Mechanical properties of rubberized lightweight aggregate concrete

Construction and Building Materials 147 (2017) 264–271

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Mechanical properties of rubberized lightweight aggregate concrete Nathan M. Miller, Fariborz M. Tehrani ⇑ Department of Civil and Geomatics Engineering, California State University Fresno, CA 93740, United States

h i g h l i g h t s
Tire-derived aggregates (TDA) contribute to sustainability of concrete applications.
Mechanical properties of light-weight aggregate concrete with TDA are investigated.
Experimental studies include six mix designs containing 0–100% of TDA substituion.
The effect of TDA on compressive, tensile, flexural, and impact tests are reported.
Results provide insights on the toughness and ductility of TDLWAC.

a r t i c l e

i n f o

Article history: Received 11 August 2016 Received in revised form 13 April 2017 Accepted 16 April 2017

Keywords: Ductility Toughness Compression test Static modulus of elasticity Splitting tensile strength Flexural strength Lightweight aggregate Rubberized concrete Tire-derived aggregate

a b s t r a c t A detailed investigation of rubberized lightweight aggregate concrete was conducted using 38 cylindrical and 36 beam specimens. Six mix designs, incorporated in the study, contained rubber replacement ratios from 0% to 100% by volume replacement of a lightweight expanded-shale coarse aggregate. The objective of this study is to investigate mechanical properties of lightweight tire-derived aggregate concrete, including compressive strength, modulus of elasticity, splitting-tensile strength, flexural strength, and flexural toughness. Further, an impact test was conducted using a falling weight to investigate dynamic properties of specimens subjected to flexure. Results suggest tire-derived aggregates reduces the mechanical strength of specimens, but, enhances ductility and toughness of materials. These enhancements are valuable for dynamic applications of lightweight concrete. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Many researchers cited in this section have been intrigued by the concept of adding a flexible material such as rubber to a material that is typically known for its rigidity, such as concrete. The development of a concrete performing with ductile behavior has been the object of ambition for many researchers. Other motivations stem from the fact that if aggregates often used in rubberized concrete (tire derived aggregates) can be incorporated into the concrete matrix, there exists a potential to divert a significant amount of waste materials away from landfills. According to the Environmental Protection Agency (EPA), the United States alone generates 289 million scrap tires annually. Beyond the amount of waste alone, the EPA provides that stockpiled waste tires can pose significant health and safety hazards including rodent and mosquito habitation which can facilitate ⇑ Corresponding author. E-mail address: [email protected] (F.M. Tehrani). http://dx.doi.org/10.1016/j.conbuildmat.2017.04.155 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.

the spread of disease and an increased risk of fire [1]. More recently, the EPA published that leather and rubber accounted for 6.18 million tons of waste after the recycling rate of 44.6% had been accounted for [2]. Countless researches conducted since the early 1990s concern rubberized normal weight aggregate concrete. Although few have shown an increase in rubber content improves durability, compression strength has been observed to decrease as rubber content is increased [3–8]. Other common properties such as the static modulus of elasticity [9–11], splitting tensile strength [4,9,11], and static flexural strength [4,12,13] have also been found to decrease as rubber content increases. However, while the strength properties decrease, material toughness has been observed to increase [10,13,14] which research suggests may serve as one of the most beneficial properties of this material. Due to the fact that material solidity can be used as a measure of a materials ability to absorb energy, researchers suggest it may be best suited for dynamic loading conditions. Two studies were found using a falling weight impact [11,15], two studies were found investigating the

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Nomenclature Notation Area under load-deflection curve up to 10.5 times the A10.5FC first crack deflection, N-m (lbf-in.) A5.5FC Area under load-deflection curve up to 5.5 times the first crack deflection, N-m (lbf-in.) A3.0FC Area under load-deflection curve up to 3.0 times the first crack deflection, N-m (lbf-in.) AFC Area under the load-deflection curve up to first crack deflection, N-m (lbf-in.) b Width of beam specimen, mm (in.) d Diameter of cylindrical specimen, mm (in.) d1 Depth of beam specimen, mm (in.) E Static modulus of elasticity, GPa (ksi) g Gravitational constant, m/sec2 (in./sec2) h Drop height used for impact testing, mm (in.) I20 Toughness index up to 10.5 times first crack deflection I10 Toughness index up to 5.5 times first crack deflection I5 Toughness index up to 3.0 times first crack deflection

