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Fracture behaviors of a new steel fiber reinforced recycled

aggregate concrete with crumb rubber

Y.C. Guo a,, J.H. Zhang a, G. Chen a, G.M. Chen a, Z.H. Xie b

a Faculty of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou, Chinab School of Civil Engineering and Transportation, South China University of Technology, Guangzhou, China

h i g h l i g h t s

The fracture behaviors of steel fiber-reinforced RA concrete which consists of crumb rubber were investigated.

The effects of rubber content on the fracture behaviors of the RSRAC subjected to different temperatures were analyzed.

The fracture properties were obtained in different temperatures.

The fracture toughness and fracture energy first increase and then decrease.

The thermal damage increases the fracture energy and CMODc.

a r t i c l e i n f o

Article history:

Received 16 September 2013

Received in revised form 1 November 2013

Accepted 20 November 2013

Available online 18 December 2013

Keywords:Steel fiber reinforced concrete

Recycled aggregate

Crumb rubber

Fracture properties

High temperature

a b s t r a c t

This paper investigated the fracture behaviors of a new Steel fiber reinforced Recycled Aggregate Con-

crete which consists of crumb Rubber (RSRAC). The effects of rubber content on the fracture behaviors

of the RSRAC subjected to different temperatures were analyzed. In RSRAC, the steel fiber was used to

improve the crack resistance of concrete, and the inclusion of crumb rubber is mainly for environment

protection, energy dissipation and reducing the risk of explosive spalling during exposure to high temper-

atures. A series of concrete mixes were prepared with ordinary Portland cement, recycled coarse aggre-

gates (RCA) or natural coarse aggregates (NCA), steel fiber with volume-ratio of 1% and crumb rubber

with different replacement ratios of 0%, 4%, 8%, 12% and 16% for fine aggregate (sand). The fracture prop-

erties, including fracture toughness (KIC) and fracture energy (GF), of the different concrete mixes sub-

jected to different temperatures (25 C, 200 C, 400 C and 600 C), were obtained through three-point

bending tests on 72 notched beams with sizes of 100 mm100 mm515 mm. The results indicated

that both the fracture toughness and fracture energy first increase and then decrease with increase of

the rubber content; at certain rubber content, the mixes had the highest toughness. The thermal damage

due to heating from 25 C to 600 C also obviously increased the fracture energy and critical crack mouth

opening displacement (CMODcri), but it was not the case for the fracture toughness. It demonstrated that

exposure to high temperature made all cementitious materials tested significantly more ductile and less

resistant to crack propagation.

2013 Elsevier Ltd. All rights reserved.

1. Introduction

A new type of Steel fiber reinforced Recycled Aggregate Con-

crete (RSRAC), which consists of crumb rubber, is a concrete mate-

rial patented by the authors (China invention patent No.: ZL.

201010019345.3). This new material has been coined on the fol-

lowing considerations: (1) the inclusion of recycled concrete aggre-

gate (RCA) and rubber particles is mainly for the environmental

and economic significance [13], (2) the steel fiber and rubber par-

ticles are used to improve the performances of concrete both

before [46]and after[7] exposure to different temperatures, (3)the advantageous interaction exists between steel-fiber and rubber

as mentioned in the literature[810].

Strength, stiffness, toughness and brittleness are the fundamen-

tal mechanical properties of concrete. The changes of these proper-

ties after exposure to high temperatures are of great importance

for the design of concrete structures [11]. It is, thus, more impor-

tant to investigate the mechanical properties of concrete in the

structures subjected to long-term high temperatures. More and

more attentions[12,13]have been paid to the mechanical proper-

ties of concrete at high temperature or the residual properties of

concrete after exposure to high temperatures. Moreover, the com-

pressive properties of RSRAC mixes have already been analyzed

0950-0618/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.11.075

Corresponding author. Tel.: +86 20 39322538; fax: +86 20 39322511.

E-mail address: [email protected](Y.C. Guo).

Construction and Building Materials 53 (2014) 3239

Contents lists available at ScienceDirect

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and discussed by the authors, and the relevant achievements have

been reported in a separate paper, which is under review. In addi-

tion, the poor performance of the RCA is associated with the cracks

and fissures formed in processing recycled aggregates, which

makes concrete prepared with RCA suffer brittleness problems

[14]. Therefore, the experimental research on the fracture proper-

ties of RSRAC mixes in this paper is in urgent need.

