimens for thermal compatibility evaluation.

Original binder With 5% rubber With 10% rubber 0

200 400 600 800 1000

Direct tension strength (psi)

Samples

Direct tension strength

Fig. 10.Direct tensile strength of different types of hardened epoxy resin.

Control With 5% rubber With 10% rubber 0

1000 2000 3000 4000 5000

Compressive strength (psi)

Samples

Compressive strength

Fig. 11.Test results of compressive strength.

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ability was reduced. Therefore, the compressive strength of specimens containing 10% rubber particles was slightly decreased.

6.3. Splitting tensile strength of epoxy concrete samples

Fig. 12shows the splitting tensile strength test results. These results showed a similar trend as compressive strength test results.

The specimens with 5% rubber particle modified epoxy concrete showed 8% improvement of splitting tensile strength based on the result of control samples. With the introduction of rubber particles, the internal tensile force could be released by the deforma- tion of the elastic rubber. With the 5% content of rubber particles, the specimens not only achieved a higher splitting tensile strength than control samples due to the stress release effect of rubber, but also showed smaller cracks after reaching the peak load. The rubber limited the growth of the crack, which also helped the samples to sustain the tension load after the occurrence of the crack. With more rubber particles as demonstrated by the 10% rubber samples, the stiffness of the samples could be considerably reduced, which could affect the ability to sustain the splitting tensile load. The pos- itive effect by added rubber particles could be minimized with more rubber particles, but the splitting tensile strength of specimens with 10% tire rubber still demonstrated a comparable strength as those of the control specimens.

6.4. Interface bond behavior of epoxy concrete overlays

Generally, the failure surface of the pull-off overlay samples varies, which depends on the interface bonding strength and substrate concrete tensile strength. In this test, different failure patterns were actually found. The different failure patterns are shown in Fig. 13, the interface bond failure between the epoxy overlay and the substrate concrete only occurred in control samples. Control samples #1 and #3 showed bond failure, and #2 showed substrate concrete failure. Listed in Table 2, the bond strength of these two samples of the control group was higher than the suggested strength of 250 psi by the American Concrete Insti- tute (ACI) 548 guide for polymer concrete overlay[34]. In addition, in the case of the #2 specimen in the control group, the substrate concrete tensile strength was also higher than 250 psi, which means the bond strength could be even higher than the concrete tensile strength of sample #2. This result verified that the control epoxy concrete that was prepared in this evaluation had a consid- erable high bonding strength with the substrate concrete slab surface. In addition, the failure did not occur on the epoxy concrete overlay, which also indicated that the prepared epoxy resin with aggregates has a good resistance of direct tensile load.

In regards to the epoxy concrete specimens with the added 5%

rubber, all the pull-out failures happened inside substrate concrete. There is no interface failure between rubber concrete and concrete substrate. This phenomenon indicated that the rubber modified epoxy concrete have better bonding behavior with the substrate concrete part than the control one, which would be applicable for use in concrete overlay or repairing construction.

The test results in this evaluation verified that the epoxy concrete with rubber particles have improved bonding strength with concrete substrate. The physical adhesion was increase, partially due to the rubber surface texture and friction. Therefore, the added 5% rubber particles would be beneficial when used in the epoxy concrete overlay.

6.5. Thermal conductivity

The thermal conductivity of the epoxy binder and epoxy concrete with and without solid rubbers is summarized in Fig. 14.

From the results, it can be stated that the neat epoxy binder has a low thermal conductivity of 0.225 W/mK. After introducing solid rubber particles, the thermal conductivity was slightly decreased with higher rubber contents. After adding aggregate, the thermal conductivity was increased based on epoxy resin samples since the relatively high conductivity (2.47 W/mK) of basalt rock[35].

This basalt rock is a good thermal bridge for heat conducting. How- ever, the same trend was found as thermal conductivity was Control With 5% rubber With 10% rubber

0 250 500 750 1000 1250

Splitting tensile strength (psi)

Samples

Splitting tensile strength

Fig. 12.Test results of splitting tensile strength.

Fig. 13.The observation of different failure patterns.

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reduced with the increase of solid rubbers within epoxy concrete samples. On the one hand, the low thermal conductivity of solid rubber particles might affect the thermal transferring, and on the other, the added solid rubbers could entrap more air because of the rough surface of solid rubber particles[36]. The increased rubber content may result in a higher content of air voids in the mixture than the neat epoxy. Thus, the connections for transferring heat could be interrupted with more rubber particles. The reduced thermal conductivity of solid rubber modified epoxy concrete could accomplish a better thermal insulation performance than the control epoxy concrete. The overlay of concrete bridge decks established with solid rubber modified epoxy concrete could get a better resistance for the variation of temperatures, which will reduce the rate of temperature change in the substrate concrete structures, and thus minimize the possibility of developing thermal cracks and other temperature related distresses.

