Dynamic compressive properties of lightweight rubberized concrete

Construction and Building Materials 238 (2020) 117705

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Dynamic compressive properties of lightweight rubberized concrete Thong M. Pham a, Wensu Chen a, Abdul M. Khan a, Hong Hao a, Mohamed Elchalakani b, Tung M. Tran c,⇑ a

Center for Infrastructural Monitoring and Protection, School of Civil and Mechanical Engineering, Curtin University, Kent Street, Bentley, WA 6102, Australia School of Civil, Environmental and Mining Engineering, The University of Western Australia, 35 Stirling Highway, WA 6009, Australia c Sustainable Developments in Civil Engineering Research Group, Faculty of Civil Engineering, Ton Duc Thang University, Ho Chi Minh City, Viet Nam b

h i g h l i g h t s
Rubberized concrete showed more sensitive to strain rate than that of normal concrete.
Rubberized concrete exhibited greater impact resistance as compared to normal concrete.
Compressive strength and axial strain at peak dynamic stress increase with rubber content.
Young’s modulus of rubberized concrete showed inconsistent strain rate sensitivity.

a r t i c l e

i n f o

Article history: Received 27 August 2019 Received in revised form 15 November 2019 Accepted 24 November 2019

Keywords: Rubberized concrete Strain rate SHPB Impact loading Energy absorption

a b s t r a c t This study experimentally examines the dynamic characteristics of concrete made of waste car tyres as both fine and coarse aggregates (rubberized concrete), resulting in light-weight concrete with the densities of 2350 kg/m3, 2091 kg/m3, and 1833 kg/m3 for 0%, 15%, and 30% rubber content, respectively. The dynamic compressive characteristic of the rubberized concrete was quantified by using a Split Hopkinson Pressure Bar equipment up to the strain rate of 182 s1. The experimental results have consistently shown excellent impact resistance of rubberized concrete as compared to that of normal concrete, including the progressive failure, crack propagation and normalized energy absorption. Under the same impact, rubberized concrete was still almost intact while normal concrete was fragmented into pieces. Rubberized concrete significantly slowed down the crack propagation and exhibited progressive failure as compared to normal concrete. The compressive strength and axial strain at maximum dynamic stress of rubberized concrete are sensitive to the strain rate and the sensitivity increases with the rubber content. Young’s modulus of rubberized concrete showed inconsistent strain rate sensitivity. Meanwhile, rubberized concrete was more sensitive to the strain rate than normal concrete. In addition, new equations to estimate the DIFs of rubberized concrete with different rubber contents were proposed, in which the lateral inertia confinement effect was removed. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Disposal of waste tyre rubber has become an environmental issue of great concern. Rubber crumbs can be used to replace aggregates in concrete for construction, which have been investigated in previous studies [1–6]. Rubberized concrete (RuC) is an environmental-friendly material, which has a similar ingredient as conventional concrete with coarse aggregates partially replaced by rubber aggregates. Owing to the existence of rubber, the rubberized concrete is much lighter than conventional concrete. Elcha⇑ Corresponding author. E-mail address: [email protected] (T.M. Tran). https://doi.org/10.1016/j.conbuildmat.2019.117705 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

lakani [2] reported that the rubberized concrete with 40% rubber content has the density of 1950 kg/m3, which is lower than 2450 kg/m3 of conventional concrete. It was reported that the rubberized concrete exhibits lower compressive strength but higher energy absorption capacity with the increasing rubber content [3,4]. Rubberized concrete has good energy absorption capacity against impact loads, as demonstrated in the previous studies [7,8]. Given the sound energy absorption capacity, rubberized concrete has been applied in the fields of roadside barriers and blocks [2,7,9,10]. The roadside barriers made of rubberized concrete can mitigate the injury risk to drivers and passengers by reducing the impact force and acceleration.

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crete has a lower DIF than the conventional concrete. The rubberized concrete exhibited the increasing energy absorption capacity with the rubber content within 10%. Whereas, the rubberized concrete experienced the opposite trend when the rubber content is over 10%. In this study, a total of 33 rubberized concrete specimens were prepared with partial aggregate replaced by rubber crumbs. Three rubber contents were 0%, 15% and 30% with respect to the total volume of aggregates. Both quasi-static and dynamic tests were carried out to investigate the effect of rubber content on the compressive strength of RuC. Dynamic tests were carried out by using Split Hopkinson Pressure Bar (SHPB) to obtain the dynamic compressive strength with the strain rate up to 182 s1. Failure mode, dynamic strength and energy absorption capacity were compared and analysed. The formulae of the dynamic increase factor (DIF) were derived accordingly.

