Strength reduction factors for structural rubbercrete

Abstract

Many researches have been carried out to study the fresh and hardened properties of concrete containing crumb rubber as replacement to fine aggregate by volume, yet there is no specific guideline has been developed on the mix design of the rubbercrete. The experimental program, which has been developed and reported in this paper, is designed and executed to provide such mix design guidelines. A total of 45 concrete mixes with three different water to cement ratio (0.41, 0.57 and 0.68) were cast and tested for fresh and mechanical properties of rubbercrete such as slump, air content, unit weight, compressive strength, flexural strength, splitting tensile strength and modulus of elasticity. Influence of mix design parameters such as percentage of crumb rubber replacement, cement content, water content, fine aggregate content, and coarse aggregate content were investigated. Three levels of slump value (for conventional concrete mixes) has been selected; low, medium and high slump. In each slump level, water content was kept constant. Equations for the reduction factors (RFs) for compressive strength, flexural strength, splitting tensile strength and modulus of elasticity have been developed. These RFs can be used to design rubbercrete mixes based on the conventional mix (0% crumb rubber content)

Keywords

crumb rubber / recycled tire / mix design / reduction factor / strength / modulus elasticity

Introduction

Waste tires continue to accumulate at increasing rates and their accumulation is causing a serious disposal problem. If not managed properly, the waste tires will lead to increasing environmental problems. Therefore, utilization of the crumb rubber from this scrap tires as sustainable building materials in the construction industry helps preserve the natural resources and also helps maintain the ecological balance. Previous research studies conducted on the properties of fresh and hardened concrete containing crumb rubber (rubbercrete), led to the following conclusions. Due to the low specific gravity of crumb rubber particles; the unit weight of the rubbercrete decreases as the percentage of crumb rubber replacement increases [1–5]. The non polarity of crumb rubber causes water to be repelled and the air to consequently be trapped on the surface; the air content in the rubbercrete increases as the rubber content increases [1,6,7]. The slump value of conventional concrete can be improved by replacing part of the fine aggregate with crumb rubber [1,2,8–13]. The compressive and flexural strength decrease as crumb rubber content increases [1,8,14–21]. Since the modulus elasticity (ME) of concrete depends on the modulus elasticity of the aggregates and their volumetric proportion in the matrix; the ME of rubbercrete decreases as the crumb rubber content increases [11,13,22,23]. Rubbercrete does not exhibit brittle failure under compression or splitting tensile forces and it exhibits high capacity for absorbing plastic energy under both compression and tension loading. Also the rubbercrete possesses higher toughness with most of the total energy generated being plastic [11–13,16,19,24–27]. The rubbercrete still can be used in the mild environment, whereas the addition of rubber to concrete will not dramatically affect the durability of concrete [28,29].

Research significance

Although rubbercrete is being used for variety construction applications [7,10,18,30,31], clear design guideline of rubbercrete mixes have not been developed yet.

The main objective of this paper is to assess mix design and mechanical properties to develop models for the determination of the strength and modulus elasticity reduction factors for rubbercrete containing crumb rubber up to 30% as partial replacement of fine aggregate by volume.

Experimental program

Investigation parameters

For the purpose of the current investigation the following mix design parameters have been examined: percentage of crumb rubber replacement, cement content, water content, fine aggregate content, and coarse aggregate content. Three levels of slump value (for conventional concrete mixes) has been selected; low, medium and high slump. In each slump level; water content was kept constant, while three different water to cement ratio (w/c) were selected.

Materials and mix proportions

The materials used in the concrete mixes were ordinary Portland cement (ASTM Type 1), river sand, mineral coarse aggregate with maximum size of 10 mm, maximum 30 mesh size of crumb rubber and potable water. The river sand properties namely specific gravity, water absorption and fineness modulus were 2.23, 2.5% and 2.63, respectively. The crumb rubber had the specific gravity of 0.54 and the fineness modulus of 2.36. The sieve analysis of the crumb rubber was carried out according to Florida Method of test for testing of ground tire rubber, (FM 5–559 [32]), whereas for fine and coarse aggregates, the sieve analysis was carried out in accordance with the requirement of ASTM C136 [33]. The particle size distributions of fine aggregate, coarse aggregate and crumb rubber from sieve analysis are shown in Fig. 1.

