Construction and Building Materials 71 (2014) 472–483

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Construction and Building Materials

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Shear transfer across a crack in high-strength concrete after elevatedtemperatures

http://dx.doi.org/10.1016/j.conbuildmat.2014.08.0740950-0618/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +86 21 65982787; fax: +86 21 65986345.E-mail address: jzx@tongji.edu.cn (J. Xiao).

Jianzhuang Xiao a,⇑, Zhiwei Li a, Jiabin Li a,b

a Department of Structural Engineering, Tongji University, Shanghai 200092, PR Chinab Institute for Structural Concrete, Graz University of Technology, Graz A8010, Austria

h i g h l i g h t s

� High temperature tests on two grades of HSC specimens were carried out.� Push-off test on HSC specimens after elevated temperatures was firstly performed.� Effect of elevated temperatures on shear transfer behavior of HSC was revealed.� Relationships between HSC shear strength and exposed temperature were proposed.

a r t i c l e i n f o

Article history:Received 8 April 2014Received in revised form 15 August 2014Accepted 24 August 2014

Keywords:High-strength concrete (HSC)Elevated temperatureShear transferCrack deformationPush-off

a b s t r a c t

This paper experimentally investigated the shear transfer behavior of high-strength concrete (HSC) acrossa crack after elevated temperatures. The compressive strength of concrete and the experienced temper-ature were the two main parameters in this study. Twenty-two uncracked push-off specimens werecasted and heated in an electrical furnace. Push-off tests were then conducted to study the shear strengthand the crack formulation and deformation of the concrete after elevated temperatures. The elevatedtemperature test results indicate that the heating process of HSC is related to the furnace chamber tem-perature, the thermal convection and the thermal radiation. The heating process can be divided into 3stages based on the heating time. Except for an exposed temperature of 200 �C, the ultimate shearstrength of HSC reduces and the corresponding crack deformation (crack slip and crack width) increaseswith the increase of the temperature. The shear brittleness of HSC decreases as the exposed temperatureincreases. The higher the strength of concrete is, the more brittle the shear transfer characteristicsbecomes. Finally, the equation for estimating the residual shear strength of HSC after elevated tempera-tures are proposed based on the statistical analysis of the test data.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, high-strength concrete (HSC) is increasinglypopular in civil engineering practice due to its higher strengthand better durability in comparison with conventional normalstrength concrete (NSC) [1]. HSC with a compressive strength of62 MPa was applied for Water Tower Place in Chicago in 1974,and the compressive strength of the concrete used in Burj Dubaireached 80 MPa [2]. However, with the increasing engineeringapplications and the deepening of related research activities, itwas noticed that HSC can be inferior to NSC in the aspects of somemechanical properties after elevated temperatures [3]. The ele-vated temperature, as one of the most severe environments, should

be taken into account in the design of HSC elements and structures[1]. The properties of NSC and HSC at elevated temperatures havebeen widely studied, and a literature review indicates that con-crete can experience drastic physical and chemical changes whenthe exposed temperature increases. The changes of the microcos-mic structure of concrete have a great influence on its macroscopicmechanical properties. For example, the interface between theaggregate and the cement paste can form micro cracks due to dif-ferent thermal expansions once the temperature exceeds 100 �C.Consequently, the bonding force between the aggregate and thecement paste reduces, and the concrete strength decreases [4].The calcium silicate hydrate (C–S–H), which provides the strengthof the cement paste, decomposes further when the exposed tem-perature is beyond 600 �C. This can result in a great reduction ofthe compressive strength of concrete [5]. At a temperature of800 �C, the CaCO3 decompose into CO2 and CaO, and the concrete

J. Xiao et al. / Construction and Building Materials 71 (2014) 472–483 473

compressive strength reduces to 20–30% of that at room tempera-ture, and the material loses its strength greatly [6]. The microcos-mic diversification of concrete can not only result in thedegradation of the compressive strength, but also reduce the ten-sile strength, the flexural strength and the elastic modulus [7].Due to the effect of the high temperature on the material proper-ties, the load bearing characteristics of HSC structural membersexposed to elevated temperatures are different from that at roomtemperature [8–10]. Up to now, however, there is only few pub-lished work that examined the shear transfer behavior of HSCacross a crack after elevated temperatures although it is an impor-tant mechanical property of this material in structural members toresist the lateral force and self-weight.

Since Birkeland and Birkeland [11] published their work in1966, a lot of studies have been undertaken to investigate the sheartransfer behavior and mechanism of concrete at room temperature.Loov and Patnaik [12] proposed an equation of the concrete shearstrength. The equation was proved to be able to provide a goodprediction of the shear strength of HSC [13]. Loov and Peng [14]further studied the effect of the concrete strength, the numberand length of the stirrups, the angle between the longitudinal axisof the beam and the shear plane on the shear transfer characteris-tics of concrete. Detailed equations for estimating the shearstrength of both NSC and HSC have been developed by Kahn andMitchell [15] based on fifty push-off specimens. The equationsindicate that the higher compressive strength and reinforcementratio lead to an improvement of the shear strength of concrete.However, the shear strength of concrete is no higher than 0.2 f 0c ,where f 0c is the cylinder compressive strength. Based on the trussmodel and a softened compressive stress–strain relation alongthe concrete struts. Hsu et al. [16] proposed a theory for predictingthe shear strength of concrete. Their theory took into account thefact that the reinforcement parallel to and near the shear planecan have a significant contribution to the shear strength of con-crete when the longitudinal reinforcement ratio is relatively low.This fact was ignored in most of the shear transfer theories. Equa-tions for the shear strength of a full range of concrete strengthswere proposed by Ali and White [17] based on the contact densitymodel. These purely analytical equations were approximated fordesign purposes. In addition to macroscopic investigations on theshear transfer behavior of concrete across a crack, studies on theshear transfer mechanism have also been carried out at the submi-croscopic level. Martin-Perez and Pantazopoulou [18] concludedthat the shear strength of concrete consists of the dowel action,the bond between the reinforcement and the concrete as well asthe aggregate interlock. The flexure of the bars makes a principalcontribution to the dowel action. The bond between the reinforce-ment and the concrete is affected by the reinforcement type andthe concrete strength. Aggregate interlock occurs because the pro-truding aggregates on one side of the shear surface squeeze thecement pastes on the other side to resist the shear deformation.The primary source of the shear transfer of concrete is due to theaggregate interlock, as demonstrated by many investigators[19,20].