Various impact tests were conducted to study energy absorption capacity of rubberized concrete under dynamic loading. Recently, Pham et al. [11] studied the impact performance of RuC columns. It was demontrated that the column using rubberized concrete has much higher energy absorption capacity under impact loading. With the rubber content changed from 0% to 30%, the impact energy absorption capacity of the column increased by 63%. The displacement experienced by the RuC columns was nearly double than that of the plain concrete column before failure. Gupta et al. [12] conducted impact tests on rubberized concrete with 25% rubber fiber by drop-weight and reported the increased energy absorption by adding rubber content. As reported by Donga et al. [13], adding rubber can enhance impact resistance capacity of RuC by up to 60%. Atahan and Yücel [14] conducted impact tests on rubberized concrete by using Instron machine. It was reported that the peak impact force can be effectively reduced and the impact duration can be prolonged by using rubberized concrete. Pham et al. [15] also conducted drop-weight tests on rubberized concrete cylinders along the axial direction. The peak impact force can be reduced by 50% and the impact duration can be prolonged by using RuC. It was also found that the impact resistance can be further improved by confining RuC cylinders via FRP sheets. Since RuC has potential applications for the impact energy absorption, it is essential to understand its mechanical properties such as compressive strength, strain, energy absorption, and modulus under dynamic loads. Intensive studies on compressive properties of concrete and concrete-like materials have been conducted under intermediate and high strain rates [16–20]. It is well known that concrete experiences different behaviors under quasi-static and dynamic loads due to the strain rate effect, which has been intensively documented [21,22]. Dynamic behaviors of concrete are affected by the inertial effect [16] and the viscosity effect (also known as Stefan effect) [23,24]. For instance, owing to the viscosity effect, the development of micro-cracks is suppressed under dynamic loading [23,25]. The effect of strain rate on concrete mechanical properties such as strength, strain and Young’s modulus has been revealed and the mechanical properties of concrete at high strain rate are also affected by the shape and content of coarse aggregates [26]. Dynamic increase factor (DIF), defined as the ratio of dynamic compressive strength with respect to quasi-static compressive strength, can be used to quantify the strength enhancement under different strain rates. Some empirical formulae [27–29] were proposed to determine the compressive strength DIF for conventional concrete. The DIF formulae were also proposed to reveal the strain rate effect on conventional concrete at the elevated temperature [30]. In addition, the effects of the factors such as various aggregates, end friction confinement and lateral inertial confinement on the dynamic compressive properties were studied by Hao et al. [16]. Rubberized concrete has similar ingredients as conventional concrete except for the inclusion of rubber aggregates. Therefore, RuC and conventional concrete might have different mechanical properties. The quasi-static mechanical properties of RuC have been well investigated [31,32]. However, the study on the dynamic compressive properties of rubberized concrete is very limited. Liu et al. [33] conducted SHPB tests on rubberized concrete to study its dynamic properties. It was reported that the rubberized con-

2. Experimental program 2.1. Mixture design and pre-treatment method Rubberized concrete consists of mortar matrix, traditional and rubber aggregates, and additive. Three RuC mixes were prepared with three rubber contents (i.e. 0%, 15%, and 30%). The details of RuC are given in Table 1 [34]. Two sets of coarse aggregates with maximum size of 10 mm and 7 mm were used in the concrete mix and silica sand was used as fine aggregates. The rubber crumbs had three sets of size 1–3 mm, 3–5 mm (to partially replace fine aggregate) and 5–10 mm (to partially replace coarse aggregate). It is noted that the replacement of both fine and coarse aggregates was adopted according to previous studies [35,36]. These studies suggested that using a combination of fine and coarse rubber aggregates results a higher strength than those with a single type of rubber aggregates. The physical properties of crumb rubber are given in Table 2. Concrete mixes were prepared in accordance with AS 1012.2 [37]. According to the previous studies [11,15], the maximum rubber content of 30% was chosen in order to achieve acceptable compressive strengths. When the rubber content is over 30%, increasing rubber content can lead to rubberized concrete with low strength, i.e. less than 10 MPa. The mix design was adopted from the previous study to reach the static compressive strength of 45 MPa, 25 MPa and 15 MPa for the rubberized concrete with the rubber contents of 0%, 15% and 30%, respectively. To improve the mechanical properties of RuC and the bonding between water cement surfaces, water soaking treatment was conducted before the RuC mixing. The water soaking treatment consists of washing rubber aggregates with water and soaking them in water for 24 h to reduce the rubber hydrophobicity. This treatment method was chosen due to its simplicity as compared to the treatment with alkaline solution of NaOH as reported in the previous study [38]. The test results showed a minor variation in the compressive strengths of concrete when using these two different treatment methods as reported in the previous study [38]. The slump test on the rubberized concrete was also carried out and the slump in this study was in the range of 130–150 mm to ensure the workability of concrete mixes.

