Stress-strain behaviour of strain-hardening cement-based composites ( SHCC ) under repeated tensile loading

This paper presents results of an experimental investigation on the behaviour of strain hardening cement-based composites (SHCC) subjected to cyclic tensile loading. A series of uniaxial tensile tests on unnotched, dog-bone shaped prisms containing 2.25% by volume of polymeric fibre were performed using both a deformation and load control testing regime, respectively. The experimental program was replenished by a number of deformation controlled monotonic tensile tests as well as tensile creep tests. Two different specimen sizes were used. The effect of the curing conditions before testing was also investigated. The results obtained from the deformation controlled tests revealed no pronounced effect of the cyclic loading on the material performance for the number of loading cycles chosen. However, cracked specimens tested under the load control regime were more prone to failure at lower strain levels. The number of loading cycles prior to failure was considerably larger in comparison to the deformation controlled cyclic tests. tion leads to the formation of another crack at the second weakest cross-section. The scenario then repeats resulting in a set of almost uniformly distributed cracks. The strain capacity is reached at the maximum load (tensile strength ft), when the localisation of the failure occurs (one main crack develops). Due to a moderate opening of a large number of fine cracks, a strain capacity of several percent can be observed. Figure1. Typical stress-strain response and crack pattern of SHCC specimens under monotonic tensile loading. SHCC, unlike common fibre reinforced concrete, is a micro-mechanically designed material. The approach for such material design was developed by Li (1993) for a composite which he called ECC (Engineered Cementitious Composite). The material used in this investigation was developed on the basis of this approach in earlier investigations by the authors; see e.g. Mechtcherine & Schulze (2005). However, a more specific term, SHCC, is used in this paper in conjunction with previous research conducted in this field. Table 1 gives the SHCC composition used for the experiments. A mix containing a combination of Portland cement 42.5 R (30% by mass) and fly ash (70% by mass) was utilized as a binder. The fine aggregate was a uniformly graded silica sand with particle sizes of 0.06 mm to 0.20 mm. Furthermore, PVA fibres, 2.25% by volume with a length of 12 mm, were applied. A superplasticizer (SP) and a viscosity agent (VA) were added to the mix in order to adjust its rheological properties. Table 1. SHCC composition used for the experiments. Cement Fly ash Silica sand Water SP VA PVA fibres [kg/m] [kg/m] [kg/m] [kg/m] [kg/m] [kg/m] [kg/m] 321.0 749.1 535.0 334.5 16.6 3.2 29.3 The average compressive strength of the SHCC was 33.7 MPa. The compressive strength was derived from 12 displacement controlled tests on cubes with a side length of 100 mm. The displacement rate of the crosshead of the loading machine was 0.01 mm/s. The findings concerning the stress-strain curves obtained and the observed crack pattern will be published elsewhere. 3 TEST SET-UP, TESTING PROCEDURE AND EXPERIMENTAL PROGRAM 3.1 Specimen geometry, casting, curing, set-up Based on the findings of previous investigations (Mechtcherine & Schulze, 2005 and Mechtcherine & Schulze, 2006), unnotched, dog-bone shaped prisms were chosen as specimens for this study. Such prisms were produced in two sizes, with identical geometrical shapes. The smaller prisms possessed a cross-section of 24 mm x 40 mm, while the larger ones had a cross-section of 60 mm x 100 mm. The gauge lengths were 100 mm and 250 mm, respectively. Figure 2 gives further geometric data for the specimens. Figure 2. Geometry of the specimen used for the tensile tests. All specimens were cast horizontally in metal forms. The moulds were stored 2 days in a climate box (T = 25°C, RH = 65%). After demoulding, the specimens were wrapped in a plastic foil (sealed condition) and stored until testing at room temperature. For some test series the specimens were stored after demoulding at the room atmosphere without any protection from desiccation (i.e. unsealed condition) in order to subsequently study on these specimens the effect of the curing conditions on the material performance under cyclic loading. All specimens were tested at a concrete age of 28 to 32 days. The uniaxial tension tests were performed with non-rotatable boundaries. The deformations were measured by means of two LVDTs fixed to the specimen as displayed in Figure 3. The specimen surfaces were covered with a thin brittle white paint in order to facilitate monitoring of the crack’s development. 100 100 60 r = 260 20 0