MECHANICAL PROPERTIES OF STRUCTURAL CONCRETE CONTAINING

RUBBER FROM SCRAP TYRES

Filipe Valério Sequeira Valadares

Extended Abstract

Civil Engineering

Coordinator

Prof. Dr. Jorge Manuel Caliço Lopes de Brito

November 2009

EXTENDED ABSTRACT

1

1. Introduction

The problem of finding a destination for scrap tyres has increased to such an extent that it cannot be ignored,

given the fact that in many countries the growing number of potential recycling routes has not matched the

increasing number of tyres that have reached the end of their useful life. Consequently, in many cases the

solution to this imbalance has been the creation of landfill sites for whole or ground tyres or simply illegal

dumping. The environmental consequences of this approach are evident in the event of fire, when large amounts

of pollutants are released into the air in the form of gases and particulate matter, and into the soil in the form of

liquids (oils). Thankfully, since 2006 European legislation has sought to put an end to this regrettable situation

through specific regulation.

Although nowadays there are countless sustainable destinations for scrap tyres, the civil construction sector has

been less ready to grasp the potential uses of this waste material. However, the market already offers some

promising possibilities: asphalt and sports paving, children’s play areas, highway crash barriers and breakwaters.

These applications show the value of new areas of research that enable us to understand the competitive uses

that can be made of scrap tyres within this sector. Within this context, the use of rubber granulate from scrap

tyres in the production of structural concrete has been studied as a useful alternative, from the perspective of

both environmental protection and the sustainability of natural reserves.

Although a relatively extensive bibliography is available nowadays on the subject, the lack of experience in

Portugal, combined with the fact that some areas have not been sufficiently clarified and weaknesses exist in

others, have led to this study. The aim is, therefore, to assess the use of rubber from scrap tyres in structural

concrete by analysing its properties, both in its fresh and hardened state. In the case of the former, workability

and bulk density were examined and, in the case of the latter, compressive strength, splitting tensile strength,

modulus of elasticity and abrasion resistance. The specimens tested varied according to the substitution

percentages (5%, 10% and 15% of the total volume of the aggregates), the size of the granulate used (fine, coarse

or both) and the processing methods (mechanical grinding or cryogenic processing). Thus, the aim was to

determine which out of the range of particle sizes produced the best results and whether the processing method

had any influence on the outcome.

There is no parallel in the literature available on the subject for the method used to incorporate the rubber into

the concrete. This is the likely cause of the innovative conclusions regarding the influence of rubber granulate size

on the performance of the concrete. In relation to abrasion resistance, only the results from CAIRNS (2004) are

available. However, given the problems described by the researcher, his conclusions should be viewed with

serious reservations and it may therefore be considered that this study introduces analysis of this property for the

first time.

MECHANICAL PERFORMANCE OF RUBBERISED CONCRETE

2

2. Experiment programme

2.1. Materials In order to carry out the work, natural calcareous aggregates were used, supplied by the company Uniconcrete. In

terms of geometry, the coarse aggregates were considered crushed and the fine rounded. Two batches of rubber

granulates were used, which had been processed in different ways. The company Biosafe supplied the ground

granulates, which were divided into 5 products of distinctive nominal sizes (0.00-0.80, 0.80-2.50, 2.50-4.00, 4.00-

7.00, 7.00-9.50). The cryogenically produced granulates were provided by the company Recipneu in 4 different

particle sizes (0.18-0.60, 0.60-1.40, 1.00-2.00, 1.50-4.70). A CEM II A-L 42,5 R cement from the SECIL cement

works in Outão, Setúbal was used as a matrix. The water came from the public mains supply.

2.2. Concrete design On the basis of Standard NP EN 206-1 (2005), we aimed to produce a concrete with average compression

resistance, tested in cubic samples of approximately 44 MPa (C 30/37) and with a workability defined as a slump

interval of 80±10 mm.