Table 1 Mix proportion of all the mixes (kg/m3) [34]. Mixes

Cement (OPC)

Water

Aggregate10 mm

Aggregate7 mm

Aggregate5 mm

Sand

Rubber crumb (1–5 mm)

Rubber crumb (5–10 mm)

0 15%RuC 30%RuC

426 426 426

205 205 205

444 377 311

306 260 214

130 111 91

843 717 500

0 45 89

0 58 116

T.M. Pham et al. / Construction and Building Materials 238 (2020) 117705 Table 2 Physical properties of crumb rubber. Mechanical Property

Value

Specific gravity (crumb Rubber) Fineness modulus (crumb Rubber) Water absorption % (crumb Rubber) Young’s modulus @100% (truck tire rubber) Young’s modulus @ 300% (truck tire rubber) Young’s modulus @ 500% (truck tire rubber) Resilience @ 23 °C (truck tire rubber) Resilience @ 75 °C (truck tire rubber) Tension strength (truck tire rubber) Break point strain (truck tire rubber)

0.54 2.36% 85% 1.97 MPa 10 MPa 22.36 MPa 44% 55% 28.1 MPa 590%

3

rate. The L/D ratio of 0.5 was applied to minimise the lateral inertial confinement in SHPB tests. The specimens were ground at both sides with the surface roughness less than 0.02 mm to minimize non-perfect contact and the frictional effect between the specimen and bars. Quasi-static compression test was conducted according to AS 1012.9 [39]. The concrete samples with the size Ø100mm200 mm were tested by using MATEST equipment as shown in Fig. 2 (L). The loading rate was 0.333 MPa/s, which corresponded to the quasi-static strain rate of 1  104/s. 2.3. Impact testing procedure

2.2. Test matrix and material properties The specimens were well prepared and tested for each concrete mix (i.e. 0%, 15%, and 30%), in which rubber aggregates were uniformly distributed in the samples as shown in Fig. 1. A total of 9 cylindrical specimens with a diameter of Ø100 mm and a length of 200 mm (Ø100mm-200 mm) were prepared for the quasistatic compressive tests. Three cylindrical specimens were tested for each rubber content. The quasi-static compressive strength can be used to calculate the DIF of compressive strength. A total of 24 cylindrical specimens with a diameter of Ø100 mm and a length of 50 mm (Ø100mm-50 mm) were prepared for SHPB compressive tests. For each rubber content, four different loading rates were used and two identical specimens were tested at each loading

Split Hopkinson Pressure Bar (SHPB) system with a diameter of 100 mm was used to conduct dynamic material tests. SHPB system has been widely used to obtain dynamic material properties [20,40]. The dynamic properties including failure progress, failure patterns, compressive strength and energy absorption capacity can be obtained from the testing data. As shown in Fig. 2 (R), Ø100mm SHPB system consists of an incident bar with the length of 5500 mm and a transmitted bar with the length of 3000 mm. The Ø100mm bars were made of stainless steels with density, Young’s modulus and elastic wave velocity of 7800 kg/m3, 240 GPa and 5064 m/s, respectively. Grease was applied at the specimen–bar interfaces before tests to minimize the end friction confinement. A pulse shaper was mounted on the impact end of the incident bar to obtain the half-sine stress

Fig. 1. Concrete cylinders with various rubber contents (left to right 0%, 15%, and 30%).

Fig. 2. Quasi-static compression test on MATEST equipment (L) and SHPB system (R).

T.M. Pham et al. / Construction and Building Materials 238 (2020) 117705

waveform. The pulse shaper could also extend the rising time of the incident pulse, which makes the stress equilibrium easier to be achieved. Circular rubber pulse shapers, which had a thickness of 3 mm and a radius of 20 mm, were used in this study for all the tests. A high-speed camera was used to record the failure progress of the specimens. All the RuC specimens with different rubber contents were tested at various pressures. As per the theory of one-dimensional stress wave propagation, the stress (r), strain rate (e_ ) and strain (e) of the specimen are derived based on the measured reflected wave (e R) and transmitted wave (e T) [41], namely


 A rðtÞ ¼ E eT ðtÞ AS 2C e_ ðtÞ ¼  0 eR L

0

4 2 0 -2 -4

-6 -8 0

ð1Þ

50

100

150

200

250

300

350

Time (µs)

ð2Þ

Stress Equilibrium (RC15_S2_0.33) Incident Stress Reflected Stress Incident + Reflected Transmitted Stress

6 4

T

e_ ðtÞdt

ð3Þ

where L and AS are the length and the cross section area of the tested specimen, respectively. A, E and C0 are the cross-section area, Young’s Modulus and the elastic wave velocity of the bars, respectively. The data is valid only when the stress equilibrium is achieved. Therefore, the stress equilibrium was checked for all the specimens. In this study, the strain rate is determined when the peak value of the strain rate plateau is achieved.