The fracture properties of concrete significantly influence the

structural behavior of concrete components at high temperatures

[11]. Studies on the fracture properties of concrete have recently

attracted more attentions. The fracture energy is defined as the en-

ergy absorbed to create a unit area of fracture surface, representing

the energy dissipation capacity of overall loading process. RILEM

[15] recommended the three-point bending method for determina-

tion of the fracture energy with specimens of notched beams. Me-

nou et al. [16] examined the residual fracture energy of cement

paste, mortar and concrete subjected to high temperature and

found that the thermal damage due to heating from 120 to

400 C increases the fracture energy by 50% compared with the ref-

erence tests at room temperature. Peng et al. [13] conducted an

experimental research to explore the relationship between explo-

sive spalling and the residual fracture properties of concrete ex-

posed to high temperatures; their results showed that the

residual fracture energy increased after heating. Nielsen et al.

[17]also suggested that the damage introduced by a temperature

within 300400 C increased the fracture energy by 50% compared

with the tests at room temperature. Since the fracture toughness

represents the resistance to instable crack propagation, namely

the resistance to brittle fracture, Hisham Abdel-Fattah et al. [18]

experimentally investigated the variation of the residual fracture

toughness of concrete with different temperatures and pointed

out that the residual fracture toughness of concrete decreases with

the increase in temperature.

This paper studies the fracture behaviors properties (including

the fracture energy GFand fracture toughnessKIC) of a new steel fi-

ber reinforced Recycled Aggregate Concrete (RSRAC) subjected to

different temperatures. From the test results, a preliminary under-standing of the fracture failure mechanism of RSRAC after exposure

to different temperatures can be achieved. This study may provide

a basis for the further research on RSRAC and its potential

applications.

2. Experimental details

2.1. General

A total of six groups of mixes, named NC-R0, RC-R0, RC-R4, RC-R8, RC-R12 and

RC-R16, were prepared using 100% recycled concrete aggregate (RCA) or natural

concrete aggregate (NCA), 1% steel fiber and rubber crumb with varied content

(0%, 4%, 8%, 12% and 16%). Each type of concrete mixes includes 12 cylinders with

dimensions of 150 mm300 mm (diameter and height) and 12 notched beams

with dimensions of 100 mm100 mm515mm, every three of which were ex-

posed to a temperature (25 C, 200 C, 400 C and 600 C). The proportions and

compressive strengths are presented in Table 1.

Crumb rubber, obtained from waste tires, has an average particle diameter of

14 20 sieve size (i.e. 0.85 1.40 mm according to ASTM-E11-09e1), a specific

gravity of 1.05, and a melting temperature of 170 C. The steel fibers were cut

and shapedfromsteelplate, witha lengthof 32mm, anaspectratio of 45anda ten-

sile strength of 600 MPa. This type of steel fibers, which were made from ordinary

steel, with a melting temperature of 1538 C and a density of 7.82 g/cm3, are loose

informat delivery (Fig. 1). In addition, a commercially available naphthalene-basedsuper-plasticizer with a solid content of 30% and a water reducing rate of 20% was

used as admixture to achieve slump of the concrete mixes around 150 mm. The

amount of plasticizer was 1.0% by weight of cement based on slump tests according

to BS 1881: Part 102 (1983). Recycled concrete aggregates, crumb rubber and steel

fibers are shown inFig. 1.

2.2. High temperature program

Among 12 cylindrical specimens in each mix, three were tested immediately

after conditioning without being unheated (at the room temperature of 25 C),

the remaining 9 specimens were divided into 3 groups and exposed to 3 tempera-

tures (200 C, 400 C and 600 C) in an electrical furnace, respectively. In the fur-

nace, the specimens were heated at a constant rate of 8 C/min, from the room

temperature to the prescribed temperatures. The temperaturetime curves used

in heating of the test specimens are showed inFig. 2, which were adopted from

the paper [13]. The target temperature was maintained for 120 min before the elec-trical furnace was turned off andthe specimenswere then naturally cooleddownto

the room temperature. During the heating period, water was allowed to evaporate

freely.

