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UV modification of tire rubber for use in cementitious composites
Accepted Manuscript UV modification of tire rubber for use in cementitious composites Gregorio Ossola, Adam Wojcik PII: DOI: Reference:
S0958-9465(14)00067-5 http://dx.doi.org/10.1016/j.cemconcomp.2014.04.004 CECO 2332
To appear in:
Cement & Concrete Composites
Received Date: Revised Date: Accepted Date:
15 August 2013 4 April 2014 22 April 2014
Please cite this article as: Ossola, G., Wojcik, A., UV modification of tire rubber for use in cementitious composites, Cement & Concrete Composites (2014), doi: http://dx.doi.org/10.1016/j.cemconcomp.2014.04.004
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UV modification of tire rubber for use in cementitious composites
Gregorio Ossola Department of Mechanical Engineering 4th Floor, Roberts Building University College London Torrington Place London WC1E 7JE United Kingdom
Adam Wojcik (corresponding author) [email protected] +447711696196
Department of Mechanical Engineering 4th Floor, Roberts Building University College London Torrington Place London WC1E 7JE United Kingdom
Abstract
The use of recycled rubber as a possible aggregate in concrete is known to result in a reduction of compressive and flexural strength. This paper summarises the results of initial studies on the effect of surface-treating rubber crumb (obtained from discarded tires) with ultraviolet (UV) radiation, with the aim of mitigating such losses. Investigation focussed on changing the surface energy, and therefore the bond strength, between cement and rubber. To identify the most effective UV wavelength for this purpose, a water retention test method was utilized, resulting in the selection of the UV-C wavelength range for treatment. Additionally, specimens containing rubber, treated for different time periods, were subjected to flexural testing. As expected, the addition of untreated crumb rubber resulted in a degradation of flexural strength, however exposure to UV-C generated, at best, values only 6% weaker than those of rubberless specimens, indicating the benefits of the investigated surface treatment.
Keywords: cement; bond strength; composite; recycled rubber; ultraviolet radiation
1. Introduction
As concern for the environment becomes increasingly important, greater emphasis has been given to recycling materials. The difficulty of recycling discarded rubber tires, however, is a major issue and derives mainly from the vulcanisation process which, on one hand, improves the durability of the tire rubber while, on the other, cross-links the elastomer, making remelting and reshaping impossible.
Presently the construction industry is a significant user of discarded rubber tires, primarily for production of asphalt based composites, as well as for heat and sound insulation in buildings and as a fuel for cement kilns. As far as the authors are aware, rubber has never been used as an aggregate for commercial-based cement composites. Its incorporation in cement deteriorates the mechanical properties of the rubber-cement composite (RCC) so created, which greatly limit the purposes for which it may be used. Eldin and Senouci [1] have quantified these losses to about 30% reduction in both the tensile and compressive strength for a 25 vol.% substitution of fine aggregate with rubber particles.
RCC – Rubber-cement composite
In previous work, researchers examined the simple mechanical addition of rubber to concrete, considering shape, size and volume fraction of the rubber as the main variables. Khaloo et al. [2], Turatsinze and Garros [3] and others, all reported dramatic reductions in strength, upon incorporation of rubber, which could not be substantially mitigated by an optimisation of the variables they studied. Such outcomes continue to preclude waste rubber aggregates from concrete for structural or high strength applications, although the ability of such composites to consume vast quantities of waste rubber remains attractive.
In identifying the reasons for the dramatic reduction in strength of rubberised concrete, compared to standard concrete, most of the authors cited here agreed that the development of stress concentrations around the rubber particles, due to the lack of bonding between cement and these particles, was a significant factor. Poorly bonded rubber particles would not be able to support any load transfer from the cement matrix and therefore would perform no function other than a space filler. Significantly however, the presence of (in effect) voids in the matrix could result in a reduction in the mechanical properties, such as flexural strength. both via the reduction in cross-sectional area of the load bearing matrix, but also via the introduction of stress concentrations. Although the latter factor might appear to only relate to flexural strength (and other tensile scenarios), compressive strength would also be expected to be affected by the presence of stress concentrations. The reasons for this are discussed later, but the published experimental work significantly supports this assertion, and all workers have reported a reduction in compressive strength [1-5].
Given the above, if the bond between rubber and matrix could be improved, a degree of load transfer to the rubber could be achieved and, in theory, the reduction in strength (both tensile and compressive) mitigated. This has been the driving force for investigation by a number of workers (as cited below) for applying surface pretreatments to rubber, and the present study had similar objectives, although via a novel approach.
An early attempt to surface treat rubber particles was by Segre and Joekes [4], whose work was later reproduced by Albano et al. [5] with modifications. For both, treatment consisted of the immersion of rubber chips in sodium hydroxide solution (NaOH) in order to improve the rubber’s hydrophilicity, so as to achieve better adhesion with the hydrated cement paste. Albano et al. modified this by using an additional silane treatment. These initial experiments showed a minor beneficial effect in raising the strength of rubberised concrete.
Later Chou [6] exposed rubber crumb to organosulfur compounds and saw, through analysis with an atomic force microscope, that interactions between rubber and cement hydration products nearly doubled, although the work did not progress to preparing and testing bulk rubberised concrete specimens.
Pelisser et al. [7] reported to have found a more effective treatment through the alkaline activation of rubber with NaOH and the use of silica fume. Unfortunately, in considering the final cost of their treatment, they dismissed this method as it would not have been viable for use in industry. It is clear that any experimental process that succeeds in improving strength must also be capable of application, both practically and economically, for it to be industrially applied. Keeping this in mind, the work presented here sought to employ ultraviolet (UV) radiation as a method of surface pre-treatment. Its effect was investigated initially via a simple water retention protocol (described below) and ultimately by a programme of bend tests, to obtain flexural strength data.
The effect of UV on polymers is well documented [8]. UV has the ability to modify the surface of polymers through a range of mechanisms. Some, like photo oxidation, generate free radicals in the presence of air (via oxygen or ozone) which can cross-link polymer chains, thereby raising stiffness and reducing toughness. Chain scission can also occur in some systems. The presence of free radicals species, polar groups, and dangling bonds, together with changes in surface free energy [9] is thought to be the reason why UV exposure is effective in also improving adhesive bonding to such surfaces. The degradation of rubber via exposure to UV radiation is well known [8] and usually manifests itself as an increase in brittleness due to further crosslinking of the elastomer structure. It was therefore hoped that UV could alter the adhesion of rubber to aqueous based systems, by changing its polar surface energy. Other surface treatments have been previously employed in order to modify adhesion to polymers in general, including electrical discharge [10] and flame-based methods, although none of these have enjoyed adoption with regards to commercial RCC materials. Assuming a beneficial outcome, UV pretreatment would provide a cost-effective method for incorporation of rubber into RCC, largely due to its clean, contactless, and easily implemented characteristics.
The authors are not aware of UV being used previously in this application and have investigated its potential over a number of years. The study described here utlized rubber in a cement matrix (i.e as an additive) as opposed to rubber as an aggregate substitute, in concrete. This differed from the cited litertaure which almost universally utilizes the latter. The justification for this was simply that the focus was on interfacial bond strength (between rubber and matrix) as the weak link in the resultant composite. Given that there is no expectation that rubber would bond directly to
aggregate, it seemed sensible to eliminate this non-influential variable from the study. The authors recognise that this may make direct comparisons with other studies difficult but it should not invalidate any benefits accrued. In any case, overall rubber volume fractions were chosen to be broadly similar (see Section 4).
The following sections describe the preliminary work undertaken to investigate the benefit of UV treatment on mechanical properties, and in particular the effect of UV wavelength and time of exposure. Further aspects of the treatment will be covered in forthcoming publications. Section 2 outlines the main features of the UV treatment process. Sections 3 and 4 describe the initial testing protocols used to evaluate the effect of UV irradiation. Section 5 constitutes a discussion of the results obtained and consideration of possible underlying mechanisms together with analysis of further tests carried out to characterize the nature of the cement-rubber interface.
2. Materials, Apparatus & Treatment
The cement used for practical experimentation was sourced from Cemex Ltd. (Cemex Rugby+ Premium Cement). This is a Portland cement containing added lime, which raises the workability. Its oxide composition by mass is given in Table 1 [11].