Voltage (V)

eðtÞ ¼

Z

Stress Equilibrium (CS_S1_0.43) Incident Stress Reflected Stress Incident + Reflected Transmitted Stress

6

Voltage (V)

4

The dynamic compressive strength of rubberized concrete was derived from the testing results by using the SHPB. It is worth mentioning that the dynamic compressive strength is only valid when the stress equilibrium condition is achieved for the testing. Therefore, the stress equilibrium was checked for all the tested specimens after removing the time lag and only those satisfy the equilibrium condition were included in this study. It is noted that rubber pulse shapers were used in these tests as suggested by the previous study by Khan et al. [40] to eliminate the high-frequency oscillation and achieve the stress equilibrium. Fig. 3 shows the

-2 -4

-8 0

50

100

150

200

250

300

350

Time (µs)

Voltage (V)

3.1. Compressive and tensile strengths under quasi-static tests

3.2. Validity and strain-rate determination of SHPB tests

0

-6

3. Experimental results and discussions

Quasi-static compression tests on rubberized concrete cylinders (100x200 mm) were carried out to estimate the compressive strength at 28 days. The compressive strengths of rubberized concrete with 0%, 15%, and 30% rubber content were 56.33 MPa (SD = 1.65), 27.96 MPa (SD = 0.09), and 16.33 MPa (SD = 1.55), respectively. The compressive strength of rubberized concrete reduced with an increase with the rubber content because of the poor bonding between the cement paste and rubber particles in the interfacial transition zone (ITZ) as also reported in previous studies [2,33,42]. The split tensile strength of the tested specimens for 0%, 15%, and 30% rubber content was 4.42 MPa, 2.55 MPa, and 2.16 MPa, respectively. It is worth mentioning that the measured density of 0%, 15%, and 30% rubberized concrete was respectively 2350 kg/m3, 2091 kg/m3, and 1833 kg/m3. In quasi-static compression tests, normal concrete failed in a more explosive manner as compared to the rubberized concrete which retained its original shape with cracks extended through the height of the specimens. The same manner was observed in the in-direct tensile tests where normal concrete was split into halves while rubberized concrete remained almost intact.

2

Stress Equilibrium (RC30_S1_0.38) Incident Stress Reflected Stress Incident + Reflected Transmitted Stress

8 6 4 2 0 -2 -4 -6 -8 -10 0

50

100

150

200

250

300

350

Time (µs) Fig. 3. Stress equilibrium of the tested specimens.

stresses including the incident stress, reflected stress, transmitted stress and the sum of the incident and reflected stresses in three specimens which represent the rubber concrete with various rubber contents (0%, 15%, and 30%). As shown in Fig. 3, the sum of the incident and reflected stresses reasonably matched with the transmitted stress, demonstrating all the tests achieved the equilibrium condition. The variation of the peak stress between the reflected wave versus the sum of the incident and reflected waves less than 5% was adopted in this study according to a suggestion from the previous study [43]. It was observed that the strain-rate of the tested specimen varied with time. Therefore, the strain-rate during the effective loading period cannot be considered as a constant, which was also discussed in previous studies [18,19,44]. There have been a few methods of determining the representative strain-rate for a given test result, i.e. Grote et al. [18] used the mean value for the strain-rate during the loading period while Zhang et al. [19] recommended that the strain-rate at the failure point should be used as the representative strain rate. In this study, the strain-rate was determined at the maximum compressive stress in the sample as shown in Fig. 4. This definition was used in previous studies to determine the strain-rate of SHPB specimens [19,44].

T.M. Pham et al. / Construction and Building Materials 238 (2020) 117705

Stress and strain rate

Stress or strain rate

200

Stress (MPa) Strain rate (1/s)

150

100

50

0 50

100

150

200

250

300

Time (µs) Fig. 4. Determination of strain rate.

3.3. Failure processes and failure modes The progressive failure of all the specimens was recorded by using a high-speed camera. Fig. 5 shows the progressive failure of the specimens with different rubber contents. Cracks initiated from both sides of the specimens and developed into the mid region. This observation again confirmed the stress equilibrium was achieved in these samples. Afterwards, some more cracks initiated within the mid-region and developed arbitrarily. It can be seen that rubberized concrete developed cracks earlier than that of normal concrete due to its lower static compressive strength, i.e. 15% rubberized concrete showed 10 minor cracks at 50 ls while only two minor cracks were found on the normal concrete at 60 ls. However, rubberized concrete slowed down the crack development as shown in Fig. 5. The number of cracks in 15% rubberized concrete at 200 ls was less than that of the normal concrete at