6.6. Thermal compatibility

In this investigation, the thermal compatibility behavior of the two types of epoxy overlay systems were evaluated after five freeze-thaw cycles. The specimens were evaluated after five freeze-thaw cycles are shown in Fig. 15. From the top view of two samples, no aggregate or epoxy resin delamination was found, the entire epoxy concrete coating layers kept the same condition as the original samples before subjecting to freeze-thaw cycles. Sim- ilarly, no cracks were imaged near the interface between the overlays and the substrate concrete blocks through the side view.

Therefore, both control and 5% rubber modified samples passed the thermal compatibility test as defined in the ASTM C884 standard[30]. As expected, after introducing 5% tire rubber particles,

the compatibility was not affected obviously. The added soft rubber particles could be easily deformed when the volume of epoxy concrete changed under different temperatures. The flexibility of epoxy concrete could be improved with the introduction of elastic tire rubbers. Therefore, the addition of solid rubber particles might help to improve the thermal compatibility between layers. As the result shown, the added rubber particles will not affect the thermal compatibility between the epoxy coating layer and the substrate concrete layer. The good thermal compatibility will prevent the interface bond from temperature-changing related damages and improve the durability of the structure.

6.7. Water absorption rate

The 24-hour water absorption rate for every specimen was calculated, each is listed inTable 3. Four specimens were prepared for each epoxy concrete sample type. Generally, the water absorption rate of epoxy polymer concrete is lower than 0.5%. In the case of the control specimens, only the sample 3-C showed a weight change after emerging from under water with a very low water absorption rate of 0.146%. After introducing the rubbers, the water absorption rate still maintained in a favorable level. The maximum water absorption rate of the sample 1-R5 of 5% solid rubber modified group was only 0.232%. With 10% rubber particles, the water absorption rate was still very comparable and similar to those of the control and 5% rubber specimens. The results are reasonable based on the two aspects. On the one hand, the hydrophobicity of solid rubber particles[37]: the introduced rubber will not absorb any water. On the other hand, when the fine rubber particles were introduced, some air might store in the rubbers, even if the reac- tion between the hardener and the epoxy resins generated heat and could accelerate the releasing of the air, the air content could still be higher than control samples. However, due to the consistent structure and hydrophobic properties of hardened epoxy resins, the possibility of establishing connected pours structures in the polymer resins matrix is low[38], which is very different from the conventional Portland cement concrete. Therefore, the solid tire rubber modified epoxy concrete still had an excellent waterproofing ability. The presented results are also related to the thermal conductivity of solid rubber modified epoxy concrete samples in the way that the thermal conductivity was slightly decreased by the addition of solid rubber particles in the specimens since the relatively high air voids content that caused by the addition of fine rubber particles.

6.8. Statistical analysis of mechanical properties of solid rubber epoxy concrete

In order to evaluate the statistically significant on the strength properties of solid rubber modified epoxy polymer concrete, the one-way ANOVA test was performed among the test data that obtained from three different rubber contents (0%, 5%, and 10%) [39,40]. Large F value indicates that the variation of the rubber Table 2

The test results of internal bonding behavior.

Area (mm²) Peak Load (kg) Fracture Mode Bond strength (psi) Tensile strength (psi)

Control

#1 1924.4 669 Bond failure at interface 494.3 N/A

#2 1813.3 570 Failure in substrate N/A 446.9

#3 1915.9 399 Bond failure at interface 296.1 N/A

With 5% rubber

0.225 0.214 0.207

0.663

0.645

0.629

Control With 5% rubber With 10% rubber 0.15

0.30 0.45 0.60 0.75

Thermal conductivity (W/m*K)

Specimen types

Epoxy binder Epoxy concrete

Fig. 14.The thermal conductivity test results.

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Fig. 15.Observation of specimens after five freeze-thaw cycles.

Table 3

The test results of water absorption rate.

