Farhan, Ahmed Hilal and Dawson, Andrew Robert and Thom, Nicholas Howard and Adam, Sarhat Mustafa and Smith, Martin John (2015) Flexural characteristics of rubberized cement-stabilized crushed aggregate for pavement structure. Materials & Design, 88 . pp. 897-905. ISSN 0261-3069
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Flexural characteristics of rubberized cement-stabilized crushed aggregate
for pavement structure
Ahmed Hilal Farhan*, 1, 2, Andrew Robert Dawson1, Nicholas Howard Thom1, Sarhat
Adam1, Martin John Smith1
Accepted manuscript
http://dx.doi.org/10.1016/j.matdes.2015.09.071
0264-1275/© 2015 Elsevier Ltd. All rights reserved.
1
Flexural characteristics of rubberized cement-stabilized crushed aggregate1
for pavement structure2
3
Ahmed Hilal Farhan*, 1, 2, Andrew Robert Dawson1, Nicholas Howard Thom1, Sarhat4
Adam1, Martin John Smith15
1 School of Civil Engineering, Faculty of Engineering, University of Nottingham, University Park,6
Nottingham NG7 2RD, UK, Tel: +44 (0) 7448461314, E-mail: [email protected]
2 Civil Engineering Department, College of Engineering, Al-Anbar University, Iraq8
*corresponding author9
10
11
Abstract12
The purpose of this paper is to investigate the flexural characteristics and to quantitatively13
study the flexural-induced cracking of reference and rubberized cement stabilized aggregate14
mixtures. Four volumetric replacement percentages (0%, 15%, 30% and 45%) of 6 mm15
fraction size were used. This modification was found to affect the material strength16
detrimentally. However, material toughness was improved and stiffness was reduced. The17
latter findings were supported by quantitative assessment of the fractured surfaces which18
revealed more tortuous and rougher cracking as a result of rubber content increasing. This, in19
turn, may ensure a good load transfer across the cracks after their formation. Overall, using20
rubber in pavement construction is a sustainable solution that ensures consumption of large21
quantities of these waste materials. At the same time, it may considered as a promising22
method to reduce cracking tendency and sensitivity which may improve shrinkage, thermal23
and fatigue performance.24
25
26
Keywords: cement stabilized aggregate; rubberized cement stabilized mixture; flexural27
behaviour; fractured surface characterization; ultrasonic testing28
29
2
1. Introduction30
The increased number of tyres stockpiled every year has created a serious economic and31
environmental problem. Disposal sites may become places for breeding of creatures that32
spread many diseases and can cause significant fire hazard (Zheng et al. 2008). Consequently,33
to deal with these problems and to save natural resources, many studies (Khatib and Bayomy34
1999; Güneyisi et al. 2004; Papakonstantinou and Tobolski 2006; Balaha et al. 2007; Zheng et35
al. 2007; Khaloo et al. 2008; Taha et al. 2008; Zheng et al. 2008; Topçu and Bilir 2009;36
Nguyen et al. 2010; Pelisser et al. 2011; Najim and Hall 2012; Eiras et al. 2014) have been37
conducted to investigate the properties of rubberized concrete. How feasible is it to use waste38
tyres in concrete structure by replacing the fine, coarse or fine and coarse aggregate with39
rubber particles of different sizes and shapes? In general, the above researchers’ findings40
revealed that replacing natural aggregate with rubber particles decreased the strength,41
although some researchers (Jingfu et al. 2009) reported a slight increase in tensile strength.42
However, less stiffness, less brittleness and improvement in toughness was obtained as results43
of aggregate replacement by coarse or fine rubber particles (Balaha et al. 2007; Taha et al.44
2008).45
46
In pavement structures, it is customarily to use cement stabilized aggregate as a base or47
subbase course to increase the structural capacity of that structure, namely in terms of strength48
and stiffness. Cement stabilized aggregate (CSA) is a mixture of aggregate, cement and a49
small amount of water (Lim and Zollinger 2003). Unlike normal concrete, cement stabilized50
aggregate contains a low amount of cement and is constructed by rolling. In spite of the51
similarity between cement stabilized aggregate and roller compacted concrete in terms of52
construction method, the latter usually contains cement contents approaching that used in53
normal concrete (PCA 2005) .54
55
3
Different testing methods are used to characterize the stabilized aggregate mixtures. These56