5

the same time instant. The number of cracks in the 15% rubberized concrete specimen at 1800 ls was still even less than that of normal concrete at 200 ls. Eventually, normal concrete was shattered into small pieces because of its brittle nature while 15% rubberized concrete specimen broke into relatively larger pieces. Unfortunately, the progressive failure of 30% rubberized concrete was not properly recorded. Under the high loading rate, it could be seen that the control sample exploded into pieces indicating the brittle collapse. However, for rubberized concrete, especially at higher strain rate, it experienced less brittle damage and such damage propagated from the edges then towards the centre of the specimen as shown in Fig. 5. When stress increased, cracks start to propagate and rubber tends to resist the crack from expanding, i.e. it acts as a crack arrestor. The previous study has shown that the difference in the elastic moduli between rubber and concrete may lead to a reduction in the crack velocity [45] because of the damping effect of rubber. So overall it shows that the increase in the rubber content enhances the impact resistance of rubberized concrete. The final failure modes of the rubberized specimens were very different with various rubber contents and strain rates as shown in Fig. 6. It can be seen that the normal concrete at the strain rate of 116 s1 was disintegrated into numerous fragments and with the increase in the strain rate it shattered into even smaller pieces. This failure mode of normal concrete was also consistent with the experimental results reported in the previous studies [19,43,44,46]. Meanwhile, rubberized concrete with 15% and 30% rubber contents showed better behaviour under high impact loads. The 15% rubberized concrete exhibited larger fragments under similar strain rate as compared to the normal concrete as shown in Fig. 6. For instance, under the strain rate of 103 s1, 15% rubber concrete failed from the edges but the middle part was still intact. For 30% rubberized concrete, only small cracks appeared on the cir-

Fig. 5. Progressive failure of normal and rubberised concrete.

6

T.M. Pham et al. / Construction and Building Materials 238 (2020) 117705

Fig. 6. Failure modes of rubberized concrete with different rubber contents.

3.4. Stress-strain curves and energy absorption To investigate the dynamic compressive strength, stress-strain curves of different specimens under various strain rates are derived from the testing data. Figs. 7–9 show the stress-strain

Normal concrete

120

Strain rate 116 /s 100

Stress (MPa)

cumference and the specimen fairly kept its cylindrical shape with much less fragments. As the strain rate increased to 125 s1, the normal concrete was shattered into smaller pieces while the 15% rubberized concrete only broke into a few large pieces. At the same strain rate, 30% rubberized concrete outperformed the former two specimens and only the outer edge was slightly damaged. Even at the higher strain rate of 151 s1 the 30% rubberized concrete specimen showed cracks near the circumference region while the middle part was still intact. This observation shows that the damage of rubberized concrete under impact loads reduces with the increase in the rubber content and the difference becomes more apparent with 30% rubber content. In the meantime, the increase of the loading rate (strain-rate) significantly affected the failure mode of normal concrete but it showed less influence on the rubberized concrete. The excellent performance of rubberized concrete can be explained as follows: (1) due to the crack arresting characteristic of rubber aggregates as reported in the previous study [45] and (2) bridging effect of coarse rubber aggregates between cracks as they elongate between the cracks reducing the intensity of cracks [47]. This finding suggests that rubberized concrete is highly suitable for structures subjected to impact or blast loadings.

Strain rate 140 /s

80 60 40 20 0 0.00

0.50

1.00

1.50

2.00

2.50

Strain (%) Fig. 7. Stress-strain curves of normal concrete at different strain-rates.

curves of the specimens with various rubber contents at different strain rates. It can be seen that the compressive strengths of all the concrete mixes increase with the strain rate. This observation supports the previous understanding of the strain rate effect on the concrete-like materials [19,46,48]. When concrete-like materials are subjected to high impact loads, the induced cracks more likely propagate through shorter and straighter paths which cut through stronger sections, i.e. coarse aggregates, and thus result in a higher strength. Whereas, in static testing, cracks in concrete

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T.M. Pham et al. / Construction and Building Materials 238 (2020) 117705

15% Rubberized concrete 120 Strain rate 103 /s Strain rate 131 /s Strain rate 150 /s Strain rate 165 /s

100

Stress (MPa)

80

60

40

20

0 0.00

0.50

1.00

1.50

2.00

2.50

3.00

Strain (%) Fig. 8. Stress-strain curves of 15% rubberized concrete at different strain-rates.

