Pore size distribution is an important factor that affects the moisture diffusion and permeability properties of cement-based materials. For cement paste, it is assumed that the mesopores (100 nm–0.01 mm) and macropores (0.01 mm–1 cm) can be neglected compared to the micropores ($<100\mathrm{nm}$ ). Based on the surface energy theory, moisture chemical potential of pore water is introduced to explain the liquid-gas equilibrium in pores with different radius. Using chemical potential as an intermedium, a quantitative relationship between micropore size distribution and water adsorption isotherms can be established. The micropore size distribution can be treated as an explanation of the moisture adsorption behavior, and by using adsorption isotherms, the micropore size distribution can be estimated conveniently. Mortar and concrete can be regarded as a combination of cement paste and aggregates; thus, besides the micropores, the mesopores and macropores are also taken into consideration. Finally, the general empirical estimation equations for pore-size distribution are developed for cement, mortar, and concrete, which can be used for refined modeling and simulating of durability-related issues, such as frost action, water permeability, and drying shrinkage.

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Strengthening by fiber-reinforced polymers (FRPs) is one of the most common solutions to the ageing infrastructures. However, long-term durability of such systems under different environmental conditions needs to be understood properly before widely adopting these methods in the field. This paper presents research outcomes from an experimental program carried out to determine the influence of moisture on the durability of the bond between FRP and concrete along with its constituent materials. Performance was evaluated through single lap shear bond tests and various kinds of tests on the resin samples, including water absorption, mechanical characterization, and glass transition temperature ($T_g$ ) analysis. The specimens were exposed to continuous water immersion and wet-dry cycles for a maximum period of up to 24 and 18 months, respectively. The results show some deterioration on the material and the bond properties in both exposure conditions. The bond strength decreased up to 32 and 12% for high-strength and normal-strength concrete substrates, respectively, after continuous immersion, indicating the key role of the substrate in the durability of bonds. As a result of exposure, failure propagated towards the primer-concrete interface region. The failure mode changed from concrete cohesion to primer-concrete mixed failure for normal-strength concrete and from mixed failure to complete adhesion failure in case of high-strength concrete. These shifts in failure patterns are mainly due to destruction of the adhesion bond caused by water molecules at the interface. However, it is found that the loss of the adhesion bond is somehow compensated by the good mechanical interlocking obtained by proper surface roughness condition of the substrate. Finally, the effects of water were also investigated in interfacial bond stress-slip laws and fracture energies.

This paper presents an experimental study on the shear behavior of RC members fully wrapped with polyethylene terephthalate (PET) fiber-reinforced polymer (FRP) composites, which are a new type of FRP material characterized by a much larger rupture strain (LRS) compared with conventional FRPs (i.e., made of carbon, glass, and aramid fibers). A total of 10 PET fully-wrapped RC beams, which were designed to fail in shear and with different shear-span to effective-depth ratios, transverse reinforcement ratios and shear strengthening ratios, were tested under 4-point bending loads. The overall load-deflection responses and the shear deformation of the beams as well as the strain development of the transverse steel reinforcement and the FRP jackets were carefully observed. Based upon the extensive strain measurements, the shear contributions by concrete, FRPs, and transverse reinforcement are differentiated. It was found that the use of PET FRP composites as the jacket material of RC members could shift the mode of shear failure from a brittle one to an ideal ductile one whereas the ultimate state of the members is no longer caused by FRP fracture. In order to efficiently predict the shear strength of RC members wrapped by LRS FRPs, the effective strain in LRS FRPs and the degradation of concrete at the peak member shear strength should be appropriately considered.

Some natural fibers such as jute which have high tensile strength can be applied in natural fiber-reinforced polymer (FRP). Natural cellulose fibers are hydrophilic, resulting in lower reinforcement performance. Heat treatment can reduce water content in natural fibers and possibly improve the properties of natural FRP in terms of enhancing fiber and polymer composite action. This paper presents an improvement of jute fiber–reinforced polymer (JFRP) composites by heat treatment. The heat treatment conditions (i.e., temperature and duration) were considered. Coupons of heated JFRPs were tested to investigate their tensile properties. The application of JFRP on confinement was investigated by compression tests on concrete cylinders with JFRP wrapping. The experimental results indicated that a suitable heat treatment condition of 80°C for 24 h can significantly increase the tensile strength of JFRP coupons and the compressive strength of JFRP-confined concrete. The testing results of concrete confinement can be used to validate the existing predicting equations for confined compressive strength. In addition, a new equation was proposed for the effect of heat treatment of JFRP.