Wedge splitting test on fracture behaviour of ultra high strength concrete

Construction and Building Materials 18 (2004) 359–365

Wedge splitting test on fracture behaviour of ultra high strength concrete b ¨ ¨ b Jianzhuang Xiaoa,*, Holger Schneiderb, Cindy Donnecke , Gert Konig

b

a Department of Building Engineering, Tongji University, Shanghai, PR China Institute for Structural Concrete and Building Materials, Leipzig University, Germany

Received 25 March 2003; received in revised form 31 March 2004; accepted 2 April 2004

Abstract The fracture behaviour of ultra high strength concrete (UHSC) with axial compressive strength more than 140 MPa is presented in this paper. Based on the wedge splitting tests with six cubical specimens, the fracture behaviour of UHSC with and without coarse aggregates is investigated. The common fracture mechanical parameters such as fracture energy, crack opening displacement (COD), the characteristic length and the fracture toughness by non-linear fracture mechanics (NLFM) theory are experimental determined. Through finite element analysis with suggesting linear and bilinear models for closing force s(w) vs. crack width with regard to the wedge splitting tests, the relationships between splitting force and COD are simulated and verified with the tested ones. It is concluded that the COD, characteristic length and fracture toughness of UHSC can be improved obviously with the contribution of coarse aggregates. 䊚 2004 Elsevier Ltd. All rights reserved. Keywords: Ultra high strength concrete; Fracture behaviour; Wedge splitting test; Coarse aggregate; Fracture mechanics; Finite element analysis

1. Introduction Concrete has been one of the most commonly used building materials in the world. It might be concluded that, from old Roman times to the 21st century, concrete is both one conventional building material and one kind of brand new one w1x. The exploring of research and applications on concrete materials seems never to cease. The development of concrete material may be marked and divided into several stages. The first is the traditional normal strength concrete (NSC). Usually, there are only four kinds of ingredients composed of the concrete, which are cement, water, fine aggregates and coarse aggregates. With the increasing development of civil engineering, such as high-rise buildings and long-span bridges, higher compressive strength concrete is needed. When the compressive strength of concrete is higher than 50 MPa, it is usually defined as high strength concrete (HSC). At the very beginning, the easiest way to reach high compressive strength was to reduce the water–cement ratio. Therefore, in HSC, the fifth ingredient, a water reducing agent or superplasticizer, is *Corresponding author. Tel.: q86-21-65983422; fax: q86-2165986345. E-mail address: [email protected] (J. Xiao).

indispensable. However, sometimes the compressive strength is not as important and necessary as some other properties, such as low penetrability, fine durability and excellent workability. Thus high performance concrete (HPC) was proposed and widely studied at the end of the last century. Compared with NSC, the other two ingredients, admixtures and additives, are added into the mix. Frequently, there are three kinds of admixtures, including silica fume, fly ash, and blast furnace slag. Due to the above three admixtures being waste material or by-product of industry production, HPC is considered as green high performance concrete (GHPC). Nowadays, the ultra high strength concrete (UHSC) with axial compressive strength more than 140 MPa was successfully developed in some research institutes, such as the Institute for Structural Concrete and Building Materials, Leipzig University w2x. This may be another milestone of the research and development on the concrete material. With the increasing development of UHSC and the ripe application of fracture mechanics to concrete, studies on the fracture behaviour of UHSC are required for the deep understanding before adopted in civil engineering together with provision for problems such as cracking in UHSC elements and structures. This paper will try to study the fracture behaviour of UHSC,

0950-0618/04/$ - see front matter 䊚 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2004.04.016

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Table 1 Physical properties of cementitious material and aggregate Material

Cement 42.5R

Silica fume

Quartz powder

Quartz sand

Crushed basalt

Size Specific gravity (kgym3)

-100 mm 3050

0.1–1 mm 2300

-10 mm 2630

0.3–0.8 mm 2650

2–5 mm 3100

Table 2 Mixture proportion of ultra high strength concrete (kgym3) Category

Cement

Water

Quartz sand

Crushed basalt

Silica fume

Super plasticizer

Quartz powder

Total weight

UHSC-1 UHSC-2

665 540

178 163

1019 440

0 1027

200 162

23 16.8

285 231

2370 2580

Table 3 Mechanical behaviour of ultra high strength concrete Category

Axial compressive strength f c,cyl.100*300 (Nymm2)

Splitting tensile strength f sp, cyl.100*200 (Nymm2)

Elastic modulus (Nymm2)