30% Rubberized concrete

80

Strain rate 105 /s 70

Strain rate 128 /s Strain rate 151 /s

Stress (MPa)

60

Strain rate 182 /s

50 40 30 20 10 0 0.00

0.50

1.00

1.50

2.00

2.50

3.00

Strain (%) Fig. 9. Stress-strain curves of 30% rubberized concrete at different strain-rates.

usually form at weak sections and then connect together, leading to failure. The induced cracks are thus long and have an arbitrary path. Dynamic increase factor (DIF) is usually quantified in order to evaluate the increase in the dynamic compressive strength with respect to the static compressive strength for all the concrete mixes with varying rubber contents. DIFs at different strain rates of all the mixes are given in Table 3. The stress strain curves show that normal concrete had the highest dynamic compressive strength followed by 15% and 30% rubberized concrete when compared at the same strain rates because the static strength of the normal concrete was higher than that of the rubberized concrete. However, the DIF of 30% rubberized concrete was the highest followed by those for 15% rubberized concrete and then normal concrete. This observation indicates that the DIF increases with the rubber content and concrete becomes more sensitive to strain rate effect as the rubber content increases. Hence, it also suggests that the addition of rubber in the concrete mix improves the mechani-

cal strength under high loading rates. This increase of the DIF can be explained by the crack arresting mechanism of rubber. When high impact load is applied, cracks start to initiate which passes through the rubber aggregates thus reduces the crack velocity through stress relaxation [35]. In addition, when subjected to dynamic loading, stress wave propagation in rubberized concrete generates friction in the crack interface and any other imperfection and thus results in a partial loss of the impact energy. However, as mentioned earlier, previous studies have reported that an increase in the dynamic compressive strength is not only due to the strain-rate effect especially under high impact loading. Lateral inertial confinement and the end friction produced between the impact bar and specimen also contributes to the increase in the dynamic compression strength, which can lead to overestimating the DIF [44]. In this study, the end friction was minimized by using grease at the two ends of the specimens. Meanwhile, the samples were not tested above the strain-rate of

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T.M. Pham et al. / Construction and Building Materials 238 (2020) 117705

Table 3 DIF of rubberized concrete with different rubber contents. Strain-rate 116 125 140 165 – – –

Normal concrete

1

s s1 s1 s1

Strain-rate

1.49 1.52 1.69 2.30* – – –

103 119 131 150 165 – –

15% RuC

1

s s1 s1 s1 s1

Strain-rate

2.36 2.68 2.54 2.87 3.25 – –

105 106 128 151 161 176 182

30% RuC

1

s s1 s1 s1 s1 s1 s1

2.53 2.87 3.21 3.56 3.23 3.64 3.86

* The DIF was adopted from the tests by Hao and Hao [44].

200 s1 because the effect of the lateral confinement and the end friction becomes inevitable above this strain rate [40]. But according to previous studies [49,50], there is still contribution of lateral inertial confinement to the DIF that should be eliminated to obtain the true material strain rate effect. The previous study on the DIF of roller compacted concrete, the authors have suggested that the contribution of lateral inertia effect of /100x50 samples ranges between 4 and 13% up to 80 s1 [20] while the corresponding number for / 100x100 was 13.68% at 200 s1 as reported by Hao et al. [49]. Since the strain rate in this study ranged from 103 s1 to 182 s1, a reduction factor of 10% caused by the lateral inertia resistance is removed from the testing results. In addition, the energy absorption, which is defined as the area underneath the stress-strain curve of the tested specimen, is computed and shown in Fig. 10. As can be seen from the figure, the energy absorption increased with the strain-rate for the same rubber content while adding rubber to concrete reduced the energy absorption of the rubberized concrete, however, the increment in the energy absorption capacity of the rubber concrete with strain rate is more prominent with the increase in rubber content. For example, when the strain rate increases from 103 s1 to 150 s1,

the improvement of the energy absorption of the 15% and 30% rubberized concrete was 18% and 117%, respectively. This observation agrees well with the observation from our previous study [15] but it is different from the conclusion drawn from other studies [12,45]. In our previous study, Pham et al. [15] carried out dropweight tests on RuC cylinders and suggested that RuC reduced the maximum impact force, which in turn also led to a reduction in energy absorption capacity although the impact duration was extended, while Gupta et al. [12,45] found rubberized concrete increased the energy absorption. It is worth mentioning that other previous studies [9,38] used different types of rubber aggregates, i.e. arbitrary shape and long strip rubber fiber, which may result in distinguished effect. This controversy observation can be explained by combining two effects of rubber aggregates. Firstly, adding rubber improves the energy absorption capacity under high loading rates [12,45], and the rubberized concrete shows more sensitivity to strain-rate. It means the dynamic increase factor of rubberized concrete is higher than that of normal concrete. However, the inclusion of rubber content simultaneously scarifies the compressive strength, which leads to lower energy absorption. Therefore, if the improvement of the energy absorption due to

1,400

15% Rubberized concrete

1,400

1,354

1,133

1,000

1,000

951

881

600 400 200 0 103

600

1,400

400

1,200

200 0 116 Strain rate s-1

140

1,204

1,037

800

800

Energy Absorption (kN/m2)

Energy Absorption (kN/m2)

1,200

Normal concrete

Energy Absorption (kN/m2)

1,200

131 150 Strain rate s-1

165

30% Rubberized concrete

1,000 837 800

698 604

600 400

322

200 0 105

Fig. 10. Energy absorption at different strain-rates.