Table 1 Cement composition [11] % SiO2
17.6
Al2O3
4.3
Fe2O3 CaO
61.3
MgO
1.1
2.9
SO3
3.1
Cl equiv. Na2O Loss on ignition
-
0.05
Free CaO
2.7
0.7 8.7
The rubber was sourced from Crumb-Rubber UK in the form of fine, high quality, rubber crumb. This originates from used car tires and buffings from truck tires created during retreading. Tires are reduced to crumb through a process of initial shredding, granulating, and milling, by a variety of commercial organisations. During the size reduction process the crumb may be passed through equipment to remove metal, fibres, and any stones. The resulting product is then
processed by Crumb-Rubber UK to create even finer milled particles, with exceptionally low quantities of contaminants. The general composition of the tire crumb as received by Crumb-Rubber UK is shown in Table 2 [12].
Table 2 General composition of tyre crumb [12] % mass Elastomers
45 - 50
Carbon Black
20 - 25
Metal
12 - 25
Textile
5 - 10
After processing, which is entirely mechanical in nature, a high quality regular sized product is created with particle sizes in a range from 420 to 840 µm, with about 50 % of the material sized at 620 µm. A typical particle is shown in Fig. 1.
Fig. 1. SEM image of a typical rubber particle.
70 60
% of batch
50
BATCH 1
BATCH 2
BATCH 3
BATCH 4
BATCH 5
BATCH 6
40 30 20 10 0 pan
180
250
425
600
840
1000
2000
2800
4000
Particle Diameter (µm) Fig. 2. Size scatter of rubber particles [13].
Use of a consistent and well characterized crumb rubber for this study was aimed at minimising variables and batchto-batch differences that could otherwise influence the mechanical properties of the RCC and/or mask the effect of the UV treatment. Fig. 2 indicates the spread of the final crumb size within, and between, individual batches.
The mass ratio for cement to water, used for the mechanical test specimens, was 3.1 : 1 (i.e. water/cement ratio of 0.32). Theoretically a 4:1 ratio is known to yield better mechanical properties [14], although this creates a very stiff and unworkable cement paste. However, given that achievement of maximum strength was not the objective of this study, an initial 4:1 ratio was selected, successively adding water until adequate workability was obtained. This was found to correspond to the final 3.1:1 ratio.
In order for the UV pretreatment to have a clear impact upon the measured strength, 15 vol.% of rubber was utilised. This rationale is further explained in Section 4 below. Additionally, no inorganic aggregate was incorporated, for similar reasons.
UV exposure was achieved using a simple light box apparatus, incorporating an array of linear light sources similar in dimension to conventional fluorescent tubes. A range of sources was employed in an attempt to determine the optimum UV frequency. These had a spectral response in either the UV-A, B or C range; all had a wattage of 18 W,
and were 610 mm long and of 26 mm in diameter. Spectral responses were obtained from the manufacturers from which it was determined that the UV-A sources emitted light in a range between 320 and 400 nm with their peak at 360 nm [15], the UV-B sources emitted only 12 % of their light in the UV-B spectrum and the rest in the UV-A spectrum [16] and the UV-C sources emitted the vast majority of their electromagnetic radiation at a wavelength of 253.7 nm [17].
Rubber crumb was treated by spreading it out on a reflective metal substrate to an even thickness of approximately 2 mm. This ensured that as much surface area of each rubber particle was exposed to the UV radiation as was possible without mechanical rotation. Crumb was exposed for set periods of time and then rapidly (within 10 minutes of final exposure, to avoid any possible temporal effects) incorporated into the cement paste, which was then subsequently cast into test specimens. Although UV exposure should result in no rise in temperature of the rubber particles (beyond the variations normally expected in ambient temperature), the time elapsed between the end of UV exposure and subsequent experimental procedures (e.g. incorporation/mixing with cement) allowed for the rubber particles to return to ambient temperature.
As a prelude to mechanical testing, and to minimise the number of specimens to be tested, the effect of UV wavelength was determined by using a simple water retention test as a proxy. The justification for this is described in the following section.
3. Water Retention Test
It is well known that physical bonding (e.g. via van der Waals interactions) is at the heart of most adhesion mechanisms [18]. Chemical bonds formed by reaction can occur in some adhesive-adherent systems, but are generally not the norm. Polar interactions can often be enhanced by surface pretreatments. The level of polar interaction can often be established by characterizing the degree of wetting that is achievable between adhesive-adherent pairs [20]. In the case of cement bonding to rubber, therefore, the extent of wetting by water or other aqueous liquids (in effect the hydrophilicity) could, in theory, indicate the likelihood of an adhesive bond between the two components.
Surface wetting is usually measured by optical determination of contact angle [7], however for this work it was more appropriate to develop a simpler methodology based on the ability of rubber crumb to retain water after total
immersion. Crumb was UV-treated, and weighed batches were immersed in an excess of distilled water, stirring to ensure coverage. Given that the UV-C fluorescent tubes were designed to emit a single frequency, the rubber should not experience any temperature change during exposure. Nonetheless, immersion occurred within 10 minutes of final exposure to UV allowing for cooling to the ambient. The resulting mix was drained, and immediately weighed to determine the retained water. A protocol was applied to ensure commonality between the batches in terms of immersion time, level of stirring and drainage time.
The results of the water retention test for various UV exposure times and principal source wavelengths are shown in
% mass increase vs control specimen
Fig. 3.
35% 30% 25% 20% UV - A UV - B UV - C
15% 10% 5% 0% 5
10
15
20
25
30
-5% -10%
Exposure time (hrs)
Fig. 3. Average water retention results for all UV ranges (Initial rubber mass of 100g, 3 tests per condition/bar). Results are relative to a control specimen with no UV exposure.
The absence of a trend between increasing exposure time and water retention for UV-A and UV-B indicates that these two wavelength ranges have very little or no effect on the level of rubber wettability. Indeed the water retention test results for UV-A and UV-B all lie close to, or within, a ±6% scatter in retention that was observed in the control batches (no UV) (average water retention for the control batch was 39.5g).
UV-C, on the other hand, presented a continuous increase in the water retention with time of exposure showing the benefits of the treatment, at least in terms of wettability. This was in line with expectations, given that UV-C implies higher energy levels due to its higher frequency, and hence a better ability to create dangling bonds and polar species.
All subsequent mechanical testing of RCC was therefore carried out using UV-C treated rubber.
4. Mechanical Testing
Flexural testing of RCC specimens was chosen as the most appropriate means of characterising the effect of rubber incorporation and pretreatment. A compression test would have been more conventional, however the quantity of treated rubber required (given the number of specimens needed for adequate statistical rigour) would have been prohibitive. A flexural test, however, generates failure via tensile stress states, and may at first seem inappropriate for a material principally used in compression. However, given that the UV pretreatment was designed to improve bond strength between the cement matrix and the rubber aggregate, a tensile regime was deemed a better indicator of its effect. It should also be noted that, in any case, tensile forces are in operation during pure compression tests [20, 21]. This implies that a link can be drawn between the projected compressive strength of a RCC and its tensile behaviour. This is discussed further below.
To achieve a reliable average for the flexural strength, 50 specimens were tested, with 10 specimens for each of the 5 conditions. Two control conditions were tested as a comparison for the UV-treated specimens: one not containing any rubber and the other containing rubber as supplied, without any pre-treatment. The three UV-C treated conditions required the production of specimens containing rubber exposed to UV-C between 20 and 60 hours in 20 hour intervals, in order to obtain an initial idea of the relationship between time of exposure and flexural strength. This number of specimens was deemed sufficient to justify the calculation of a Weibull distribution, in order to give an indication of the variability in the flexural strength.
As the objective of these tests was not to find the optimal volume fraction of rubber for use in concrete, any arbitrary percentage could have been selected. Preliminary experiments confirmed that the loss in strength of the rubber-cement composite increased with volume fraction of fine (un-treated) rubber particles, in keeping with published literature, in
the range up to 50 vol.%. Khatib and Bayomy [22] suggested that rubber content should not exceed 20% of the total aggregate volume, as otherwise the strength is too severely degraded through loss of the supporting cement matrix. Therefore to ensure sensitivity to the effect of the pretreatment for the rubber-cement composites, a volume fraction of 15% was chosen as a compromise between generating sufficient weakness (to be subsequently mitigated), without so gross a deterioration in strength (which could overwhelm and mask any benefit). Moreover, a comparison with the relevant published literature shows that such a rubber volume fraction (in a cement matrix) is broadly similar to that adopted by other workers (e.g. [1, 3, 22]) when testing rubber as a replacement for aggregate, (i.e. when calculated as a volume fraction of the overall concrete mixture).
Each specimen was cured in air for a period of 7 days, at which point the cement matrix would have achieved about 85 % of its final strength [23]. This allowed testing to be expedited. Standard curing protocols (e.g. ASTM C109) normally require specimens to be cured in water, however the aim of the testing protocols adopted by the authors was to emphasize the role of interfacial bonding, rather than achieve the best strength values for the rubber-cement composite, and given the comparative nature of the study, the mode of curing was not critical (as long as it was consistent).