include uniaxial compressive testing (Lim and Zollinger 2003), direct tensile testing (Shahid57
1997), indirect tensile testing (Hudson and Kennedy 1968; Khattak and Alrashidi 2006) and58
flexural testing (Disfani et al. 2014; Arulrajah et al. 2015). The tensile properties in terms of59
flexural strength and indirect tensile are considered critical for pavement design (Xuan et al.60
2012). However, the flexural testing mode is most preferable test (Arulrajah et al. 2015) used61
since it is the most simulative to what actually happening in the field (Arnold et al. 2012). In62
the Mechanistic- Empirical (M-E) pavement design guide, CSA layers are usually designed to63
resist tensile fatigue failure at the bottom of that layer. This requires estimation of flexural64
strength as an important parameter. Therefore, reduction of stiffness without significant65
strength loss, which rubber addition might achieve, could be an attractive option as it would66
be likely to reduce the applied stress and, hence, extend fatigue life.67
68
A very limited number of studies have been conducted regarding the effect of rubber on the69
behaviour of cement stabilized aggregate intended to be used for pavement structure (Guo et70
al. 2013). Since the construction of highways consumes large quantities of natural resources71
as compared with other engineering applications, using crumb rubber in this application is a72
highly sustainable option (Cao 2007; Barišić et al. 2014). Furthermore, it may mitigate the 73
disadvantages of cement stabilization regarding high stiffness and brittleness. Another74
motivation comes from the fact that since the mix will be compacted, this might ensure a75
good interaction between the rubber and the natural aggregate particles as compared with wet-76
cast concrete mixtures. For the above reasons, the purpose of this paper is to study the effect77
of crumb rubber on the flexural properties of rubberized cement stabilized aggregate (R-CSA).78
79
80
81
82
4
2. Experimental Program83
2.1 Properties of materials used84
The aggregate used in this study is a crushed limestone, with a nominal maximum size of 2085
mm, obtained from Dene quarry in Nottinghamshire, UK. This was collected in different sizes86
(20mm, 14mm, 10mm, 6mm and dust (< 6mm)). These were dried then the gradation for each87
fraction size was estimated by sieve analysis in accordance with BS EN 933-1:2012. The88
crumb rubber (Figure 1) was sourced from J Allcock and Sons Ltd. in Manchester, UK. Its89
gradation is presented in Figure 2. Two reasons are behind selecting this size. Firstly, from the90
economic point of view, this size is cheaper and commonly available as compared with finer91
ones (Najim 2012). Secondly, initial examination showed that the gradation of this size is92
similar to that of the 6 mm natural aggregate fraction (Figure 2) which would be likely to93
enable replacement of some of the latter size without a big change in volumetric relationships94
of the mix. Hence its use would permit estimation of the effect of rubber replacement alone.95
The specific gravity of the rubber was adopted as a 1.12 as measured by (Najim and Hall96
2012). An Ordinary Portland cement CEM I 52.5 R conforming to BS EN 197-1: 2000 was97
used as the binding agent. Potable tap water is used across this investigation.98
99
2.2 Mixture design100
To constitute the required Cement bound granular mixture (CBGM) gradation as stated in BS101
EN 14227-1:2013- [CBGM2-0/20] , all five aggregate fractions sizes were combined together102
in different proportions using the trial and error method (Garber and Hoel 2009). These103
proportions are 11%, 20%, 11%, 13% and 45% for 20mm, 14mm, 10mm, 6mm and dust,104
respectively. The final gradation after blending all sizes in these proportions is illustrated in105
Figure 3. The cement content used to stabilize the aggregate mixture was 5% of the dry106
weight of aggregate. Optimum water content as a percentage of the dry weight of cement and107
aggregate was estimated through the compactibility test in accordance with BS EN 13286-108
4:2003. This was done by constructing a water content-maximum density relationship as109
5
presented in Figure 4. Since the gradations of 6 mm natural aggregate and crumb rubber are110
similar, it was decided to replace the former by the latter volumetrically to help ensure the111
same packing. Thus, investigate the effect of aggregate type. Three volumetric replacement112