128 151 Strain rate s-1

182

9

T.M. Pham et al. / Construction and Building Materials 238 (2020) 117705

be suggested that rubberized concrete absorbs more energy than normal concrete at the same compressive strength. Pervious research by Elchalakani [2] showed the RuC with 60 MPa compressive strength can be achieved by adding silica fume.

Table 4 Energy absorption of rubberized concrete with different rubber contents. Rubber content (%)

Strain rate (s1)

Energy absorption (kN/m2)

Energy absorption per MPa (kN/m2/MPa)

0

116 140 103 131 150 165 105 128 151 182

1133 1354 881 951 1037 1204 322 604 698 837

20 24 32 34 37 43 20 37 43 51

15

30

3.5. Rate effects on dynamic properties The experimental results have confirmed that rubberized concrete is sensitive to strain rate, in which the higher rubber content the higher sensitivity as shown in Fig. 12. The increase of DIF is almost linearly proportional to the rubber content within a wide range of strain rates from 103 s1 to 162 s1. For example, at the strain rate of 124–131 s1, the DIF of rubberized concrete increased from 1.52 to 2.54 and 3.21 corresponding to the rubber content of 0%, 15%, and 30%, respectively. The rate dependence of concrete behaviour is associated with two mechanisms: (1) the dependence of fracture process on the rate of crack opening and (2) the viscoelastic deformation effect on the intact cement paste [51,52]. These two mechanisms are important to concrete behaviour but the first dominates at high strain rates, i.e. under impact and blast loads. Najim and Hall [35] carried out experimental testing on the flexural toughness and found that rubberized concrete showed not only a significant increase in the toughness indices but also improvement in crack mouth opening displacement. Meanwhile, Gupta et al. [45] used scanning electron microscopy (SEM) technique to investigate the microcrack propagation and failure of rubberized concrete. These authors mentioned that the mismatch of the elastic modulus between rubber and cement paste may make cracks to pass through rubber aggregates and in turn reduce crack velocity. Their results clearly showed that rubberized concrete reduced the crack opening. Therefore, it indicates that rubberized

strain rate effect by adding rubber is less than the loss by reducing the compressive strength, the energy absorption of rubberized concrete is less than that of normal concrete. Therefore, the energy absorption capacity of all the specimens is normalized with their compressive strengths and shown in Table 4 and Fig. 11. As can be seen clearly from the figure that the normalized energy absorption of rubberized concrete was significantly higher than that of normal concrete, i.e. at the strain rate of 141–150 s1, the normalized energy absorption of rubberized concrete was 54–79% higher than that of normal concrete, which are 24 kN/m2/MPa, 37 kN/m2/ MPa, and 43 kN/m2/MPa for the specimens with 0%, 15%, and 30% rubber content, respectively. For rubberized concrete with different rubber contents, the normalized energy absorption of 30% rubberized concrete was higher than that of 15% rubberized concrete at almost all strain rates except those at 103 s1. Therefore, it can

Energy Absorption (kN/m2/MPa)

60

Normal concrete 50 40

43

40 32

34

37

30 20

10 0 103

30

24

20 10

0 116 Strain rate s-1

140

131 150 Strain rate s-1

165

60

20 Energy Absorption (kN/m2/MPa)

Energy Absorption (kN/m2/MPa)

60

15% Rubberized concrete 50

30% Rubberized concrete

51

50 43 37

40 30 20

20

10 0 105

128 151 Strain rate s-1

Fig. 11. Normalized energy absorption at different strain-rates.

182

10

T.M. Pham et al. / Construction and Building Materials 238 (2020) 117705

Strain rate sensitivity 5

103-116 s-1 125-131 s-1 140-151 s-1 165-182 s-1

DIF

4 3 2 1 0% RC

15% RC

30% RC

Fig. 12. Strain rate sensitivity of rubberized concrete.

concrete possesses higher resistance to loading at high loading rates as also observed in the present experimental testing. Liu et al. [33] found that rubberized concrete was less sensitive to strain-rate than normal concrete. This finding is different from the observation in this study where rubberized concrete was found more sensitive to the strain-rate than normal concrete. Liu et al.

[33] also observed the DIF significantly increased with the size of rubber aggregates. They used rubber aggregates with a maximum size of 2 mm while the maximum size of 10 mm was used in this study. This difference may explain the significantly higher DIF of rubberized concrete observed in this study as compared to that of normal concrete. The relationship between the DIF and strain rate for the specimens with various rubber contents is presented in Fig. 13. The figure has shown that the DIF increases with the rubber content and rubberized concrete is more sensitive to strain rate than normal concrete. The previous models [48,49] for normal concrete were also plotted to compare with the DIF of rubberized concrete. As can be seen that the DIF of rubberized concrete was higher than that of normal concrete. However, this study covers only a limited range of strain rates from 103 s1 to 182 s1. More experimental testing on rubberized concrete at strain rate out of this range is needed.