Metal moulds were employed to cast the specimens, with internal dimensions of 100 x 50 x 17.5 mm. A silicone mould release was employed, and cured specimens were level ground to produce better consistency. The specimens were loaded in 3-point bend at a cross-head speed of 1 mm/min in a Hounsfield H5KS universal tester. Circular anvil supports (ground steel) were used to minimise any crushing effects. An anvil spacing of 75 mm was employed for practical reasons. Although standard flexural testing protocols (e.g. ASTM C348) suggest a smaller span, the authors did not feel that the value adopted would significantly alter the failure mode of the specimens and adversely impact the results.
Initial results from testing of all specimens are given in Table 4. The height of each specimen was measured postfailure across the fractured cross-section.
The flexural strength was calculated using: σf = (3PL)/(2bh^2)
where σf = flexural strength (MPa), P = fracture load (N), L = distance between anvil supports (mm), b = base length of the specimen (mm), and h = height of the specimen (mm).
Fig. 4 and Table 4 show the average values for the flexural strength (σf) for the different durations of UV-C exposure along with additional data, such as the variability of the average values. It is important to note that in Table 4 the average flexural strength is taken as the mean of all the individual flexural strength values calculated for every specimen tested, and not by using the tabulated mean values of maximum force (Pmax) and height. To aid interpretation of Fig. 4, the key to the specimen codes is contained in Table 3.
Table 3 Key to specimen codes Specimen code
vol.% of rubber
Treatment Curing time time (hrs) (days)
R0ND7
0
n/a
7
R15ND7
15
0
7
R15U20D7
15
20
7
R15U40D7
15
40
7
R15U60D7
15
60
7
5. Discussion
From the mechanical testing data gathered, it can be seen that the result of the addition of untreated rubber to the cement mixture causes an immediate loss in flexural strength, as was expected from the published literature. This behaviour supports the hypothesis that untreated rubber acts as a void inside the cement and therefore creates stress concentrations at the rubber-cement interface. As treatment time increases, the flexural strength clearly improves until reaching a plateau at approximately 40 hours. As a check, a separate side study also indicated that similar levels of improvement (percentage-wise) were detected even if the rubber volume fraction was raised to 35 vol. %. These results are not reported here in detail. Drawing a link to the water retention test results and assuming water retention is a valid proxy for surface adhesion, this appears to affirm that an improvement in the bonding between cement and rubber is leading to an improved flexural strength. As was discussed earlier, this is highly likely to be due to the ability of rubber particles to transfer and support load once a degree of bonding between them and the matrix has been
achieved. In effect the rubber can be thought of as acting to increase the fracture toughness of the RCC by created load-bearing bridges across the pore-like defects formed via the inclusion of the crumb.
A noticeable reduction in the scatter of the flexural strength data on addition of rubber crumb can also be seen, commensurate with the introduction of a more uniform defect population. The rubber crumb creates this uniformity by generating voids of similar size in the matrix. This effect can also be seen in the Weibull modulus figures.
Flexural strength (MPa)
7.5 7.0 6.33
6.5
5.96
6.0 5.5
5.94
5.21 4.98
5.0 4.5 4.0
R
R 15 U
60 D 7
D 7 15 U 40
R 15 U 20 D 7
D 15 N R
R
0N
D
7
7
3.5
Specimen code Fig. 4. Average flexural strength for each condition.
Table 4 Summary of average data gathered Specimen code
Exposure time (hrs)
P max (N)
h (mm)
σf (MPa)
St. Dev.
% change over % change rubberless over 0 hrs
Weibull Modulus
R0ND7
n/a
805.63
17.68
6.33
0.73
0.0%
27.0%
8.31
R15ND7
0
647.46
17.55
4.98
0.25
-21.2%
0.0%
18.91
R15U20D7
20
676.04
17.13
5.21
0.35
-17.7%
4.6%
14.39
R15U40D7
40
803.71
17.78
5.96
0.52
-5.8%
19.6%
10.73
R15U60D7
60
878.46
17.60
5.94
0.51
-6.1%
19.2%
10.63
Whilst the changes in chemical nature of the treated rubber crumb were not investigated in this study, some idea of what may be occuring can be found in the literature [24]. UV radiation contains energies which are of an appropriate level to break chemical bonds in polymers and so create a range of effects [8, 25]. Free radical species can easily be created especially in the presence of ozone (which can form when air is irradiated with UV). These can initiate further chemical reactions including cross-linking and the creation of new functional groups in the surface regions of an irradiated polymer. Such groups would be expected to alter the surface energy of the polymer, and thereby its “wettability”, either by presenting a more chemically reactive surface or simply through enhanced polar interactions. The latter mechanism provides a convincing explanation for the increase in water retention seen in this work, after UV exposure. Similarly, by association, the improvement in flexural strength can be ascribed to the same surface effects, particularly those resulting in dipole interactions, which are commonly regarded as the principal determinants of the strength of adhesive bonds [18].
Given that such interactions are limited to surface regions, it would be expected that the benefit of an UV pretreatment would reach a saturation level, at which point all the possible polar sites had been created. From the flexural strength data obtained (Fig.4) this indeed seems to be the case as no benefit in flexural strength accrues beyond the 40 hour exposure.
It may be possible to improve upon the 40 hour strength data if it transpires that the use of a light box exposure method only treats a proportion of the available surface of individual crumb particles. This may well be the case, and an UV apparatus that continually rotates the crumb has been designed to optimise the exposure methodology [26].
Fig. 5 compares the percentage change in flexural strength, normalised against the flexural strength of the R15ND7 (no UV exposure) specimens, in order to better illustrate the improvement in strength obtained via the UV-C pretreatment. As can be seen, the R15U40D7 and R15U60D7 treated specimens display an improved average flexural strength of about 20% over that of the specimens containing untreated rubber. When compared with cement containing no rubber, which is an average of 27% stronger, it is clear that the UV treatment has restored much of the original strength lost in creating the RCC. Indeed the recovery in strength now lies within the standard deviation scatter band of the rubberless cement specimens (see Fig. 5).
40% 35%
% change in flexural strength
30% 25% 20%
20%
19%
15% 10% 5%
5%
0% -5%
0 0%
10
-10%
20
30
40
50
60
70
UV-C treatment time (hrs)
Fig. 5. Change in flexural strength of RCC with respect to UV-C treatment time (normalized against untreated rubber RCC). Dotted line represents baseline strength for cement containing no rubber.
Khaloo et al [2] and Turatsinze and Garros [3] observed during their tests, that rubberised cement specimens still had some residual strength after initial failure. This was also observed in the present work in all specimens containing rubber, whilst the rubberless specimens failed catastrophically, as was expected.
Once the matrix has failed, rubber particles on the newly created cracked surfaces still hold the two faces together, as the crack will have effectively propagated around them. The residual strength will therefore be linked to the strength required for the pull-out of each of these particles and this, in turn, involves debonding from the cement matrix. As a consequence, the improvement of the bond strength through the use of the UV-C pre-treatment should also have beneficial effects on the fracture toughness of the RCC, in addition to its flexural strength.
An additional element that can be gathered from Table 4 (as noted earlier), is that there seems to be a reduction in the standard deviation of the flexural strength of the specimens and hence an improvement in the Weibull Modulus when rubber is incorporated in the cement matrix. This reduction in scatter then seems to worsen as the UV-C treatment time is raised. The initial reduction is most likely a consequence of simply introducing an array of similarly sized
defects (i.e. the rubber crumb) within a brittle matrix. Assuming that these are larger than the intrinsic defects within the cement (i.e. its “natural” porosity), the flexural strength would be expected to drop, and the Weibull Modulus rise, depending upon the uniformity of the dimensions of the crumb. It should be noted that the square root dependency within the Griffith relationship would also be expected to lead to a reduction in the scatter in flexural strength with an increase in crack size (for a constant scatter in defect size). The reduction in the Weibull Modulus with increasing UV-C treatment time is more difficult to explain, although it may also have its roots in the non-linearity of the Griffith relationship, if it is assumed that enhanced cement-rubber bonding causes a reduction in the “effective” crack size.
Clearly, therefore, the UV-C pre-treatment has been beneficial, whether it is due to a reduction in “effective” crack size (via improved bonding between the matrix and rubber), or a change in the “effective” fracture toughness of the composite (via crack bridging of existing flaws by the bonded rubber particles), or via a rise in global fracture toughness (due to energy absorbtion processes during localised debonding, pull-out etc).