percentages were taken into considerations. These are 15%, 30% and 45% of the 6 mm113
aggregate fraction volume, which are equivalent to 2.1%, 4.2% and 6.2% of the total volume114
of the aggregate, respectively. To produce comparable mixtures, the quantity of water and115
cement was kept constant for all replacement levels. Conventionally (and for the reference116
mix) water and cement contents were computed as percentages of the dry weight of aggregate117
in the conventional mix. As rubber replacement would change this weight due to its low118
specific gravity, mix design maintained the same volumetric proportions and hence the same119
opportunities for packing of aggregate with the same surrounding mortar. Table 1 shows the120
investigated mixtures which are designated as follows: C5R0 for the reference mixture121
without rubber whereas C5R15, C5R30 and C5R45 are used to describe the mixtures122
containing 15%, 30% and 45% rubber replacement, respectively.123
124
2.3 Mixing and sample preparation125
All mixing was conducted in a pan mixture with a capacity of 0.1 m3. In terms of mixing126
sequence, the cement and dust were firstly mixed together until a uniform colour was127
achieved then this was added to the rest of aggregate and mixed thoroughly for one minute.128
After that, mixing for another two minutes was done after adding the required quantity of129
water. A standard 100 mm x 100 mm x 500 mm steel mould was used for manufacturing130
prisms. In order to achieve a uniform density, regular dimensions and a smooth surface for131
accurate testing, a mould extension was fabricated to fit on top of the mould and used so that132
more than a 100 mm height was achieved. The specimen was then sawn down to 100 mm133
height. After placement in the mould, the mixture was compacted in three layers using a134
Kango 368 vibrating hammer fitted with a 100 mm x 100 mm square tamper. The compaction135
time was 60 second per layer and each was scarified prior to compacting the next layer.136
6
Triplicate samples were manufactured for each mix. All manufactured samples were left137
inside their moulds and covered with wet paper and polythene sheets to prevent moisture loss.138
After 24 hours, they were demoulded and wrapped with nylon film and placed in wet139
polythene bags and closed tightly. Then, they were moved to the humid room and kept for 28140
days.141
142
2.4 Testing procedures143
Flexural testing was conducted in accordance with BS 188: Part 118:1983. A 200 KN144
capacity closed loop deflection controlled Zwick 1484 universal testing machine was used for145
the static flexural testing program. Four point bending test configuration was used for146
prismatic specimens spanning 300 mm. To obtain the post-peak load-deflection behaviour,147
the test was conducted under deformation control at a stroke rate of 0.05 mm/min and148
corresponding deflection was measured at mid-span of beams using two linear variable149
differential transducers (LVDTs). These were mounted utilizing a yoke arrangement. The150
average value from these two LVDTs was used as a deflection at each load application.151
Figure 5 illustrates the flexural testing configuration.152
153
The ultrasonic pulse velocity (UPV) and dynamic modulus of elasticity were measured non-154
destructively in accordance with ASTM C597 using the PUNDIT- plus apparatus. This was155
done by measuring the stress wave speed by the direct transmission method from transducer156
to receiver. Then, from the measured UPVs values, the dynamic modulus of elasticity (Ed)157
was also calculated as follows (Mardani-Aghabaglou et al. 2013):158
159
ܧ = ρ UPV2(1+ν)(1-2ν)
1 − ν
160
where ρ and ν are the density and Poisson`s ratio, respectively. The bulk density of the161
fabricated beams was estimated using the water displacement method.162
7
3. Results and discussion163
3.1 Bulk density and rubber distribution164
Table 1 shows the effect of rubber substitution on the bulk density of test samples. It can be165
clearly seen that, as expected, replacement of natural aggregate with crumb rubber causes a166
decrease in the bulk density since the specific gravity of rubber particles is much lower than167
that of natural aggregate. This can also be attributed to the higher elasticity of rubber particles168
which might have a detrimental effect on the effectiveness of the compaction. The rate of169
decrease in density was about 16 Kg/m3 for every 1% increase in rubber content which is170
