The ultrasonic transmission method based on embedded piezoelectric transducers is employed to continuously monitor the cracking process of different strength notched concrete specimens under three-point bending. For comparison, the strain gauge technique is applied simultaneously. An ultrasonic damage index is employed to analyze the onset and propagation of cracking in concretes of three different strength levels. It is found that the damage process could be characterized by three typical stages, i.e., undamaged stage, progressive stage and failure stage, by this damage index. For estimating the onset of cracking, the embedded ultrasonic method is more accurate and sensitive than strain gauge technique. With the increase of strength level of concrete, the difference in initial cracking loads between the results obtained by the embedded ultrasonic method and strain gauge technique is reduced. An obvious decreasing trend in the duration of progressive damage stage with the increase of strength level is confirmed with embedded ultrasonic method.
Alkali Silica Reaction (ASR) is a long term chemical reaction causes swelling and damage to the concrete material in the form of cracking and material deterioration. In this research a chemo-thermo-mechanical ASR finite element numerical code is developed to model and analyse this phenomenon in concrete dams. It considers the effects of variables such as temperature, humidity, non-uniform time-dependent material degradation and 3D stress confinement on ASR evolution. While the structural behaviour of ASR affected structures under monotonic and quasi-static loading has been extensively investigated over the last decades, limited research has addressed the effect of dynamic loads on structures affected by ASR. The combined effect of old and new cracks under dynamic excitation may cause dam failure. The SU-ASR (Stellenbosch University ASR FE code) model developed, is used to analyse and predict dynamic behaviour of the Kleinplaas dam which is located on the Jonkershoek River near Stellenbosch in the Western Cape province of South Africa. The combined ASR and seismic actions based on the state of the structure at the end of the long-term ASR analysis are investigated and comparisons are made through a comprehensive study of the damage development and crest displacement. The development of the cracks in the coupled analysis of the ASR and seismic load is significantly different from the cracks in the dam when only ASR is considered.
Damage of PC tendons in a prestressed concrete structure need to be detected before such damage accumulates to cause a serious failure. That, however, is quite a difficult task because the PC tendons are invisible from outside and the damage location cannot be known beforehand. Phase space analysis based on vibration data is a novel method for damage detection. An earlier study by the authors demonstrated that Change of Phase Space Topology (CPST) was effective in identifying the existence of PC tendon damage. However, CPST from impact hammer test, which is a more practical method, did not show a trend as obvious as that of the free vibration test. As it is known that PC tendon damage affects several modes of vibrations and that the hammer can be used for excitation to the higher modes, it may be possible to improve the capability of impact hammer test by considering CPST separately in different frequency ranges. As in the previous study, the current study conducted experiments on PC tendon damage, but with an increased number of accelerometers for constructing mode shapes and investigated CPST further in different frequency ranges. The results still revealed that CPST was more sensitive to damage than the parameters from modal-based analysis. CPST in different frequency ranges can improve results from impact hammer test and has the capability to identify roughly the damage locations.
The calculation of crack width in thick, restrained, reinforced concrete (RC) members has relevance, either at the design stage or during the assessment of existing structures. This type of structural element exhibits a complex serviceability behaviour, due to the nonlinear self-induced deformations caused by cement hydration and shrinkage, and also the interaction between primary cracks and secondary cracks which do not fully penetrate in the cross section. In this context, this paper presents a staggered thermo-hygro-mechanical (THM) analysis methodology, based on the finite element method (FEM), for calculation of the long-term development of self-induced deformations, stresses and cracks, since casting, at the macro scale. A comprehensive approach is followed, in which the mechanical material models are defined as a function of the calculated thermal and hygral fields. This analysis methodology is applied in the study of the crack formation in end restrained slab-like RC members, with a thickness of 50 cm, a parametric analysis in conducted in order to gain insight about the influence of some relevant structural and material variables.