The characteristics of the aggregate interlock vary with the con-crete compressive strength. The aggregate particles in NSC can bepushed out from the cement paste through the crack propagation[19]. However, the cement paste in HSC has a higher strength thanthe aggregate, which results in the aggregate fracture and the crackacross aggregate particles. Consequently, the shear strength startsto reduce due to the serious fracture of the aggregate particleswhen the concrete compressive strength reaches a certain value[21]. Hence, there is a complex relation between the aggregateinterlock and the compressive strength of concrete. Xiao et al.[22] investigated the shear transfer performance of recycled aggre-gate concrete (RAC). The results have showed that the aggregate

interlock in RAC is weakened because of a layer of old and weakmortar wrapping the recycled coarse aggregate. It was observedthat concrete containing river gravel exhibited more favorableaggregate interlock behavior compared to that mixed with lime-stone aggregate. In addition, the volume of aggregate was alsofound to be an influencing factor of the shear strength of concrete[23].

As described above, the elevated temperatures can result in adeterioration of the cement paste, which is much stronger thanthe aggregates in HSC at room temperature. The aggregate evendecomposes beyond 800 �C. It is assumed that the aggregate inter-lock characteristics may change when the exposed temperatureincreases. Thus the shear transfer of HSC across a crack can changeand degrade. However, no detailed studies on the shear transfer ofHSC after elevated temperatures have been reported in literature.Therefore, a comprehensive study is required to investigate theshear transfer and the degradation mechanism of HSC after ele-vated temperatures. This paper presents such an experimentalinvestigation on the shear transfer characteristics of HSC across acrack after elevated temperatures.

2. Research significance

The shear transfer behavior of HSC after a fire is an importantindicator for the fire resistance requirement of HSC elements andstructures. The change of the aggregate interlock is the main rea-son characterizing the shear transfer behavior of HSC at roomand elevated temperature. Elevated temperatures have adverseeffect on the cement paste and aggregate, hence the correspondingcharacteristics of the aggregate interlock can be changed. Thispaper aims to study the shear transfer behavior and mechanismof HSC after elevated temperatures. This study may provide usefuldata for the assessment of some mechanical performance of HSCelements after a fire.

3. Test programme

3.1. Materials

The specimens consisted of two concrete types, labeled as L (lower strength)-series and H (higher strength)-series. PO42.5 and PO52.5 Portland cement wereused for L-series and H-series specimens, respectively. Slag powder was used forboth series, and silica fume was only mixed into H-series. The maximum aggregatesizes of the siliceous crushed stones with continuous grading were 25 mm and20 mm for L- and H-series, respectively. The fine aggregate for both series was riversand with a fineness modulus of 2.70. Table 1 describes the chemical composition ofHSC mixture. The mixture proportions of HSC are presented in Table 2.

3.2. Specimen preparation

The specimens for both L- and H-series were similar to those used in the push-off tests recently conducted by Xiao et al. on RAC [22]. The dimensions of all push-off specimens were 150 � 400 � 600 mm3, as shown in Fig. 1. Four closed stirrups(diameter 8 mm) with a spacing of 70 mm across the shear plane were used to sim-ulate the lateral constraint for the shear plane. The yield strength of the stirrups is325 MPa at room temperature. All specimens had eight steel bars with a diameter of14 mm as longitudinal reinforcements (parallel to the shear plane). The reinforce-ment cages were assembled carefully, and were then placed inside of woodenmoulds, as shown in Fig. 2.

Each series of specimens was subdivided into four groups, designated as 20,200, 400 and 800. These numbers represent the scheduled exposed highest temper-ature. The group 20 included 2 specimens and the other groups consisted of 3 ones.Therefore, the specimens were designated by the series name followed by the groupnumber and a letter. For example, L-200-a indicated the first specimen of lowerstrength concrete, which was exposed the highest temperature of 200 �C. In total,there were 8 groups of specimens.

The specimens were compacted by a poker vibrator. Three cubes of150 � 150 � 150 mm3 for L- and H-series were respectively casted, for determiningthe cube compressive strength of the concrete. Three K-type thermocouples wereinstalled into one of every three specimens of group 200, 400 and 800 before thesetting of the HSC specimens. Fig. 3 displays the thermocouple locations with anX-designation and a reference number. Thermocouples 1, 2 and 3 recorded the

Table 1Chemical composition of HSC mixture (% by mass).