consistent with the findings of Khatib and Bayomy (1999) who investigated the replacement171
with rubber (albeit in conventional concrete) of fines-only, only-coarse fraction and of both172
fine and coarse fractions.173
174
Regarding the rubber distribution, Figure 6 illustrates the rubber distribution (at mid height of175
the sample) for different investigated mixtures. For C5R30 mix, rubber distribution is shown176
at three levels of the prism height (top, mid, and bottom). As shown in the latter figure, a177
uniform rubber distribution was achieved for all replacements. Unlike the normal wet-cast178
concrete mixtures where the rubber may float upwards due to the vibration as well as the179
consistency of this mixture, compacted CSA with relative dry consistency shows a fairly180
uniform rubber distribution across the individual levels. This, in turn, might ensure a uniform181
stress distribution inside the mixture since, otherwise, the accumulation of rubber due to non-182
uniform distribution might cause stress concentrations and accelerate sample failure183
accordingly.184
185
3.2 Flexural properties186
3.2.1 Flexural strength187
Tests were conducted at 28-day age and triplicate specimens were used for each mix. The188
flexural strength was computed utilizing the formula ௦ܨ = /ܮ ℎଶwhere Fs, L, P, b, h are the189
8
flexural strength, span, ultimate load, width of the prism and height of the prism, respectively.190
Figure 7 illustrates the effect of rubber replacement on flexural strength. A clear reduction in191
strength is seen as the amount of replacement increases. It seems that the introduction of192
rubber has an adverse effect on the aggregate interlocking, which can be considered, in this193
type of mixture, as the main factor for frictional resistance development. The latter is the194
mechanism by which the compacted mixtures resist applied traffic loading. In addition, the195
reduction in flexural strength can be attributed also to the drop in both tensile and196
compressive strengths due to replacement of natural aggregate of high strength by the softer197
rubber particles.198
199
3.2.2 Flexural stiffness200
Flexural stiffness as reported by (Turatsinze and Garros 2008) can be calculated as a slope201
(∆P/∆δ) of the linear part of load-deflection curve based on the 30% pf the ultimate load and 202
its corresponding deflection (Arnold et al. 2012) . Figure 7 clearly shows that there was a203
reduction in stiffness of the mixture as a result of rubber replacement. This can be attributed204
to the lower modulus of elasticity of the rubber particles. This confirms previous findings for205
concrete mixtures (Turatsinze and Garros 2008; Najim and Hall 2012).206
207
3.2.3 Flexural toughness208
Toughness can be considered as an indication of the ability of material to absorb energy209
(Erdem 2012) or, in other words, it is an expression of the energy required to fail the210
specimen. Regarding normal concrete, a limited number of researches have been conducted to211
quantify the toughening effect due to rubber replacement (Najim and Hall 2012). Toughness212
was estimated from the area under the load-deflection curves (Figure 8) based on the ASTM213
C 1018 method which is most widely used. This specification defines the toughness in terms214
of three indices (I5, I10 and I20) which were calculated by dividing the area under the load-215
9
deflection curve at deflections of 3, 5.5 or 10 times the first crack deflection, respectively, by216
the area under the curve up to the deflection when the first crack was observed.217
The conclusion that can be drawn from Figure 9 is that the replacement of natural aggregate218
by rubber enhances the toughness of the resulted mixtures. This might be partly because219
rubber particles contribute to delay the micro-crack initiation by stress relaxation (Najim and220
Hall 2012) and/or due to lengthening of the crack path by propagation through the rubber221
particles. Many authors (Toutanji 1996; Aiello and Leuzzi 2010; Guleria and Dutta 2011;222
Najim 2012) have reported similar improvement in toughness due to the inclusion of rubber223
in concrete mixtures. In addition, this means improvement in deformability of material and,224
hence, the formation of a more ductile material (Topcu 1995). In fact, this may ensure a225
mixture with less sensitivity to fatigue cracking which is, as reported by (Brown 2012), the226
main mode of failure in bound base courses of pavement structures. The investigation of227
