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: evxahfa@nottingham.ac.uk7

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

References1018

Aiello, M. A. and F. Leuzzi (2010). "Waste tyre rubberized concrete: properties at fresh and hardened1019

state." Waste Manag 30(8-9): 1696-1704.1020

Arnold, G., C. Morkel, et al. (2012). Development of tensile fatigue criteria for bound materials.1021

Arulrajah, A., M. M. Disfani, et al. (2015). "Modulus of rupture evaluation of cement stabilized1022

recycled glass/recycled concrete aggregate blends." Construction and Building Materials 84:1023

146-155.1024

Balaha, M. M., A. A. M. Badawy, et al. (2007). "effect of using ground waste tire rubber on the1025

behavior of concrete mixes." Indian journal of Engineering and Materials Science 14: 427-435.1026

Barišić, I., S. Dimter, et al. (2014). "Strength properties of steel slag stabilized mixes." Composites 1027

Part B: Engineering 58: 386-391.1028

Beshr, A. A. A. and I. M. Abo Elnaga (2011). "Investigating the accuracy of digital levels and1029

reflectorless total stations for purposes of geodetic engineering." Alexandria Engineering1030

Journal 50(4): 399-405.1031

Brown, S. F. (2012). "An introduction to asphalt pavement design in the UK." Proceedings of the ICE-1032

Transport 166(4): 189-202.1033

Cao, W. (2007). "Study on properties of recycled tire rubber modified asphalt mixtures using dry1034

process." Construction and Building Materials 21(5): 1011-1015.1035

Chupanit, P. and J. R. Roesler (2008). "Fracture energy approach to characterize concrete crack1036

surface roughness and shear stiffness." Journal of Materials in Civil Engineering 20(4): 275-1037

282.1038

CloudCompare (2013). 3D point cloud and mesh processing software1039

Open Source Project, EDF.1040

Disfani, M. M., A. Arulrajah, et al. (2014). "Flexural beam fatigue strength evaluation of crushed brick1041

as a supplementary material in cement stabilized recycled concrete aggregates."1042

Construction and Building Materials 68: 667-676.1043

Eiras, J., F. Segovia, et al. (2014). "Physical and mechanical properties of foamed Portland cement1044

composite containing crumb rubber from worn tires." Materials & Design 59: 550-557.1045

Erdem, S. (2012). Impact load-induced microstructural damage of concrete made with1046

unconventional aggregates. PhD thesis, University of Nottingham.1047

Garber, N. J. and L. A. Hoel (2009). Traffic & highway engineering, Cengage Learning.1048

Guleria, S. and R. Dutta (2011). "Unconfined compressive strength of fly ash–lime–gypsum composite1049

mixed with treated tire chips." Journal of Materials in Civil Engineering 23(8): 1255-1263.1050

Güneyisi, E., M. Gesoğlu, et al. (2004). "Properties of rubberized concretes containing silica fume." 1051

Cement and Concrete Research 34(12): 2309-2317.1052

Guo, H. M., H. Zhu, et al. (2013). "The Applied Research of Waste Crumb Rubber in Road Base."1053

Applied Mechanics and Materials 253: 317-321.1054

Guo, L.-P., W. Sun, et al. (2007). "Study on the flexural fatigue performance and fractal mechanism of1055

concrete with high proportions of ground granulated blast-furnace slag." Cement and1056

Concrete Research 37(2): 242-250.1057

Hassan, N. A. (2012). Microstructural characterization of rubber modified asphalt mixtures PhD, The1058

university of Nottingham.1059

Hudson, W. R. and T. W. Kennedy (1968). An indirect tensile test for stabilized materials, Center for1060

Highway Research, University of Texas at Austin.1061

Jingfu, K., H. Chuncui, et al. (2009). "Strength and shrinkage behaviors of roller-compacted concrete1062

with rubber additives." Materials and Structures 42(8): 1117-1124.1063

Khaloo, A. R., M. Dehestani, et al. (2008). "Mechanical properties of concrete containing a high1064

volume of tire-rubber particles." Waste Management 28(12): 2472-2482.1065

Khatib, Z. K. and F. M. Bayomy (1999). "Rubberized Portland cement concrete." Journals of Materials1066

in Civil Engineering, ASCE 11: 206-213.1067

Khattak, M. J. and M. Alrashidi (2006). "Durability and mechanistic characteristics of fiber reinforced1068

soil–cement mixtures." International Journal of Pavement Engineering 7(1): 53-62.1069