free vibration using an impulse hammer [8,16], and a single study was found investigating the behavior of a full scale traffic barrier subject to a non-severe collision impact [6]. Few researches analyzing the properties of rubberized lightweight aggregate concrete created using rubber aggregates as replacement for lightweight mineral aggregates have been found [17,18]. By studying six mechanical properties that are of common interest for concrete, the investigation that follows was conducted to further the understanding of this material that has been researched by few. 2. Research significance Presence of lightweight aggregate has the potential to alter the mechanical properties of rubberized concrete. The substitution of natural normal-weight aggregates with lightweight aggregates, such as expanded shale, has the potential to expand applications of rubberized concrete. This study is directed towards the advancement of existing literature on mechanical properties of rubberized lightweight-aggregate concrete including: compressive, splittingtensile, flexural strength, flexural toughness and impact resistance. 3. Experimental procedure Cylinder and beam specimens were cast containing various amount of crumb rubber, tire-derived aggregate (TDA), by volume replacement of the coarse lightweight aggregate (LWA). The constituents in the mix included the expanded shale lightweight coarse aggregate, natural sand fine aggregate, cement, and water. The target strength for the control mix was 21 MPa (3 ksi). The TDA was then added by volume replacement of the lightweight coarse aggregate. Replacement ratios of 0% to 100% in 20% increments were used in the investigation for both cylinder and beam specimens. Cylinders were used in testing compressive strength, static modulus of elasticity, and splitting-tensile strength. Beam specimens were used to examine flexural strength, toughness, and response to an impact flexure test. 3.1. Materials The mix constituents included lightweight coarse aggregates, fine aggregates, cement, and water. Tire derived aggregates were later added by volume replacement of the lightweight coarse

L1 L2 m P R10,20 R5,10 T d1

e1 e2 r1 r2 x

Cylinder length, mm (in.) Specimen clear span, mm (in.) Mass of falling weight, kg (lb) Peak applied load, kN (lbf) Residual strength factor 2 Residual strength factor 1 Splitting-tensile strength, MPa (psi) Deflection corresponding to peak static load, mm (in.) Lower bound of strain used for the calculation of the modulus of elasticity Upper bound of strain used for the calculation of the modulus of elasticity Lower bound of normal stress used to calculate the modulus of elasticity, MPa (psi) Upper bound of normal stress used to calculate the modulus of elasticity, MPa (psi) Observed static load for static flexure test, kN (lbf)

aggregate. The coarse aggregate used in the procedure consisted of expanded shale produced by Utelite Corporation, which is classified to be their structural medium grade. These materials have unit dry weight of nearly 750 kg/m3 (46.8 pcf) and water content of 7.3%. Table 1 provides the gradation report for the expanded shale as published by the manufacturer. Natural sand as well as type I and type II cement blend were applied. The cement blend was used due to its availability, with no research suggesting this would adversely affect the rubberized concrete specimens. Tap water was incorporated in the procedure for all concrete specimens. The TDA, provided by West Coast Rubber Recycling located in Hollister, California, was produced using mechanical shredding and of comparable size to the mineral aggregate. The source of these materials is a combination of car and truck tires. The steel fibers were removed from the rubber during the manufacturing process; however, textile fibers remained mixed within the rubber particles (see Fig. 1). The unit weight of TDA was nearly 560 kg/m3 (35.2 pcf). Table 1 provides the sieve analysis for the material. No additional mixtures were used in the designs, and no pretreatment of the rubber was conducted prior to incorporating it into the mix. Throughout the investigation, all mix design quantities were held constant with the exception of the lightweight coarse aggregate and the tire derived aggregates. Fig. 2 shows all six mix designs used. These values have been adjusted for water absorption of materials, when applicable. 3.2. Specimens Both cylinder specimens, 0.15 m (6 in.) diameter and 0.30 m (12 in.) height, and beam specimens 0.15 m (6 in.) square size and 0.53 m (21 in.) length, were used for testing, in accordance to ASTM C39 and C78. Plastic, single use concrete cylinder molds, were used to cast cylindrical specimens. For the beam specimens,

Table 1 Gradation report for lightweight expanded shale aggregate (LWA) and rubber particles (TDA). Sieve Size mm (in.)