Although pure compression tests were not performed, the results from the flexural test can be considered a valid representation of the behaviour of the material under compression. This is due to the fact that the crack initiation and failure mechanisms of brittle materials in compression are initially very similar to the ones under tensile conditions, assuming a population of crack-like defects. Griffith [20] and Inglis [21] showed that in such situations it is not the theoretical compressive strength of the material that governs failure in compression, but rather its tensile strength. In a component undergoing compression, tensile stresses develop in the proximity of a pore or defect, as shown in Fig. 6. Therefore, under a global compressive load, it is a tensile force that causes initial crack propagation and failure although following initiation, the failure mode may then alter, hence generating a difference between the ultimate tensile strength, and the “practical” compressive strength. This therefore makes it possible to extrapolate the behaviour of a “defective” brittle material in compression, from its behaviour under a tensile or flexural test. Thus the improvement found in the flexural strength of the RCC after UV-C irradiation would also be indicative of a beneficial effect upon the compressive strength.
P σy
τxy
σt
σx
τyx
Fig. 6. Stresses on an element around a defect in compression (adapted from Griffith [20] and Lajtai [27]), where τ refers to shear, σx and σy to compressive stresses and σt to tensile stresses respectively.
A microstructural investigation of the fracture surface of the mechanical test specimens was also carried out in an attempt to understand the nature of the cement-rubber interface. Given an improvement in the bonding between rubber and cement that was deduced from the mechanical testing, SEM fractography (Fig. 7) could reasonably be expected to reveal changes in the degree of cement debris on the surface of the rubber particles. Additionally poor adhesion (hence poor wetting) might be predicted to affect the topography of the interface region, for example, by creating interfacial voids. The occurrence of these would be expected to reduce as the bonding improved. The microstructural analysis indeed detected discernable differences in the fraction of residual cement debris on exposed rubber particles together with distinct evidence for tighter rubber-cement contact in UV-treated specimens. These observations therefore supported the flexural testing results. The existence of debris suggests that the adhesive bond strength (i.e. that between rubber and cement) had improved to the point at which it was stronger than the cohesive strength between the weaker cement particles. Small amounts of residual cement debris were, nevertheless, present on the surface of untreated rubber (Fig. 7a) and this is probably the result of statistical weaknesses in the cement-cement bonds.
Fig. 7. SEM fractographs of representative specimens, all showing particulate matter (lighter spots) on embedded rubber particles (central objects). a) R15ND7 (no UV) and b) R15U60D7 (60 hrs UV).
The fractography results prompted additional testing to further confirm that the strength improvements measured in the mechanical tests were due to increases in the adhesive bond strength. A simple pull-out test (Fig. 8) was devised for this purpose with the aim of isolating the rubber-cement interface, and therefore characterizing the changes in rubbercement bond strength, due to the UV pre-treatment. This test was performed by embedding nitrile rubber rod in cement (cured and held in a dedicated specimen holder) and measuring the load required to shear the rod from the holder. Nitrile rubber was selected as it is nominally of a similar chemical make-up to tire rubber, and should therefore behave similarly with exposure to UV. Bulk mechanical properties were also similar. Standard nitrile rod for the fabrication of bespoke O-rings was utilized. This had a smoother surface texture than the tire rubber crumb, which served to remove the effect of any mechanical interlocking, thereby ensuring that the pull-out load measured was mainly a function of bond strength.
Two conditions were employed, a set of nitrile controls (no UV-C treatment) and nitrile specimens treated for 30 hours with UV-C. To help minimise statistical error, 7 specimens for each condition were employed. The same testing machine and cross-head speed adopted for the earlier 3-point flexure tests were utilised for the pull-out testing.
F
L
d
Fig. 8. Pull-out test specimen schematic.
The shear failure stress was calculated using: τs = F/(πdL)
where τs = shear failure strength (MPa), F = maximum load (N), d = diameter (mm) and L = length of the embedded rod (mm).
Both the length of the embedded rod and the diameter were maintained constant throughout all specimens and were 40 mm and 4 mm, respectively.
Table 5 Summary pull-out test results E xposure Tim e (hrs)
F (N)
Extension (mm)
τs (MPa)
0
11.47
11.24
0.014
30 Δ%
18.59 62.1%
13.21 17.6%
0.022 62.1%
Results from the pull-out tests indicated a dramatic improvement in the shear failure stress after surface treatment with UV-C. The shear stress should, in theory, be at least representative of the adhesive bond strength between the rubber and the cement, although in practice there will also be a frictional contribution (which would act to raise the failure load). Conversely the low stiffness of the rubber, in combination with its Poisson’s ratio, would act to create a significant cleavage component at the interface between free and embedded rubber and this would help the rubber pull
away from the surrounding cement matrix and would therefore act to reduce the required pull-out force. The cleavage behaviour may be assumed to be a constant factor as the bulk mechanical properties of the rubber (such as stiffness) are unchanged by the UV treatment. Similarly, the frictional component should not vary with irradiation, given that the UV-C treatment did not appear to alter the rubber’s surface roughness, and hence the frictional coefficient should remain unchanged. It is therefore reasonable to assume that the increase in the shear failure stress is mainly due to an enhancement of the adhesive bond strength between rubber and cement, and that this is central to the improvement in flexural strength of the RCC seen in this work, following UV-C irradiation of the rubber phase.
6. Conclusions
The work presented here has confirmed the previously observed detrimental effect upon strength, generated by the insertion of rubber in a cement matrix, however, using a UV based pre-treatment on the rubber crumb has been shown to reduce the majority of these strength losses. This is largely thought to be due to an improvement in interfacial bonding between the cement matrix and the rubber. By means of optimising the UV treatment, either by changes in the wavelength employed, and/or by modifying the apparatus to enhance treatment uniformity, it is believed that further benefits may be forthcoming. The effect of the UV-C pre-treatment on flexural strength should also translate into an improvement in compressive strength, although the level of mitigation might be different due to the variation in crack propagation modes, under compression, once failure has initiated, and supplementary work is being carried out in order to further clarify this, and other aspects influencing the rubber-cement interface, such as alternative and combinatorial pretreatments to improve upon the observed results.
Acknowledgement
The authors would like to thank M. Aitken of Crumb-Rubber UK for providing them with the rubber crumb used in this study.
References
[1] Eldin NN and Senouci AB. Rubber-tire particles as concrete aggregate. J Mater Civil Eng 1993; 5 : 478-496.
[2] Khaloo AR, Dehestani M and Rahmatabadi P. Mechanical properties of concrete containing a high volume of tirerubber particles. Waste Manage 2008; 28 : 2472-2482. [3] Turatsinze A and Garros M. On the modulus of elasticity and strain capacity of Self-Compacting Concrete incorporating rubber aggregates. J Res Con Rec 2008; 52 : 1209-1215. [4] Segre N and Joekes I. Use of tire rubber particles as addition to cement paste. Cem Concr Res 2000; 30 : 14211425. [5] Albano C, Camacho N, Reyes J, Feliu JL and Hernandez M. Influence of scrap rubber addition to Portland I concrete composites: Destructive and non-destructive testing. J Compos Struct 2005; 71 : 439-446. [6] Chou LH. On improvements of rubberized concrete with organosulfur compounds. Adv Mater Res 2011; 156 : 1459-1462. [7] Pelisser F, Zavarise N, Longo TA and Bernardin AM. Concrete made with recycled tire rubber: Effect of alkaline activation and silica fume addition. J Clean Produc 2011; 19 : 757-763. [8] Rabek JF. Photodegradation of Polymers: Physical Characteristics and Applications. first ed. Berlin: Springer, 1996. [9] Mathieson I, Bradley RH. Improved adhesion to polymers by UV/ozone surface oxidation. Int J Adhesion and Adhesives 1996; 16 : 29-31 [10] Weibel DE, Michels AF, Horowitz F, Cavalheiro RDS, Mota GVdS. Ultraviolet-induced surface modification of polyurethane films in the presence of oxygen or acrylic acid vapours. Thin Solid Films 2009; 517 : 5489-5495. [11] Cemex Ltd. - Brookbanks, P., Personal communication. Rugby+ Premium Cement. [12] Crumb-Rubber UK, 2012. Product Specification. [online] Available at: < http://www.crumb-rubber.co.uk/ > [Accessed 19 December 2012]. [13] Crumb-Rubber UK - Carroll, N., Personal communication. [14] Powers TC. The nonevaporable water content of hardened Portland-cement paste - Its significance for concrete Research and Its Methods of Determination. ASTM Bulletin 1949; 158 : 68. [15] Koninklijke Philips Electronics N.V, 2009. TL-D 18W BLB SLV Product Description. [online] Available at: [last accessed 15 January 2010]. [16] Arcadia, 2012. Arcadia D3 Forest Lamp T8 Product Description. [online] Available at: < http://www.arcadiareptile.com/products/d3-reptile-lamp-2/> [last accessed 4 September 2012].
