Tie Jun Zhao

Chloride Penetration into Concrete

Reinforced concrete structures may be exposed to an aggressive environment. In this case combined mechanical and environmental actions may act simultaneously and their synergetic influences have to be taken into consideration. In this contribution results of tests on two different types of concrete and mortars are presented and discussed. Capillary absorption of not carbonated and carbonated concrete has been determined. The influence of mechanical load on capillary suction has been studied separately. A moderate compressive load reduces the coefficient of capillary suction, while capillary suction is enhanced by higher mechanical loads as micro-cracks are formed. Chloride diffusion has been measured by means of the diffusion cell test. The diffusion coefficient observed in carbonated concrete is roughly speaking one order of magnitude higher than the value measured on not carbonated concrete.

Effect of Compressive Strain on Electrical Resistivity of Carbon Nanotube Cement-Based Composites

Multi-walled carbon nanotube (MWCNT) fiber reinforced cement-based composites (MWFRC) with 0.1 wt.% and 0.5 wt.% weight concentration of MWCNT (wcM)were prepared, associated with the reference. The electrical resistances and compressive strains of these cured nanocomposites under cyclic uploading/unloading were real-time collected, to explore their stress/strain-sensitive properties. Results reveal as follows, there is no self-sensing trait for the reference, but exists good piezoresisitivity and high strain sensitivity (above 110) for MWFRC; the fractional change in resistivity (Δρ) regularly descends or ascends following the compressive stress, or the longitudinal strain of MWFRC; the resistance caused by pore electrolyte polarization has obvious impact on the time-stability of the Δρ trendline of MWFRC with 0.1 wt.% wcM, similar to the reference, but negligible effect on that of MWFRC with 0.5 wt.% wcM.

Chloride Penetration Resistance of Concrete Structures under Compressive Load

The mechanism of concrete durability under uniaxial compressive load has been studied through chloride penetration experiments in this contribution. It has been found that there is a good corresponding relationship between chloride penetration resistance of concrete and the applied compressive stress level. With increasing of the applied compressive load, chloride penetration depth and content both decreased firstly. After the compressive load up to a critical level, they began to increase quickly. The apparent diffusion coefficient and chloride concentration at the surface also increase with the same rule. It is shown that chloride penetration of concrete is a complex process. So the effect of mechanical load should be considered as an important factor for prediction of service life of reinforced concrete structures in marine environment.

Analyses of the Protective Effect of Polyethylene-Triethoxysilane and Triethoxysilane Emulsion Coatings on Concrete Substrate

Water could transport aggressive substances such as carbon-dioxide, sulfates and chloride ions in reinforced concrete structures. By penetration, the stability of the whole structure is influenced. This paper deals about waterproof effect and chloride resistance of two kinds of silane coatings, polyethylene-triethoxysilane and triethoxysilane emulsions on concrete substrate along with the mechanism of the hydrophobic action. The results, in terms of visual observations, chloride test and water absorption test, showed that the two silane coatings could reduce the chloride penetration and the water absorption efficiently. The comparison between the two indicates that polyethylene-triethoxysilane penetrates little when applied on the substrate, but it performs better waterproof effect and is more efficient to chloride penetration.

Water Repellent Treatment on Lime/Lime-Fly Ash/Lime-Cement Mortar with and without Carbonation

Water repellent treatment (WRT) has proved to be effective to prevent porous materials from water penetration and consequently can improve the durability of structures. In this contribution, three types of mortar, which are pre-carbonated and non-carbonated, had been water repellent treated by silane gel with usage of 400 g/m2, to investigate into the influence of WRT on carbonated and non-carbonated mortar. Results indicate that carbonation reaction increased the compressive strength of all three types of mortar. Silane gel penetrated to a comparatively higher depth for the carbonated mortar. WRT reduced the absorbed water and capillary absorption coefficient greatly, both for non-carbonated mortar to 2 % ~ 7 % and for carbonated mortar to 22 % ~ 66 %, compared with non-treated ones. WRT is still a feasible method for porous materials with some carbonation to prevent from water penetration. However, the efficiency of WRT on reducing capillary absorption became much lower compared with the non-carbonated mortar. Capillary absorption curves of non-WRT mortar, both carbonated and non-carbonated, could be well fitted by a hyperbolic function of square root of time. For the treated mortar, the absorption curves could be described as a linear equation before carbonation, but an exponential function when the mortar carbonated.