LWA Retained (%)

TDA Retained (%)

12.7 (1/2) 9.5 (3/8) 4.75 (3/16) 2.36 (3/32) 1.18 (3/64)

0 5.66 72.8 20.89 0.35

0 0.38 77.82 20.73 0.38

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Fig. 1. Crumb rubber manufactured through mechanical shredding of recycled tires.

Fig. 2. Six mix designs used for current investigation.

a combination of plastic beam molds and wooden beam molds were used (see Fig. 3). A total of 36 beams and 38 cylinders were cast, which corresponds to three beams and three cylinders for each mix design for each test and two extra control cylinders. All specimens were cast on the same day and all molds were stripped 24 h after casting. After removal from the mold, the specimens were placed in the moist curing room with observed temperature of 23 °C (73 °F) and humidity of 95%. They remained there, undisturbed, for the remainder of the curing process. Specimens were removed from the curing room for test preparation 32 days after casting (see Fig. 3). Strain gages were attached to both cylinders used for compression testing and beams used for static flexure testing. Four strain gages were used for each specimen. For cylindrical specimens, the strain gages were attached at mid-height and at 90 degree intervals around the cylinder. For beam specimens, the strain gages were attached at mid-span. Two were attached 0.013 m (0.5 in.) from the extreme compression edge, and two were attached 0.013 m (0.5 in.) from the extreme tension edge. 3.3. Items of investigation

the age of the concrete was more than the standard 28-day, the relative comparison between results of various mix designs was not expected to be impacted substantially. The test was carried out using a 500 kN (120 kip) Tinius Olsen manually operated universal testing machine. Cylinders were placed between a rigid bottom bearing block and a spherically mounted top bearing block, in accordance with ASTM standard C39 (see Fig. 4). The compression load was applied at a rate slower than the ASTM C39 rate, 0.24 MPa (35 psi) per second. This was conducted to allow adequate reporting of the strain gages, which record a single data point every second. The load-deformation relationship was recorded directly by the universal testing machine and exported for the calculations of compression strength and static modulus of elasticity. The static modulus of elasticity was determined using ASTM C469 and is defined by the slope of the linear elastic portion of the stress-strain relationship. The stress-strain relationship was calculated using the load-deflection data recorded during each test. In accordance with ASTM C469, the peak stress and strain for the calculation was taken at 40% of the first peak prior to the start of cracking. The lower bound for the stress-strain relationship was taken as the first point when the stress-strain relationship became recognizable linear. This did not occur at the start of the test due to the testing equipment, but did occur shortly after the start of each test. For both the compression strength and static modulus of elasticity, the results were gathered and averaged for each rubber replacement ratio. The splitting-tension strength was also tested 35 days after the specimens were cast, in accordance with ASTM C496. The test was conducted using the same universal testing machine as the compression test, and the load was applied at a rate of 48.9 kN (11 kip) per second. This is the lower end of the acceptable load rates per the ASTM standard. The test called for a 25.4 mm (1 in.) strip of plywood to be used as a bearing strip at the bottom and top of each specimen, and the strips are to be replaced with each test. Due to the availability of materials, a 2.54 mm (1 in.) wide steel was used in lieu of the plywood strips. After observing the failure modes for each specimen, it was determined that the

Compression testing occurred on day 35 after casting, due to unpredicted delays, using the procedures of ASTM C39. Although,

Spherical bearing at plate

Strain gage attachment

Rubber plate to distribute load over uneven surface

Fig. 3. Specimens after removal from curing room.

Fig. 4. Compression test set up.