Mix proportions

Concrete mix proportions used in this study are shown in Table 1. Control concrete mixes (0% of crumb rubber content) have been designed according to ACI 211.1–91. For control mixes; each water to cement ratio has been selected (through trial mixes) to achieve a 28-days compressive strength target of 40, 30 and 20 MPa for water cement ratios of 0.41, 0.57 and 0.68, respectively. As indicated in Table 1 mixes 1–15 have lower slump (25–50 mm), mixes 16–30 have medium slump (75–100 mm) and mixes 31–45 have higher slump (150–175 mm).

Preparation and casting of specimens

All mixes were tested for fresh and hardened properties. Fresh property tests included slump test, unit weight and air content while the hardened property tests included compressive strength, flexural strength, splitting tensile strength and modulus elasticity at the age of 28 day. All specimens were demolded 24 h after casting and then cured in proper curing tank until the day of testing. Table 2 shows the specimen size and the Standards which had been followed for testing.

Results and discussions

Fresh properties

Workability, unit weight and air content of the rubbercrete mixes were investigated. The results are presented in the following sections.

Slump

The slump tests have been carried out for all mixes and the results are represented in Fig. 2. The results revealed that the increase in crumb rubber content in the mix resulted in increasing of slump of the mixtures. Generally, it can be concluded that concrete containing crumb rubber produced a workable mix in terms of ease of handling, placement and finishing in comparison with the control mix. This was due to tendency of the crumb rubber which repels water and entraps air into its surface. This water will contribute to enhancing the workability of rubbercrete in comparison to conventional concrete mix. The introduction of higher content of crumb rubber actually enhances the workability of concrete.

Unit weight and air content

The results show that the unit weight of concrete decreases and the air content increases with the increase of the crumb rubber content for all mixes. The phenomenon of reduction in the unit weight in crumb rubber concrete can be explained by the flocculation of the crumb rubber particles during the mixing of concrete where higher rubber content creates large voids and lead to higher porosity and due to lower density of crumb rubber compared to the fine aggregate. This can be seen from the relationship between unit weight and air content of the concrete presented in Figs. 3–5. There is a reduction in unit weight with the increase of crumb rubber content due to increase of air content in the mixes. The best fit lines representing the relationships between unit weight and air content, respectively with crumb rubber content can be expressed generally in the following regression models (Figs. 3–5):

(1)U_{\mathrm{w}}=-C_{\mathrm{1}}R+C_{\mathrm{2}},

(2)A_{\mathrm{c}}=C_{\mathrm{3}}R+C_{\mathrm{4}},

where U_w = unit weight of fresh concrete; A_c = air content of fresh concrete; R = the percentage of crumb rubber replacement; and C₁, C₂, C₃, C₄ = constants.

Mechanical Properties

The results of the strength are reported in Table 3 as an average of 3 specimens. The strength (compressive, flexural and splitting) and modulus elasticity are found to decrease with the increase of crumb rubber content. For the same amount of crumb rubber replacement, the strengths are found to decrease with the increase of water cement ratio. The main reason of strength reduction was due to the macroporosity of the rubbercrete. The higher air content in the rubbercrete mixtures occurred because crumb rubber particles have a tendency to repel water. Therefore, when fine aggregate was replaced by crumb rubber particles, the crumb rubber attracts air. On the other hand, the air content inside the rubbercrete depends on the internal pressure inside the mixture. Therefore, the macroporosity (air content and large pores), which describes the hydrophobic nature of rubber to entrap air, helps to increase the air content when the fine aggregate is replaced with crumb rubber particles.

Reduction factors (RFs) for mechanical properties

The correlation between the reductions factors (RFs) with respect to the percentage of crumb rubber replacement have been made through regression analysis to simulate the reduction in hardened rubbercrete properties. RF can be defined as ratio between rubbercrete property (compressive strength, flexural strength, splitting tensile strength and modulus elasticity) to the property of the conventional (control) concrete. The general mathematical model adopted in this investigation has been suggested by Khatib and Bayomy [1] and is presented as Eq. (3).