2.3. Methods of three-point bending tests

A three-point bending method was used in the study to determine the fracture

performance of the new concrete material RSRAC in accordance with the recom-

mendation of RILEM Fracture Mechanics Committee (TC50-FMC) [15]. As showed

inFig. 3(a), the notched beams used for the three-point bending test had dimen-

sions of 100 mm100 mm515mm anda span of 400 mm; a notch with a depth

of 30mm (a0/h= 0.3) was located in the mid-span place. The test was conducted on

a closed-loop Electro Hydraulic universal testing machine with a 500-kN capacity

and three control modes: load control, displacement control and strain control. In

the study of the fracture properties, a 50-kN load cell with the precision of 1 N

and a 50-mm displacement transducer (LVDT) with the accuracy of 0.01 mm were

applied to obtain the load and deflection at the mid-span respectively, while the

crack mouth opening displacement (CMOD) was measured with a 10-mm clip-on

gages with the accuracy of 0.001 mm. All the data were recorded via the synchro-

nous collection system of TDS-530. During the loading process, a constant displace-

ment rate of 0.05 mm/min with the central deflection as the control parameter was

applied until the final failure of the specimen. In addition, a testing strategy used in

the three-point bending method [Fig. 3(b)] was designed to eliminate the negative

effect of compressive plastic deformation on the compressed parts of specimens

(e.g. the pedestal and actuator head) on the measurement. An advantage of the de-

signed testing strategy is that the reference points located on the neutral axis of the

notched beams were applied to measure the mid-span deflection in order to re-

move the compressive plastic deformation on the other parts of specimens from

the measured deflection. As a result, the measured values are authentic deflection

of the tested beams. Both the specially designed testing system and the precision

of measurement ensure the accuracy of the loaddeflection (Pd) curves and

load-CMOD (P-CMOD) curves. The specimen and set-up of three-point-bending test

are shown inFig. 3.

Table 1

Mix proportions and compressive strengths.

Mix Compressive strength (MPa) Mix proportions (unit weight:kg/m3)

W/C W OPC S NCA RCA AW SF R WRA

NC-R0 56.52 0.35 170 485 645 1052 78 4.5

RC-R0 51.41 0.35 170 485 645 954 37 78 4.5

RC-R4 49.06 0.35 170 485 625 954 37 78 7.9 4.5

RC-R8 39.41 0.35 170 485 605 954 37 78 15.7 4.5

RC-R12 37.61 0.35 170 485 585 954 37 78 23.6 4.5

RC-R16 35.88 0.35 170 485 565 954 37 78 31.5 4.5

Note: NC = natural concrete, RC = recycled concrete, R0, R4, R8, R12 and R16 for volume substitution ratio of rubber is 0%, 4%, 8%, 12% and16%, W/C= water/cement

ratio(mass), W = water, OPC = ordinary Portland cement, S = sand, NCA = natural coarse aggregate, RCA = recycled concrete aggregate, AW = additional water, SF = steel fiber,R = crumb rubber, WRA = naphthalene-based high-range water-reducing admixture.

Y.C. Guo et al. / Construction and Building Materials 53 (2014) 3239 33
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2.4. Determination of the fracture energy

The fracture energy, defined as the total energy dissipated over a unit area of

the cracked ligament, is obtained from the work done by the force (the area under

the loaddeflection curve). The fracture energy of the notched beams includes 4

parts (Fig. 4)[11], which can be expressed as:

W W0 W1 W2 W3 2-1

Here, W0 is theworkdoneby the external force P, (i.e. theenveloping area of the

loaddeflection curves) which is recorded by the data acquisition system (DAS); W1is the work done by the self-weight of the beams before application of the external

force P, but neglected in this paper as it is extremelysmall; W2and W3arethe addi-

tional work done during the loading process by the self-weight of the beams, in

whichthereexists a equationofW2 0:5 mgd0; W3, the end part ofthe curves after

d0, cannot be measured in tests. The computational steps of the fracture energy are

as follow:

(1) W0 Rd0

0 Pdd.

(2) W2 0:5mgd0, in which mg (g= 9.81m/s2) is the self-weight of the beams

scaled byS/L of the total weight (hereinafter referred to scaled weight).

(3) Calculating the additional work W3 done by scaled weight. First, the

descending branch of the loaddeflection (Pd) curve is artificially extendedby curve-fitting based on the power function method[19]:

(a) Recycled concrete aggregates

(b) Crumb rubber

(c) Steel fibers

Fig. 1. Material components.

