J. Cent. South Univ. Technol. (2008) 15: 569-573 

DOI: 10.1007/s11771-008-0107-7

Complete splitting process of steel fiber reinforced concrete at

intermediate strain rate

LUO Zhang(罗 章)¹, LI Xi-bing(李夕兵)², ZHAO Fu-jun(赵伏军)³

(1. Department of Architectural Engineering, Hunan Institute of Engineering, Xiangtan 411104, China;

2. School of Resources and Safety Engineering, Central South University, Changsha 410083, China;

3. School of Energy and Safety Engineering, Hunan University of Sciences and Technology, Xiangtan 411201, China)

Abstract: The complete splitting process of steel fiber reinforced concrete (SFRC) at intermediate strain rate was studied by experiment. The basic information of a self-developed SFRC dynamic test system matching with Instron 1342 materials testing machine was given, and the experiment principle and the loading mode of cubic split specimen were introduced. During the experiment, 30 cubes of 150 mm×150 mm×150 mm and 36 cubes of 100 mm×100 mm×100 mm, designed and prepared according to C20 class SFRC with different volume fractions of steel fiber (0, 1%, 2%, 3%, 4%) were tested and analyzed. At the same time, the size effect of SFRC at intermediate strain rate was investigated. The experimental study indicates that SFRC size effect is not influenced by the loading speed or strain rate. When the steel fiber content increases from 0 to 4%, the splitting strength of SFRC increases from 100% to 261%, i.e. increasing by 161% compared with that of the common concrete. The loading rate increases from 1.33 kN/s to 80.00 kN/s, and the splitting tensile strength increases by 43.55%.

Key words: steel fiber reinforced concrete; intermediate strain rate; splitting test; size effect

                                                                                                                                                                       
1 Introduction

In general, solid materials are not sensitive to low strain rate and very sensitive to high strain rate, while the intermediate strain rate lies in a transition region where solid materials become sensitive to strain rate. Therefore, exploring the strain rate correlation of the mechanical properties at intermediate strain rate is very necessary, and is of great value for the project application^[1]. For example, a load corresponding to the strain rate from 10^-3/s to 10^-1/s is equivalent to the role of earthquake. So, studying the strain rate correlation of those mechanical properties of steel fiber reinforced concrete (SFRC) and its stress—strain behavior can provide an important theoretical basis for SFRC structural seismic design and dynamic security assessment of SFRC structure^[2-3].

Recent years, SFRC is gradually accepted as a composite material for its excellent properties of high tensile strength, high shear strength, high toughness, and good crack-prevention ability, impact durability, fatigue durability, and so on. There are many research results on SFRC mechanical properties now^[4-6]. SFRC tensile test is more difficult than compression test, more time- spending and greater data discretion. As concrete materials (including SFRC) are inherent in cracks, their structural damages often occur because of the inadequate tensile strength. Scientific researchers and engineering technicians at home and abroad pay great attention to the concrete material tensile deformation and relate issues^[7-11]. The strengthening function of SFRC is mainly originated from the tensile strength improvement but not the compressive strength. The complete tensile damage process of SFRC known as the complete stress—strain process, is the macro-expression of internal micro-damage mechanism of material, and is an important component of the theory to research and analyze SFRC strength. To study the entire tensile damage process laws of SFRC is the foundation to analyze the tensile properties of SFRC, which can also be expected to be more effective to evaluate the steel fiber reinforcement than to study laws of compression damage.

Taking a panoramic view to the research results in the past 20 years gained by many domestic and foreign experts on the SFRC’s complete tension process, it is not difficult to find that the mostly used axial tensile tests were carried out by using reformed test installation^[12-15], which are high in equipment requirement, large in experiment errors and low in success rate. And even up to now, there still exist many difficulties in SFRC tensile experiment. For example^[2], 1) multi-phase composite structure and randomly distributed micro-cracks and gas pores make the physical alignment difficult, even impossible, while the loading processes; 2) inevitable stress concentration makes the fracture strain often fall

over the measure point, resulting in the experiments in a very low success rate; 3) deformation measurement of the experimental process is difficult. But the split tension method has the advantages of simple management and high success rate, so it has become the alternative standard method for concrete-like material axial tension test at present^{[5-6, 15]}. In this work, the split tension process experiment based on a self-developed dynamic test system was used to study the complete tension process and the mechanical properties of SFRC at intermediate strain rate. The effect of shape and size of the steel pad on splitting tensile strength, and the quantitative relationship between the specimen size and the tensile strength at intermediate strain rate were investigated.