[17] Koninklijke Philips Electronics N.V, 2011. TUV 15W SLV Product Description. [online] Available at:
Fig. 1. SEM image of a typical rubber particle [13]. Fig. 2. Size scatter of rubber particles [14]. Fig. 3. Average water retention results for all UV ranges (Initial rubber mass of 100g, 3 tests per condition/bar). Results are relative to a control specimen with no UV exposure. Fig. 4. Average flexural strength for each condition. Fig. 5. Change in flexural strength of RCC with respect to UV-C treatment time (normalized against untreated rubber RCC). Dotted line represents baseline strength for cement containing no rubber. Fig. 6. Stresses on an element around a defect in compression (adapted from Griffith [21] and Lajtai [29]), where τ refers to shear, σx and σy to compressive stresses and σt to tensile stresses respectively. Fig. 7. SEM fractographs of representative specimens, all showing particulate matter (lighter spots) on embedded rubber particles (central objects). a) R15ND7 (no UV) and b) R15U60D7 (60 hrs UV). Fig. 8. Pull-out test schematic.
Table 1 – Cement composition [12]. Table 2 – General composition of tyre crumb [13]. Table 3 – Key to specimen codes. Table 4 – Summary of average data gathered. Table 5 – Summary of pull-out test results.
S0958-9465(14)00067-5 http://dx.doi.org/10.1016/j.cemconcomp.2014.04.004 CECO 2332
To appear in:
Cement & Concrete Composites
Received Date: Revised Date: Accepted Date:
15 August 2013 4 April 2014 22 April 2014
Please cite this article as: Ossola, G., Wojcik, A., UV modification of tire rubber for use in cementitious composites, Cement & Concrete Composites (2014), doi: http://dx.doi.org/10.1016/j.cemconcomp.2014.04.004
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UV modification of tire rubber for use in cementitious composites
Gregorio Ossola Department of Mechanical Engineering 4th Floor, Roberts Building University College London Torrington Place London WC1E 7JE United Kingdom
Adam Wojcik (corresponding author) [email protected] +447711696196
Department of Mechanical Engineering 4th Floor, Roberts Building University College London Torrington Place London WC1E 7JE United Kingdom
Abstract
The use of recycled rubber as a possible aggregate in concrete is known to result in a reduction of compressive and flexural strength. This paper summarises the results of initial studies on the effect of surface-treating rubber crumb (obtained from discarded tires) with ultraviolet (UV) radiation, with the aim of mitigating such losses. Investigation focussed on changing the surface energy, and therefore the bond strength, between cement and rubber. To identify the most effective UV wavelength for this purpose, a water retention test method was utilized, resulting in the selection of the UV-C wavelength range for treatment. Additionally, specimens containing rubber, treated for different time periods, were subjected to flexural testing. As expected, the addition of untreated crumb rubber resulted in a degradation of flexural strength, however exposure to UV-C generated, at best, values only 6% weaker than those of rubberless specimens, indicating the benefits of the investigated surface treatment.
Keywords: cement; bond strength; composite; recycled rubber; ultraviolet radiation
1. Introduction
As concern for the environment becomes increasingly important, greater emphasis has been given to recycling materials. The difficulty of recycling discarded rubber tires, however, is a major issue and derives mainly from the vulcanisation process which, on one hand, improves the durability of the tire rubber while, on the other, cross-links the elastomer, making remelting and reshaping impossible.
Presently the construction industry is a significant user of discarded rubber tires, primarily for production of asphalt based composites, as well as for heat and sound insulation in buildings and as a fuel for cement kilns. As far as the authors are aware, rubber has never been used as an aggregate for commercial-based cement composites. Its incorporation in cement deteriorates the mechanical properties of the rubber-cement composite (RCC) so created, which greatly limit the purposes for which it may be used. Eldin and Senouci [1] have quantified these losses to about 30% reduction in both the tensile and compressive strength for a 25 vol.% substitution of fine aggregate with rubber particles.
RCC – Rubber-cement composite
In previous work, researchers examined the simple mechanical addition of rubber to concrete, considering shape, size and volume fraction of the rubber as the main variables. Khaloo et al. [2], Turatsinze and Garros [3] and others, all reported dramatic reductions in strength, upon incorporation of rubber, which could not be substantially mitigated by an optimisation of the variables they studied. Such outcomes continue to preclude waste rubber aggregates from concrete for structural or high strength applications, although the ability of such composites to consume vast quantities of waste rubber remains attractive.
In identifying the reasons for the dramatic reduction in strength of rubberised concrete, compared to standard concrete, most of the authors cited here agreed that the development of stress concentrations around the rubber particles, due to the lack of bonding between cement and these particles, was a significant factor. Poorly bonded rubber particles would not be able to support any load transfer from the cement matrix and therefore would perform no function other than a space filler. Significantly however, the presence of (in effect) voids in the matrix could result in a reduction in the mechanical properties, such as flexural strength. both via the reduction in cross-sectional area of the load bearing matrix, but also via the introduction of stress concentrations. Although the latter factor might appear to only relate to flexural strength (and other tensile scenarios), compressive strength would also be expected to be affected by the presence of stress concentrations. The reasons for this are discussed later, but the published experimental work significantly supports this assertion, and all workers have reported a reduction in compressive strength [1-5].
Given the above, if the bond between rubber and matrix could be improved, a degree of load transfer to the rubber could be achieved and, in theory, the reduction in strength (both tensile and compressive) mitigated. This has been the driving force for investigation by a number of workers (as cited below) for applying surface pretreatments to rubber, and the present study had similar objectives, although via a novel approach.
An early attempt to surface treat rubber particles was by Segre and Joekes [4], whose work was later reproduced by Albano et al. [5] with modifications. For both, treatment consisted of the immersion of rubber chips in sodium hydroxide solution (NaOH) in order to improve the rubber’s hydrophilicity, so as to achieve better adhesion with the hydrated cement paste. Albano et al. modified this by using an additional silane treatment. These initial experiments showed a minor beneficial effect in raising the strength of rubberised concrete.
Later Chou [6] exposed rubber crumb to organosulfur compounds and saw, through analysis with an atomic force microscope, that interactions between rubber and cement hydration products nearly doubled, although the work did not progress to preparing and testing bulk rubberised concrete specimens.
Pelisser et al. [7] reported to have found a more effective treatment through the alkaline activation of rubber with NaOH and the use of silica fume. Unfortunately, in considering the final cost of their treatment, they dismissed this method as it would not have been viable for use in industry. It is clear that any experimental process that succeeds in improving strength must also be capable of application, both practically and economically, for it to be industrially applied. Keeping this in mind, the work presented here sought to employ ultraviolet (UV) radiation as a method of surface pre-treatment. Its effect was investigated initially via a simple water retention protocol (described below) and ultimately by a programme of bend tests, to obtain flexural strength data.
The effect of UV on polymers is well documented [8]. UV has the ability to modify the surface of polymers through a range of mechanisms. Some, like photo oxidation, generate free radicals in the presence of air (via oxygen or ozone) which can cross-link polymer chains, thereby raising stiffness and reducing toughness. Chain scission can also occur in some systems. The presence of free radicals species, polar groups, and dangling bonds, together with changes in surface free energy [9] is thought to be the reason why UV exposure is effective in also improving adhesive bonding to such surfaces. The degradation of rubber via exposure to UV radiation is well known [8] and usually manifests itself as an increase in brittleness due to further crosslinking of the elastomer structure. It was therefore hoped that UV could alter the adhesion of rubber to aqueous based systems, by changing its polar surface energy. Other surface treatments have been previously employed in order to modify adhesion to polymers in general, including electrical discharge [10] and flame-based methods, although none of these have enjoyed adoption with regards to commercial RCC materials. Assuming a beneficial outcome, UV pretreatment would provide a cost-effective method for incorporation of rubber into RCC, largely due to its clean, contactless, and easily implemented characteristics.
The authors are not aware of UV being used previously in this application and have investigated its potential over a number of years. The study described here utlized rubber in a cement matrix (i.e as an additive) as opposed to rubber as an aggregate substitute, in concrete. This differed from the cited litertaure which almost universally utilizes the latter. The justification for this was simply that the focus was on interfacial bond strength (between rubber and matrix) as the weak link in the resultant composite. Given that there is no expectation that rubber would bond directly to
aggregate, it seemed sensible to eliminate this non-influential variable from the study. The authors recognise that this may make direct comparisons with other studies difficult but it should not invalidate any benefits accrued. In any case, overall rubber volume fractions were chosen to be broadly similar (see Section 4).