fatigue characteristics has been undertaken but will be presented in a separate paper.228
229
3.3 Ultrasonic pulse velocity and dynamic modulus of elasticity230
Figure 10 shows the relative dynamic modulus of elasticity and ultrasonic pulse velocity231
(UPV) of rubberized mixtures with respect to the reference one. This figure revealed that a232
decrease in wave velocity by 3.6%, 4.7% and 7.9% occurred when the 6 mm fraction233
aggregate was replaced by 15%, 30% and 45% crumb rubber particles, respectively. There234
was also a commensurate decrease in the compacted dynamic modulus of elasticity values by235
7.7%, 10.65% and 18.5%. These changes may be because of the interlocking of rubber236
particles with natural aggregate causing loss of contact points between stiff aggregate237
particles which in turn would affect the transmission of ultrasonic wave. In addition, the lower238
modulus of elasticity of the rubber particles relative to that of the conventional aggregate may239
explain the reductions. Similarly, in concrete technology, where the UPV and dynamic240
modulus of elasticity is frequently used for assessing concrete quality non-destructively, many241
10
authors (Zheng et al. 2008; Najim and Hall 2012) have reported similar behaviour for242
rubberized normal concrete.243
244
Figure 11 shows that the flexural strength is fairly well correlated with the UPVs. One245
application of this relationship is the possibility to estimate the flexural strength (and hence246
the stress ratio) for the flexural fatigue test. Mechanistic pavement design is significantly247
dependent on the fatigue performance of the bound mixtures, and the stress ratio at which248
they are called to operate plays a crucial role. Stress ratio can be defined as the ratio between249
the applied flexural stress and strength. In the latter test, the conventional approach is to250
measure the flexural strength of the mixture based on the static flexural test then to estimate251
the stress ratios to be applied to the different specimens in flexural fatigue tests. However, due252
to the heterogeneity of CSA, this approach does not necessarily ensure that a representative253
flexural strength is obtained. (Sobhan and Das 2007) tried to overcome this by correlating the254
flexural strength of full-sized beams to the flexural and compressive strengths of beams and255
cubes sawn from fatigue failed specimens. In this way they were able to estimate the actual256
flexural strength and, hence, stress ratio of the fatigued specimens. In this paper, the257
suggested correlation between flexural strength and UPV provides a means of better258
estimating the flexural strength, non-destructively, for the same specimen to be used for the259
fatigue test instead of totally relying on estimation of this parameter based on static flexural260
testing. This methodology may also help to reduce the cost and time of specimen sawing and261
testing.262
263
4 Quantification of the flexural induced cracks264
To provide more understanding regarding the effect of rubber on the behaviour of CSA and265
the mechanism of its failure, a quantitative evaluation of the flexural induced fractured266
surfaces was performed in terms of surface tortuosity and volumetric surface texture ratio267
(VSTR). Tortuosity can be defined as the ratio between the actual crack length and the268
11
projection of that crack (Hassan 2012). In this paper, the earlier definition was extended to269
evaluate the 3D tortuosity as the ratio of the surface area of the fractured surface to the270
projection of that surface. The surface area was calculated according to the methodology271
presented in (Chupanit and Roesler 2008). The VSTR is the surface parameter that can be272
calculated from the volume between the actual surface and the mean plane of the surface as273
shown below (Chupanit and Roesler 2008)274
275
VSTR =∑│R୧��A୧│ + ∑│S୨A୨│
∑(A୧+ A୨)
276
where Ri=distance above the mean plane; Sj= distance below the mean plane; Ai or Aj =277
projected area of each small element (1 mm2).To estimate above surface parameters, it278
necessary to acquire the fractured surface in terms of xyz coordinates to use them in surface279
creation. This was done utilizing the photogrammetry procedure280
281
4.1 Photogrammetric procedure282
It is well known that many methods are available for characterizing and modelling 3D283
surfaces, for example, laser scanning or photogrammetry. There are two main approaches that284