Table 2.1 – Composition of control concrete (C)

Cement 0.147

Agg

rega

tes

0 0.25 0.021

0.25 0.5 0.048

0.5 1 0.055

1 2 0.064

2 4 0.073

4 5.6 0.039

5.6 8 0.045

8 11.2 0.045

11.2 16 0.092

16 22.4 0.114

22.4 25.4 0.045

Water 0.196

Voids 0.015

1.000

Rubber was incorporated into the concrete through volumetric substitution of the aggregates. This procedure was

separate from the rubber processing procedure and involved varying percentages and particle sizes used to

create 13 batches.

The percentages were calculated at 5%, 10% and 15% of the total aggregate volume. In relation to particle size,

ground rubber was incorporated into the fine, coarse and mixed aggregates. Fine aggregates should be

understood as particles able to pass through a 4 mm mesh sieve and coarse aggregates as the remaining material

in the sieve that could pass through a 11.2 mm mesh, in the case of rubber granulate, or a 25.4 mm mesh in the

case of mineral aggregates. This disparity in the maximum size of the coarse particles was due to a lack of larger-

sized ground rubber granulate. As cryogenic processing produces smaller-sized granulates, the analysis was only

carried out on the smallest particle sizes within the three substitution ratios described.

EXTENDED ABSTRACT

3

As both the fine and coarse mixtures were subdivided into various particle size fractions, it is necessary to clarify

how the substitutions were carried out. The underlying concept was to minimise any discontinuity in the grading

curve of the aggregates, which similarly affected the intervals defined for the sieves that were used. This meant

that, for example, to substitute a particular percentage of fine aggregate, all the particle size fractions that were

less than 4 mm were affected to the extent that each contributed towards defining the standard curve.

Finally, to maintain the same level of workability, which was expected to be affected by an increase in the amount

of rubber incorporated, the w/cm ratio was varied.

Table 2.2 – Main characteristics of the composition of the different concretes produced

Identification C F05C/F05C* F10C/F10C* F15C/F15C* FC05C FC10C FC15C C05C C10C C15C

% of aggregates substituted

0.00 5.00 10.00 15.00 5.00 10.00 15.00 5.00 10.00 15.00

% of fine aggregates substituted

0.00 12.25 24.15 35.77 6.13 12.08 17.88 0.00 0.00 0.00

% of coarse aggregates substituted

0.00 0.00 0.00 0.00 4.22 8.53 12.92 8.49 17.15 25.94

w/cm ratio 0.43 0.43 0.45 0.47 0.43 0.45 0.47 0.44 0.46 0.48

2.3. Testing of aggregates The aggregates and granulates were characterised on the basis of the following tests:

sieve analysis – NP EN 933-1 (2000) and NP EN 933-2 (1999);

bulk density and water absorption – NP EN 1097-6 (2003);

apparent bulk density – NP EN 1097-3 (2003) (coarse aggregates and granulates only);

shape index – NP EN 933-4 (2002) (coarse aggregates and granulates only);

Los Angeles abrasion test – LNEC E237 (coarse aggregates and granulates only).

2.4. Testing of fresh concrete The following tests were carried out on the fresh concrete:

slump test (Abrams cone) – NP EN 12350-2 (2002);

bulk density– NP EN 12350-6 (2002).

2.5. Testing of hardened concrete The following tests were carried out on the hardened concrete:

compressive strength at 7, 28 and 56 days – NP EN 12390-3 (2003);

splitting tensile strength – NP EN 12390-6 (2003);

modulus of elasticity at 28 days – LNEC E397;

abrasion resistance – DIN 52108 (2002).

MECHANICAL PERFORMANCE OF RUBBERISED CONCRETE

4

3. Experiment results and discussion

3.1. Properties of aggregates and granulates Table 3. shows the results of the tests carried out on the mineral aggregates and rubber granulates.

