Recommendation of RILEM TC 246-TDC : test methods to determine durability of concrete under combined environmental actions and mechanical load

The combination of environmental actions and mechanical load has obvious synergetic effects on the durability of concrete. But these effects have been widely neglected so far. For a realistic service life prediction the effect of an applied mechanical load on chloride penetration has been taken into consideration as a first and important step in RILEM TC 246-TDC since 2011. This recommendation focuses on the test method to determine the effect of applied compressive stress and tensile stress on chloride diffusion. It includes detailed experimental procedure to receive consistent results of chloride profile and the apparent chloride ion diffusion coefficient of concrete under compressive and tensile stress

Test methods to determine durability of concrete under combined environmental actions and mechanical load: final report of RILEM TC 246-TDC

At present several methods are available to predict the durability of reinforced concrete structures. In most cases, one dominant deterioration process such as carbonation or chloride penetration is taken into consideration. Experimental results as well as observations in practice show that this is not a realistic and certainly not a conservative approach. In order to test more realistically, RILEM TC 246-TDC, founded in 2011, has developed a method to determine the durability of concrete exposed to the combined action of chloride penetration and mechanical load. In this report, a test method is presented which allows determination of realistic diffusion coefficients for chloride ions in concrete under compressive or tensile stress. Comparative test results from five different laboratories showed that the combination of mechanical and environmental loads may be much more severe than a single environmental load without mechanical loading. Modelling and probabilistic analysis also showed that the obvious synergetic effects cannot be neglected in realistic service life prediction

INFLUENCE OF POROSITY AND WATER CONTENT ON THE DIFFUSIVITY OF CO2 AND O2 THROUGH HYDRATED CEMENT PASTE

Knowledge of the diffusivity of CO2 and 02 is of considerable importance for a quantitative assessment of the carbonation and corrosion of the reinforcement in concrete. A special measuring cell which allows the simultaneous determination of the effective diffusion coefficients of the two gases as a function of the relative humidity has been developed. Measurements have been carded out on carbonated discs of hydrated cement paste prepared with water/cement ratios between 0.4 and 0.8. The results show that, if the water/cement ratio is increased from 0.4 to 0.8, diffusion coefficients increase more than ten times. On the other hand, only little influence of the relative humidity between 50-90% the diffusivity has been observed on samples in adsorption equilibrium. The variation of diffusivity as a function of water content and porosity is explained by the characteristic microstructure, which has been characterized by water adsorption isotherms and mercury intrusion porosimetry measurements. Finally, a model with two levels in the microstructure is proposed to describe CO2 diffusion in a carbonating material

Explanation of size effect in concrete fracture using non-uniform energy distribution

A local fracture energy model originally proposed to explain the influence of fracture process zone (FPZ) on fracture energy of cementitious materials is further developed in this study. By assuming a bilinear distribution for the fracture energy distribution, the ligament-dependentfracture energy Gf is obtained. The analytical expression of Gf contains two important parameters: the intrinsic size-independent fracture energy Gf and a reference ligament size at which determines the inte section of the twolinear fracture energy functions. It is shown that the ligament-dependent Gf approaches the size-independent GF asymptotically. As a result, Gf can be determined from the ligament-dependent Gf results. It is also found that while the reference ligament size aI* is influenced by the specimen geometry, size and loading conditions, thederived fracture energy Gf is virtually constant. The present local fracture energy distribution model is also discussed and compared with the original local fracture energy model

Boundary effect on specific fracture energy of concrete

The influence of specimen back-face boundary on the shape and size of the crack-tip fracture process zone during crack growth is related to the fracture energy distribution along the crack path. The governing mechanism responsible for the size effect on the specific fracture energy of concrete is the height variation of the crack-tip fracture process zone. Such a height variation exists in the boundary zone where the development of the crack-tip fracture process zone is limited due to the confined space and sharp stress gradient. The reduction in the fracture process zone height leads to a decreasing specific fracture energy distribution along the crack path in the back-face boundary region. The adhesive thickness effect on the critical energy release rate of the adhesive joint sandwiched between two non-yielding substrates and the un-cracked ligament effect on the large scale yielding of polymers and metals provide further proof to the relationship between the height variation of the crack-tip plastic zone and the specific fracture energy

Asymptotic analysis of boundary effects on fracture properties of quasi-brittle materials

Composite materials such as concrete and ceramics with coarse material microstructures exhibit size-dependent fracture properties when the size of damage zone or fracture process zone (FPZ) around a crack tip is comparable to the structural or specimen size. These materials do not comply with the linear elastic fracture mechanics (LEFM), and therefore, are referred to as quasi-brittle materials. This size-dependence of fracture behaviour has been attributed to the interactions between the crack tip, fracture process zone and specimen boundaries [1,2]. It was pointed out recently that when a crack tip is close to a free boundary or a bimaterial interface, the stress/strain fields around the crack tip and associated damage zone will interact with the boundary, and the fracture behaviour of the structure is changed [1-5]. Therefore, the size effect in fracture of quasibrittle materials is in fact, due to the influence of specimen boundaries and/or bimaterial interfaces. This means that the size effect is a boundary effect in reality