Damage Evaluation of Full-Scale RC Slab by Ultrasonic Tomography

There have been reports of road bridges that deteriorated after their RC slabs had been repaired. The insufficient removal of damaged materials is said to be the cause of such post-repair deterioration. Consequently, there is a call for a survey method that can detect deteriorated materials of bridges with high accuracy. Towards the development of such a method, this report focuses on ultrasonic tomography, because it can detect damage to the RC slabs of road bridges over relatively large areas. We examined the applicability of ultrasonic tomography to a method of assessing the RC slab damage by a test that uses a full-scale RC slab specimen that had been deteriorated by repeated loading on a wheel-tracking test machine. The results show that ultrasonic tomography is well able to detect areas of punching shear damage but not areas of horizontal cracking whose causes include both punching shear and compressive stress

Experimental investigation on shear strengthening of corroded reinforced concrete columns by pet fibers with large fracturing strain

The seismic strengthening of the concrete columns improperly designed or constructed is in an urgent need. There has been an enormous interest in the research and application of conventional fiber reinforced polymers (FRPs) in RC column seismic retrofitting. However, due to low fracturing strain capacity of conventional FRPs, the fiber materials tend to fail sooner due to fiber breakage. New fiber materials such as polyacetal fiber (PAF), polyethylene naphthalate (PEN) and polyethylene terephthalate (PET) have properties of large fracturing strain and low stiffness in comparison to aramid, carbon, and glass fibers. In this paper, an experimental study is presented on the influence of PET warping on shear capacity, ductility and energy absorptivity of RC columns with stirrup corrosion before strengthening. The experimental program involved an electrochemical process to accelerate the migration of chlorides from an external electrolyte into the tested columns, a wetting–drying cycle process with a controlled current to speed up the corrosion of the stirrup in the tested columns, the strengthening of corroded columns with PET warping, and a Pseudo static test to determine the shear capacity of the tested beams. The shear performance of PET wrapped RC columns with different corrosion levels in stirrups, including the yield strength, the peak strength, the ductility ratio and the energy dissipation ability was examined and the related mechanism was discussed

Basic Examination on Assessing Mechanical Properties of Concrete That Has Suffered Combined Deterioration from Fatigue and Frost Damage

An examination including an evaluation of mechanical properties of concrete that had suffered combined deterioration from fatigue and frost damage was done using cylindrical specimens. The order of deterioration and degree of deterioration of the specimens were used as variables. The examination clarified that certain mechanical properties of concrete that had undergone combined deterioration were able to be evaluated by measuring the propagation speed of ultrasonic waves. The decrease in the elastic modulus and the ultrasonic velocity that accompanies the increase in the number of freeze–thaw cycles was smaller in the specimens that experienced fatigue first and then frost damage than in the specimens that experienced frost damage only. The values for mechanical properties (e.g., compressive strength, elastic modulus, and shrinkage strain) of the specimen that experienced frost damage first and then fatigue had already greatly decreased after the application of freeze–thaw cycles. Therefore, the decrease in the mechanical properties was small even with increases in the number of loading cycles. The decrease in the fatigue life of the specimen in the fatigue test was proportional to the deterioration from the preceding frost damage

Tensile Bond Between Substrate Concrete and Normal Repairing Mortar under Freeze–Thaw Cycles

Concrete patch repair has long been used to repair the damaged concrete structures. In cold regions, freeze– thaw cycle is one of the major damage factors. Not only the material itself is damaged by freeze–thaw cycles, but also the adhesive interface, which is regarded as the weakest part of composite system, degrades under freeze–thaw cycles. Air entraining agent has long been used to increase the freeze–thaw resistance of concrete materials. However, the effect of air entraining agent on the adhesive interface under freeze–thaw cycles has not been explored. The degradation mechanism and failure mode of concrete repair system have not been studied, either. In this study, to investigate the effects of water–cement ratio of substrate concrete and air entraining agent in substrate concrete and repairing mortars, three kinds of substrate concrete were casted and repaired by two kinds of ordinary Portland cement mortar. With certain number of freeze–thaw cycles up to 150 cycles, through splitting prism test, the splitting tensile strength and failure mode of composite specimens were experimented. The relative dynamic elastic modulus and splitting tensile strength of substrate concretes and repairing mortars were obtained as well. Results showed that air entraining agent in the repairing mortar greatly influenced the adhesive tensile strength under freeze–thaw cycles. The water–cement ratio and air entraining agent of substrate concrete insignificantly affected the adhesive interface, but affected the splitting tensile strength and the freeze– thaw resistance of substrate concrete, and thus affected the failure mode of composite specimens

Moisture and Temperature Effects on Interface Mechanical Properties for External Bonding