can be used to process digital images to generate 3D surfaces using either algorithms from285
photogrammetry or the structure from motion (SfM) technique which has been largely286
developed by the computer vision community. Using close range photogrammetry, it is287
possible to reach an estimated accuracy of better than 1mm with non-metric cameras288
(Remondino and Fraser 2006). There are a number of commercial photogrammetric software289
packages accurate 3D surface measurements. In SfM however, often the visualisation and290
automation of 3D model is more important than the accuracy. Similar to stereo-291
photogrammetry, SfM can use a set of images acquired with a consumer grade camera to292
generate a 3D surface. The main difference compared to photogrammetry is that image293
acquisition can be more flexible in relative position and attitude of the images. The294
12
procedures that both photogrammetry and SfM share are; camera calibration, image matching,295
and inverse triangulation with bundle adjustment for 3D point cloud generation. Figure 12296
shows this sequence of activities.297
298
The photogrammetric and SfM processes were used to generate the 3D surface modelling of299
fractured concrete samples in terms of a cloud of X, Y, Z coordinated points. To minimize the300
processing time and to help extraction of the coordinates defining the fractured surface, the301
sides of each sample were painted white and placed on a white background. This helped the302
process of removing unwanted point as it is easier to differentiate points on the fractured303
surface from those on the white surfaces. A datum is required to define the coordinate system304
and four specially designed markers of 0.5x0.5 cm were placed on each of the samples as305
shown in Figure 13 and surveyed using a reflectorless total station in order to produce306
coordinates so they could be used as control points in the image orientation process, see307
Figure 12.308
309
Eight fractured samples were modelled and transformed to their ‘total station' coordinate310
system utilizing the aligning tool in Cloud Compare (CC), (CloudCompare 2013) . Cloud311
compare provides details of the transformation process such as the transformation matrix,312
RMSE of the transformation and the recovered scale. With 4 control points on each sample313
the RMSE values of the residuals give a quality estimate of transforming each sample to local314
‘total station’ coordinate system This gives an indication of the estimated quality of the315
coordinates defining the surface. RMSE values for different scanning are tabulated in Table 2.316
As can see from the latter table, significant differences among RMSE values exist between317
different samples of the same mixture especially for C5R0 and C5R15. This can be attributed318
to the process of surveying and measuring targets or the difference in the angle of image319
capturing during image acquisition process, where the RMSEs is a scanning-dependent320
parameter. However, as can be seen from Table 2 all the Root Mean Square Error (RMSE)321
13
values are well below 1mm with an average RMSE value of 0.64mm. This was considered322
acceptable for the purposes of these experiments. Based on the work by Beshr and Abo323
Elnaga (2011), RMSE values of less than 0.2mm are achievable for the coordinates of the324
marks based on a range of less than 2m, and an inclination of less than 45°, so there appears325
to be the possibility of further improving the technique in the future.326
327
4.2 Fractured surface quantification findings328
329
Based on the above procedure the 3D digital fractured surfaces were constructed from the xyz330
coordinates for each surface as shown Figure 14. From these surfaces, the tortuosity and331
VSTR were estimated as mentioned earlier. Figure 15 reveals that the inclusion of rubber332
causes greater tortuosity and VSTR for all replacement levels. This can be explained as333
follows. An increase in the embedded rubber particles increases the number of weak points334
because of the large differences between moduli of rubber and adjacent particles. This, in335
turn, may causes cracks to divert via these weak points, hence changing (and may be336
lengthening) the crack path so that it becomes more tortuous. However, amongst the337
rubberized mixtures, the smaller differences between the value of both VSTR and tortuosity338
might be because the local distribution of the crumb rubber inside the sample. In addition,339