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replacement was acceptable (see Fig. 5). The load-deflection relationship, recorded by the universal testing machine, was used to determine the splitting tensile strength of each specimen. The results were grouped, averaged, and compared to the common relationships which relate compressive strength to splittingtensile strength. The third test conducted was a static flexure test taking place on day 36 after specimens were cast. This test was carried out in accordance with ASTM C78 for simple beams with third-point loading. The beams had a simple span of 0.456 m (18 in.) with load points at 0.15 m (6 in.) and 0.30 m (12 in.). Fig. 6 displays the test set up of the static flexure test. Loads were applied at a rate of 1 mm (0.039 in) per minute. The load rate was kept at a minimum in order to allow adequate data acquisition from strain gages. The data was then used to calculate the modulus of rupture in accordance with ASTM C78 and the flexural toughness in accordance with ASTM C1018-97. The final test conducted was the impact flexure test. The specimens were subject to third point loading with the span lengths as a static flexure test. However, the load was applied using an 1110N (250-lbf) falling weight as opposed to the universal testing machine. The weight was lifted to the desired height using an electronic winch and released using a quick release pin to impact the specimen. The drop heights were predicted for each rubber replacement ratio using minimum strengths and corresponding deflections observed during the static tests. The static strength and deflection was then used to determine a corresponding drop height using a simplified energy Eq. (6) allowing suitable drop heights to be approximated. The maximum drop height was then rounded in order to be easily measured using a tape measure,

Steel bearing strips

Impact plates

Accelerometers

Ears used to stabilize impact head

Impact head to provide loading at third points

Apparatus anchored to strong floor to reduce rebound and vibration

Edge supports

Fig. 7. Impact flexure test set up.

and divided into thirds. Three different drop heights were used for each rubber replacement ratio. The first three drops occurred at a height that was one-third that of the calculated maximum, and the next three drops occurred at a height of two-thirds of the maximum. The final three drops occurred at the predicted maximum height. The instrumentation used for this test was a Sensr GPI-L triple axis programmable accelerometer that was mounted to the impacting plate. The accelerometer recorded the accelerationtime history for the impact event at a sampling rate of 100 points per second. This data was then used to calculate the impulse force generated by each impact. Fig. 7 shows the test set-up for the impact flexure test. 4. Analytical procedure

Fig. 5. Splitting-tension test set up.

Fig. 6. Static flexure test set up.

All calculations, with the exception of the impact flexure test, were carried out in accordance with the ASTM standards for each respective test. The compressive strength was calculated in accordance with ASTM C39. The same load-deformation relationship was also used to calculate the static modulus of elasticity. The stress-strain relationship was calculated from the elastic-linear load-deformation relationships. The stress-strain relationship could then be used to calculate the static modulus of elasticity in accordance with ASTM C469. The splitting-tension strength was calculated using ASTM C496. It is determined using the maximum applied load and the specimen’s length and diameter. The loaddeflection relationship from the static flexure test was used to calculate modulus of rupture which is a measure of the materials flexural strength. The calculation was carried out in accordance with ASTM C78. The same load-deflection relationship was also used to calculate the flexural toughness of each specimen. This was done in accordance with ASTM C1018-97, which provides equations for the calculation of three different toughness indices and two residual strength factors. The toughness indices are calculated using ratios of areas under the load-deflection diagram, and the residual strength factors were calculated from the toughness indices. The calculations were carried out in accordance with the ASTM standard using the following equations;

268

I5 ¼

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A3:0FC AFC

ð1Þ

I10 ¼

A5:5FC AFC

ð2Þ

I20 ¼

A10:5FC AFC

ð3Þ

R5;10 ¼ 20ðI10  I5 Þ

ð4Þ

R10;20 ¼ 10ðI20  I10 Þ

ð5Þ

The last test that was conducted was the impact flexure test which utilized a 1220 N (250 lbf) falling weight. The drop height was calculated using the minimum flexural strength and deflection that was observed during the static test. The following simplified energy equation was used to predict the necessary drop heights for each rubber replacement value;