In order to develop rational guidelines for strengthening by external bonding, it is necessary to clarify longterm performance of interfacial bonding property. In this paper, moisture effects on bonding properties at FRP–concrete interface and temperature/moisture effects on bonding properties at PCM–concrete interface are presented. Shear bond strength of FRP–concrete interface is affected by moisture because resin–concrete adhesion strength is affected by moisture. Among tested CFRP external bonding systems, wet-layup CFRP systems all show the strength reduction, while prefabricated CFRP systems all show the strength increase after immersion. The bond stress–slip relationship and interfacial fracture energy also change, which can explain the change in shear bond strength. The reduction in shear bond strength does not show clear dependency on resin strength/stiffness reduction. The observed big variation in shear bond strengths reported in past studies can be explained by difference in interface roughness, since chemical bond is affected by moisture but mechanical bond is not. Tension/shear bond strength of PCM–concrete interface is affected significantly by temperature but less by moisture. The tensile/shear bond strength is less than tensile/shear strength of constituent materials (PCM and concrete). The reduction in tension/shear bond strength depends on the reduction in tensile/shear strengths of PCM and concrete and then estimated by proposed equation which is a function of PCM and concrete strength. The ultimate loads of beams strengthened by PCM overlay can be predicted by the proposed model in which the proposed tensile/shear bond strength is applied. Under high temperature, the failure mode can be changed from flexure/shear failure to debonding failure due to the reduction in shear bond strength

Electrical Response of Mortar Saturated with NaCl Solutions under Freeze–Thaw Cycles

This paper presents the test results of electrical response of mortar saturated with sodium chloride (NaCl) solutions under freeze–thaw cycles (FTCs). To quantitatively evaluate the salt frost damage of mortar based on its electrical response, mesoscale samples are prepared to assure the uniform pore solution concentration. The reduction of electrical resistivity shows the same tendency with elastic modulus, but with less degree. The investigation shows that electrical resistivity of mortar decreases with temperature and the phase changes can be observed based on their relationship. The freezing and thawing points decreasing with increment of solution concentration can be found, but their variations with FTCs are not significant. Basically, along with frost damage development, the electrical resistivity of mortar at 23 and -28°C is decreasing with FTCs. However, for lower water-to-cement ratio and higher NaCl concentration solution exposed samples, contrary tendency are observed. In addition, with FTCs, there is no clear change for the activation energy of DI water case, whereas the decreasing tendency is observed in the cases of 5 and 15% NaCl solution. Therefore, the electrical properties are important for understanding the salt frost damage, but a comprehensive parameter to quantify the damage is still in need

FEM Analysis of Structural Behaviour of Reinforced Concrete Beam under the Effect of Frost Damage

Minimizing frost damage is one of the important durability issues for concrete structures in cold and wet areas. This paper performed a Finite Element Model (FEM) analysis on the structural behavior of reinforced concrete beam under the effect of frost damage. The frost damage was considered as degradations in two constituents: concrete material and bond between concrete and reinforcement. The constitutive models of deteriorated concrete material were proposed using a mesoscale simulation approach (Rigid Body Spring Model). DIANA FEA program was adopted for the structural analysis and experiments were also conducted for the verification. It was clarified that the structural performance of reinforced concrete beam under frost damage could be well evaluated by the FEM analysis. The assessment of the structural behavior of concrete members with frost damage would be achieved with this method

Effect of PET Wrapping on Shear Performance of Corroded Reinforced Concrete Columns

In this paper, an experimental study is presented on the influence of PET warping on shear performance, such as diagonal shear cracking load, peak load, and energy dissipation of RC columns with stirrup corrosion before strengthening. The experimental program involved an electrochemical process to accelerate the migration of chlorides from an external electrolyte into the tested columns, a wetting–drying cycle process with a controlled current to speed up the corrosion of the stirrup in the tested columns, the strengthening of corroded columns with PET warping, and a pseudostatic test to determine the shear capacity of the tested beams. The shear performance of PET wrapped RC columns with different corrosion levels in stirrups, including the diagonal cracking strength, the peak strength, the ductility ratio, and the energy dissipation ability, was examined and compared

Effect of Multi- Steel Bolt Anchorages on Composite Beams

Using fiber-reinforced polymer (FRP) to retrofit or strengthen the concrete structures is an attractive option in construction areas nowadays. However, premature debonding failures limit the efficacy of fiber utilization. It is presently accepted that anchorage system is an attractive option to solve this problem. Much efforts has been made through experimental testing and numerical modeling to investigate the anchorage systems, meanwhile various systems were created and developed. However, researches on the mechanism of the anchorage systems are still too rare to build a countable and union design guideline with respect to different premature debonding failure modes. The present paper focused on two commonly documented anchorage methods: steel bolt anchorage and CFRP end wrapping anchorage and conducted a specially design experiment to further analyze the mechanism of effect of both systems on premature debonding failures (concrete cover separation and IC debonding). Results show that CFRP wrapping and Steel bolts can both effectively stop or suppress the propagation of IC debonding. And the ultimate load is effected by the finally failure mode, which could be changed with different height of steel bolt