0

100

200

300

400

500

600

700

0 50 100 150 200 250 300

Time (min)

T

emperature(C)

200400600

CCC

Fig. 2. Temperaturetime curves in heating of the test specimens.

(a) Notched beam of three-point-bending test

(b) Set-up of three-point-bending test

(c) A complete loading system in 500 kN testing machine

Clip-on

Gages=400 mm

l=515 mm

57.557.5

t=100

h=100

P

a0=30 mm

LVDT

Load

Cell

Fig. 3. specimen and set-up of three-point-bending test.

34 Y.C. Guo et al. / Construction and Building Materials 53 (2014) 3239


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P bdk P 0b; k> 0

lnP b k ln d2-2

(4) The reliability of the fitting curves is represented by the reliability coeffi-

cient, R2, and thespecific fitting parameters (i.e. b, k, R2) are listedin Table 2.

Thus, W3can be calculated by the following formula (2-3).

W3

Z 1d0

b

dkdd

b

k 1dk10

2-3

(5) The real fracture energy of NC-R0 and RSRAC mixes is obtained in accor-

dance to the formula (24) after W0, W2 andW3 are obtained. Here, Aligis

the area of ligament, namely Alig= t(h-a0).

GFW0 W1 W2 W3

Alig2-4

The calculated results of the fracture energy of NC-R0 and RSRAC mixes with

various contents of crumb rubber are summarized in Table 2. It should be noted

that no more than one testing curve was rejected as they are significantly different

from the remaining two. Effective cross-sectional size and non-uniformed distribu-

tion of steel fiber contribute to the discreteness. As a result, each listed value is the

average of two or three measurements.

2.5. Determination of the fracture toughness

The fracture toughness (KIC) of concrete is another important parameter to de-

scribe the fracture performance of concrete, it reflects the ability of concrete mate-

rial to resist crack extension, namely the ability of resisting brittle fracture. The

fracture toughness KICof a concrete material is calculated by formula (25) as in

ASTM E399-74 (American Society for Testing and Material).

KICPmaxS

th3=2

f a

h

2-5

where Pmaxis the vertical peak load;h,tand Sare respectively the height, width and

the span of the specimens (Fig. 3);a0is the notch depth; fahis the geometric shape

factor, calculated by formula (2-6).

f a

h

2:9

a

h

1=2 4:6

a

h

3=2 21:8

a

h

5=2 37:6

a

h

7=2 38:7

a

h

9=22-6

The inclusion of steel fiber introduces the anchoring force between steel fiber and

concrete matrix, which causes the fracture process zone of steel fiber reinforced con-

crete larger than that of plain concrete. Theformula of calculating fracture toughness

recommended by ASTM, however, is based on the plain concrete material. The influ-

ence of fracture process zone on adhesive fracture toughness of steel fiber reinforced

concrete is included by the replacement ofawiththe effective crack length acin for-

mula (2-6)[20].The effective crack length (ac) was calculated byac= a0+Dac. When

the testing load (P) reaches its maximum (Pmax), the crack mouth opening displace-

ment gets its critical value (CMODc), and the real length of pre-crack also develops

from the initial value (a0) to the critical effective crack length (ac). Hence according

to the linear asymptotic superposition principle,accan be calculated by a LEFM for-

mula (2-7)[21]:

ac2

ph h0 arctan

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffitECMOD

c

32:6Pmax 0:1135

s h0 2-7

whereh0is the thickness of steel sheet used to set up the clip-on gages on the crack

mouth (i.e. additional thickness); (CMOD)c is the critical value of the crack mouth

opening displacement;Eis the modulus of rupture, expressed by formula (2-8):

E 1

tci3:70 32:60tan2p

2a0 h0h h0

2-8

whereci CMODi

Pi(case in the table) is a initial value determined at a arbitrary point

(P, CMOD) on the ascent stage ofP-CMOD curves (Fig. 6). The calculated parameters

and results of the fracture toughness (KIC) of NC-R0 and RSRAC mixes are summa-

rized inTable 3.