(3)RF=a+b{\left(I-R\right)}^{\mathrm{m}},

where RF = reduction factor; R = rubber content; volumetric ratio by total aggregate; a,b and m = functional parameters; for conventional concrete mix (0% crumb rubber content) the condition a + b = 1 holds.

However, assumptions have been made by Khatib and Bayomy [1] that all mixes must have the same constituents and volumetric content. In the following sections; the same mathematical model has been examined and statistically tested for mixes with different w/c for every level of slump (low, medium and high). In view of the fact that the modulus elasticity is an important concrete property; the suitability of the suggested model has been extended to include the reduction in the modulus elasticity of rubbercrete.

Reduction factor in compressive strength (CRF)

Figure 6 graphically illustrates the reduction in compressive strength for concrete containing crumb rubber as partial replacement to fine aggregate. The developed CRF models for low, medium and high slump mixes are shown as following:

(4)CR{F}_{\mathrm{l}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r}}=\mathrm{0.422}+\mathrm{0.605}{\left(\mathrm{1}-R\right)}^{\mathrm{15}},

(5)CR{F}_{\mathrm{m}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{u}\mathrm{m}}=\mathrm{0.327}+\mathrm{0.691}{\left(\mathrm{1}-R\right)}^{\mathrm{14}},

(6)CR{F}_{\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}}=\mathrm{0.379}+\mathrm{0.619}{\left(\mathrm{1}-R\right)}^{\mathrm{18}}.

It can be seen that an increase in crumb rubber content from 0% to 30% maintained a polynomial relationship for all level of slump mixes. The R² for lower medium and high levels are 0.98, 0.97 and 0.97, respectively.

Reduction factor in flexural strength (FRF)

Figure 7 illustrates the reduction in flexural strength for all level of mixes. The developed FRF models for low, medium and high slump are shown in the following equations:

(7)FR{F}_{\mathrm{l}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r}}=\mathrm{0.473}+\mathrm{0.550}{\left(\mathrm{1}-R\right)}^{\mathrm{16}},

(8)FR{F}_{\mathrm{m}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{u}\mathrm{m}}=\mathrm{0.343}+\mathrm{0.665}{\left(\mathrm{1}-R\right)}^{\mathrm{15}},

(9)FR{F}_{\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}}=\mathrm{0.402}+\mathrm{0.589}{\left(\mathrm{1}-R\right)}^{\mathrm{18}}.

Figure 3 also demonstrated the same trend of polynomial relationship for all level of slump mixes. The R² for lower medium and high levels are 0.78, 0.97 and 0.97, respectively.

Reduction factor in splitting tensile strength (SRF)

The polynomial relationships between strength reduction factors in splitting tensile strength with crumb rubber content for all level of slump mixes are shown in Fig. 8. The R² for lower medium and high levels are 0.91, 0.95 and 0.97, respectively. The developed SRF models for splitting tensile strength for all level of slump mixes are given below as:

(10)STR{F}_{\mathrm{l}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r}}=\mathrm{0.377}+\mathrm{0.626}{\left(\mathrm{1}-R\right)}^{\mathrm{8}},

(11)STR{F}_{\mathrm{m}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{u}\mathrm{m}}=\mathrm{0.502}+\mathrm{0.521}{\left(\mathrm{1}-R\right)}^{\mathrm{12}},

(12)STR{F}_{\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}}=\mathrm{0.508}+\mathrm{0.514}{\left(\mathrm{1}-R\right)}^{\mathrm{18}}.

Reduction factor in modulus elasticity (ERF)

The relationships between reduction factors for modulus elasticity with crumb rubber content are shown in Fig. 9. The R² for lower medium and high levels are 0.96, 0.98 and 0.97, respectively. The developed ERF models for modulus elasticity for all level of slump mixes are given below:

(13)ER{F}_{\mathrm{l}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r}}=\mathrm{0.236}+\mathrm{0.790}{\left(\mathrm{1}-R\right)}^{\mathrm{15}},

(14)ER{F}_{\mathrm{m}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{u}\mathrm{m}}=\mathrm{0.218}+\mathrm{0.775}{\left(\mathrm{1}-R\right)}^{\mathrm{10}},

(15)ER{F}_{\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}}=\mathrm{0.248}+\mathrm{0.720}{\left(\mathrm{1}-R\right)}^{\mathrm{18}}.