Table 3.1 – Results of tests on mineral aggregates and rubber granulates

Sand Crushed gravel Rubber granulate

Fine Coarse 1,18-4,00

mm 4,00-6,30

mm 4,75-12,50

mm 9,50-25,00

mm Cryogenic Ground

Bulk density of dry particles (kg/m

3)

2613 2400 2216 2560 2443 2601 1074 1010

Bulk density of saturated particles with dry surface

(kg/m3)

2621 2419 2244 2593 2475 2626 1091 1028

Water absorption (%) 0.29 0.80 1.23 1.28 1.32 0.97 1.60 1.78

Apparent bulk density (kg/m

3)

1410 1543 1472 1437 1422 436-472 359-497

Los Angeles abrasion text (%)

- - 23 29 31 - 0

Shape index (%) - - 18 18 14 - 8-15

As expected, the results in Table 3. show that there is a substantial difference in the bulk density of materials of

different origins (mineral aggregates and rubber granulates). Consequently, as density depends heavily on bulk

density, parallel results were recorded for this property. However, it should be noted that coarse sand stood out

in terms of its high figures, probably due to greater continuity in the grading curve.

With regard to water absorption after 24 h immersion, the figures inTable 3. for mineral aggregates, unlike those

for the granulates, were as expected (lower values – less than 1.4%). In the case of rubber, the non-negative

values show that the methodology produced results that did not match the hydrophobic properties of the

material. The cause of this discrepancy lies in the methodology described in Standard NP EN 1097-6 (2003) which

stipulates a subjective procedure that is difficult to apply: the sample must be left exposed to the air until the

water particles disappear but the material must always appear damp.

The results of the Los Angeles abrasion test show that all the materials used complied with the limits set for the

application of structural cements. The mineral aggregates gave results of 23% to 31%. The rubber granulate

behaved differently, registering zero wear due to reduced rigidity which dissipated the energy of the impact from

the spheres, thus reducing the effects of attrition. It was therefore concluded that this test is not suitable for

assessing rubber granulate.

The shape index figures showed a similar geometry for the various coarse mineral aggregates and a highly

significant variation for the rubber granulate. Despite these results and given other factors, it can argued that its

influence on the mechanical properties of the concrete is negligible.

3.2. Properties of fresh concrete

3.2.1. Workability

In interpreting the results for the workability of the different mixtures shown in Figure 3.1, each curve is

composed of 3 points. The first, which has a low w/cm ratio, corresponds without exception to the 5% aggregate

substitution, whilst the second and third correspond to the 10% and 15% substitutions, respectively.

EXTENDED ABSTRACT

5

Figure 3.1 – Results of slump test

Figure 3.1 shows that although the rubber negatively affects the workability of the cement into which it is

incorporated, alteration of the w/cm ratio is an effective procedure for countering the problem. In fact, slump

figures within the recognized interval were obtained, regardless of the substitution percentage, the size of the

granulate used and the method used to process the rubber.

3.2.2. Bulk density

Figure 3.2 shows the bulk density percentage for the fresh concrete in relation to the standard concrete and in

terms of the aggregate substitution ratios. If the aim is to determine any result as an absolute value, the value for

the control concrete (C) is 2392 kg/dm3.

Figure 3.2 – Results of bulk density testing of fresh concrete

Figure 3.2 confirms that there is a reduction in the bulk density of fresh concrete as the volume of incorporated

granulate increases. This trend is evident when the clear difference in density between the mineral and rubber

aggregates is taken into consideration. In addition, by establishing the substitution percentage, it can be seen that

loss of bulk density depends on the size of the granulate used. This is due to the hydrophobic properties of rubber

which, when associated with larger specific surface areas in the fine mixtures, leads to the introduction of greater

air content and consequently lower bulk density in fresh concrete.

MECHANICAL PERFORMANCE OF RUBBERISED CONCRETE

6

3.3. Properties of hardened concrete

3.3.1. Compressive strength In order to understand the influence of the aggregate substitution percentage on the compressive strength of

concrete at 28 days, the test results are shown in Figure 3.3. The control concrete (C) registered an average value

of 55.5 MPa for this property. This data enables any other given value to be determined using this graph.