Effect of Octyl-Triethoxysilane Emulsion on Protection of Concrete

In this study, octyl-triethoxysilane emulsion was prepared using octyl-triethoxysilane monomer. The protective effect of the silane monomer and emulsion was investigated on the concrete with different water to cement ratios (0.4 and 0.5). The results showed that octyl-triethoxysilane emulsion displayed efficient protection of concrete. Octyl-triethoxysilane emulsion acquired excellent penetration depth (> 3.7 mm), the water absorption coefficient was reduced by 83.4%, and the chloride ion diffusion coefficient was only 1.8 × 10−12 m2 s−1, reduced by 71.3%. The emulsion also showed good resistance to carbonization and freezing-thawing. The carbonation depth of concrete was reduced by 42%, while the silane monomer has little effect on carbonization. Good protection performance was obtained in the freezing-thawing cycle tests. The relative dynamic elastic modulus was increased by 27%; the mass loss was reduced by 49% after 300 cycles. The protective effect of silane emulsion was also related with the water cement ratio of concrete and the test method. Silane monomer had good waterproof effect and chloride resistance, but poor resistance to carbonization and freezing-thawing. Silane emulsion was a better choice for concrete protection, for it displayed lower rate of volatilization, insignificant toxicity, and better retainment of the active ingredient.

Improvement on Mechanical and Piezoresistivity Properties of Cementitious Binder by Using Surface Oxidized Multi-Wall Carbon Nanotubes

Two types of the cementitious binders (MWNT/CB, AMNT/CB) filled with 0.5 wt.% multi-wall carbon nanotube (MWNT), and acid-treatment MWNT (AMNT) were prepared. The surfaces of MWNT and AMNT were firstly characterized with TEM micrograph and FT-IR spectrum before and after acid oxidation. The electrical resistivities (ρ) and the longitudinal strains (εl) of two cured nanocomposites under cyclic compressive stress (σ) are real-time collected, in order to investigate their piezoresistivity properties, associated with mechanical behaviors. On one hand, acid treatment can enhance interface bonding between AMNT and cementitious binder decorated with some hydrophilic groups, resulting in some mechanical improvements; on the other hand, it causes better distribution and more network pathway, AMNT/CB has resultantly balanced piezoresistivity property with regular and consistent variation of ρfollowing the σ, or the εl, and fixed stress/strain sensitivity (around 62), compared to that of MWNT/CB.

Damage and Chloride Penetration of Cement Mortar under Conventional Triaxial Compression

In this paper, the influence of conventional triaxial compression on damage and chloride ion penetration of cement mortar are investigated. Conventional triaxial compression experiment was carried out with confining loading of 10Mpa. And the stress-strain curve was measured when axial stress was 50%, 80% and 100% of peak axial loading, and 80% axial loading post-maximum. Then the damaged cement mortars was stored in Qingdao sea water for 30 days, and the free chloride content as well as chloride diffusion coefficient of damaged mortars were quantitatively determined. The experimental results show that the cement mortar is compacted in the end, and shear damaged in the middle under conventional triaxial compression. Compared to cement mortar under uniaxial compression, its compressive strength and axial deformation increases by 1.94 times and 5.6 times when cement mortar under conventional triaxial compression. When the axial stress is less than 48% of peak axial loading, and the axial deformation is less than 0.63mm, the mortar is compacted and its relative dynamic elastic modulus increases with raising axial loading and deformation. The free chloride content in the pressure-bearing side is higher than that in the bottom side. And the free chloride content in the interior of mortar increases with raising axial loading. The chloride diffusion coefficient and axial loading are related in quadratic function. When the axial deformation of mortar is higher than 0.72mm and 1.57mm, the chloride diffusion coefficient of non-load mortar, is less than that of loaded mortar in the pressure-bearing side, and in the bottom side, respectively.

Inner Damage and Anti-Chloride Penetration of High-Performance Concrete under Axial Tensile Load

The total porosity of high performance concrete specimens after different level uniaxial tensile loading were measured to reflect the damage degree of mechanical load to the microstructure of the concrete. Meanwhile, considering the environmental characteristics of the ocean tidal zone, chloride penetration tests of the concrete were carried out with salt solution capillary absorption method. Based on the profile of chloride measured from specimens, chloride diffusion coefficients of the concrete under uniaxial tensile load with different levels are determined further by Fick’s second law. Results show that both of total porosity and chloride diffusion coefficient of the concrete are increased significantly after short-term tensile loading. And the evolutions of the porosity and chloride diffusion coefficient are similar with the development of micro-cracks under uniaxial tensile load.

Corrosion Rate Evolution in Concrete Structures with Hydrophobic Agents Exposed to the Marine Environment

In many cases, service life of reinforced concrete structures is severely limited by chloride penetration until the steel reinforcement. Today, concrete with high resistance with respect to chloride penetration can be produced by internal hydrophobic treatment. The aim of this study was to fill this gap in regards to reinforced concrete structures inserted in a marine environment. Results indicated the efficacy of the hydrophobic agents in cases where capillary suction is the mechanism of water penetration. However, when the transport mechanism is permeability this product is not advisable. Moreover, it was demonstrated that the chloride diffusion coefficient is reduced by the hydrophobic agents, and the corrosion rate of reinforcement could be well protected in the treated concrete with hydrophobic agents of silane. The durability of reinforced structures can be considerably increased and can be accurately designed by the application of an appropriate and optimized protective layer.