From Figs. 6–9, the results indicated that the reduction factor for compressive strength, flexural strength, splitting tensile strength and modulus elasticity for all level of slump mixes increase with increasing of crumb rubber replacement from 0% to 30%. The parameters of the proposed models and the statistical test results for all level of slump mixes are shown in Table 4. From the regression analysis, it is found that the RF_s for all the concrete properties showed the highest value of R² and adjusted R². This implies that the proposed models, (Eq. (4)–(15)) can fit the obtained data perfectly. The exponent m reflect the downward curvature degree, therefore, it represents the sensitivity degree of the mix to the loss in strength due to the incorporation of crumb rubber as partial replacement of the fine aggregate by volume. It was noted that the m values for concrete properties in concrete with high slump values were higher compared to other mixes. This indicated that this mix is more sensitive to loss of strength than other mixes due to the higher ratio of crumb rubber volume to the total volume of aggregate.

Conclusions

The following conclusions have been drawn from this research:

1) Concrete containing crumb rubber (rubbercrete) achieves a workable mix in terms of ease of handling, placement and finishing in comparison to conventional concrete mix.

2) The unit weight of the rubbercerete decreases as the crumb rubber content increases due to increase of the air content inside the mix and lower density of crumb rubber.

3) The tendency of the crumb rubber to repel water and entrap air leads to reduction in strength of the rubbercrete compared to the conventional concrete mix.

4) Models of reduction factors (RFs) for compressive strength, flexural strength, splitting tensile strength and modulus elasticity for concrete (rubbercrete) as replacement to fine aggregate by volume were developed and statistically tested. Rubbercrete mixes with higher volume of crumb rubber to the volume of aggregatep are more sensitive to the loss of strength.

5) These RFs can be used to design rubbercrete mixes based on the properties of conventional concrete (0% content of crumb rubber). For example; if rubbercrete with compressive strength of 25 MPa is required then the compressive strength of the control mix (before adding crumb rubber) should be; 25/CRF (RF for compressive strength) considering the desired slump level and the percentage of crumb rubber content replacement.

References

[1] Khatib Z K, Bayomy FM. Rubberized portland cement concrete. Journal of Materials in Civil Engineering, 1999, 11(3): 206–213

[2] Batayneh M K, Marie I, Asi I. Promoting the use of crumb rubber concrete in developing countries. Journal of Waste management, 2008, 28: 2171–2176

[3] Güneyisi E, Gesoğlu M, ÖzturanT. Properties of rubberized concretes containing silica fume. Journal of Cement and Concrete Research, 2004, 34(12): 2309–2317

[4] Sukontasukkul P, Chaikaew C. Properties of concrete pedestrian block mixed with crumb rubber. Journal of Construction and Building Material, 2006, 20(7): 450–457

[5] Khatib Z K, Bayomy F M. Rubberized Portland Cement Concrete. Journal of Materials in Civil Engineering, 1999, 11(3): 206–213

[6] Siddique R, Naik T R. Properties of concrete containing scrap tire rubber– an overview. Journal of Waste Management, 2004, 24(6): 563–569

[7] Turatsinze A, Garros M. On the modulus of elasticity and strain capacity of self compacting concrete incorporating rubber aggregate. Journal of Resources Conservation and Recycling, 2008, 52(10): 1209–1215

[8] Toutanji H A. The use of rubber tire particles in concrete to replace mineral aggregates. Journal of Cement & Concrete Composites, 1996, 18(2): 135–139

[9] Raghavan D, Huynh H, Raghavan D. Workability, mechanical properties and chemical stability of a recycled tyre rubber-filled cementitious composite. Journal of Materials Science, 1998, 33(7): 1745–1752

[10] Bignozzi M C, Sandrolini F. Tyre rubber waste recycling in self-compacting concrete. Journal of Cement and Concrete Research, 2006, 36(4): 735–739