Constituent PO42.5 Portland cement PO52.5 Portland cement Slag powder Silica fume Crushed stone

CaO 73.402 61.401 47.384 0.433 5.343SiO2 20.251 27.264 28.476 96.631 63.206Al2O3 2.788 7.131 13.884 0.871 15.279Fe2O3 2.410 2.794 0.427 0.098 6.745MgO 0.459 0.441 9.062 1.555 3.113Na2O 0.116 0.189 0.433 0.082 3.462K2O 0.574 0.780 0.334 0.330 2.852

Table 2Mixture proportions of HSC (kg/m3).

Series Cement Slag powder Silica fume Crushed stone Sand Water Superplasticizer

L 412.50 137.50 – 1046.00 614.00 181.50 2.13H 406.00 145.00 29.00 1114.60 654.60 136.30 14.50

400

V-slot

600

AA

B

400

120

Section A-A

125

125

2525

150

15

300

Section B-B

(a) Drawing (b) Photo

Fig. 1. Dimensions of push-off specimen (unit: mm).

(a) Drawing (b) Photo

8 14

120

80

110 100 110400

Fig. 2. Reinforcement details of push-off specimen (unit: mm).

400

300

300

DD400

150

570

150

3313 2

Section D-D

(a) Drawing (b) Photo

Fig. 3. Thermocouple locations of push-off specimen (unit: mm).

474 J. Xiao et al. / Construction and Building Materials 71 (2014) 472–483

Fig. 5. Specimens and steel cage.

J. Xiao et al. / Construction and Building Materials 71 (2014) 472–483 475

temperatures at 5 mm, 33 mm (the longitudinal reinforcement) and 75 mm (themid-depth of the specimen) depth from the concrete surface, respectively.V-grooves at both the front and rear faces of specimens were developed by layingthe v-slot wood bars when HSC specimens were casted, which resulted in a shearplane area Ac = 36,000 mm2 (120 � 300 mm2). The width and length of the shearplane were respectively about 5 and 12 times of the maximum aggregate size,and the aggregate interlock of the shear plane was able to work. The wood barwas moved away after hardening of the HSC. The shear reinforcement ratio isdefined as: q ¼ Ay=Ac ¼ 402=36000 ¼ 1:12%, where Ay is the cross-sectional areaof all stirrups. After 24 h, the specimens were demolded and moved to the curingroom with a temperature of 20 ± 2 �C and more than 90% relative humidity.

3.3. Elevated temperature test programme

At a curing age of 28 d, all specimens including the cubes were removed fromthe curing room and were dried at room temperature for 60 days. During this per-iod, the room temperature was 25–30 �C. Then compression tests on cubes and ele-vated temperature tests on push-off specimens were carried out, respectively. Theaverage compressive strengths of the cubes fcu for L- and H-series were 64.7 MPaand 94.0 MPa, respectively. The elevated temperature test was performed in anelectrical furnace with 650 � 650 � 1050 mm3 volume and 36 kW maximumpower, shown in Fig. 4. To avoid the damage of the heating wires within furnacewalls owing to a possible spalling of the HSC, all specimens were put in a specialsteel bar cage, see Fig. 5.

The specimens were heated with a heating rate of 5 �C/min. Two thermocoupleswere installed in the furnace wall and the chamber, respectively. The temperatureswere automatically collected and recorded for every 30 seconds. Once the targettemperatures (200, 400 and 800 �C) of furnace walls (heating wires) were reached,the temperatures were kept constant until the temperature differences between themid-depth temperatures of specimens and the target temperatures were smallerthan 10%. The hold time at heating for L-200, L-400, L-800, H-200, H-400 and H-800 were 680, 560, 715, 640, 495 and 715 min, respectively. Then the vent holeat the top of the furnace was opened and the elevated temperature test was ended.

3.4. Push-off test programme

The push-off test was carried out after the elevated temperature test and alltests completed within two weeks. All push-off tests were conducted in a testingmachine with a capacity of 3000 kN. The specimens were placed on the testingmachine in the vertical direction. The top and bottom of the specimens were sup-ported on a knife-edge and a roller, respectively. Thus the specimens can movefreely in the horizontal direction to impose vertical concentrated loads on the shearplane. The crack slips were measured by two linear variable displacement transduc-ers (LVDTs) attached to one side of the specimens. There were three LVDTs on theother side to measure the crack opening widths. The locations of the LVDTs onthe specimens were shown in Fig. 6. At each second, the concentrated load andthe crack deformation (crack slip and width) readings were recorded automaticallyby the computerized data acquisition system. The push-off test loading sketch isshown in Fig. 7. The load was applied with a displacement control at a rate of0.05 mm/min until the vertical concentrated load tends to be a constant.

Fig. 4. Electrical furnace.

4. Results and discussion

4.1. Thermal response

As described in Sections 3.2 and 3.3, the thermocouples mea-sured temperatures of 5 locations, which were 5 mm, 33 mm,75 mm depth from the specimen surface, as well as at the furnacewall and in the chamber. The temperatures were averaged fromthe specimens of the same group. The development of the temper-atures in group 200, 400 and 800 is displayed in Fig. 8.

In Fig. 8, each subgraph shows the temperatures of 5 locationsfor a typical group. The temperatures exhibit the similar trendfor all groups. However, the temperatures of H-800 specimensmay not be fully reliable due to some surface spalling of the spec-imen. From the surface to the mid-depth of the specimens, therecorded temperatures decreased. Based on the heating time, tem-perature variations in the specimens can be considered as a three-stage process:

(1) The temperatures increased slowly from about zero to60 min. This is likely due to the small temperature differencebetween the furnace chamber (wall) and the specimen sur-faces. The equations for the thermal convection and thethermal radiation are described by [24]

q ¼ hðTw � Tf Þ ð1Þ

U ¼ e1A1r½ðT1 þ 273Þ4 � ðTf þ 273Þ4� ð2Þ

where q is the heat flux (W/m2); h is the coefficient of thermalconductivity (W/(m2 K)); Tw and Tf are the temperatures of the airin the furnace chamber and the specimen surfaces (�C), respec-tively; U is the heat flow (W); e1 is the emissivity; A1 is the areaof radiant surface (m2); r is Stefan–Boltzmann constant(5.67 � 10�8 W/(m2 K4)); T1 is the temperature of the furnace wall(�C). In Eqs. (1) and (2), q and U decrease if the temperature differ-ence between the furnace chamber (wall) and the specimen surfacebecomes small.

