Compared with most construction materials, concrete is considered as a brittle material, and its brittleness increases with the compressive strength. For super-high-strength concrete, failure can be sudden, explosive and disastrous. Also the tensile strength is not proportionally increased. Therefore, it is necessary to carry out research on the brittleness of concrete in order to establish parameters for assessing the brittleness, find ways to improve the brittleness and tensile strength, and eventually design and manufacture concrete materials with high strength and low brittleness. In this study, strengthening and toughening effects of polymer materials on the high performance concrete (HPC) were investigated. The HPC was manufactured using ordinary Class 52.5 N Portland cement, silica fume and superplasticizer. The adopted polymers included the styrene-butadiene-rubber (SBR) latex, polyvinylidene chloride (PVDC), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) with contents of 1.5%, 3% and 5% in weight of cement content. The measured material and fracture properties included compressive and tensile strengths, modulus of rupture, Young's modulus, fracture energy, fracture toughness and brittleness. The test results at 28 days indicate that the addition of 1.5% and 3% SBR, PVDC, LLDPE and HDPE into the HPC could largely improve the compressive strength by up to 15.7%, while the addition of 5% SBR, LLDPE and HDPE did not show any enhancement except for 5% PVDC which increased the compressive strength by 10.9%. The tensile strength was considerably increased for all dosages of polymers, with the maximum increases of 72.7% and 83.2% for 3% SBR and 1.5% LLDPE, respectively. The fracture energy were also enhanced by adding 1.5% SBR and all dosages of LLDPE, with a maximum increase of 24.3%, while there were no indications of enhancement for other dosages of polymers. The modulus of rupture, fracture toughness and Young's modulus were not improved for lower dosages of polymers but slightly decreased for higher dosages. The brittleness decreased monotonically with increasing amount of LLDPE, but it increased with increasing amounts of SBR, PVDC and HDPE.

Failure criteria play a vital role in the numerical analysis of reinforced concrete structures. The current failure criteria can be classified into two types, namely the empirical and theoretical failure criteria. Empirical failure criteria normally lack reasonable theoretical backgrounds, while theoretical ones either involve too many parameters or ignore the effects of intermediate principal stress on the concrete strength. Based on the octahedral shear stress model and the concrete tensile strength under the state of triaxial and uniaxial stress, a new failure criterion, that is, the simplified unified strength theory (UST), is developed by simplifiing the five-parameter UST for the analysis of reinforced concrete structures. According to the simplified UST failure criterion, the concrete strength is influenced by the maximum and intermediate principal shear stresses together with the corresponding normal stresses. Moreover, the effect of hydrostatic pressure on the concrete strength is also taken into account. The failure criterion involves three concrete strengths, namely the uniaxial tensile and compressive strengths and the equal biaxial compressive strength. In the numerical analysis, a degenerated shell element with the layered approach is adopted for the simulation of concrete structures. In the layered approach, concrete is divided into several layers over the thickness of the elements and reinforcing steel is smeared into the corresponding number of layers of equivalent thickness. In each concrete layer, three-dimensional stresses are calculated at the integration points. For the material modelling, concrete is treated as isotropic material until cracking occurs. Cracked concrete is treated as an orthotropic material incorporating tension stiffening and the reduction of cracked shear stiffness. Meanwhile, the smeared craclc model is employed. The bending reinforcements and the stirrups are simulated using a trilinear material model. To verify the correctness of the simplified UST failure criterion, comparisons are made with concrete triaxial empirical results as well as with the Kupfer and the Ottosen failure criteria. Finally, the proposed failure criterion is used for the flexural analysis of simply supported reinforced concrete beams. Also conducted are the punching shear analyses of single- and multi-column-slab connections and of half-scale flat plate models. In view of its accuracy and capabilities, the simplified UST failure criterion may be used to analyse beam- and slab-type reinforced concrete structures.

Nowadays, thin-walled load bearing structures can be realised using textile reinforced concrete (BRAMESHUBER and RILEM TC 201-TRC [1]). The required tensile strength is achieved by embedding several layers of textile. By means of the laminating technique the number of textile layers that can be included into the concrete could be increased. To further increase the first crack stress and the ductility as well as to optimize the crack development, fine grained concrete mixes with short fibres can be used. By a schematic stress-strain curve the demands on short fibres are defined. Within the scope of this study, short fibres made of glass, carbon, aramid and polyvinyl alcohol are investigated in terms of their ability to fit these requirements. On the basis of these results, the development of hybrid fibre mixes to achieve the best mechanical properties is described. Additionally, a conventional FRC with one fibre type is introduced. Finally, the fresh and hardened concrete properties as well as the influence of short fibres on the load bearing behaviour of textile reinforced concrete are discussed.