DIF 15%RuC ¼ 104 ðlog e_d Þ  0:027ðlog e_d Þ þ 3:594 for 165s1 2

> e_d > 100s1

30% RuC 15% RuC 0% RuC DIF Hao and Hao (2014) DIF CEB (1993) Fit line (30% RuC) Fit line (15% RuC) Fit line (0% RuC)

4.5 4.0 3.5

DIF

ð4Þ

3.0 2.5 2.0 1.5 1.0 50

500

Strain rate 1/s Fig. 13. Comparison of DIF with different rubber contents.

0% rubberised concrete

35

15% rubberised concrete

Young's modulus (GPa)

30

30% rubberised concrete

25 20 15 10 5 0 10

100

Strain rate 1/s Fig. 14. Young modulus of rubberized concrete with different strain rates.

1000

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T.M. Pham et al. / Construction and Building Materials 238 (2020) 117705

0.6

0% rubberised concrete 15% rubberised concrete

Axial strain (%)

0.5

30% rubberised concrete

0.4

0.3

0.2

0.1

0 10

100

1000

Strain rate 1/s Fig. 15. Axial strain at peak stress of rubberized concrete with different strain rates.

2

DIF 30%RuC ¼ 4  106 ðlog e_d Þ þ 0:012ðlog e_d Þ þ 1:474 for 182s1 > e_d > 100s1

ð5Þ

The fitted empirical relations of the testing data in this study are given above. These empirical relations can be used to estimate the DIF of rubberized concrete when predicting the dynamic responses of structures under high-rate loadings. Meanwhile, the effect of the strain rate on the Young’s modulus and ultimate compressive strain of concrete is also important and worth further investigations. A previous study has stated that Young’s modulus of concrete increased with the strain rate but at a lower level than that of the compressive strength [21]. Hao and Hao [44] also confirmed that Young’s modulus of normal concrete and fiber reinforced concrete increases with the strain rate but at a less ratio as compared to that of the compressive strength. The Young’s modulus of all the tested specimens are presented in Fig. 14. As can be seen that the variation of Young’s modulus is quite arbitrary and no definite statement can be made. There is no consensus on the strain rate sensitivity of the Young’s modulus in which different observations and statements were reported in the literature [21]. For instance, many previous studies [21,40,44] have found that Young’s modulus of normal concrete increases with strain rate while another study stated that an increase of Young’s modulus is pseu-do-strain-rate sensitivity [53]. In the meantime, the axial strain at peak stress shows a minor increase with the strain rate but the enhancement is not clear as also mentioned in the previous study. Among those specimens, the axial strain at peak stress of rubberized concrete exhibited a consistent enhancement with strain rate while the trend of normal concrete was unclear as shown in Fig. 15. It is worth mentioning that inconsistent observations were reported in the literature, i.e. some previous studies have stated that the axial strain at the maximum stress increased with the strain rate [18,54] while other studies found it remained almost constant [55] or even declined when the strain rate increased [56,57]. Further investigations on the effects of strain rate on Young’s modulus and axial strain at peak load are indeed necessary.

4. Conclusions This study examines the dynamic compressive behavior of rubberized concrete with various crumb rubber contents up to 30% by

using SHPB tests. The findings from this study can be summarized as follows: 1. Rubberized concrete shows great impact resistance under high loading rate. Under the same impact, rubberized concrete still almost intact while normal concrete was fragmented into pieces. Rubberized concrete significantly slowed down the crack propagation and progressive failure as compared to normal concrete. 2. The experimental results consistently showed that rubberized concrete is sensitive to the strain rate. The higher rubber content, the more sensitive it had to strain rate. 3. The lateral inertia resistance was quantified from previous studies and then was removed from the experimental results and the DIF formulae for rubberized concrete were proposed. 4. The absorbed energy normalized using compressive strength of rubberized concrete was 54–79% higher than that of normal concrete. The experimental results showed that rubberized concrete is clearly sensitive to the strain rate. However, this study only covers a range of strain rate from 103 s1 to 182 s1. Although the results shine some lights on the dynamic material performance of rubberized concrete, further studies on the dynamic material properties of rubberized concrete in a wider strain rate range are deemed necessary.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement The financial support from the Australian Research Council (Australia) Laureate Fellowships FL 180100196 is acknowledged. The last author acknowledges the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement no. 777823. The authors would like to thank Adrian Jones from Tyrecycle for donating rubberized aggregates.

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