220

160

220

110180110

Fixed points

LVDTSteel bar

Sheet glass

Welding pointfor both bars

220

8022

0

110 180 110

80

Sheet glass

Fixed points

LVDTSteel bar

(a) Two LVDTs for the crack slips (b) Three LVDTs for the crack widths

Fig. 6. Locations of the LVDTs (unit: mm).

Fig. 7. Push-off test setup and specimen.

476 J. Xiao et al. / Construction and Building Materials 71 (2014) 472–483

(2) The temperatures increased rapidly from about 60 to360 min. This can be explained as follows. The temperaturedifference between the furnace chamber (wall) and the spec-imen surfaces becomes greater, and the corresponding q andU increase.

(3) The temperatures increased very slowly after 360 min, andthe temperatures in the specimens hardly reach the temper-atures of the furnace chamber. As a similar explanation, thetemperatures of specimen surfaces are close to those of fur-nace chamber (wall). Consequently, q and U are almostequal to zero, and the specimens cannot receive further heat.Besides the thermal convection and the thermal radiation,the migration of moisture and the thermal conductivity vari-ations in the HSC also influence the heating rate [25,26]. Inbrief, the heating process reflects the combined effects ofthe furnace chamber (wall) temperature and the physicalas well as chemical variations of the HSC.

4.2. Elevated temperature test observations

The specimens of group 400 and 800 released water vapor dur-ing the elevated temperature test. When water vapor emittedthrough the vent hole, the temperatures at the mid-depth of thespecimens were about 80 �C. As the temperature at the mid-depthof the specimens reached about 220 �C, the water vapor disap-peared. At 120 min after heating, the first explosive spalling soundwas heard. The temperatures of the furnace chamber and the mid-depth of the specimen at the moment were 430 �C and 105 �C,respectively. The explosive spalling sounds sustained approxi-mately 1 h.

After the elevated temperature test, specimens were carefullyobserved and photographed. A surface spalling of H-800 specimenoccurred and some steel bars were naked, as shown in Fig. 9. Thiscan be attributed to the fact that specimens of H-800 had greatercompactness and higher temperature. The greater compactnessresults in lower permeability, and a spalling occurs when the porepressure exceeds its tensile strength [27]. The fracture sections ofthe spalling after 800 �C were observed by scanning electronmicroscope (SEM) to study the changes of the HSC at the submicro-scopic level after elevated temperatures. The interface between theaggregate and the cement paste exhibited pronounced cracks.Some cracks even passed through the coarse aggregates. Fig. 10clearly shows the cracks of HSC at the submicroscopic level after800 �C. With the development of the cracks, the bonding forcebetween the aggregate and the cement paste reduces, and subse-quently the strength and elasticity modulus of HSC decrease. Fur-thermore, the shear transfer characteristics of HSC also change.

The spalling of H-800 specimen happened on the outer surfaceof the specimen. The shear plane of the specimen was located inthe middle of both V-grooves and the shear plane was 15 mm dis-tant from the outer surface of the specimen (as can be seen inFig. 1). In addition to the restraint of the stirrup, the shear planeof the specimen remained intact. Before the push-off tests, theouter surface of H-800 specimens was repaired by mortar. Themixture proportion of the used mortar was cement: slag powder:silica fume: sand: water = 1:0.36:0.07:1.61:0.43. The mortar didnot influence the shear strength of the specimen due to integrityof the shear plane. The repair process is shown in Fig. 11. Otherspecimens showed no explosive spalling except for some randomhair-like cracks. Also the mass and the surface color of specimenschanged, as listed in Table 3. The mass loss is mainly due to twopossible reasons. The first one is the elimination of the free waterby evaporation, and the loss of chemically bound water [28]. The

00

50

100

150

200

250Furnace chamber

Furnace wall

33mm depthMid-depth

Tem

pera

ture

(°C

)Te

mpe

ratu

re (°

C)

Tem

pera

ture

(°C

)

Tem

pera

ture

(°C

)Te

mpe

ratu

re (°

C)

Tem

pera

ture

(°C

)

Time (minutes) Time (minutes)

5mm depth

00

50100150200250300350400450 Furnace chamber Furnace wall

33mm depth

Mid-depth5mm depth

0

(a) L-200 (b) L-400

0100200300400500600700800900 Furnace chamber Furnace wall

Mid-depth

33mm depth

5mm depth

0

50

100

150

200

250

Mid-depth

33mm depth5mm depth

Furnace chamberFurnace wall

(c) L-800 (d) H-200

050

100150200250300350400450 Furnace chamber

Furnace wall

Mid-depth

33mm depth

5mm depth

120 240 360 480 600 720

0

Time (minutes)120 240 360 480 600 720 0

Time (minutes)120 240 360 480 600 720

0

Time (minutes)120 240 360 480 600 720

120 240 360 480 600

Time (minutes)0 120 240 360 480 600

0100200300400500600700800900 Furnace chamber Furnace wall

Mid-depth

33mm depth

5mm depth

(e) H-400 (f) H-800

Fig. 8. Temperature development during the elevated temperature test.

