Behaviour of Strain-hardening Cement-based Composites (SHCC) under monotonic and cyclic tensile loading: Part 1 – Experimental investigations

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Abstract

This first part of the project at hand presents the results of experiments performed on a Strain-hardening Cement-based Composite (SHCC) in order to investigate the specific behaviour of such materials under monotonic and cyclic tensile loading. The testing programme included uniaxial tension tests on dumbbell specimens, tension tests on single fibres, single-fibre pullout tests, and optical investigations. All mechanical tests were performed under deformation-controlled loading regime. Optical investigations of the SHCC microstructure provided detailed insights into the failure mechanisms observed in the mechanical tests. The results obtained can serve as a reliable basis for the development of corresponding constitutive relationships relevant to SHCC. Since the modelling to be presented in the second part of this paper is based on a multi-scale approach, the experimental results are discussed in particular with respect to the identification and description of the determinant physical phenomena influencing material performance at different levels of observation.

Introduction

This paper addresses the Strain-hardening Cement-based Composites (SHCC) which exhibit strain-hardening, quasi-ductile behaviour due to the bridging of fine multiple cracks by short, well-distributed fibres. The favourable mechanical properties of this material offer many possible applications in new and old structures as well as in the strengthening and repair of structural elements made of reinforced concrete or other traditional materials [1], [2].

The characteristic behaviour of SHCC under monotonic tensile loading is shown in Fig. 1 and can be described as follows: Microscopic defects trigger the formation of matrix cracks at so-called first-crack stress σ1. As the first crack forms, the fibres bridge the crack, transmitting tensile stresses across the crack surfaces. The applied load must be increased in order to force further crack formation. This leads to the subsequent development of another crack at the second weakest cross-section. The scenario then repeats itself, 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, namely when 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.

The behaviour of SHCC under tension brought about by monotonic, quasi-static loading has been studied intensively during the last few years; see, for example, Mechtcherine [3]. However, in practice the majority of concrete structures are exposed to more or less severe cyclic loadings such as traffic loads, temperature changes, wind gusts, sea waves in some cases, vibrations due to the operation of machinery, or in extreme circumstances earthquake. Appropriately enough, a thoroughgoing understanding of the behaviour of SHCC under fatigue is indispensable to the safe and economical design of structural members and building components for which such materials might be used.

As yet only a few investigations of SHCC behaviour under cyclic loading have been performed. Fukuyama et al. [4] investigated the cyclic tension–compression behaviour of two SHCC materials, which possessed a strain capacity of 0.5% and 1.0%, respectively. Only about five cycles were needed until the strain capacity was exhausted, while the cyclic tension response accurately reflected the corresponding curve obtained from a monotonic tension test. In contrast, Douglas and Billington [5] found that the envelope of the stress–strain curve from the cyclic tests lay below the relationship measured in the monotonic regime. The difference was particularly pronounced in the experiments with high strain rates. The SHCC investigated showed a strain capacity of approximately 0.5% when subjected to monotonic, quasi-static loading.

Mechtcherine and Jun [6] investigated SHCC with a strain capacity clearly higher than 2%. Different loading routines were applied: deformation-controlled monotonic and cyclic tests as well as load-controlled cyclic and creep tests. In addition, the effects of specimen size and curing conditions were investigated. Simultaneously Mechtcherine [3] discussed the appropriateness of different testing techniques for uniaxial tension experiments. Furthermore, a series of fibre pullout tests was performed by Jun and Mechtcherine [7], [8] in order to investigate the characteristic material behaviour on this level of observation and to establish the accurate testing technique. Based on this information, the most appropriate testing procedures were accepted as the basis of further experimental investigations.

This paper gives an overview of the experimental results obtained in uniaxial monotonic and cyclic tension tests on SHCC. In order to understand the specific material behaviour of SHCC better, single-fibre pullout tests and single fibre tension tests under monotonic and cyclic loading as well as the accompanying optical investigations were performed additionally. The results obtained can serve as a general basis for describing and explaining the mechanical performance of SHCC. The discussion of the experimental findings is conducted with special regard to the development of a multi-scale material model for SHCC under monotonic and cyclic tensile loading (this model is introduced in the second part of this paper). The major feature and advantage of such a multi-scale modelling approach is in establishing a link between basic physical mechanisms acting at the micro-level and material response observed at the macro-level. The model should not only provide a physically sound basis for deriving constitutive relations, but should also enable a comprehensive analysis of the material behaviour leading to more purposeful material design. Particular attention during the evaluation of the experimental results is therefore paid to the various specific phenomena observed on different scales of observation.

Section snippets

Material under investigation

SHCC, unlike common, fibre-reinforced concrete, is a micromechanically designed material. One approach to such material design was developed by Li [9]. Material composition used in this investigation was developed on the basis of this particular approach by Mechtcherine and Schulze [10], [11]. The material chosen showed stable strain-hardening behaviour in earlier studies by the authors. With regard to its mechanical performance under monotonic tensile loading, it can be considered to be

Specimen geometry, casting, curing, experimental setup

Based on the findings of previous investigations [10], [11], unnotched, dumbbell shaped prisms were chosen as specimens for this study. Such prisms had a cross-section of 24 mm × 40 mm. The gauge length was 100 mm. Fig. 2a gives further geometric data for the specimens and shows the test setup.

All specimens were cast horizontally in metal forms. The moulds were stored 2 days in a climatic box (T = 25 °C, RH = 65%). Investigations on material shrinkage indicated that the development of shrinkage

Fibre characteristics, test setup

Twelve millimeter-long PVA fibres (Kuraray Co., Ltd., Kuralon K-II REC15) with a specified diameter of 40 μm were used as dispersed reinforcement for the SHCC investigated. Single fibres were tested in order to provide their basic mechanical characteristics which are essential for the material modelling. Moreover, additional information was expected from this tension testing with respect to the behaviour of fibre under cyclic loading. Still further, due to the specific test setup the results

Specimen description, casting, curing, test setup

The test setup was based on the approach presented by Kabele et al. [12]. The fibre was inserted into a hollow medical cannula with a blunt tip. The position of the fibre was then fixed with wax when the desired embedment length was attained. The embedded length was subsequently measured by using images made by a high-resolution camera. The constant diameter of the cannula was used as a reference “scale”. Finally, the frame with a number of cannulae was fixed to the mould, and the SHCC matrix

Optical investigations

Optical investigations of the microstructure were performed on the cracked SHCC surfaces and on pullout specimens after mechanical testing by using ESEM. The purpose of these observations was to clarify the pronounced differences in pullout behaviour as observed in the experiments. Matrix cylinders used in the pullout testing were split into two pieces in order to gain access to the embedded fibre and to the fibre–matrix interface. Additionally, the fibres pulled out of the matrix were

Summary and outlook

The results of the experimental investigation and the conclusions drawn can be summarised as follows.

Uniaxial tensile tests under deformation-controlled monotonic and cyclic loading regimes provided detailed characteristics of the specific behaviour of SHCC. The results observed did not show any pronounced effect of loading conditions on material performance. The analysis of the hystereses of the stress–strain curves showed a pronounced decrease in the material stiffness with an increasing

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