3. Results and discussion

The fracture properties of mixes including NC-R0 and RSRAC are

determined by the three-point bending tests on centrally notched

beams (seeFig. 3) according to the RILEM fracture mechanics of

concrete (TC50-FMC) [15]. The measured loaddeflection (Pd)

curves and load-crack mouth opening displacement (P-CMOD)

curves are presented inFigs. 5 and 6 respectively.

The fracture energy (GF) and fracture toughness (KIC) of concrete

mixes are calculated (seeTable 4) and analyzed based on Pd and

P-CMOD curves. It should be noted that each value in Table 4 is the

average calculated from two or three specimens in a group.

3.1. Fracture energy

The fracture energy of each concrete mix, including both un-

heated and heated specimens, was shown inTable 4. The effects

W2

W0

0

W3W1

P P

Pmax

c

Fig. 4. Determination of fracture energy of concrete by means of three-point

bending method.

Table 2

Parameters of the fitted cure and values of the fracture energy.

Specimens a0(mm) mg (N) d0(mm) b k R2 W0(Nm) W2 (Nm) W3(Nm) Alig(m

2) GF(N/m)

NC-R0-T25-1 30.6 91.433 3.345 1.9991 1.1807 0.9746 11.5341 0.1529 8.8944 0.00306 6725.96NC-R0-T25-3 31.2 92.195 3.498 2.0525 1.1953 0.9789 10.8925 0.1612 8.2295 0.00312 6180.53

Average 6453.24

RC-R0-T25-1 31.4 91.814 5.001 3.2077 2.7761 0.9801 11.3739 0.2296 0.1035 0.00314 3728.35

RC-R0-T25-2 30.8 91.433 3.245 6.5399 7.7801 0.9727 9.5236 0.1484 0.0003 0.00308 3140.35

Average 3434.35

RC-R4-T25-2 31.7 88.766 9.000 2.5172 1.4368 0.9797 17.9374 0.3994 2.2071 0.00317 6480.74

RC-R4-T25-3 29.5 88.385 7.707 1.2241 1.3028 0.9834 16.2312 0.3406 2.1783 0.00295 6355.95

Average 6418.35

RC-R8-T25-1 30.8 86.099 8.851 2.9284 1.3969 0.9845 23.1908 0.3810 3.1052 0.00308 8661.37

RC-R8-T25-2 29.5 86.099 8.392 2.0653 1.3455 0.9814 20.6545 0.3613 2.8664 0.00295 8095.66

RC-R8-T25-3 29.8 85.718 8.958 2.5054 1.3559 0.9856 21.2141 0.3839 3.2259 0.00298 8330.19

Average 8362.41

RC-R12-T25-1 29.6 83.433 9.000 1.9526 1.3523 0.9906 12.8667 0.3754 2.5558 0.00296 5337.13

RC-R12-T25-3 31.6 84.957 9.000 3.0155 1.4515 0.9937 14.0124 0.3823 2.4766 0.00316 5339.03

Average 5338.08

RC-R16-T25-1 30.2 83.052 8.841 2.1846 1.7259 0.9878 10.3178 0.3671 0.6186 0.00302 3742.90

RC-R16-T25-2 31.9 84.195 8.931 2.8587 1.7336 0.9845 11.8971 0.3760 0.7819 0.00319 4092.45

Average 3917.68

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0

1

2

3

4

5

6

7

0 2 4 6 8 10 12

Deflection (mm)

L

oad(P,

kN)

NC-R0-T25

RC-R0-T25

RC-R4-T25

RC-R8-T25

RC-R12-T25

RC-R16-T25

(a) Load-deflection curves of mixes at room temperature

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12

Deflection (mm)

Load

(P,

kN)

NC-R0-T200

RC-R0-T200

RC-R4-T200

RC-R8-T200

RC-R12-T200

RC-R16-T200

(b) Load-deflection curves of mixes exposed to 200 C

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12

Deflection (mm)

Load(P,

kN)

NC-R0-T400

RC-R0-T400

RC-R4-T400

RC-R8-T400

RC-R12-T400

RC-R16-T400

(c) Load-deflection curves of mixes exposed to 400 C

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12

Deflection (mm)

Load

(P,

kN)

NC-R0-T600

RC-R0-T600

RC-R4-T600

RC-R8-T600

RC-R12-T600

RC-R16-T600

(d) Load-deflection curves of mixes exposed to 600 C

Fig. 5. Measured loaddeflection curves of mixes.