xd1 ¼ mgh

ð6Þ

The drop heights used are provided in Table 2. The accelerationtime history was recorded and used to calculate the force-time relationship, and later integrated using the trapezoid approximation to determine the net impulse for each drop for each specimen. 5. Experimental results and discussions 5.1. Compressive strength and static modulus of static modulus of elasticity The compression strength of the rubberized lightweight aggregate concrete was observed to decrease as rubber content increased. The control mix reached a compressive strength of 23.4 MPa (3388 psi) which was beyond the target strength. As can be seen from Fig. 8, an inflection point occurs between 40% and 80% rubber replacement. This was found to be unique to rubberized lightweight aggregate concrete. Much of the previous research found for rubberized normal-weight aggregate concrete suggests a non-linear but consistent decrease in compression strength as rubber content increases. Research also suggests a boundary condition exists at 60% rubber replacement between a concrete controlled specimen and a rubber controlled specimen. It is believed that this quality is displayed in the rubberized lightweight aggregate concrete by the inflection point. The failure mode observed for the control mix was a type 3 failure as defined by ASTM C39 which is characterized by vertical cracks that propagated from top to bottom. The failure of control specimens were rapid with little warning prior to complete failure. As the rubber content increased, the failure of the specimens became more gradual. The failure mode of cylinders with high rubber contents were characterized by diagonal cracking that started at the top of the cylinder and propagated to the cylinder edges. It was observed that the cylinders with higher rubber contents exhibited greater transverse deformation prior to failure, and the

Table 2 Calculated drop heights for falling weight impact test. Rubber Content %

Static Load kN (lbf)

Deflection at Static Load mm (in.)

Equivalent Drop Height mm (in.)

0% 20% 40% 60% 80% 100%

27.71 (6229) 24.12 (5422) 20.00 (4496) 19.78 (4446) 14.56 (3273) 13.55 (3046)

1.9 2.3 1.9 1.7 1.9 2.1

48 50 34 29 24 25

(0.0762) (0.0915) (0.0753) (0.0653) (0.0727) (0.0810)

(1.899) (1.984) (1.354) (1.161) (0.952) (0.987)

Fig. 8. Relationship of compressive strength to rubber content (1 MPa = 145 psi).

rubber particles were able to hold the cylinders intact once the peak load was reached. This is displayed in Fig. 9. The collected from this test was also used to determine the static modulus of elasticity. It was determined that the stress-strain relationship of the control specimen did not fit the ASTM standard calculation which requires the 40% of the peak strength to be the upper bound used in calculation. As a result, the initial linearelastic relationship or the linear elastic relationship prior to the first noticeable crack was used for calculation. A similar inflection point was observed between 40% and 60% rubber replacement. Fig. 10 provides the relationship measured for static modulus of elasticity rubber content. 5.2. Splitting-tensile strength During the splitting-tensile test, all specimens exhibited the characteristic splitting-tensile failure mode characterized by a single crack developing down the center of the cylinder before failure (see Fig. 11). While a decrease in peak strength was observed, the failure was more gradual. Specimens still developed the same crack down the center of the cylinder; however, the time it took for the crack to propagate through the cylinder increased. The relationship between splitting-tensile strength and rubber content was also more linear than that of compression strength or static modulus of elasticity (see Fig. 12). Furthermore, it could also be seen that the rubber content was evenly distributed throughout the cylinders (see Fig. 13). 5.3. Static flexure strength and flexural toughness The flexural strength of all specimens was measured through the calculation of the modulus of rupture, later recognized to share a similar relationship with rubber content as other mechanical properties. The value of flexural strength decreased nonlinearly as the rubber replacement ratios varied from 0% to 100%. Once again, an inflection point was observed between 40% and 60% rubber replacement. Fig. 14 provides a display of the relationship that exists between modulus of rupture and rubber content. In addition, this test was used to calculate the flexural toughness in accordance with ASTM C1018-97 through the use of three toughness indices and two residual strength factors. For rubberized lightweight aggregate concrete, there was no evidence suggesting an increase in toughness at rubber replacement values less than 80%. However, flexural toughness was significantly increased at rubber replacement values of 80% and 100%. This differs from previous research found, which suggest a systematic increase in

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Fig. 9. Greater transverse deflection observed in rubberized concrete specimens with 100% rubber content (left) in comparison with conventional concrete (right).

Fig. 10. Relationship of modulus of elasticity to rubber content (1 MPa = 145 psi). Fig. 12. Relationship of Splitting-tensile strength to (1 MPa = 145 psi). 0% 20% 40% 60% 80% 100% Rubber contents.

rubber

content

The results are provided in Table 3, and the results for toughness index I5 are provided in Fig. 15. 5.4. Impact flexure test

Fig. 11. Comparison of splitting-tensile load-deformation curves for low and high rubber content samples.

flexural toughness as rubber content increases. Only the first index could be calculated for all of the rubber replacement values. The second and third toughness indices could only be calculated for a total of three specimens of 80% and 100% rubber. Out of all of the specimens tested, only one specimen with 100% rubber replacement had a post-peak behavior that allowed for the calculation of all three toughness indices and both residual strength factors.