Poisson’s ratio

UHSC-1 UHSC-2

149.0 147.7

8.0 8.6

47 000 52 837

0.18 0.18

Table 4 Classification of the specimens Category

UHSC-1

Specimens

UHSC-1a

UHSC-2 UHSC-1b

UHSC-1c

compared with HSC, in order to investigate and understand its basic mechanical behaviour, such as strength and brittleness. Usually, four types of test method w3–6x are recommended to measure the fracture parameters for Mode I crack. These are the method of three-point bend test on a notched beam, wedge splitting test on cubical or cylindrical specimens, single notched plate tensile test and double notched tensile test. A possible problem associated with the use of the three-point bend beam is that when the size of the beam tested is relatively large, the effect of self-weight of the beam on material fracture parameters should be very carefully evaluated. While for direct uniaxial tensile test, both single and double notched, it is not very easy to carry out. Hence the ‘compact’ three-point bend beam, that is wedge splitting test is used to investigate the fracture behaviour of ultra high strength concrete in this paper.

UHSC-2a

UHSC-2b

UHSC-2c

and coarse aggregates (if any), respectively. The main physical properties of the ingredients are described in Table 1. A superplasticizer is used in the concrete mixtures to obtain the slump of more than 250 mm and the flowing value of more than 650 mm. Quartz powder is adopted as one kind of micro filler, with such a grain diameter that fit between cement and silica fume. Altogether two categories of concrete mixtures are prepared. The mixture proportions and their basic mechanical behaviours of the hardened concrete are described in Tables 2 and 3, respectively. The water to binder (cement and silica fume) ratio in UHSC-1 and UHSC-2 is 0.206, and 0.232, respectively. After casting in a steel mould, all the specimens are stored at the laboratory environment (20"5 8C, 80"15% RH) and demoulded after 48 h, and then put the specimens in the water at 20 8C, curing for 28 days. 2.2. Test specimens

2. Test program 2.1. Mixture of UHSC The cementitious materials used in this study are normal Portland cement (CEM I 42.5R) and silica fume. Quartz sand and crushed basalt are adopted as the fine

Altogether, six specimens are designed and cast. The classification of the specimens is listed in Table 4. For each kind of concrete, three specimens with the dimension of 100=300=300 mm, as shown in Fig. 1, are prepared for wedge splitting test w6x. One day before testing, the specimen is notched with a diamond saw.

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Where u is the wedge angle indicated in Fig. 2b and f is the coefficient of friction ranging from 0.1 to 0.5% for different roller bearings used w7x. In the condition of this test, us158 and suppose fs0, then Fsps1.866Fv. 3. Test results 3.1. Test phenomena

Fig. 1. The size and geometry of specimens.

The notch depth and width are 95"1 mm and 3.5"0.5 mm, respectively. 2.3. Test set-up A closed-loop servo controlled hydraulic jack with a maximum capacity of 100 kN is used to control the crack opening displacement (COD) at constant speed, i.e. the rate is fixed at approximately 0.02–0.03 mmy min. The test set-up is shown in Fig. 2. Two highprecision displacement gauges are situated (see Fig. 2a) just at the mouth of the starter crack. The final value of COD is taken as the mean value of the readings from the two gauges. From Fig. 2b, according to the equilibrium condition, the relationship between vertical load Fv and splitting force Fsp is given as Fsps

Fv . 2(1qf cotanu)tanu

(1)

Before the peak load, the crack propagates slowly and stably; whereas after the peak load it behaves the opposite. The failure photos of two kinds of concrete are displayed in Figs. 3 and 4, respectively. The final crack orientation and appearance of UHSC-1 are similar to that of UHSC-2. The comparison on the surface of concrete after crack opening is shown in Fig. 5. From Fig. 5, it may be noted that for concrete without coarse aggregates, the surface is smoother, which implies that the effect of crack bridging and crack deflection would not be significant in the fracture process zone (FPZ). In fact, the scope and development of FPZ will strongly depend on the size and shape of coarse aggregates. It will be verified quantitatively by the fracture parameters described in the following section. 3.2. Test curves of splitting force-COD In order to compare efficiently and coincidentally, all the curves are limited and cut at the point of 5% peak load in the descending branch. The splitting force-COD test curves of UHSC-1 and UHSC-2 are described in Figs. 6 and 7, respectively. The effect of coarse aggregates could be found out from the curves illustrated in Fig. 7, compared with those in Fig. 6. It may be noted that the coarse aggregates can increase both the peak

Fig. 2. Test set-up.