0

1

2

3

4

5

6

7

8

0 2 4 6 8 10 12

CMOD (mm)

Load(P,

kN)

NC-R0-T400

RC-R0-T400

RC-R4-T400

RC-R8-T400

RC-R12-T400

RC-R16-T400

(c) Load-CMODcurves of mixes exposed to 400 C

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12

CMOD (mm)

Load(P,

kN)

NC-R0-T600

RC-R0-T600

RC-R4-T600

RC-R8-T600

RC-R12-T600

RC-R16-T600

(d) Load-CMODcurves of mixes exposed to 600 C

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12

CMOD (mm)

Load(P,

kN)

NC-R0-T25

RC-R0-T25

RC-R4-T25

RC-R8-T25

RC-R12-T25

RC-R16-T25

(a) Load-CMODcurves of mixes at room temperature

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12

CMOD (mm)

Load(P,

kN)

NC-R0-T200

RC-R0-T200

RC-R4-T200

RC-R8-T200

RC-R12-T200

RC-R16-T200

(b) Load-CMODcurves of mixes exposed to 200

Fig. 6. Measured Load-CMOD curves of mixes.



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of temperature and rubber content on the fracture energy of con-

crete mixes were shown in Fig. 7(a and b), respectively. It can be

seen fromTable 4andFig. 7(a) that, the temperature has remark-

able effects on the fracture energy of concrete mixes. Averagely

speaking, the fracture energy of the concrete mixes increases

1.80 times after exposure to 200 C, which further increases 2.81

and 2.78 times after exposure to 400 C and 600 C respectively.

This was consistent with the results of Peng et al. [13], Menou

et al.[16]and Nielsen et al.[17]on the fracture energy. Exposure

to high temperature, on the one hand, may degrade the compres-sive strength of concrete mixes for the severe thermal decomposi-

tion (seeTable 4); on the other hand, it can improve the fracture

properties, especially the fracture energy [seeFig. 7(a)]. The main

mechanism responsible for this observation is that thermal dam-

age makes the cracks develop along a tortuous rather than a sharp

routine as at room temperature [16], which dissipates more

energy.

It should be noted that the fracture energy gradually becomes

flat when the temperature exceeded 400 C [see Fig. 7(a)]. The frac-

ture energy of several concrete mixes (e.g. NC-R0, RC-R4 and RC-

R8) slightly decreases, while the others kept rising with the tem-perature increasing from 400 C to 600 C.

Table 3

Parameters and values of fracture toughness of mixes.

Specimens a0(mm) Ci (lm/KN) h0(mm) mg (N) Pmax (kN) dc(mm) CMODc (mm) E(Gpa) ac(mm) KIC (MPa m1/2)

NC-R0-T25-1 30.6 6.925 1.45 91.433 6.021 0.372 0.118 19.143 49.867 2.036

NC-R0-T25-3 31.2 7.138 1.45 92.195 6.545 0.423 0.158 19.176 53.579 2.502

Average 2.269

RC-R0-T25-1 31.4 6.947 1.45 91.814 6.192 0.652 0.301 19.914 65.491 3.701

RC-R0-T25-2 30.8 6.996 1.45 91.433 5.653 0.482 0.343 19.152 68.107 3.764

Average 3.732RC-R4-T25-2 31.7 7.086 1.45 88.766 6.521 0.514 0.408 19.839 69.015 4.504

RC-R4-T25-3 29.5 6.971 1.45 88.385 6.385 0.448 0.319 17.934 64.340 3.641

Average 4.073

RC-R8-T25-1 30.8 7.752 1.45 86.099 6.821 0.591 0.435 17.284 67.366 4.397

RC-R8-T25-2 29.5 6.978 1.45 86.099 6.778 0.608 0.412 17.916 67.194 4.338

RC-R8-T25-3 29.8 7.563 1.45 85.718 7.065 0.791 0.378 16.797 64.385 4.033

Average 4.256

RC-R12-T25-1 29.6 7.219 1.45 83.433 5.261 0.358 0.345 17.410 67.861 3.467

RC-R12-T25-3 31.6 6.989 1.45 84.957 5.662 0.386 0.371 20.007 69.753 4.037

Average 3.752

RC-R16-T25-1 30.2 6.945 1.45 83.052 5.079 0.502 0.323 18.685 68.417 3.426

RC-R16-T25-2 31.9 7.139 1.45 84.195 6.074 0.531 0.302 19.903 65.814 3.676

Average 3.551

Table 4

The experimental and calculated results of three-point bending tests.