2 Experimental

2.1 Experimental device

The SFRC split tension complete process experiment at intermediate strain rate used Instron 1342 servo-controlled hydraulic materials test machine as a loading device. Because the original dynamic strain collecting device of the test machine cannot meet the expected experiment precision, after a lot of test comparisons, the author took DH-5932 data incepting and analyzing instrument and DH-3840 programmable amplifier (made by Donghua Testing Technology Development Co. Ltd. Jiangsu, China) as recording devices for dynamic strain, and after assistant design, constituted a dynamic testing system based on Instron materials test machine. The experimental design principle is shown in Fig.1.

2.2 Specimen preparation

Two groups of SFRC cubic specimens were prepared according to the technological requirement of C20 class SFRC. The 1st group was 100 mm×100 mm×100 mm in size and used to investigate the effect of steel fiber’s volume fraction and loading rate on SFRC’s splitting tensile strength with a total number of 36; the 2nd one was 150 mm×150 mm×150 mm in size with a total number of 30 and mainly used to investigate the effect of steel-pad’s shape and size on SFRC splitting tensile strength. The amounts of steel fiber of all specimens were 0, 1%, 2%, 3% and 4% (volume fraction), respectively, and the length of each fiber was 34 mm.

2.3 Experimental method

As shown in Fig.2(a), before loading, two pieces of plywood pad with 12 mm in width, 3-4 mm in thickness and 100 mm in length were put on the top and under the bottom of 100 mm×100 mm×100 mm specimen to improve the loading and stress distribution states, respectively, then data were incepted/processed after loading.

Fig.1 Dynamic loading and data incepting system

Fig.2 Schemes of splitting tension test method: (a) Using 100 mm×100 mm×100 mm SFRC specimen to investigate fiber’s volume fraction and loading rate on splitting tensile strength; (b) Using 150 mm×150 mm×150 mm SFRC specimen to investigate effect of steel-pad’s shape and size on splitting tensile strength

While as shown in Fig.2(b), when 150 mm×150 mm×150 mm specimen was tested to investigate the effect of pad shape and size on splitting tensile strength, the cylinder steel pad of d4 mm×150 mm and plane steel pad of 4 mm×8 mm×150 mm were used instead.

3 Results and discussion

3.1 Effect of fiber’s volume fraction and loading rate on splitting tensile strength

Table 1 shows the splitting tensile strength R_t of 100 mm×100 mm×100 mm SFRC specimen with different volume fractions (φ_f) of steel fiber at invariant compressive loading rate of 32.00 kN/s (160 kN/5 s) or strain rate of 10^-2 s^-1. These splitting tensile strengths (R_t) are calculated by using the formula R_t=2P/(πa²), where P stands for the splitting force, i.e. the compressive load of the Instron loading system, which is automatically recorded by the measuring system; a is the side length of the cubic specimen.

Table 1 Test results of 100 mm×100 mm×100 mm specimens with different volume fractions of steel fiber

It can be seen from Table 1 that the splitting tensile strength is increased with the increase in steel fiber’s volume fraction. When the steel fiber’s volume fraction increase from 0 up to 4%, the splitting strength of SFRC increases from 100% to 261%, i.e. increased by 161% compared with the common concrete (φ_f=0).

Table 2 shows the effect of loading rate on splitting tensile strength for 100 mm×100 mm×100 mm SFRC specimens.

Table 2 Effect of loading rate on splitting tensile strength of 100 mm×100 mm×100 mm SFRC specimens with steel fiber’s volume fraction of 2%

Table 2 indicates that the splitting tensile strength increases about 8% per step with the loading rate increases step by step from 1.33, 2.67, 5.33 to 10.66 kN/s, and then increases quickly from 10.66 kN/s to 32.00 and 80.00 kN/s.

3.2 Effect of steel-pad’s shape and size on splitting tensile strength

Fig.3 shows the results of the test concerning the splitting tensile strength and two kinds of steel-pads using the 150 mm×150 mm×150 mm SFRC specimens with different steel fiber’s volume fraction.

Fig.3 Effect of steel-pad’s shape on splitting tensile strength at intermediate strain rate (loading rate: 32.00 kN/s)

Fig.4 shows the test results of the effect of steel- pad’s size (length) on splitting tensile strength of 150 mm×150 mm×150 mm SFRC specimen.