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J. Xiao et al. / Construction and Building Materials 18 (2004) 359–365

Fig. 3. Photos of failure for UHSC-1.

Fig. 4. Photos of failure for UHSC-2.

Fig. 5. Crack surface appearance.

load and ultimate deformation ability, which could be explained as the contribution of aggregate bridging and some influence of aggregate interlock. This actually leads to a longer softening displacement. 3.3. Fracture parameters All the test results, including the peak load, the COD corresponding to the peak load (peak COD), the ultimate COD and the area enveloped by the curves, are summarized and listed in Table 5. Moreover, the mean value of the test result for each group of concrete is calculated

and described in Table 6. From Table 6 and compared UHSC-1 with UHSC-2, the coarse aggregate has obvious effects on both the peak load and the value of COD, especially to the mean value of ultimate COD. It can be said that there is much more prominent influence for coarse aggregates on the post-peak behaviour than prepeak performance. Two kinds of enveloped mean area are described in Table 6. Compared these two mean area values, it would be also concluded that the coarse aggregates are more efficient to post-peak behaviour. The maximum value of fracture energy is calculated by dividing the area surrounded by the splitting force

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Table 6 Summary of the mean value for test results Category Mean Mean Mean Mean Mean

UHSC-1 UHSC-2 UHSC-1:UHSC-2

peak load (kN) peak COD (mm) ultimate COD1 (mm) area one2 (kN mm) area two3 (kN mm)

9.945 0.060 0.184 0.939 0.586

12.073 0.096 0.560 2.122 1.416

1:1.21 1:1.60 1:3.04 1:2.26 1:2.42

The same as Table 5.

Table 7 Fracture parameters Fig. 6. Splitting force-COD curves of UHSC-1.

Parameters

Formula

UHSC-1

UHSC-2

UHSC-1:UHSC-2

GF (Nym) Gf (Nym) Lch (mm) K (MPa m1y2)

A1yht A2yht GFEyft2 (GFE)1y2

56.91 35.52 41.79 1.64

128.61 85.82 91.88 3.26

1:2.26 1:2.42 1:2.20 1:1.99

GF is fracture energy; Lch is characteristic length; K is fracture toughness.

if the coarse aggregate has been added. From Table 7, it would be found that the ratio of Gf to GF is approximately 0.6 for UHSC. This is close to the conclusion of test results for other kinds of concrete w8x. 3.4. Comparison analysis Fig. 7. Splitting force-COD curves of UHSC-2.

Fsp vs. crack opening displacement curve up to 5% peak load point by the area of the ligament (which is 100=165 mm). The main related fracture parameters are described in Table 7. It can be seen from Table 7 for concrete with crushed basalt coarse aggregates, the fracture energy is over 100% higher than that of concrete without coarse aggregates. Usually, the brittleness of concrete is evaluated by using the characteristic length. The characteristic length (lch), the fracture toughness (K) and the ultimate COD all tend to increase with the mixture of coarse aggregates. This means that the ductility may be improved by the more non-homogeneity

The values of fracture energy and fracture toughness for two high strength concrete mixtures were published in Ref. w9x and listed here in Table 8. The test results from this paper are also described and compared in Table 8. From Table 8, it can be demonstrated that although tensile strength of ultra high strength concrete is much higher than that of normal high strength concrete, the fracture energy is decreased, while the fracture toughness is increased, in UHSC without coarse aggregates. For UHSC with coarse aggregates, of which the maximum diameter is 5 mm only, both the fracture energy and fracture toughness are improved obviously compared with normal HSC.

Table 5 Summary of test results Category

UHSC-1

Specimens Peak load (kN) Peak COD (mm) Ultimate COD1 (mm) Area one2 (kN mm) Area two3 (kN mm)

UHSC-1a 9.289 0.060 0.180 1.117 0.766

1

UHSC-2 UHSC-1b 11.358 0.078 0.192 1.030 0.511

UHSC-1c 9.189 0.043 0.177 0.671 0.482

Ultimate COD refers the COD with splitting force equals 5% of peak load. Area one refers all the area beneath the curve from zero to ultimate COD. 3 Area two refers the area beneath the curve from peak COD to ultimate COD. 2

UHSC-2a 11.158 0.104 0.596 2.026 1.356

UHSC-2b 12.659 0.108 0.631 2.463 1.581

UHSC-2c 12.402 0.076 0.452 1.876 1.312

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364 Table 8 Comparison of fracture parameters Specimen geometry

wyc

Cement (kgym3)

da (mm)