[11] Khaloo A R, Dehestani M, Rahmatabadi P. Mechanical properties of concrete containing a high volume of tire-rubber particles. Journal of Waste Management, 2008, 28(12): 2472–2482

[12] Mohammed B S, Azmi N J, Abdullahi M. Evaluation of rubbercrete based on ultrasonic pulse velocity and rebound hammer tests. Construction and Building Materials Journal, 2011, 25(3): 1388–1397

[13] Mohammed B S, Azmi N J. Failure mode and modulus elasticity of concrete containing recycled tire rubber. Journal of Solid Waste Technology Management, 2011, 37(1): 16–24

[14] Mohammed B S, Anwar Hossain K M, Eng Swee J T, Wong G, Abdullahi M. Properties of crumb rubber hollow concrete block. Journal of Cleaner Production, 2012, 23(1): 57–67

[15] Fattuhi N I, Clark L A. Cement based materials containing shredded scrap truck tire rubber. Journal of Construction and Building Materials, 1996, 10(4): 229–236

[16] Kaloush E K, Way G B, Zhu H. Properties of crumb rubber concrete. Journal of the Transportation Research Board, 2005, 1914(1): 8–14

[17] Al-Akhras N M, Smadi M M. Properties of tire rubber ash mortar. Journal of Cement and Concrete Composites, 2004, 26(7): 821–826

[18] Turgut P, Yesilata B. Physico mechanical and thermal performance of newly developed rubber added bricks. Journal of Energy and Building, 2008, 40(5): 679–688

[19] Eldin N, Senouci A B. Rubber-tire particles as concrete aggregate. Journal of Materials in Civil Engineering, 1993, 5(4): 478–496

[20] Ganjian E, Khorami M, Maghsoudi A A. Scrap tyre rubber replacement for aggregate and filler in concrete. Journal of Construction and Building Materials, 2009, 23(5): 1828–1836

[21] Segre N, Joekes I. Use of tire rubber particles as addition to cement paste. Journal of Cement and Concrete Research, 2000, 30(9): 1421–1425

[22] Zheng L, Sharon Huo X, Yuan Y. Experimental investigation on dynamic properties of rubberized concrete. Journal of Construction and Building Materials, 2008, 22(5): 939–947

[23] Hernández-Olivares F, Barluenga G. Fire performance of recycled rubber filled high strength concrete. Journal of Cement and Concrete Research, 2004, 34(1): 109–117

[24] Topçu I B. The properties of rubberized concrete. Journal of Cement and Concrete Research, 1995, 25(2): 304–310

[25] Papakonstantinou C G, Tobolski M J. Use of waste tire steel beads in portland cement concrete. Journal of Cement and Concrete Research, 2006, 36(9): 1686–1691

[26] Snelson D G, Kinuthia J M, Davies P A, Chang S R. davies, P.A., and Chang, S.R. Sustainable construction: composite use of tyres and ash in concrete. Journal of Waste Management, 2009, 29(1): 360–367

[27] Khaloo A R, Dehestani M, Rahmatabadi P. Mechanical properties of concrete containing a high volume of tire-rubber particles. Journal of Waste Management, 2008, 28(12): 2472–2482

[28] Ganjian E, Khorami M, Maghsoudi A A. Scrap-tyer-rubber replacement for aggregate and filler in concrete. Journal of Construction and Building Materials, 2009, 23(5): 1828–1836

[29] Huynh H, Raghavan D. Durability of simulated shredded rubber tire in highly alkaline environments. Journal of Advanced Cement Based Materials, 1997, 6(3–4): 138–143

[30] Sukontasukkul P. Use of crumb rubber to improve thermal and sound properties of pre-cast concrete panel. Journal of Construction and Building Materials, 2009, 23(2): 1084–1092

[31] Mohammed, B. S. Structural behavior and m–k value of composite slab utilizing concrete containing crumb rubber. Journal of Construction and Building materials, 2010, 24: 1214–1221

[32] Florida Method of Test for Testing of Ground Tire Rubber. 2004, FM 5–559

[33] American Society for Testing and Materials. Standard test method for sieve analysis of fine and coarse aggregates. ASTM C136. Annual Book of ASTM Standards, 1985

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