(a) H-800-a (b) H-800-b (c) H-800-c

Fig. 9. Specimen spalling.

J. Xiao et al. / Construction and Building Materials 71 (2014) 472–483 477

Cement paste

Crack

Aggregate

Cement paste

Crack

Aggregate

(a) Crack between aggregateand cement paste

(b) Crack passing through aggregate

Fig. 10. SEM crack photos of HSC after 800 �C.

(a) Specimen during repairing (b) Specimen after repairingFig. 11. Repair process of specimen.

478 J. Xiao et al. / Construction and Building Materials 71 (2014) 472–483

second reason is the surface spalling of the specimen (especially forH-800 specimens) [29]. Except for L-800, the percentage of themass loss of L-series is greater than that of H-series because thewater content of L-series is higher. H-800 specimens exhibit thegreatest mass loss due to the surface spalling. The elevated tem-peratures bleached the colors of the specimens out.

Table 3Mass and surface color variation of specimens after elevated temperatures.

Specimen Mass variation

Unheated (kg) Heat-treated (kg) Percentage of mass loss (%

L-200-a 89.86 86.82 3.38L-200-b 89.82 86.84 3.32L-200-c 89.96 86.98 3.31

L-400-a 90.50 85.72 5.20L-400-b 87.60 83.68 5.30L-400-c 90.54 85.96 5.06

L-800-a 90.50 83.59 7.64L-800-b 87.60 80.84 7.72L-800-c 87.85 81.10 7.68

H-200-a 92.10 90.08 2.19H-200-b 93.10 91.58 1.63H-200-c 92.22 90.46 1.91

H-400-a 92.04 88.20 4.17H-400-b 91.52 87.36 4.55H-400-c 92.60 88.68 4.23

H-800-a 93.50 82.50 11.76H-800-b 92.50 80.90 12.54H-800-c 91.95 74.09 19.42

4.3. Shear stress–crack deformation curves

The shear stress on the shear plane is defined as s = P/Ac, whereP is the vertical concentrated load. The ultimate shear stress istermed as the shear strength. The crack slip and crack width arethe relative movement between the shear faces in the vertical

Color variation

) Average percentage of mass loss (%) Unheated Heat-treated

3.34 Gray white Light gray

5.18 Oyster white

7.68 Ivory

1.91 Graphite gray Mouse gray

4.32 Umbra gray

14.58 Light gray

J. Xiao et al. / Construction and Building Materials 71 (2014) 472–483 479

and transverse directions, respectively. The shear cracks occurredalong the shear planes or the V-grooves. A typical shear crack isshown in Fig. 12. The shear stress–crack slip and shear stress–crackwidth curves of the 8 groups of specimens are shown in Figs. 13and 14, respectively. For convenience, the data are expressed asthe average values of the measurements of the LVDTs for the samespecimen. Although some scattering for each group exhibits, thetrends are in good agreement with each other.

By comparing Figs. 13 and 14, one can find that the crack slipsand the crack widths have the similar trend except for the initialdeformation. There are nearly no crack slips in both series priorto the reaching of the ultimate shear stress at room temperatureand at 200 �C. Similar results were reported by Hofbeck et al.[30] and Mattock et al. [31]. However, unlike the initial crack slip,the initial crack width begins to develop at a load level of about50% of the ultimate load, as shown in Fig. 14a and b.

The crack deformation (crack slip and crack width) occurredearlier when the specimen exposed a higher temperature. Thusas the elevated temperature increases, the initial stiffnessdegrades. It can also be found that the post-peak shear stressdecreases rapidly at room temperature, however the slope of thecurve beyond the ultimate stress decreases with the increase ofthe elevated temperature. When the temperature exceeds 200 �C,the ultimate shear stress su decreases and the corresponding crackslip su increases. Briefly speaking, elevated temperature leads to amore ductile shear transfer behavior of HSC. This is most likelydue to the strength degradation of HSC resulted from the elevatedtemperature. A similar tendency of the typical load–deformationcurve was also observed for HSC under uniaxial compression load-ing after elevated temperatures [29]. With the increase of the ele-vated temperature, the cracks develop along the interfacesbetween the aggregate and the cement paste or across the aggre-gate, as shown in Fig. 10. In addition, the cement paste softensdue to the decomposition of C–S–H [4]. Hence, the aggregate inter-lock resistance of HSC reduces.

The crack slip and the crack width can induce a tensile stress inthe steel bar, which in turn creates a clamping force in the concreteacross the crack [30]. The yield strength of the stirrups and the

Fig. 12. Typical crack pattern of push-off specimen.

bond between the reinforcement and the concrete also decreasesafter elevated temperatures. When the reinforcement across thecrack yields, the limit state is reached and the shear strength canbe calculated [32]. The weakness in the bond between the rein-forcement and the concrete can result in a reduction of the lateralconstraint stiffness. As a result, the resistance against the deforma-tion of the shear plane decreases after elevated temperatures.

It should be noted that the curves of H-series shown in Figs. 13aand b and 14a and b exhibit evident plateaus after the ultimateshear stress. This can be explained through a comparison of theshear stress–crack deformation curves of L-20 and H-20. As thecracks development along the shear planes, the crack deformationof L-20 increases. Consequently, the shear stress decreases gradu-ally. In contrast, the stiffness of the concrete with H-series is higherthan that of L-series. Although the cracks occurred, the crack defor-mation of H-20 slightly increased. The shear resistance remainsnearly the same, until the two halves of the specimen entirely sep-arate. Once a crack crossing the last connection at the shear plane,the crack deformation increases quickly. This process formed a pla-teau, followed by a sharp drop of the shear stress. The plateauappeared suddenly during the push-off tests, and nearly no datapoint could be collected during this plateau.