Fig.4 Effect of steel-pad’s length on splitting tensile strength of 150 mm×150 mm×150 mm SFRC specimen at intermediate strain rate (loading rate: 32 kN/s)

From Fig.3 and Fig.4, it can be found that both the pad’s shape and size affect the splitting tensile strength to a certain degree. And the common trend is that the effect of pad’s shape is larger than that of the pad’s size (length); the larger the volume fraction of steel fiber, the larger the splitting tensile strength. In addition, by comparing the splitting tensile strength in Table 1 and Fig.3, it may be found another trend as that the bigger the specimen, the larger the splitting tensile strength. For example, the splitting tensile strength of the 100 mm×100 mm×100 mm specimen is about 0.806 times as large as that of the 150 mm×150 mm×150 mm specimen under the same other conditions, which is consistent with the empirical coefficient in Ref.[7].

3.3 Effect of calculation method on splitting tensile strength

To improve the loading and the stress distribution state so that the specimen will not be prematurely damaged due to the local high compressive stress concentration in the loading zone, it is necessary to place two pieces of plywood pads with about 12 mm in width and 3-4 mm in thickness on the top and under the bottom side of the cubic specimen and the crosshead/ supporting platform of the loading system respectively during test. Because the steel fiber’s volume fraction is not high (only up to 4%), the tensile strength of SFRC specimen was still smaller than its compressive strength, thus a buffer zone was formed in the loading zone and the strain distribution was changed dramatically. With the loading continued cracks initiated and developed in the middle of the loading zone and followed by splitting. Because the horizontal tensile strain led to the specimen to split, so horizontal stress should be used to compare the splitting tensile strength of the SFRC specimen with the related data of beam-shaped specimen that used bending tension mode. According to the relative elasticity principles, the strain distribution on the splitting surface of the SFRC specimen can be known as shown in Fig.5, and then the splitting tensile strength can be calculated by using the formula of R_t=2P/(πa²).

Fig.5 Scheme of stress distribution on splitting surface of SFRC cubic specimen under vertical compressive load

Incidentally, though the concrete is not real elastic material, in recent years, the splitting tensile strength calculation formula is often based on elasticity theory, and the result of the nonlinear finite element analysis based on the elastic-plastic constitutive model is similar to the result of elasticity formula. This shows that the elasticity formula reflecting the tensile strength of concrete materials is better, therefore, the result calculated by the elasticity formula for the tensile strength of SFRC pseudo-plastic materials is undoubtedly much better than that by the plastic theory. In addition, the variation coefficient of the result of splitting tensile test is small, and the tensile strength tested has a good correlation with the volume fraction at the slenderness ratio of the steel fiber.

3.4 Effect of pad’s shape and size on splitting tensile strength

In splitting tension test of SFRC specimen, the concrete under the pad was in compression state, and the pad functioned as a wedge, resulting in stress concentration in the concrete. The larger the dimension of the pad, the larger the pressure on surface, particularly for the plane pad. Generally, for the pads with the same cross-sectional shape, the one with larger size will need larger load to make the specimen damaged, so the measured splitting tensile strength will be also larger evidently. The experiment results of SFRC specimen at intermediate strain rate in this work confirmed this fact. Moreover, the hardness/softness of the pad material also affects the splitting tensile strength. Hard pad can lower the strength; the soft pad can also affect the measured results because of the deformation itself. What’s more, the top and bottom pads must be parallel to each other and placed on the same vertical plane, otherwise, the discrete degree of the measured results will increase, and the measured strength values are also larger, which can be directly identified according to material mechanics.

To inspect the effect of pad’s shape and size on the splitting tensile strength of SFRC, in this work, we used two different shapes of steel pad and the same specimen of 150 mm×150 mm×150 mm to carry out splitting tension test. The results indicate that their effect is obvious. In China, it is required to use cubic specimen to test tensile compressive strength as the standard. In the norm about splitting tensile strength test it is uniformly required to select the side length of 150 mm as a standard cubic sample size, but the size and shape of the pad do not have a unified standard. Adoption of uniform size and shape of the pad is very necessary, because it allows the results to be comparable. Analysis shows that the splitting tensile strength is lower than axial tensile strength when the size of the pad is smaller; when the size of the pad is larger the splitting tensile strength is higher than axial tensile strength. So it is possible to determine a pad with appropriate size and shape so as to make the splitting tension experiments using 150 mm×150 mm×150 mm cubic specimen and axial tensile experiments using 150 mm×150 mm×150 mm get the same results. Ref.[7] shows that splitting tensile strength and axial tensile strength are the closest when using 8 mm width plane plywood pad. This result is worthy of great attention. In Ref.[7] static loading was studied for common concrete. About the dynamic loading and the situation of SFRC in the same case still need a lot of experimental work.

