To characterize the nonlinearity of thermal expansion coefficients dependent on the moisture content of hardened cement paste, samples of hardened cement paste were used to experimentally determine the coefficient of temperature dependency of relative humidity, the hygrothermic coefficient, defined as (∂h/∂T)θ, where T = temperature, h = relative humidity, and θ = moisture content. The relative humidity at which an equilibrium state was reached at 25°C and 35°C during the first desorption process of saturated samples was measured. The relationship between the relative humidity values and the corresponding hygrothermic coefficients was found to have a convex profile with approximately three- to four-fold higher values than those measured during the adsorption of dried samples.
In order to inhibit water penetration into concrete, hydrophobic surface impregnation is generally used to protect concrete, but there is concern that water repellency may decrease greatly when cracks occur. This study aims to develop an internal hydrophobic cementitious material by mixing several hydrophobic agents. Two mixing methods of the agents, direct mixing and sand spraying, were carried out. It was found that mortar with sand sprayed with silicone-based agent had a relatively high contact angle and hydrophobicity under wetting and drying cycles even if cracks occurred, although compressive strength decreased to some extent. Further, mortar with the addition of an alkyl-modified siloxane agent could greatly inhibit the water penetration after partial drying, and drying shrinkage was also found to be smaller than that of normal mortar.
This study aims to evaluate the effect of Shirasu substitution on the chloride penetration resistance in OPC paste and mortar. To evaluate said effect, a salt water immersion test was carried out. The results showed that use of Shirasu promotes high chloride resistance. This was considered to be due to the pozzolanic reaction leading to the formation of new hydrates and hence densification of the matrix and fixation of chloride ion. The pozzolanic reaction rate increased under saline condition. Further, higher content of fine particles in Shirasu was found to result in higher pozzolanic reaction rate and chloride resistance.
This paper presents the results of the investigation of a new method for evaluating the quality of the upper surface of concrete slabs with a surface water absorption test (SWAT) developed by the authors. Methods to measure the moisture content of concrete at the depth of 5mm from the surface were investigated considering parameters such as W/C, type of cement and curing conditions. It was found that the moisture content of concrete at the depth of 5 mm from the casting surface can be detected by the HI-100 moisture tester (manufactured by Kett) and the concrete can be judged to be sufficiently dry when the count value of the device is below 120. When the count value was below 120, the quality of the concrete at the upper surface of the slab could be evaluated by using the cumulative water absorption amount over a duration of 100 seconds. In conclusion, the count value of 120 can be used as an index for detecting the decrease in scaling resistance of concrete due to micro-cracking at the surface.
Thin and lightweight structural concrete can be achieved by using fiber reinforced Porosity Free Concrete, which has extremely high compressive strength exceeding 300 N/mm2. In this study, to investigate the structural behavior of thin and lightweight prestressed segmental concrete beams using externally prestressed fiber reinforced Porosity Free Concrete, bending tests on beams with high prestress were conducted. In these experiments, the loading capacity of a fiber reinforced Porosity Free Concrete beam with 40 N/mm2 prestress was dramatically improved compared with a beam with 20 N/mm2 prestress. The analytical results obtained with the non-linear finite element method showed good agreement with the experimentally obtained yielding capacity. Prestressed concrete members consisting of thin thickness fiber reinforced Porosity Free Concrete segments were found to have potential for use in concrete structures through the application of high prestress.
The authors developed a seismic retrofitting method using precast panels and high strength fiber reinforced mortar for the purpose of construction labor saving. There are few cases of the use of high strength fiber reinforced mortar to increase seismic resistance, and the shear failure mechanism when such method is used was not clear. This paper describes an experimental study that was carried out with a retrofitting beam specimen to clarify the shear failure mechanism when this method is used. As a result, it was confirmed that the developed seismic retrofitting method improved the shear strength of the beam specimen, and the mode of fracture was found to be shear fracture due to yielding of the connecting steel used for panel assembly. The influence of each member of the reinforcement on shear strength was also clarified.
Several RC beam-column joint shear strength calculation methods incorporating assumptions related to the concrete compressive strut and the effective compressive strength have been investigated. However, these methods did not consider the inelastic behavior of columns and beams around beam-column joints. Therefore, a method that incorporates inelastic behavior of columns and beams was developed for theoretical calculation of beam-column joint shear strength. This proposed method determines the compressive strut width of concrete at joints as a function of the compressive regions of columns and beams at a critical section. The calculation results obtained with the proposed method were confirmed to agree well with the experiment results.
This report describes the study of a method to automatically detect the locations of deformations in a structure based on the surface temperature distribution of the structure measured by infrared thermography. This method uses the two-dimensional Gabor Wavelet transform filter, which is effective for extracting local periodical and directional characteristics of transient signals in steady-state signals to detect inherent shape changes in the surface temperature distribution related to specific deformations in a structure. Cracks in a concrete structure were selected as deformations, and the surface temperature distribution of the structure containing cracks was obtained by the finite element method, an experiment, and actual measurement. The efficacy and function of this inspection method was examined using these data.
This research aims to identify defective areas inside existing concrete structures based on impact response characteristics. An impact response test was conducted on small test slabs containing a number of artificial defects by using an impulse hammer with force sensor and an acceleration sensor. Various properties of defects in terms of their influence on impact response characteristics were investigated, and the frequency response function (transfer function) was determined to be an important index of evaluation of defective areas. Evaluation of defective areas with minimum diameter of 200 mm and maximum depth of 70 mm was carried out by applying frequency response function data to Self-Organizing Map (SOM). Further, this method was applied to existing concrete structures and it was shown to be effective for evaluation of defective areas inside existing concrete structures.
In the current surface water absorption test proposed by the authors, the surface water absorption rate at 10 minutes obtained from the measurement result is used as the criterion for evaluating cover concrete quality. In this study, it was experimentally confirmed that the surface water absorption rate at 10 minutes and the amount of absorption in 10 minutes are in a linear relationship for concretes with W/C of 55%, 65%, and 75% and with various extents of deterioration caused by accelerated freezing and thawing, and thus that evaluation of the extent of deterioration due to freezing and thawing of cover concrete with W/C in the range of 55% to 75% is possible with the current criteria. Shortening of the measurement time was also investigated and new evaluation criteria for measurement durations between 1 and 10 minutes were proposed. The evaluation results yielded by the newly proposed criteria are the same as the results achieved by the current criteria.
A new shear strengthening retrofit method has been developed to provide existing structures with shear reinforcement installed from one side even under constraints such as ground in full contact with the back of the structure to be reinforced. In the newly developed method, shear reinforcing bars with a hex nut provided at one end to function as an anchor plate are integrated with the existing concrete structure by use of grout designed specifically for the new method so as to improve shear capacity. Newly devised jigs for grout injection and shear reinforcing bar insertion make easy and high-quality construction possible. Loading experiment results confirmed that the new method improves shear capacity and allows the use of a shear capacity evaluation method based on conventional calculation formulas.