As described above, a higher concrete strength leads to a morebrittle shear transfer at room temperature. Even if the HSC experi-ences elevated temperatures, the shear failures of H-series are alsomore brittle than those of L-series, as shown in Figs. 13 and 14.However, the elevated temperatures reduce the disparity of thebrittleness between both series.

Mansur et al. [32] and Emiko et al. [33] divided the shear stress–crack deformation curves of precracked push-off specimens intosome stages. In this investigation, the complete deformationresponse of the specimens with a certain elevated temperaturecan also be divided into 4 stages, as shown in Fig. 15. Stage I rep-resents the elastic stage, during which the cracks do not form(for the crack width) or the cracks do not develop insufficiently(for the crack slip). Through comparing Figs. 13a and 14a, it shouldbe noted that the development of the crack width is more sensitiveto that of the crack slip. Once the cracks form, the crack widthcurves will enter stage II immediately. It seems possible that thecrack slip is restricted by the aggregate interlock. Even if the cracksform, the crack slip does not develop to stage II simultaneously. Forthis reason, the end of stage I represents either the formation of thecrack (for the crack widths) or sufficient development of the crack(for the crack slips). In stage II, the shear stress continues toincreases with a lower slope, until the yielding of the stirrups. Atthis stage, due to the yielding of the stirrups, the dowel action ofthe bars becomes less significant [34], and the aggregate interlockand the friction between the two opposite planes provide the mainshear resistance of the HSC. After the peak stress, the deformationresponse enters stage III. The aggregate interlock reduces gradu-ally, and the interlocking force disappears and the dowel actionof the bars provides the residual shear stress [32]. At this moment,the friction of the cement paste in the shear plane could also con-tribute to the residual shear stress. Comparing Figs. 13 and 14 withFig. 15, however, not all specimens could clearly exhibit the com-plete four stages. For example, the shear stress–crack slip curveof specimen H-20-a in Fig. 13a can hardly exhibit stage II due tothe higher shear brittleness of HSC with series H.

4.4. Main test results

Table 4 describes the main test results in detail, including theultimate shear load Pu, the ultimate shear stress or the shearstrength su, the crack slip su and the crack width wu at the ultimateshear load. To compare more clearly, the variations of the shearstrength su and the corresponding crack deformation (crack slip

0123456789

10111213

H-20-a

H-20-b

L-20-b

Shea

r stre

ss (M

Pa)

Crack slip (mm)

L-20-a

0123456789

1011121314

H-200-bH-200-aH-200-c

L-200-c

L-200-b

Shea

r stre

ss (M

Pa)

Crack slip (mm)

L-200-a

(a) L-20 and H-20 (b) L-200 and H-200

0123456789

1011

H-400-bH-400-c

H-400-a

L-400-c

L-400-b

Shea

r stre

ss (M

Pa) L-400-a

0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0

Crack slip (mm) Crack slip (mm)0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0

0

1

2

3

4

5

6

H-800-c

H-800-b

H-800-a

L-800-c

L-800-b

Shea

r stre

ss (M

Pa)

L-800-a

(c) L-400 and H-400 (d) L-800 and H-800

Fig. 13. Shear stress–crack slip curves.

0.0 0.5 1.0 1.50123456789

10111213

H-20-b

H-20-a

L-20-b

Shea

r stre

ss (M

Pa)

Crack width (mm)

L-20-a

0123456789

1011121314

H-200-cH-200-b

H-200-a

L-200-cL-200-b

Shea

r stre

ss (M

Pa)

Crack width (mm)

L-200-a

(b) L-200 and H-200(a) L-20 and H-20

0123456789

1011

H-400-b

H-400-a

H-400-c

L-400-c

L-400-b

Shea

r stre

ss (M

Pa)

Crack width (mm)

L-400-a

0.0 0.5 1.0 1.5

0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 2.5 3.00

1

2

3

4

5

6H-800-b H-800-b

H-800-a

L-800-c

L-800-b

Shea

r stre

ss (M

Pa)

Crack width (mm)

L-800-a

(c) L-400 and H-400 (d) L-800 and H-800

Fig. 14. Shear stress–crack width curves.

480 J. Xiao et al. / Construction and Building Materials 71 (2014) 472–483

su and crack width wu) with the temperature are also drawn inFigs. 16 and 17, respectively.

With the increase of the elevated temperature, the shearstrength of specimens with L-series is found to decrease linearly.

However, the shear strength of H-series increases by 8.18% whenthe temperature rises from 20 �C to 200 �C. This phenomenonmay be due to the removal of the absorbed moisture. As the tem-perature increases from 100 �C to 200 �C, the cement gel stiffens

Stage ΙV

Stage Ι

Stage ΙΙStage Ш

Formation of the crack or sufficientdevelopment of the crack (for crack slip)

Yielding of reinforcement

Loss of interlocking force

Crack slip or crack width

Shea

r stre

ss

Fig. 15. Complete deformation response of push-off specimen.

0 200 400 600 800 10000123456789

101112131415

Shea

r stre

ngth

τu (M

Pa)

Temperature (°C)

L-seriesH-series

Fig. 16. Shear strength su–temperature curves.