each surface parameter gave different ranking where C5R15 has the larger VSTR while340
C5R30 has the larger tortuosity. (Chupanit and Roesler 2008) have reported the same341
behaviour and they attributed that to unavoidable differences in the scale and resolution when342
scanning to assess both surface parameters. The practical implication of tortuosity increase343
means production of tougher and less brittle materials as observed by (Guo et al. 2007) who344
introduced a brittleness parameter as the inverse of the tortuosity value (brittleness parameter345
=A/As, where As and A represent the surface area of the fracture surface and the projection of346
that surface, respectively). The rougher crack is likely to ensure good load transfer efficiency347
14
across the crack after its formation (Vandenbossche 1999; Vandenbossche et al. 2014) from348
which greater material toughness results.349
350
In support of the above hypothesis that the rubber deflects the crack route through the351
mixture, the amount of rubber per perpendicular unit length across the fractured surface was352
determined by an image processing technique utilizing the ImageJ software. Firstly, the353
fracture surfaces images were captured using a high resolution camera. Then these were354
converted from RGB mode to an 8 bytes greyscale. ImageJ tools were then utilized to conduct355
thresholding (Figure 16) and eventually estimation of the rubber content across this surface.356
The resulting measurement was compared with the quantity used which was known from the357
mix design. It was found, as shown in Figure 17, that the quantity of rubber visible on the358
crack surface is more than that used. Furthermore, this difference increases as rubber content359
increases. The above results confirm that the cracks propagate around rubber particles which360
support the conjecture of longer crack paths. This explanation is further supported by the361
measured toughness index changes.362
363
5. Conclusions364
In this paper, the effect of rubber replacement on the properties of cement stabilized aggregate365
with emphasis on flexural properties was investigated. In the light of the findings the366
following conclusions can be drawn:367
1. Flexural strength, density and stiffness were reduced as a result of rubber replacement.368
However, more ductile mixtures were produced as confirmed by toughness improvement.369
This was further confirmed by ultrasonic testing where both dynamic modulus of370
elasticity and UPV decreased due to the introduction of rubber particles. This indicates371
that both the studied mixture and normal concrete exhibit similar trends of behaviour372
when rubber is added.373
374
15
2. Quantitative assessment of the fractured surfaces revealed that as the rubber replacement375
level increased, the tortuosity and the VSTR of the crack surfaces increased. This means376
more fracture energy was dissipated. These, combined with an evaluation of the rubber377
content on fractured surfaces, may lead to an improved understanding of the mechanism378
of the failure.379
380
3. A good correlation was found between flexural strength and UPV values. This may381
enable accurate estimation of the static flexural strength of the material non-destructively.382
This in turn will help to overcome problems associated with the heterogeneity of this type383
of cementitious mixture when performing fatigue tests.384
385
4. Use of the crumb rubber in compacted cement stabilized mixtures aggregate does not386
experience the same segregation difficulties as encountered in conventional concrete after387
casting and vibration.388
389
5. In spite the decline in the flexural strength of the mixtures due to rubber incorporation,390
the use of rubber will be justified since this detrimental impact on the flexural strength391
may be balanced by the other advantages like improvement in the cracking pattern392
(achieving good load transfer across the crack) and ,importantly, disposing of the waste393
materials. Furthermore, reduction in stiffness will reduce the stress experienced at the394
bottom of the stabilized layer.395
396
6. More investigation is required to assess the performance of rubberized cement stabilized397
mixtures in terms of fatigue performance and durability so as to evaluate and validate398
their use.399
400
401
16
Acknowledgements402
The support from Higher Committee of Education Development in Iraq (HCED) by providing403