39

Lim, S. and D. G. Zollinger (2003). "Estimation of the compressive strength and modulus of elasticity1070

of cement-treated aggregate base materials." Transportation Research Record: Journal of the1071

Transportation Research Board 1837(1): 30-38.1072

Mardani-Aghabaglou, A., Ö. Andiç-Çakir, et al. (2013). "Freeze–thaw resistance and transport1073

properties of high-volume fly ash roller compacted concrete designed by maximum density1074

method." Cement and Concrete Composites 37: 259-266.1075

Najim, K. B. (2012). Determination and enhancement of mechanical and thermo-physical behaviour1076

of crumb rubber-modified structural concrete. PhD thesis, The University of Nottingham.1077

Najim, K. B. and M. R. Hall (2012). "Mechanical and dynamic properties of self-compacting crumb1078

rubber modified concrete." Construction and Building Materials 27(1): 521-530.1079

Nguyen, T., A. Toumi, et al. (2010). "Mechanical properties of steel fibre reinforced and rubberised1080

cement-based mortars." Materials & Design 31(1): 641-647.1081

Papakonstantinou, C. G. and M. J. Tobolski (2006). "Use of waste tire steel beads in Portland cement1082

concrete." Cement and Concrete Research 36(9): 1686-1691.1083

PCA (2005). "soil cement technology for pavement different products for different applications."1084

Portland Concrete Association(illinois).1085

Pelisser, F., N. Zavarise, et al. (2011). "Concrete made with recycled tire rubber: Effect of alkaline1086

activation and silica fume addition." Journal of Cleaner Production 19(6-7): 757-763.1087

Remondino, F. and C. Fraser (2006). "Digital camera calibration methods: considerations and1088

comparisons." International Archives of Photogrammetry, Remote Sensing and Spatial1089

Information Sciences 36(5): 266-272.1090

Shahid, M. A. (1997). "Improved Cement Bound Base Design for Flexible Composite Pavement." PhD1091

Thesis, University of Nottingham.1092

Sobhan, K. and B. M. Das (2007). "Durability of soil–cements against fatigue fracture." Journal of1093

Materials in Civil Engineering 19(1): 26-32.1094

Taha, M. M. R., A. S. El-Dieb, et al. (2008). "Mechanical, Fracture, and Microstructural Investigations1095

of Rubber Concrete." Jounal of Materials in Civil Engineering, ASCE 20: 640-649.1096

Topcu, I. B. (1995). "The properties of rubberized concrete." Cement and Concrete Research 25(2):1097

304-310.1098

Topçu, İ. B. and T. Bilir (2009). "Experimental investigation of some fresh and hardened properties of 1099

rubberized self-compacting concrete." Materials & Design 30(8): 3056-3065.1100

Toutanji, H. A. (1996). "The Use of Rubber Tire Particles in Concrete to Replace Mineral Aggregates."1101

Cement & Concrete Composkes 18: 135-139.1102

Turatsinze, A. and M. Garros (2008). "On the modulus of elasticity and strain capacity of self-1103

compacting concrete incorporating rubber aggregates." Resources, conservation and1104

recycling 52(10): 1209-1215.1105

Vandenbossche, J. M. (1999). "Estimating potential aggregate interlock load transfer based on1106

measurements of volumetric surface texture of fracture plane." Transportation Research1107

Record: Journal of the Transportation Research Board 1673(1): 59-63.1108

Vandenbossche, J. M., M. Barman, et al. (2014). "Using Surface Texture Measurements of a Crack1109

Surface to Establish a Joint Spring Stiffness Representing the Shear Transfer Capacity."1110

Transportation Research Record: Journal of the Transportation Research Board.1111

Xuan, D., L. Houben, et al. (2012). "Mechanical properties of cement-treated aggregate material–a1112

review." Materials & Design 33: 496-502.1113

Zheng, L., X. Sharon Huo, et al. (2007). "Experimental investigation on dynamic properties of1114

rubberized concrete." Construction and Building Materials 22(5): 939-947.1115

Zheng, L., X. Sharon Huo, et al. (2008). "Strength, Modulus of Elasticity, and Brittleness Index of1116

Rubberized Concrete." Journal of Civil Engineering , ASCE.1117

1118

1119