Sample types Original weight (g) After 24 hr weight (g) Weight change (g) Water absorption rate (24 hr) Control

1-C 57.0 57.0 0 0%

2-C 51.2 51.2 0 0%

3-C 68.4 68.5 0.1 0.146%

4-C 63.9 63.9 0 0%

With 5% rubber

1-R5 86.1 86.3 0.2 0.232%

2-R5 74.4 74.5 0.1 0.134%

3-R5 87.0 87.0 0 0%

4-R5 73.6 73.7 0.1 0.135%

With 10% rubber

1-R10 60.5 60.5 0 0%

2-R10 59.5 59.6 0.1 0.168%

3-R10 60.0 60.1 0.1 0.167%

4-R10 61.5 61.5 0 0%

Table 4

ANOVA results for compressive strength (a^{= 0.05).}

Source of Variation SS(Sum of squares) df(Degree of freedom) MS(Mean square) F P-value F-critical

Between Groups 447,037.6 2 223,518.8 15.53181 0.001207 4.256495

Within Groups 129,519.3 9 14,391.04

Total 576,557 11

Table 5

ANOVA results for splitting tensile strength (a^{= 0.05).}

Source of Variation SS(Sum of squares) df(Degree of freedom) MS(Mean square) F P-value F-critical

Between Groups 1,886,707 2 943,353.3 10.91935 0.003919 4.256495

Within Groups 777,535.5 9 86,392.83

Total 2,664,242 11

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contents makes significant on the properties of epoxy concrete. The

a

level that selected in this investigation is 0.05, which represents 95% level of confidence.

The ANOVA results are summarized inTables 4 and 5, which correspond with the compressive strength and splitting tensile strength, respectively. FromTable 4, it is obvious that theFvalue is much greater than the F_critical value for the

a

level selected (0.05). Meanwhile, theP_valueis smaller than

a

= 0.05. It could be concluded that the application of solid rubber particles has significant effect on the compressive strength of ordinary epoxy polymer concrete. Thus, the 9% increase in the compressive strength after adding 5% rubbers has 95% level of confidence.

Regards to the splitting tensile strength, as shown in theTable 5, theFvalue is also larger than theFcriticalvalue, which could be the evidence to support that the splitting tensile was affected after introducing solid rubbers. In addition, theP_valueis also smaller than

a

= 0.05. The 8% enhancement in the splitting tensile strength after adding 5% solid rubber particles has 95% level of confidence.

7. Conclusions

This study evaluated the mechanical and thermal properties of rubber modified epoxy concrete with lab tests. The performance of rubber-modified epoxy concrete samples were compared with control specimens. The SEM morphology was also captured to investigate the rubber effect on the interface microstructure. Sev- eral conclusions can be addressed as follows:

1. The microstructure observations through SEM images showed that the good bond behaviors between rubber particle and epoxy resin. But there are also some gaps existed due to added rubber particles.

2. The added rubber particles can improve the mechanical performance of the epoxy concrete materials, especially on compressive strength and splitting tensile strength. Specifically, the compressive strength and the splitting tensile strength of epoxy composite specimens with 5% rubber particles were increased approximately 9% and 8% when compared with that of control specimens at 95% level of confidence, respectively. Further- more, the performance of the samples with 10% rubber particles is worse than that of specimen with 5% rubber particles. The added rubber will slightly decrease the direct tensile strength of epoxy resin and the declination for samples with 5% rubber particles is within 5%.

3. The added rubber particles has transformed the fracture mode from ‘‘Bond failure at the interface” to ‘‘Bond failure in the substrate concrete”. The transformation demonstrates the bonding strength between solid rubber modified epoxy overlay and concrete substrate is stronger than the direct tensile strength of the concrete.

4. The added rubber particles will decrease the thermal conductivity and have no obvious effect on the thermal compatibility. The thermal conductivity of the epoxy concrete with 5% and 10%

rubber particles was decreased from 0.663 W/(m*k) to 0.645 and 0.629 W/(m*k), respectively. The rubber modified epoxy overlay system can also pass the thermal compatibility test rec- ommended by ASTM C884 standard. Additionally, both control and rubber modified specimens showed very low water absorption rate (<0.5%). The thermal- and moisture-related durability can be guaranteed with solid rubber modified epoxy concrete overlay system.

In summary, the recycling of solid rubber tires into epoxy concrete overlay or repairing systems can not only improve the mechanical and thermal properties, but also help to reduce the

environmental impact related to the landfilling of waste tire rubbers. The service life of existing concrete bridge structures could be extended with application of solid rubber modified epoxy concrete overlay, followed by reduced maintenance cost for concrete bridges. In the future study, the feasible methods for removing the entrapped air by rubbers will be developed, and the related properties will also be studied to furtherly improve the performance of solid rubber modified epoxy concrete overlay system.

Conflicts of interest

None declared.

Acknowledgement

The authors would like to appreciate Dr. Stanley J. Vitton, a faculty in Department of Civil and Environmental Engineering at Michigan Technological University for the thermal conductivity equipment support. The authors also like to thank Dr. Xingfeng Xie, a faculty in School of Forest Resources and Environmental Science at Michigan Technological University for the interface bond strength testing equipment support.

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