The following sections describe the preliminary work undertaken to investigate the benefit of UV treatment on mechanical properties, and in particular the effect of UV wavelength and time of exposure. Further aspects of the treatment will be covered in forthcoming publications. Section 2 outlines the main features of the UV treatment process. Sections 3 and 4 describe the initial testing protocols used to evaluate the effect of UV irradiation. Section 5 constitutes a discussion of the results obtained and consideration of possible underlying mechanisms together with analysis of further tests carried out to characterize the nature of the cement-rubber interface.
2. Materials, Apparatus & Treatment
The cement used for practical experimentation was sourced from Cemex Ltd. (Cemex Rugby+ Premium Cement). This is a Portland cement containing added lime, which raises the workability. Its oxide composition by mass is given in Table 1 [11].
Table 1 Cement composition [11] % SiO2
17.6
Al2O3
4.3
Fe2O3 CaO
61.3
MgO
1.1
2.9
SO3
3.1
Cl equiv. Na2O Loss on ignition
-
0.05
Free CaO
2.7
0.7 8.7
The rubber was sourced from Crumb-Rubber UK in the form of fine, high quality, rubber crumb. This originates from used car tires and buffings from truck tires created during retreading. Tires are reduced to crumb through a process of initial shredding, granulating, and milling, by a variety of commercial organisations. During the size reduction process the crumb may be passed through equipment to remove metal, fibres, and any stones. The resulting product is then
processed by Crumb-Rubber UK to create even finer milled particles, with exceptionally low quantities of contaminants. The general composition of the tire crumb as received by Crumb-Rubber UK is shown in Table 2 [12].
Table 2 General composition of tyre crumb [12] % mass Elastomers
45 - 50
Carbon Black
20 - 25
Metal
12 - 25
Textile
5 - 10
After processing, which is entirely mechanical in nature, a high quality regular sized product is created with particle sizes in a range from 420 to 840 µm, with about 50 % of the material sized at 620 µm. A typical particle is shown in Fig. 1.
Fig. 1. SEM image of a typical rubber particle.
70 60
% of batch
50
BATCH 1
BATCH 2
BATCH 3
BATCH 4
BATCH 5
BATCH 6
40 30 20 10 0 pan
180
250
425
600
840
1000
2000
2800
4000
Particle Diameter (µm) Fig. 2. Size scatter of rubber particles [13].
Use of a consistent and well characterized crumb rubber for this study was aimed at minimising variables and batchto-batch differences that could otherwise influence the mechanical properties of the RCC and/or mask the effect of the UV treatment. Fig. 2 indicates the spread of the final crumb size within, and between, individual batches.
The mass ratio for cement to water, used for the mechanical test specimens, was 3.1 : 1 (i.e. water/cement ratio of 0.32). Theoretically a 4:1 ratio is known to yield better mechanical properties [14], although this creates a very stiff and unworkable cement paste. However, given that achievement of maximum strength was not the objective of this study, an initial 4:1 ratio was selected, successively adding water until adequate workability was obtained. This was found to correspond to the final 3.1:1 ratio.
In order for the UV pretreatment to have a clear impact upon the measured strength, 15 vol.% of rubber was utilised. This rationale is further explained in Section 4 below. Additionally, no inorganic aggregate was incorporated, for similar reasons.
UV exposure was achieved using a simple light box apparatus, incorporating an array of linear light sources similar in dimension to conventional fluorescent tubes. A range of sources was employed in an attempt to determine the optimum UV frequency. These had a spectral response in either the UV-A, B or C range; all had a wattage of 18 W,
and were 610 mm long and of 26 mm in diameter. Spectral responses were obtained from the manufacturers from which it was determined that the UV-A sources emitted light in a range between 320 and 400 nm with their peak at 360 nm [15], the UV-B sources emitted only 12 % of their light in the UV-B spectrum and the rest in the UV-A spectrum [16] and the UV-C sources emitted the vast majority of their electromagnetic radiation at a wavelength of 253.7 nm [17].
Rubber crumb was treated by spreading it out on a reflective metal substrate to an even thickness of approximately 2 mm. This ensured that as much surface area of each rubber particle was exposed to the UV radiation as was possible without mechanical rotation. Crumb was exposed for set periods of time and then rapidly (within 10 minutes of final exposure, to avoid any possible temporal effects) incorporated into the cement paste, which was then subsequently cast into test specimens. Although UV exposure should result in no rise in temperature of the rubber particles (beyond the variations normally expected in ambient temperature), the time elapsed between the end of UV exposure and subsequent experimental procedures (e.g. incorporation/mixing with cement) allowed for the rubber particles to return to ambient temperature.
As a prelude to mechanical testing, and to minimise the number of specimens to be tested, the effect of UV wavelength was determined by using a simple water retention test as a proxy. The justification for this is described in the following section.
3. Water Retention Test
It is well known that physical bonding (e.g. via van der Waals interactions) is at the heart of most adhesion mechanisms [18]. Chemical bonds formed by reaction can occur in some adhesive-adherent systems, but are generally not the norm. Polar interactions can often be enhanced by surface pretreatments. The level of polar interaction can often be established by characterizing the degree of wetting that is achievable between adhesive-adherent pairs [20]. In the case of cement bonding to rubber, therefore, the extent of wetting by water or other aqueous liquids (in effect the hydrophilicity) could, in theory, indicate the likelihood of an adhesive bond between the two components.
Surface wetting is usually measured by optical determination of contact angle [7], however for this work it was more appropriate to develop a simpler methodology based on the ability of rubber crumb to retain water after total
immersion. Crumb was UV-treated, and weighed batches were immersed in an excess of distilled water, stirring to ensure coverage. Given that the UV-C fluorescent tubes were designed to emit a single frequency, the rubber should not experience any temperature change during exposure. Nonetheless, immersion occurred within 10 minutes of final exposure to UV allowing for cooling to the ambient. The resulting mix was drained, and immediately weighed to determine the retained water. A protocol was applied to ensure commonality between the batches in terms of immersion time, level of stirring and drainage time.
The results of the water retention test for various UV exposure times and principal source wavelengths are shown in
% mass increase vs control specimen
Fig. 3.
35% 30% 25% 20% UV - A UV - B UV - C
15% 10% 5% 0% 5
10
15
20
25
30
-5% -10%
Exposure time (hrs)
Fig. 3. Average water retention results for all UV ranges (Initial rubber mass of 100g, 3 tests per condition/bar). Results are relative to a control specimen with no UV exposure.
The absence of a trend between increasing exposure time and water retention for UV-A and UV-B indicates that these two wavelength ranges have very little or no effect on the level of rubber wettability. Indeed the water retention test results for UV-A and UV-B all lie close to, or within, a ±6% scatter in retention that was observed in the control batches (no UV) (average water retention for the control batch was 39.5g).
UV-C, on the other hand, presented a continuous increase in the water retention with time of exposure showing the benefits of the treatment, at least in terms of wettability. This was in line with expectations, given that UV-C implies higher energy levels due to its higher frequency, and hence a better ability to create dangling bonds and polar species.
All subsequent mechanical testing of RCC was therefore carried out using UV-C treated rubber.
4. Mechanical Testing
Flexural testing of RCC specimens was chosen as the most appropriate means of characterising the effect of rubber incorporation and pretreatment. A compression test would have been more conventional, however the quantity of treated rubber required (given the number of specimens needed for adequate statistical rigour) would have been prohibitive. A flexural test, however, generates failure via tensile stress states, and may at first seem inappropriate for a material principally used in compression. However, given that the UV pretreatment was designed to improve bond strength between the cement matrix and the rubber aggregate, a tensile regime was deemed a better indicator of its effect. It should also be noted that, in any case, tensile forces are in operation during pure compression tests [20, 21]. This implies that a link can be drawn between the projected compressive strength of a RCC and its tensile behaviour. This is discussed further below.
To achieve a reliable average for the flexural strength, 50 specimens were tested, with 10 specimens for each of the 5 conditions. Two control conditions were tested as a comparison for the UV-treated specimens: one not containing any rubber and the other containing rubber as supplied, without any pre-treatment. The three UV-C treated conditions required the production of specimens containing rubber exposed to UV-C between 20 and 60 hours in 20 hour intervals, in order to obtain an initial idea of the relationship between time of exposure and flexural strength. This number of specimens was deemed sufficient to justify the calculation of a Weibull distribution, in order to give an indication of the variability in the flexural strength.