The impact flexure test was conducted using the same thirdpoint loading as the static flexure test, and the load was applied using 1110 N (250 lbf). The acceleration-time history was collected using an accelerometer which allowed the force-time relationship and net impulse for each drop to be calculated. The maximum netimpulse prior to failure was isolated for each specimen of the rubber replacement values. The drop at failure was neglected due to the fact that the way the weight came to rest, skewed the acceleration-time history which in turn skewed the net impulse results. The maximum impulse calculated for each specimen was then grouped by rubber content and averaged. The results are provided in Table 4, which shows no noticeable trend between rubber content and the maximum net impulse calculated. This is likely due to the limitations of the test itself. One limitation of the impact test was the lack of stability in the test system itself. For the weight drop apparatus, the impacting plate is guided by three slender steel tubes using linear bearings. The lack of rigidity of the frame allowed for a significant amount of sway during testing. Without measuring the strain of the frame

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0%

20%

40 % 60% Rubber contents

80 %

100%

Fig. 13. Dispersion of particles within cementitious matrix.

Table 4 Impact flexure test results.

Fig. 14. Relationship of modulus of rupture to rubber content (1 MPa = 145 psi).

Table 3 Flexural toughness results. Rubber content (%)

Average I5

I10

I20

R5,10

R10,20

0 20 40 60 80 100

1.070 1.048 1.039 1.055 2.030 2.420

0 0 0 0 2.456 3.065

0 0 0 0 0 3.399

0 0 0 0 4.532 15.915

0 0 0 0 0 0.851

Rubber Content (%)

Max. Net Impulse N-sec (lbf-sec)

0 20 40 60 80 100

536 442 441 486 494 482

(121) (99) (99) (109) (111) (108)

50.8 mm (2 in) and error in measuring the drop height, significant errors occur when comparing the test results. Compounding these issues is the lack of data acquisition equipment available for the test. The only instrumentation used was the accelerometer, which limits the amount of analysis that can be carried out once testing is complete. Because deflection of the beam was not monitored, it is unknown whether the plate bounced or remained in contact with the beam specimen during the impact event, affecting the analysis drawn from the data. The final limitation of this test was due to the methodology itself. The drop heights varied over the rubber content specimens based on the static testing to ensure multiple drops for each specimen prior to failure. However, due to the fact that there is no direct comparison between rubber replacement values, the comparison of results become increasingly difficult. 6. Future research Further research of rubberized lightweight aggregate concrete is desirable as previous research found investigating the use of rubber particles as replacement for lightweight mineral aggregates. Previous research surrounding rubberized normal-weight aggregate concrete suggest that the material may be a suitable element for crash barriers, vibration mitigation, and other dynamic loading conditions. Due to the limitations of the dynamic testing, this study cannot definitively prove or disprove any of those claims for rubberized lightweight aggregate concrete. Thus, testing both small scale and full scale should be conducted regarding the practical application of this material. The increased toughness at high rubber content suggests that this material has a greater capacity for energy absorption than plain concrete, making it beneficial for applications where energy absorption over strength is a primary concern.

Fig. 15. Relationship of flexural toughness index I5 to rubber content.

itself, it is difficult to measure how the sway is affecting the impact test. Another shortcoming of the testing machine itself was the 1110 N (250 lbf) weight itself. With drop heights of less than

7. Conclusions From the results of this experimental study on the mechanical properties of rubberized lightweight aggregate concrete, the following conclusions can be drawn:

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The static mechanical properties decrease as rubber content increases, with an inflection point for static properties between 40% and 80% rubber content.
The flexural toughness increases at rubber replacement values of 80% and 100%, showing negligible change at values below 80%.
Results suggest the material to be suitable for applications where energy absorption is a primary concern at high rubber replacement values.

[6] [7] [8]

[9] [10]

Acknowledgments The authors wish to express their sincerest gratitude to the Fresno State Foundation for the financial support of this research, to Utelite Company for donating lightweight aggregates, and to West Coast Rubber Recycling for providing discounted materials.

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[12] [13] [14]

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