Age (days)

fc (Nymm2)

ft (Nymm2)

GF (Nym)

K (MPa m1y2)

HSC Cubic w9x

0.47 0.48 0.47 0.48 0.206 0.232

400 400 400 400 665 540

20.0 12.5 20.0 12.5 0 5.0

45 150 45 150 28 28

51 55 51 55 149.0 147.7

3.7 4.5 3.7 4.5 8.0 8.6

68.7 68.0 87.5 75.9 56.91 128.61

0.92 0.11 1.10 1.37 1.64 3.26

HSC Cylinder w9x UHSC Cylinder

4. Test simulation 4.1. Finite element simulation The basic concept of finite element (FE) analysis method is quite familiar to structural engineers. The constitution relationships of materials, the equilibrium conditions and the deformation compatibility functions should be assumed as known. Generally, it is easy to apply FE method in a continuum. For a cracked concrete element, additional equations to model the crack initiation, propagation etc. should be induced. Fortunately, the application of fracture mechanics can fulfil this objective. Usually, discrete crack approach and smeared crack approach are mainly adopted in the finite element simulation w7x. 4.2. The relationship of closing stress-width for fracture process zone According to the CEB-FIB Model Code w10x, a bilinear curve for s(w), as given in Fig. 8 and Eq. (2), when calculation of UHSC with coarse aggregates is proposed by finite element method.

w1s

GFy22wcŽGFykd. 150ŽGFykd.

0.95

TftyŽftys1.wyw1 sŽw.sU T

s1ys1Žwyw1.yŽwcyw1.

0.95

(3)

Where GFs128.61 Nym, wcs0.11 mm and coefficient kds4 for maximum aggregate size das5 mm. For simulating the fracture behaviour of UHSC without coarse aggregates, a linear model is proposed as sŽw.sftŽ1ywywc..

S

V

MPa; w1 is suggested to be 0.0154 mm; and wc is assumed to be 0.11 mm. Base on the above assumptions, the simulation analysis can be undertaken by using the SNAP (Structural Non-linear Analysis Program) w11x. The comparison of splitting force-COD curves between the test and calculation results for UHSC with coarse aggregates is displayed in Fig. 9. It can be illustrated from the figures that the bilinear model is suitable for the fracture behaviour simulation of ultra high strength concrete with coarse aggregates. Moreover, the suggested parameters of s1, w1 and wc are in well accordance with the global rules recommended by CEB-FIP w10x, such as w1 is close to the value given by the Eq. (3) as follows:

(4)

for wFw1 for w)w1 (2)

Where, f ts8.0 MPa, and wc is assumed to be 0.013 mm. Again, by SNAP, the comparison of splitting forceCOD curves between the test and calculation results for

Where, f t equals 8.6 MPa; s1 is supposed to be 1.07

Fig. 8. The relationship of syw.

Fig. 9. The comparison between test and calculation of UHSC-2.

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confinement and proper stirrups confinement, should be necessary to obtain satisfactory ductility behaviour. Acknowledgments In this study, Dipl.-Ing Jianxin Ma helped to prepare the casting and curing of ultra high strength concrete specimens. His valuable advice and technical help are gratefully acknowledged. German Academic Exchange Service (DAAD) is also gratefully acknowledged, which granted the first author the study visit in the Institute for Structural Concrete and Building Materials of Leipzig University. Fig. 10. The comparison between test and calculation of UHSC-1.

References UHSC without coarse aggregates is shown in Fig. 10. It can be noted from the figures that the linear model seems to simulate the fracture behaviour of UHSC without coarse aggregates well, compared with the bilinear model mentioned above. 5. Conclusions and suggestions 5.1. Conclusions In general, the fracture behaviour of UHSC is not ideal, it depicts much brittleness despite its high compressive strength. The fracture energy, COD, characteristic length and fracture toughness of ultra high strength concrete are improved with the contribution of coarse aggregates. Compared with test results, the calculation results indicate that the bilinear model of s(w) may be adopted to simulate the fracture behaviour of UHSC with coarse aggregates by finite element method well; whereas for UHSC without coarse aggregates, the linear model is much more suitable. 5.2. Suggestions It is recommended that coarse aggregates, even with small size of diameter, should be mixed in the UHSC to improve its fracture behaviour. When UHSC are be used in structures, it is strongly suggested that fine confinements, such as steel hoop

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