As the objective of these tests was not to find the optimal volume fraction of rubber for use in concrete, any arbitrary percentage could have been selected. Preliminary experiments confirmed that the loss in strength of the rubber-cement composite increased with volume fraction of fine (un-treated) rubber particles, in keeping with published literature, in
the range up to 50 vol.%. Khatib and Bayomy [22] suggested that rubber content should not exceed 20% of the total aggregate volume, as otherwise the strength is too severely degraded through loss of the supporting cement matrix. Therefore to ensure sensitivity to the effect of the pretreatment for the rubber-cement composites, a volume fraction of 15% was chosen as a compromise between generating sufficient weakness (to be subsequently mitigated), without so gross a deterioration in strength (which could overwhelm and mask any benefit). Moreover, a comparison with the relevant published literature shows that such a rubber volume fraction (in a cement matrix) is broadly similar to that adopted by other workers (e.g. [1, 3, 22]) when testing rubber as a replacement for aggregate, (i.e. when calculated as a volume fraction of the overall concrete mixture).
Each specimen was cured in air for a period of 7 days, at which point the cement matrix would have achieved about 85 % of its final strength [23]. This allowed testing to be expedited. Standard curing protocols (e.g. ASTM C109) normally require specimens to be cured in water, however the aim of the testing protocols adopted by the authors was to emphasize the role of interfacial bonding, rather than achieve the best strength values for the rubber-cement composite, and given the comparative nature of the study, the mode of curing was not critical (as long as it was consistent).
Metal moulds were employed to cast the specimens, with internal dimensions of 100 x 50 x 17.5 mm. A silicone mould release was employed, and cured specimens were level ground to produce better consistency. The specimens were loaded in 3-point bend at a cross-head speed of 1 mm/min in a Hounsfield H5KS universal tester. Circular anvil supports (ground steel) were used to minimise any crushing effects. An anvil spacing of 75 mm was employed for practical reasons. Although standard flexural testing protocols (e.g. ASTM C348) suggest a smaller span, the authors did not feel that the value adopted would significantly alter the failure mode of the specimens and adversely impact the results.
Initial results from testing of all specimens are given in Table 4. The height of each specimen was measured postfailure across the fractured cross-section.
The flexural strength was calculated using: σf = (3PL)/(2bh^2)
where σf = flexural strength (MPa), P = fracture load (N), L = distance between anvil supports (mm), b = base length of the specimen (mm), and h = height of the specimen (mm).
Fig. 4 and Table 4 show the average values for the flexural strength (σf) for the different durations of UV-C exposure along with additional data, such as the variability of the average values. It is important to note that in Table 4 the average flexural strength is taken as the mean of all the individual flexural strength values calculated for every specimen tested, and not by using the tabulated mean values of maximum force (Pmax) and height. To aid interpretation of Fig. 4, the key to the specimen codes is contained in Table 3.
Table 3 Key to specimen codes Specimen code
vol.% of rubber
Treatment Curing time time (hrs) (days)
R0ND7
0
n/a
7
R15ND7
15
0
7
R15U20D7
15
20
7
R15U40D7
15
40
7
R15U60D7
15
60
7
5. Discussion
From the mechanical testing data gathered, it can be seen that the result of the addition of untreated rubber to the cement mixture causes an immediate loss in flexural strength, as was expected from the published literature. This behaviour supports the hypothesis that untreated rubber acts as a void inside the cement and therefore creates stress concentrations at the rubber-cement interface. As treatment time increases, the flexural strength clearly improves until reaching a plateau at approximately 40 hours. As a check, a separate side study also indicated that similar levels of improvement (percentage-wise) were detected even if the rubber volume fraction was raised to 35 vol. %. These results are not reported here in detail. Drawing a link to the water retention test results and assuming water retention is a valid proxy for surface adhesion, this appears to affirm that an improvement in the bonding between cement and rubber is leading to an improved flexural strength. As was discussed earlier, this is highly likely to be due to the ability of rubber particles to transfer and support load once a degree of bonding between them and the matrix has been
achieved. In effect the rubber can be thought of as acting to increase the fracture toughness of the RCC by created load-bearing bridges across the pore-like defects formed via the inclusion of the crumb.
A noticeable reduction in the scatter of the flexural strength data on addition of rubber crumb can also be seen, commensurate with the introduction of a more uniform defect population. The rubber crumb creates this uniformity by generating voids of similar size in the matrix. This effect can also be seen in the Weibull modulus figures.
Flexural strength (MPa)
7.5 7.0 6.33
6.5
5.96
6.0 5.5
5.94
5.21 4.98
5.0 4.5 4.0
R
R 15 U
60 D 7
D 7 15 U 40
R 15 U 20 D 7
D 15 N R
R
0N
D
7
7
3.5
Specimen code Fig. 4. Average flexural strength for each condition.
Table 4 Summary of average data gathered Specimen code
Exposure time (hrs)
P max (N)
h (mm)
σf (MPa)
St. Dev.
% change over % change rubberless over 0 hrs
Weibull Modulus
R0ND7
n/a
805.63
17.68
6.33
0.73
0.0%
27.0%
8.31
R15ND7
0
647.46
17.55
4.98
0.25
-21.2%
0.0%
18.91
R15U20D7
20
676.04
17.13
5.21
0.35
-17.7%
4.6%
14.39
R15U40D7
40
803.71
17.78
5.96
0.52
-5.8%
19.6%
10.73
R15U60D7
60
878.46
17.60
5.94
0.51
-6.1%
19.2%
10.63
Whilst the changes in chemical nature of the treated rubber crumb were not investigated in this study, some idea of what may be occuring can be found in the literature [24]. UV radiation contains energies which are of an appropriate level to break chemical bonds in polymers and so create a range of effects [8, 25]. Free radical species can easily be created especially in the presence of ozone (which can form when air is irradiated with UV). These can initiate further chemical reactions including cross-linking and the creation of new functional groups in the surface regions of an irradiated polymer. Such groups would be expected to alter the surface energy of the polymer, and thereby its “wettability”, either by presenting a more chemically reactive surface or simply through enhanced polar interactions. The latter mechanism provides a convincing explanation for the increase in water retention seen in this work, after UV exposure. Similarly, by association, the improvement in flexural strength can be ascribed to the same surface effects, particularly those resulting in dipole interactions, which are commonly regarded as the principal determinants of the strength of adhesive bonds [18].
Given that such interactions are limited to surface regions, it would be expected that the benefit of an UV pretreatment would reach a saturation level, at which point all the possible polar sites had been created. From the flexural strength data obtained (Fig.4) this indeed seems to be the case as no benefit in flexural strength accrues beyond the 40 hour exposure.
It may be possible to improve upon the 40 hour strength data if it transpires that the use of a light box exposure method only treats a proportion of the available surface of individual crumb particles. This may well be the case, and an UV apparatus that continually rotates the crumb has been designed to optimise the exposure methodology [26].
Fig. 5 compares the percentage change in flexural strength, normalised against the flexural strength of the R15ND7 (no UV exposure) specimens, in order to better illustrate the improvement in strength obtained via the UV-C pretreatment. As can be seen, the R15U40D7 and R15U60D7 treated specimens display an improved average flexural strength of about 20% over that of the specimens containing untreated rubber. When compared with cement containing no rubber, which is an average of 27% stronger, it is clear that the UV treatment has restored much of the original strength lost in creating the RCC. Indeed the recovery in strength now lies within the standard deviation scatter band of the rubberless cement specimens (see Fig. 5).
40% 35%
% change in flexural strength
30% 25% 20%
20%
19%
15% 10% 5%
5%
0% -5%
0 0%
10
-10%
20
30
40
50
60
70
UV-C treatment time (hrs)
Fig. 5. Change in flexural strength of RCC with respect to UV-C treatment time (normalized against untreated rubber RCC). Dotted line represents baseline strength for cement containing no rubber.
Khaloo et al [2] and Turatsinze and Garros [3] observed during their tests, that rubberised cement specimens still had some residual strength after initial failure. This was also observed in the present work in all specimens containing rubber, whilst the rubberless specimens failed catastrophically, as was expected.
Once the matrix has failed, rubber particles on the newly created cracked surfaces still hold the two faces together, as the crack will have effectively propagated around them. The residual strength will therefore be linked to the strength required for the pull-out of each of these particles and this, in turn, involves debonding from the cement matrix. As a consequence, the improvement of the bond strength through the use of the UV-C pre-treatment should also have beneficial effects on the fracture toughness of the RCC, in addition to its flexural strength.