a scholarship for this research is gratefully acknowledged. Thanks are also due to Dr. Luis404
Neves for his valuable advices.405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
17
Figure Captions422
Fig.1. Appearance of rubber used in this study.423
Fig.2. Individual grain size distribution of the 6 mm fraction size and rubber.424
Fig.3 Mixture gradation after blending all different sizes.425
Fig.4 Water content- dry density relationship.426
Fig.5 Flexural test configuration.427
Fig.6 Distribution of rubber for different replacement levels: (a) C5R15-mid (b) C5R30-top (c)428
C5R30-middle (d) C5R30-bottom (e) C5R45-mid.429
Fig.7. Effect of rubber replacement on flexural strength and stiffness.430
Fig.8. Load-deflection curves for investigated mixtures.431
Fig.9. Effect of rubber replacement on toughness indices.432
Fig.10. Ultrasonic dynamic moduli and UPVs for different mixtures.433
Fig.11 Relationship between UPV and flexural strength.434
Fig.12. A flow diagram of the photogrammetric and structure from motion processes.435
Fig.13. Fixing markers on the fractured surfaces.436
Fig.14. Samples of fractured surfaces scan: a. C5R0, b. C5R30.437
Fig.15. VSTR and tortuosity of investigated mixtures.438
Fig.16. Analysis of rubber quantity through fractures surface: a. captured image, b.439
thresholded image and c. thresholding process.440
Fig.17. Effect of replacement level on amount of rubber across the fractured surfaces.441
18
Table 1: Investigated mixtures details and designation442
Mixture
Cement content(% by
wt. of aggregate and
cement of control mix)
Rubber content(%by
volume of 6mm
fraction size)
Water (% wt. of
aggregate and
cement of control
mix)
Bulk
Density
Kg/m3
C5R0 5 0 4.62529.647
(0.1%)
C5R15 5 15 4.62494.5
(0.17%)
C5R30 5 30 4.62456.433
(0.2%)
C5R45 5 45 4.62418.533
(0.24%)
Values in the brackets are the coefficient of variation.443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
19
Table2: RMSE of the residuals at the control points on the fractured surface463
464
SampleID
C5R0-1 C5R0-2 C5R15-1 C5R15-2 C5R30-1 C5R30-2 C5R45-1 C5R45-2
RMSE(mm)
0.54 0.20 0.68 0.44 0.84 0.82 0.88 0.73
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
20
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
Figure 1: Appearance of rubber used in this study.
21
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
Figure 2: Individual grain size distribution of the 6 mm fraction size and rubber
22
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
Figure 3: Mixture gradation after blending all different aggregate sizes.
23
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
Figure 4: Water content- dry density relationship.
24
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
Figure 5: Flexural test configuration
25
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
Figure 6: Distribution of rubber for different replacement levels: (a) C5R15-mid (b) C5R30-
top (c) C5R30-middle (d) C5R30-bottom (e) C5R45-mid
26
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
Figure 7: Effect of rubber replacement on flexural strength and stiffness
27
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
Figure 8: Load-deflection curves for investigated mixtures.
28
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
Figure 9: Effect of rubber replacement on toughness indices
29
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
Figure 10: Ultrasonic dynamic moduli and UPVs for different mixtures
30
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
Figure 11: Relationship between UPV and flexural strength.
31
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
Figure 12 A flow diagram of the photogrammetric and structure from motion processes.
32
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
Figure 13: Fixing markers on the fractured surfaces.
33
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
Figure 14: Samples of fractured surfaces scan: a. C5R0, b. C5R30
(a)
34
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
(b)
35
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
Figure 15: VSTR and tortuosity of investigated mixtures
36
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
Figure 16: Analysis of rubber quantity through fractures surface: a. captured image,
b. thresholded image and c. thresholding process
37
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
Figure 17: Effect of replacement level on amount of rubber across the fractured surfaces.
38
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