Figure 3.3 – Results of compressive strength test (28 days)

The results show that, regardless of whether the substitution model refers to fine or coarse mixtures or both,

compressive strength is heavily affected by an increase in the volume of rubber incorporated. This behaviour can

be explained by comparing the physical properties of the rubber and mineral aggregates, specifically with regard

to their significantly different rigidity. Due to this, the rubber particles tend to act as voids, leading to the

formation of high tension around their periphery and consequently creating fissures which, if they spread enough,

lead to the premature rupturing of specimens (KHALOO (2008), GUNEYISI (2004), BIEL (2004)). Analysis of Figure

3.3 also supports the argument that the use of smaller-sized rubber granulates produces worse results than the

use of larger-sized granulates. The simultaneous substitution of both fine and coarse aggregates apparently

results in intermediate values that are very close to those obtained with the exclusive use of coarse aggregate.

Given this, it can be argued that this behaviour results from a close correlation between the compressive strength

and compactness of the mixtures. Therefore, as this latter property depends on the air content introduced, it is to

be expected that the size of the rubber particles will affect compressive strength in inverse proportion to its effect

on loss of bulk density. In addition, for the same reasons, (different amounts of air introduced), compressive

strength results were observed that were apparently better for FC* mixtures than for FC mixtures.

The evolution of compressive strength in all the mixtures is shown in the graph below. In Figure 3.4, each

substitution model is represented in the 3 curves. The curve with the highest compressive strength value refers to

the 5% percentage substitution, whilst the intermediary and lower values refer to the 10% and 15% models

respectively.

EXTENDED ABSTRACT

7

Figure 3.4 – Evolution of compressive strength in the different mixtures

Although Figure 3.4 is not entirely conclusive with regard to the evolution of compressive strength, an analysis of

the results enables us to present the same argument as FEDROFF (1996): in younger concretes, the loss of

compressive strength in relation to the control concrete (C) is more marked.

3.3.2. Splitting tensile strength

In order to understand the influence of the substitution percentage on splitting tensile strength and to determine

which size of rubber granulate produces the best performance, the results for this test are presented in Figure

3.5. To determine the results as absolute values, the value for the control concrete (C) is 3.4 MPa.

MECHANICAL PERFORMANCE OF RUBBERISED CONCRETE

8

Figure 3.5 – Results of splitting tensile strength test (28 days)

As with the trend shown for compressive strength, the results in this case also reveal a substantial reduction in

performance as the percentage of incorporated rubber increases, in any of the substitution models. The

underlying reasons for this behaviour are identical to those which explain the loss in compressive strength. Thus,

the analogy with voids caused by the differing rigidity of the materials, leading to increased tension at the

periphery of the rubber particles, explains the trend shown in this data. In addition, the behaviour observed for

the different sizes of granulate is identical to the results observed for compressive strength, i.e. better

performance is evident in the CC mixtures in comparison with the FC mixtures. The results for the FCC specimens

were not entirely conclusive, but it can be argued that they also behave similarly or, in other words, produce

intermediate values. The existence of anomalous results (FC05C-FC10C) may be caused by the sensitivity of the

testing to possible imperfections in the cement matrix.

3.3.3. Modulus of elasticity

The relative loss of modulus of elasticity in all the mixtures in relation to the control concrete (C), is shown in

Figure 3.6. In order to determine absolute values, the value for the control concrete (C) is 40.6 GPa.