0 200 400 600 800 1000

0.0

0.5

1.0

1.5

2.0

2.5

H-series-width

H-series-slipL-series-width

L-series-slip

L-series-width

H-series-width

H-series-slip

Cra

ck d

efor

mat

ion

(su

and

wu)

(mm

)

Temperature (°C)

L-series-slip

Fig. 17. Crack deformation (su and wu)–temperature curves.

J. Xiao et al. / Construction and Building Materials 71 (2014) 472–483 481

and the surface forces between the gel particles increase [35].Therefore, the aggregate interlock is strengthened, even if the crackoccurs between the aggregate and the cement paste. In Fig. 17, thecrack slip su of H-series increases from 20 �C to 200 �C. It seemspossible that the cracks shown in Fig. 10 formed, and the increasedslip is due to the deformation of the cracks between the aggregateand the cement paste or those in the aggregates. When the cracksclose due to the vertical concentrated loads and a firm contactbetween the aggregate and the cement paste or in the aggregatesis established, the ultimate shear load is gained. The shear strengthof H-series shows a sharp drop when the elevated temperature isbeyond 200 �C. When the temperature reaches 800 �C, comparedwith the shear strength of the specimen at 20 �C, the shear strengthfor L- and H-series was reduced by 52.97% and 56.13%,respectively.

As can be seen in Table 4 and Fig. 17, except for the crack widthwu at 200 �C, the crack deformation of the 8 groups shows a sharpincrease with the increase of the elevated temperatures. This hasbeen stated in Section 4.3. The reduction of the width wu at200 �C may be attributed to the stiffening of the cement paste, or

Table 4A summary of main results for push-off tests.

Specimen Pu (kN) Average of Pu (kN) su (MPa) Average of su (MPa) su (mm) Average of su (mm) wu (mm) Average of wu (mm)

L-20-a 353.16 357.48 9.81 9.93 0.17 0.14 0.55 0.55L-20-b 361.80 10.05 0.10 0.55

L-200-a 357.84 334.20 9.94 9.28 0.16 0.30 0.24 0.25L-200-b 303.12 8.42 0.32 0.20L-200-c 341.64 9.49 0.41 0.31

L-400-a 319.68 289.56 8.88 8.04 0.25 0.50 0.96 0.78L-400-b 259.56 7.21 0.33 0.83L-400-c 289.44 8.04 0.91 0.56

L-800-a 170.28 168.24 4.73 4.67 0.74 0.72 1.85 1.89L-800-b 152.28 4.23 0.74 1.87L-800-c 182.16 5.06 0.68 1.96

H-20-a 416.52 428.22 11.57 11.90 0.05 0.08 0.30 0.33H-20-b 439.92 12.22 0.10 0.35

H-200-a 457.20 463.20 12.70 12.87 0.32 0.18 0.25 0.23H-200-b 486.72 13.52 0.13 0.15H-200-c 445.68 12.38 0.08 0.28

H-400-a 353.16 331.80 9.81 9.22 0.27 0.24 0.46 0.39H-400-b 320.76 8.91 0.18 0.46H-400-c 321.48 8.93 0.26 0.26

H-800-a 186.12 188.04 5.17 5.22 0.36 0.55 0.81 0.88H-800-b 190.44 5.29 0.53 1.11H-800-c 187.56 5.21 0.77 0.73

0 20 40 60 80 1000

2

4

6

8

10

12

14

Eq. (7) for H-seriesEq. (7) for L-series

L-series

H-series

Shea

r stre

ngth

τT u (M

Pa)

Cylinder compressive strength f ' Tc (MPa)

Fig. 18. Relationship between sTu and f 0Tc .

482 J. Xiao et al. / Construction and Building Materials 71 (2014) 472–483

the increase in the surface forces of the cement particles [35]. Thecohesive force among the gel particles increases, which have posi-tively effects on the crack opening, thus the crack width wu

decreases. The crack deformation of H-series at any elevated tem-perature is smaller than that of L-series.

From 20 �C to 800 �C, the crack slips su of L- and H-seriesincrease by 414.29% and 587.50%, respectively. And the crackwidths wu of both series increase by 243.64% and 166.67%,respectively.

4.5. Relationship between residual shear strength and compressivestrength

For the shear strength of HSC at room temperature, Loov andPatnaik [12] recommended an equation as follows:

su=f 0c ¼ 0:573ðqf y=f 0cÞ0:456 0:3 ð3Þ

where fy is the yield strength of the stirrups (MPa); q is the stirrupratio; and f 0c is the cylinder compressive strength (MPa). An equa-tion describing the relationship between f 0c and fcu was introducedby Gu [36]:

f 0c ¼ kf cu ð4Þ

where fcu is the cube compressive strength (MPa); k is a coefficientto consider the concrete grade. When the concrete grade is lowerthan C60, the coefficient is equal to 0.79. When the concrete gradesare C60, C70 and C80, the coefficients are equal to 0.833, 0.857 and0.875, respectively. The coefficient is assumed to be 0.875 when theconcrete grade is higher than C80.

According to Eqs. (3) and (4), the measured value and the pre-dicted value of L-series are respectively 9.93 MPa and 9.25 MPa,and the measured value and the predicted value of H-series arerespectively 11.90 MPa and 11.59 MPa. The errors between themeasured shear strengths of HSC and the predicted values of bothseries are 6.85% and 2.61%, respectively. This indicates that the testdata is in good agreement with Eq. (3). However, this equation wasproved not to be suitable for the shear strength of HSC after ele-vated temperatures. As indicated in the introduction, no literaturehas been found to focus on the shear transfer characteristics ofconcrete after elevated temperatures. Therefore, more study isneeded to further validate the shear transfer of concrete after ele-vated temperatures in this paper.