Results NC-R0-T25 RC-R0-T25 RC-R4-T25 RC-R8-T25 RC-R12-T25 RC-R16-T25

(a) The experimental and calculated results of mixes at 25C

Compressive strength/(MPa) 56.52 51.41 49.06 39.41 37.61 35.88

Pmax/(kN) 6.283 5.923 6.453 6.922 5.462 5.577

dC/(mm) 0.398 0.567 0.481 0.699 0.372 0.517

CMODC/(mm) 0.138 0.322 0.364 0.395 0.358 0.313

GF/(N/m) 6453.24 3434.35 6418.35 8362.41 5338.08 3917.68

KIC/(MPa m1/2) 2.269 3.732 4.073 4.256 3.746 3.551


(b) The experimental and calculated results of mixes at 200C


Pmax/(kN) 5.416 5.929 5.247 5.781 5.126 4.903

dC/(mm) 0.672 0.982 1.051 1.061 1.077 0.741

CMODC/(mm) 0.379 1.033 0.821 0.843 0.675 0.457GF/(N/m) 8552.75 7127.89 9571.71 16544.98 13490.84 5740.96

KIC/(MPa m1/2) 2.219 3.470 3.730 3.723 3.190 2.952


(c) The experimental and calculated results of mixes at 400C


Pmax/(kN) 5.315 6.468 6.294 6.782 6.269 5.819

dC/(mm) 1.445 1.270 1.550 1.227 1.405 1.445

CMODC/(mm) 1.261 1.141 1.349 1.123 0.974 1.182

GF/(N/m) 16695.00 15412.22 18230.00 18095.25 14027.35 12755.09

KIC/(MPa m1/2) 3.344 3.861 4.344 3.317 2.203 2.089


(d) The experimental and calculated results of mixes at 600C


Pmax/(kN) 4.846 6.311 6.545 6.191 6.080 5.970

dC

/(mm) 1.155 1.155 1.611 1.870 2.105 1.920

CMODC/(mm) 0.903 1.032 1.514 1.046 1.402 1.476

GF/(N/m) 16095.19 16176.88 16585.55 16683.09 15092.3 13763.07

KIC/(MPa m1/2) 2.600 3.114 3.768 3.373 3.513 3.578

http://-/?-http://-/?-


7/8

It can be seen fromTable 4andFig. 7(b) that, for the concrete

specimens after exposure to high temperatures except for 600 C,

a full replacement of NCA by RCA only results in a clear decrease

in the fracture energy, as the descending branch ofP-CMOD curves

for the mixes prepared with RCA drops faster after peak load (see

Fig. 6). When the rubber content increased from 4% to 16%, the

fracture energy first increased and then decreased with increase

of rubber content, with RC-R8 having the highest fracture energy

at the high temperatures, indicating that an appropriate rubber

content enhances the energy absorption capacity of the concrete

but too much rubber may have a negative effect on the energy

absorption capacity. Thus, to effectively improve of the energy

absorption capacity of concrete mixes (both unheated and heated

specimens), an appropriate amount of rubber should be selected.

3.2. Fracture toughness

The effects of temperature and rubber content on the fracture

toughness of concrete mixes were shown in Fig. 8(a and b), respec-

tively. It can be seen fromTable 4andFig. 8(a) that the fracture

toughness of the concretes mixes changes obviously after exposure

to a high temperature. However, with the different replacement ra-

tios of crumb rubber, the influences of temperature on the fracture

toughness of the concretes mixes are not consistent. For RC-R8, RC-

R12 and RC-R16 specimens, the fracture energy first decreased and

then increased with increase of rubber content, with the fracturetoughness being smallest at the temperature 400 C [see

Fig. 8(a)]. For other groups of specimens, the fracture toughness

of the concretes mixes changed in a different trend, this may be

caused by the non-uniform distribution of the steel fiber in the

crack surface and worth of a separate investigation.