Figure 3.6 – Results of modulus of elasticity testing (28 days)

EXTENDED ABSTRACT

9

Given the sensitivity of the modulus of elasticity to the rigidity of the aggregates, it may be presumed that it will

decrease even more as the substitution ratio increases. In addition, for reasons associated with the compactness

of the mixtures, granulate size contributes towards variations in the results in a manner that is analogous with the

findings for compressive strength and traction. However, in this case the markedly diverging trend across the

substitution models as the volume of incorporated rubber increases suggests the existence of a variable that has

not yet been accounted for. In this context, reference should be made to the microfissures resulting from the high

tension generated at the periphery of the rubber particles, which adversely affects the continuity of the cement

matrix (FEDROFF (2006) and KHALOO (2008)). Thus it may be concluded that the reduction in the modulus of

elasticity is the result of the reduced rigidity of the rubber particles which, on the one hand, do not help reduce

the load and also adversely affect the rigidity of the cement matrix by generating early microfissures.

3.3.4. Abrasion resistance

With regard to abrasion resistance, in order to understand the influence of the variables being analysed – the

method used to process the granulate, the size of the rubber particles and the aggregate substitution percentage

- the results for abrasion resistance testing are shown in Figure 3.7. In terms of this property, the control concrete

(C) registered an abrasion loss depth of 2.6 mm.

Figure 3.7 – Results of abrasion resistance testing (after 91 days)

Figure 3.7 shows that concrete containing rubber granulate registered less wear due to the greater resistance to

abrasion of this material. As the testing progressed, the particles gradually became more prominent on the test

surface and resisted the action of the abrasion set (disc and powder) on the cement matrix. This effect, which

became more evident as the granulate size increased, together with the different degrees of compactness,

produced results that were identical to the other mechanical properties: better results for CC mixtures and worse

results for FC mixtures. For the remainder, variations in the arrangement of the rubber particles in the testing

phase prevented more conclusive results from being obtained but it may be stated that the results are

intermediate for the FCC mixtures whereas the FC* results are identical to the FC results.

4. Conclusions In this study, the mechanical performance of structural concrete containing rubber from scrap tyres was analysed.

After completing the work, the following conclusions could be drawn:

MECHANICAL PERFORMANCE OF RUBBERISED CONCRETE

10

compressive strength is heavily affected by the incorporation of rubber granulate; substitution involving

coarse granulates leads to better performance;

loss of splitting tensile strength in relation to the control concrete (C) produced values that were identical

to those of compressive strength; in this case also, the use of larger-sized granulates is associated with

better results;

the modulus of elasticity is also adversely affected by the incorporation of rubber granulate into the

concrete; however, the loss in relation to the control concrete (C) is slightly less than the loss in

compressive strength and splitting tensile strength and larger particles lead to better performance;

all the mixtures showed significant benefits in terms of abrasion resistance with the incorporation of

rubber granulate; the greater the substitution percentage and size of the rubber particles, the better the

results.

From a general perspective, in order to prevent marked reductions in the mechanical properties of structural

concrete, the use of rubber granulate is limited to percentages in the order of 5% of the total aggregate (in

terms of volume), which would appear to be the key to the solution. However, in order to mitigate this

problem, it should be noted that coarser granulates may be chosen and that this will certainly lead to better

performance, with the added benefit of being more attractive from an economic point of view.

5. References

BIEL, T; LEE, H (1994) - Use of recycled tire rubbers in concrete. Materials Engineering Conference, Utah, pp. 135-

139.

CAIRNS, R.; KEW, H.; KENNY, M. (2004) - The use of recycled rubber tyres in concrete construction. Final Report,

University of Strathclyde, Glasgow.

FEFROFF, D.; AHMAD, S.; SAVAS, B. (1996) - Mechanical properties of concrete with ground waste tire rubber.

Transportation Research Record 1532, pp. 66-72.

GUNEYISI, E.; GESOGLU, M.; OZTURAN, T. (2004) - Properties of rubberized concretes containing silica fume.

Cement and Concrete Research, 34, Pergamon pp. 2309-2317.

KHALOO, A.; DEHESTANI, M.; RAHMATABADI, P. (2008) - Mechanical properties of concrete containing a high

volume of tire-rubber particles. Waste Management, ARTICLE IN PRESS, Elsevier.