The following equation on the relationship between the cubecompressive strength after elevated temperatures and the cubecompressive strength at room temperature was developed by Xiaoand Falkner [37]:

f Tcu=f 20

cu ¼ 0:002� ðT=100Þ3 � 0:03� ðT=100Þ2 þ 0:07

� ðT=100Þ þ 0:9737 ð5Þ

where f Tcu is the cube compressive strength after elevated tempera-

tures (MPa); f 20cu is the cube compressive strength at room temper-

ature (MPa); T is the experienced elevated temperature (�C).Additionally, Eq. (4) is assumed applicable to the concrete after ele-vated temperatures. Hence, Eq. (4) becomes:

f 0Tc ¼ kf Tcu ð6Þ

where f 0Tc is the cylinder compressive strength after elevated tem-peratures (MPa); T is the experienced elevated temperature (�C).The values of k are the same as these in Eq. (4). The residual yieldstrength of reinforcing steel test after elevated temperatures wasconducted by Neves et al. [38]. Up to 500 �C, the reinforcing steelis assumed to recover their full room temperature strength. Whenthe exposed temperature is beyond 500 �C, the residual yield

strength decreases linearly with the increase of elevated tempera-tures, and it reaches 0.7 of the room temperature strength at 800 �C.

Based on the residual yield strength of stirrups, Eqs. (5) and (6)as well as the shear strengths of the concrete after elevated tem-peratures, the fitting parameters in Eq. (3) is calculated again. Inaddition, the cylinder compressive strength of HSC at room tem-perature f 020

c is considered as a multiplier. The fitting analysis isshown in Fig. 18 and the corresponding equations with a specificstirrup ratio of 1.12% for L- and H-series can be written as follows:

sTu=f 0Tc ¼ 4:92ð0:0112f T

y=f 0Tc Þ�0:98ðf 020

c Þ�1:5

ðR ¼ 0:96Þ ð7Þ

where f 020c calculated by Eq. (4) is greater than 55 MPa and less than

83 MPa; T is the experienced elevated temperature, and T is greaterthan 20 �C and less than 800 �C; f T

y is the residual yield strength ofstirrups after elevated temperatures (MPa); sT

u is the shear strengthof the concrete after elevated temperatures (MPa). In Eq. (7), theshear strength of HSC after elevated temperatures is a function ofexposed temperature, concrete strength at room temperature andstirrup yield strength. The exposed temperature influences theshear strength of HSC in the form of concrete compressive strengthafter elevated temperatures.

In Fig. 18, a basic trend shows that the shear strength of L- andH-series reduces linearly as the concrete compressive strengthafter elevated temperatures decreases. However, there is some var-iation for L- and H-series in the relationship between sT

u and f 0Tc . Ingeneral, for the same concrete compressive strength after elevatedtemperatures, the shear strength is lower if the compressivestrength at room temperature is higher.

5. Conclusions

Within the scope of this study, the following conclusions can bedrawn:

(1) During the heating phase, the higher temperature and thehigher compressive strength result in the explosive spallingof the concrete more likely. After elevated temperatures, themass of HSC decreases and the color of HSC bleaches.

(2) For both series, the crack slips occur near the ultimate shearstress of the specimens, and the crack widths occur at about50% of the ultimate shear stress at room temperature. Thenthe post-peak shear stress decreases rapidly. The appearanceof the crack deformation is earlier when the elevatedtemperature increases. Meanwhile, except for specimensexposed a temperature of 200 �C, the shear strength ofthe HSC decreases, the crack deformation increases and theslope of the shear stress–crack deformation curve afterthe ultimate stress reduces as the elevated temperature

J. Xiao et al. / Construction and Building Materials 71 (2014) 472–483 483

increases. Through a comparison of the two types of con-crete, a higher compressive strength results in more brittleshear failure, irrespective of the elevated temperature.Nevertheless, the elevated temperature can reduce the shearbrittleness of HSC.

(3) At room temperature, the shear strengths of specimens of L-and H-series are 9.93 MPa and 11.90 MPa, respectively. From20 �C to 800 �C, the shear strength of L-series reduces line-arly by 52.97%. However, the shear strength of H-seriesincreases by 8.15% from 20 �C to 200 �C firstly. Then, com-pared with the shear strength at 20 �C, it drops sharply by56.13% at 800 �C.

(4) At room temperature, the crack slip su and width wu of L-ser-ies are 0.14 and 0.55 mm, respectively. The crack slip su andwidth wu of H-series are 0.08 mm and 0.33 mm, respectively.The crack deformation shows a sharp increase with theincrease of the elevated temperature except that the crackwidths wu of L- and H-series decrease respectively by54.55% and 30.3% from 20 �C to 200 �C. By comparing withthe crack deformation at 20 �C, the crack slip su and widthwu of L-series increase respectively by 414.29% and243.64%, as well as the crack slip su and width wu of H-seriesincrease respectively by 587.50% and 166.67% at 800 �C.From the comparisons between both series, the increase ofthe concrete compressive strength reduces the shear crackdeformation of the HSC at any elevated temperature.

(5) Based on the experienced temperatures and the compressivestrength after elevated temperatures, Eq. (7) is proposed topredict the shear strength of HSC after elevated temperatures.

Acknowledgements

The authors would like to gratefully acknowledge the researchgrants from the Chinese National 973 Plan (2012CB719703), theNational Natural Science Foundation of PR China (51325802).

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