It can be seen from Table 4andFig. 8(b) that a full replacement

of NCA by RCA leads to a significant increase in the fracture tough-

ness, and that the concrete mixes with RCA have high resistance to

brittle fracture. It is well known that the concrete strength depends

on the strength of the cement paste, the aggregates and the inter-

facial bond between the cement paste and the aggregates [22]. Forconcrete mixes prepared with natural aggregates and recycled

aggregates, the difference in the peak stress and fracture toughness

might be related to the strength of the interfacial bond. During

vibration, the water inside the cement paste may move to the recy-

cled aggregates due to their high water absorption capacity, creat-

ing a relatively high w/c value in the vicinity of recycled

aggregates. As a result, a stronger bond might be formed between

the cement paste and RCA. It is also noted that when the rubber

content increases from 4% to 16%, the fracture toughness first in-

creases and then decreases with increase of rubber content, with

RC-R4 or RC-R8, having the highest fracture toughness [see

Fig. 8(b)], which indicates that an appropriate rubber content im-

proves the resistance to brittle fracture of the concrete mixes but

too much rubber content may have a negative effect on the resis-tance to brittle fracture.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

25 200 400 600

Temperatures

Fracturee

nergy(N/m)

NC-R0RC-R0RC-R4RC-R8RC-R12RC-R16

(a) Effects of temperature on the fracture energy

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

NC-R0 RC-R0 RC-R4 RC-R8 RC-R12 RC-R16

Mix

Fractureenergy(N/m)

T25

T200

T400

T600

(b) Effects of rubber content on the fracture energy

Fig. 7. Effects of temperature and rubber content on the fracture energy.

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

25 200 400 600

Temperatures

NC-R0RC-R0RC-R4RC-R8RC-R12RC-R16

(a) Effects of temperature on the fracture

toughness

1.5

2.0

2.5

3.0

3.5

4.0

4.5

NC-R0 RC-R0 RC-R4 RC-R8 RC-R12 RC-R16

Mix

Fracturetoughness(MPam

1/2)

T25

T200

T400

T600

(b) Effects of rubber content on the fracture toughness

Fracturetough

ness(MPam

1/2)

Fig. 8. Effects of temperature and rubber content on the fracture toughness.



8/8

4. Conclusions

In this paper, a series of three-point bending tests on notched

beams of 100 mm100 mm515 mm were conducted in accor-

dance to the recommendation of RILEM. The fracture energy and

fracture toughness were calculated and the effects of heating and

rubber contents on them were analyzed. The following conclusions

can be drawn from this research:

(1) A severe degradation of compressive strength has been

found for all concrete mixes after exposure to high temper-

atures, however, averagely the fracture energy of the con-

crete mixes increased 1.80 times after exposure to 200 C,

further increased to 2.81 and 2.78 times after exposure to

400 C and 600 C respectively. This indicates that the

energy dissipation capacity of concrete mixes increases as

the temperatures rise, which is opposite to the trend of the

compressive strength.

(2) The main mechanism responsible for the increase in the

fracture energy is that, the thermal damage makes the

cracks develop in a more tortuous routine rather than

sharp one as at room temperature, which dissipates more

energy.

(3) A full replacement of NCA by RCA results in an evident

decrease in the fracture energy, as after the peak load the

descending part of the fracture energy for the concrete

mixes prepared with RCA dropped faster. The fracture

energy first increased and then decreased with increase of

rubber content from 0% to 16%, with RC-R8 having the high-

est fracture energy at high temperatures.

(4) The fracture energy represents the energy dissipation capac-

ity of concrete mixes, while the fracture toughness reflects

resistance to brittle fracture of concrete mixes. A full

replacement of NCA by RCA leads to a significant increase

in the fracture toughness. While the rubber content

increased from 4% to 16%, the fracture toughness first

increased and then decreased with increase of rubber con-tent, with RC-R4 or RC-R8 having the highest fracture

toughness.

(5) Appropriate rubber content increases the ductility of the

concrete mixes but too much rubber may have a negative

effect on the ductility of the concrete mixes.

Acknowledgments

The authors gratefully acknowledge the financial support

provided by the National Natural Science Foundation (Project

Nos. 51278132, 11372076), and Science and Technology Planning

Project of Guangdong Province (2011B010400024), Technology

Planning Project of Huangpu District (201356) and Foundation of

Guangdong Provincial Department of Transport (Project Nos.

2013-02-017, 2013-04-006).

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