4 Conclusions

1) Shape of the pad has a great effect on the splitting tensile strength of SFRC, and the pad with uniform size and shape is necessary to reduce the discreteness in its measured results to a large extent.

2) Splitting tensile strength of the 150 mm×150 mm×150 mm SFRC specimen matched with d4 mm×8 mm×150 mm cylinder steel pad is close to the axial tensile strength of 100 mm×100 mm×100 mm SFRC beam.

3) At a intermediate strain rate, the effect of loading rate on splitting tensile strength of SFRC is rather small, only 7.66% for loading rate from 1.33 kN/s to 2.67 kN/s, which suggests that relevant results of size effect on the tensile strength of SFRC under static load are still effective for the dynamic load.

References

[1] ALBERTINI C, CADONI E, LABIBES K. Study of the mechanical properties of plain concrete under dynamic loading [J]. Experimental Mechanics, 1999, 39(2): 137-141.

[2] BISCHOFF P H. Impact behavior of plain concrete at high strain rates [J]. Materials and Structure, 1991, 24(1): 425-438.

[3] LUO Zhang. Study on the constitutive relationship of steel fiber reinforced concrete under intermediate strain rate [D]. Changsha: School of Resources and Safety Engineering, Central South University, 2004. (in Chinese)

[4] RIEDER K A, MINDESS S. New test method to evaluate the impact behaviour of biaxially confined concrete [J]. Materials and Structures, 1998, 31(12): 669-675.

[5] HARAJLI M H. Effect of confinement using steel FRC, or FRP on the bond stress-slip response of steel bars under cyclic loading [J]. Materials and Structures, 2006, 39(6): 621-634.

[6] GAO Dan-ying, ZHAO Jun, TANG Ji-yu. Experimental study on the dimensional effect of steel fiber reinforced high-strength concrete (SFRHSC) under splitting tension [J]. Journal of Building Materials, 2004, 7(3): 295-298. (in Chinese)

[7] ZHANG S, HAUSSLER-COMBE U, EIBL J. New approach to strain rate sensitivity of concrete in compression [J]. Engineering Mechanics, 1999, 125(12): 1403-1410.

[8] LI Xi-bing, LUO Zhang, ZHAO Fu-jun. Experimental study on the tensile behavior of steel fiber reinforced concrete under intermediate strain rate [J]. Journal of Experimental Mechanics, 2004, 19(3): 301-309. (in Chinese)

[9] EIBL J B. Schmidt-Hurtienne. Strain-rate-sensitive constitutive law for concrete [J]. Engineering Mechanics, 1999, 125(12): 1411-1420.

[10] LIN T Y, PARAMASIRAM P, LEE L. Analytical model for tensile behavior of steel fiber reinforced concrete [J]. ACI Materials Journal, 1987, 87(4): 286-298.

[11] LI Xi-bing, ZUO Yu-jun, WANG Wei-hua. Constitutive model of rock under static-dynamic coupling loading and experimental investigation [J]. Trans Nonferrous Met Soc China, 2006, 16(3): 714-722.

[12] XING Ji-bo, YU Liang-qun, ZHANG Rui-feng. Numerical modeling of progressive failure in particulate composites like sandstone [J]. Trans Nonferrous Met Soc China, 1999, 9(1): 165-171.

[13] GEORGE G N, ANTOINE E N. Strain rate effects on tensile properties of fiber reinforced concrete [C]// Materials Research Society Symposia Proceedings. Pittsburgh: Materials Research Soc, 1986, 64: 97-118.

[14] ALTUN F, HAKTANIR T, ARI K. Experimental investigation of steel fiber reinforced concrete box beams under bending [J]. Materials and Structures, 2006, 39(4): 491-499.

[15] FAWAZ Z, ELLGIN F. Fatigue failure model for fiber-reinforced materials under general loading conditions [J]. Composite Materials, 1994, 28(15): 1342-1451.

Received date: 2007-12-27; Accepted date: 2008-03-12

Corresponding author: LUO Zhang, PhD; Tel: +86-13187023052, +86-732-8680493; E-mail: Luozhang1@263.net

(Edited by YANG Hua)