0 1000 2000 3000 4000 5000 6000

Compression strain, x10 6

FIGURE 15.19 Comparison of stress-strain curves in compression of HPC and normal concrete.

15.5.2.1 Direct Tensile Strength

The direct tensile test is difficult to conduct. Due to the difficulty in testing, only limited and often conflicting data are available. For normal-strength concrete, it is often assumed that the direct tensile strength of concrete is about 10% of its compressive strength. For HPC, the ratio of tensile strength to compressive strength is greatly reduced to even 5%.

Among more recent studies on the tensile strength of concrete, a new test method for uniaxial tension test of a concrete has been developed by Li et al. [37,38]. Several measures have been taken to ensure the success of the uniaxial tension test. To avoid possible eccentricity or bending of the tension specimen, a set of special loading fixture was developed.

The setup for the uniaxial tension test is shown in Figure 15.20. Two identical loading fixtures were used: one is gripped on the actuator of the MTS machine and the other is connected to the load cell. The loading fixture contains a ball joint that is essential for protecting the tension specimen from possible bending effects. The glued specimen was connected to the loading fixture by using a 0 12 mm pin. Four SCHAEVITZ LVDTs (linear variable differential transformer), with a working range of ±0.635 mm and a designed gage length of 120 mm, were mounted on the two sides of the specimen for deformation measurement as well as test control. The LVDT holders were specially designed to allow the adjustment of entering and offsetting for the LVDTs.

Tension tests were conducted using an MTS machine with a capacity of 250 kN. The tests were carried out using a newly developed adoptive control method, which ensured the successful and stable measurement of the postpeak response. Figure 15.21 shows the tension stress-deformation curve measured using this method. It can be seen that the postpeak response can be easily recorded even for concrete at early ages [38].

15.5.2.2 Indirect Tensile Strength

The most commonly used tests for estimating the indirect tensile strength of concrete are the splitting tension test (ASTM C 496) and the third-point flexural loading test (ASTM C 78). Both the splitting tensile strength (ft) and the flexural strength or modulus of rupture (fr) are related to the compressive strength by the following general expression:

ACI Committee 363 recommends that for concrete strength up to 83 MPa, the coefficient should be taken as 7.4 for tensile strength and as 11.7 for flexural strength. Other investigators have proposed fctor fr = k^fc

FIGURE 15.21 Tension stress-deformation curves.

slightly different values for the two types of strengths. More details can be found in a previous report [39].

The results of the recent SHRP studies indicated that for the splitting tensile strength the recommendation of ACI Committee 318 is equally acceptable as that of ACI Committee 363. However, for the flexural strength (modulus of rupture), the recommendation of ACI 318 is a better representation than that of ACI Committee 363.

The study by Burg and Ost [40] showed that the average modulus of rupture and splitting tensile strength in comparison with the compressive strength were similar to the recommendation of ACI

Committee 363. The moist cured specimens consistently produced higher strength than air cured specimens.

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