An additional element that can be gathered from Table 4 (as noted earlier), is that there seems to be a reduction in the standard deviation of the flexural strength of the specimens and hence an improvement in the Weibull Modulus when rubber is incorporated in the cement matrix. This reduction in scatter then seems to worsen as the UV-C treatment time is raised. The initial reduction is most likely a consequence of simply introducing an array of similarly sized
defects (i.e. the rubber crumb) within a brittle matrix. Assuming that these are larger than the intrinsic defects within the cement (i.e. its “natural” porosity), the flexural strength would be expected to drop, and the Weibull Modulus rise, depending upon the uniformity of the dimensions of the crumb. It should be noted that the square root dependency within the Griffith relationship would also be expected to lead to a reduction in the scatter in flexural strength with an increase in crack size (for a constant scatter in defect size). The reduction in the Weibull Modulus with increasing UV-C treatment time is more difficult to explain, although it may also have its roots in the non-linearity of the Griffith relationship, if it is assumed that enhanced cement-rubber bonding causes a reduction in the “effective” crack size.
Clearly, therefore, the UV-C pre-treatment has been beneficial, whether it is due to a reduction in “effective” crack size (via improved bonding between the matrix and rubber), or a change in the “effective” fracture toughness of the composite (via crack bridging of existing flaws by the bonded rubber particles), or via a rise in global fracture toughness (due to energy absorbtion processes during localised debonding, pull-out etc).
Although pure compression tests were not performed, the results from the flexural test can be considered a valid representation of the behaviour of the material under compression. This is due to the fact that the crack initiation and failure mechanisms of brittle materials in compression are initially very similar to the ones under tensile conditions, assuming a population of crack-like defects. Griffith [20] and Inglis [21] showed that in such situations it is not the theoretical compressive strength of the material that governs failure in compression, but rather its tensile strength. In a component undergoing compression, tensile stresses develop in the proximity of a pore or defect, as shown in Fig. 6. Therefore, under a global compressive load, it is a tensile force that causes initial crack propagation and failure although following initiation, the failure mode may then alter, hence generating a difference between the ultimate tensile strength, and the “practical” compressive strength. This therefore makes it possible to extrapolate the behaviour of a “defective” brittle material in compression, from its behaviour under a tensile or flexural test. Thus the improvement found in the flexural strength of the RCC after UV-C irradiation would also be indicative of a beneficial effect upon the compressive strength.
P σy
τxy
σt
σx
τyx
Fig. 6. Stresses on an element around a defect in compression (adapted from Griffith [20] and Lajtai [27]), where τ refers to shear, σx and σy to compressive stresses and σt to tensile stresses respectively.
A microstructural investigation of the fracture surface of the mechanical test specimens was also carried out in an attempt to understand the nature of the cement-rubber interface. Given an improvement in the bonding between rubber and cement that was deduced from the mechanical testing, SEM fractography (Fig. 7) could reasonably be expected to reveal changes in the degree of cement debris on the surface of the rubber particles. Additionally poor adhesion (hence poor wetting) might be predicted to affect the topography of the interface region, for example, by creating interfacial voids. The occurrence of these would be expected to reduce as the bonding improved. The microstructural analysis indeed detected discernable differences in the fraction of residual cement debris on exposed rubber particles together with distinct evidence for tighter rubber-cement contact in UV-treated specimens. These observations therefore supported the flexural testing results. The existence of debris suggests that the adhesive bond strength (i.e. that between rubber and cement) had improved to the point at which it was stronger than the cohesive strength between the weaker cement particles. Small amounts of residual cement debris were, nevertheless, present on the surface of untreated rubber (Fig. 7a) and this is probably the result of statistical weaknesses in the cement-cement bonds.
Fig. 7. SEM fractographs of representative specimens, all showing particulate matter (lighter spots) on embedded rubber particles (central objects). a) R15ND7 (no UV) and b) R15U60D7 (60 hrs UV).
The fractography results prompted additional testing to further confirm that the strength improvements measured in the mechanical tests were due to increases in the adhesive bond strength. A simple pull-out test (Fig. 8) was devised for this purpose with the aim of isolating the rubber-cement interface, and therefore characterizing the changes in rubbercement bond strength, due to the UV pre-treatment. This test was performed by embedding nitrile rubber rod in cement (cured and held in a dedicated specimen holder) and measuring the load required to shear the rod from the holder. Nitrile rubber was selected as it is nominally of a similar chemical make-up to tire rubber, and should therefore behave similarly with exposure to UV. Bulk mechanical properties were also similar. Standard nitrile rod for the fabrication of bespoke O-rings was utilized. This had a smoother surface texture than the tire rubber crumb, which served to remove the effect of any mechanical interlocking, thereby ensuring that the pull-out load measured was mainly a function of bond strength.
Two conditions were employed, a set of nitrile controls (no UV-C treatment) and nitrile specimens treated for 30 hours with UV-C. To help minimise statistical error, 7 specimens for each condition were employed. The same testing machine and cross-head speed adopted for the earlier 3-point flexure tests were utilised for the pull-out testing.
F
L
d
Fig. 8. Pull-out test specimen schematic.
The shear failure stress was calculated using: τs = F/(πdL)
where τs = shear failure strength (MPa), F = maximum load (N), d = diameter (mm) and L = length of the embedded rod (mm).
Both the length of the embedded rod and the diameter were maintained constant throughout all specimens and were 40 mm and 4 mm, respectively.
Table 5 Summary pull-out test results E xposure Tim e (hrs)
F (N)
Extension (mm)
τs (MPa)
0
11.47
11.24
0.014
30 Δ%
18.59 62.1%
13.21 17.6%
0.022 62.1%
Results from the pull-out tests indicated a dramatic improvement in the shear failure stress after surface treatment with UV-C. The shear stress should, in theory, be at least representative of the adhesive bond strength between the rubber and the cement, although in practice there will also be a frictional contribution (which would act to raise the failure load). Conversely the low stiffness of the rubber, in combination with its Poisson’s ratio, would act to create a significant cleavage component at the interface between free and embedded rubber and this would help the rubber pull
away from the surrounding cement matrix and would therefore act to reduce the required pull-out force. The cleavage behaviour may be assumed to be a constant factor as the bulk mechanical properties of the rubber (such as stiffness) are unchanged by the UV treatment. Similarly, the frictional component should not vary with irradiation, given that the UV-C treatment did not appear to alter the rubber’s surface roughness, and hence the frictional coefficient should remain unchanged. It is therefore reasonable to assume that the increase in the shear failure stress is mainly due to an enhancement of the adhesive bond strength between rubber and cement, and that this is central to the improvement in flexural strength of the RCC seen in this work, following UV-C irradiation of the rubber phase.
6. Conclusions
The work presented here has confirmed the previously observed detrimental effect upon strength, generated by the insertion of rubber in a cement matrix, however, using a UV based pre-treatment on the rubber crumb has been shown to reduce the majority of these strength losses. This is largely thought to be due to an improvement in interfacial bonding between the cement matrix and the rubber. By means of optimising the UV treatment, either by changes in the wavelength employed, and/or by modifying the apparatus to enhance treatment uniformity, it is believed that further benefits may be forthcoming. The effect of the UV-C pre-treatment on flexural strength should also translate into an improvement in compressive strength, although the level of mitigation might be different due to the variation in crack propagation modes, under compression, once failure has initiated, and supplementary work is being carried out in order to further clarify this, and other aspects influencing the rubber-cement interface, such as alternative and combinatorial pretreatments to improve upon the observed results.
Acknowledgement
The authors would like to thank M. Aitken of Crumb-Rubber UK for providing them with the rubber crumb used in this study.
References
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Fig. 1. SEM image of a typical rubber particle [13]. Fig. 2. Size scatter of rubber particles [14]. Fig. 3. Average water retention results for all UV ranges (Initial rubber mass of 100g, 3 tests per condition/bar). Results are relative to a control specimen with no UV exposure. Fig. 4. Average flexural strength for each condition. Fig. 5. Change in flexural strength of RCC with respect to UV-C treatment time (normalized against untreated rubber RCC). Dotted line represents baseline strength for cement containing no rubber. Fig. 6. Stresses on an element around a defect in compression (adapted from Griffith [21] and Lajtai [29]), where τ refers to shear, σx and σy to compressive stresses and σt to tensile stresses respectively. Fig. 7. SEM fractographs of representative specimens, all showing particulate matter (lighter spots) on embedded rubber particles (central objects). a) R15ND7 (no UV) and b) R15U60D7 (60 hrs UV). Fig. 8. Pull-out test schematic.
Table 1 – Cement composition [12]. Table 2 – General composition of tyre crumb [13]. Table 3 – Key to specimen codes. Table 4 – Summary of average data gathered. Table 5 – Summary of pull-out test results.