The main objective of this paper is to examine the shear strength of seismic retrofitting by external reinforcement for existing RC columns. The method proposed herein is external reinforcement using divided thin steel plate with ribs and polyethylene fiber sheets with a grout mortar layer. In order to investigate the shear strength, lateral loading tests were conducted for four specimens with two different column heights and two different rib widths. The test results show that this method is very effective in enhancing the shear strength by providing ribs on steel plates. Theoretical formulas derived from the model of compressive struts are proposed to evaluate the ultimate shear capacity.
We proposed a new retrofitting element to externally retrofit existing concrete buildings. The proposed element consists of a monolithic steel column and steel beams at each story level, and looks like a tree. The tree-shaped element is connected to the exterior of the building through studs and anchors distributed only along the length of the beams, and the steel column is free from the existing building. Three retrofitted column specimens and four indirect connection specimens were tested. The effectiveness of the proposed retrofitting element and the mechanical properties of the indirect connection were discussed and investigated.
An analysis of finite elements whose sectional area was measured was conducted for corroded reinforcements extracted from three actual concrete structures. The variable factor for the analysis was the length of the finite elements. The analysis result shows that the length of finite elements remarkably influences the deformation capacity after yielding. The results of the analysis of the stress-strain curve using finite element length twice the reinforcement diameter show good agreement with experimental results. For the purpose of convenience for the finite element analysis of whole concrete structures, the analytical stress-strain curves were modeled using a bi-linear model. The results show that tensile strength and yield strength decrease proportionally with the reduction of the minimum sectional area of corroded reinforcement. The decrease in yield strain was more gradual than that of tensile and yield strength. The second stiffness of the bi-linear model increased as the minimum sectional area decreased. The ultimate strain at tensile strength declined remarkably as the minimum sectional area decreased.
Although, in the design of a structure, structural and durability performance is considered, environmental performance such as CO2 emissions is rarely considered. This study investigated measures for the reduction of the environmental impact of precast concrete production through the use of industrial waste, industrial byproducts and recycled aggregates as well as omitting steam curing on hot days. It was found that SOx emissions can be reduced by about 14% by not performing stream curing for 45.7 days in a year, and that CO2 emissions can be reduced by about 18% by replacing 20% of ordinary portland cement with fly ash and 30% of fine aggregate with molten-slag fine aggregate.
For weight reduction of CFT members, concrete-filled double skin steel tubular members, CFDT, have been developed. To ensure the composite action of CFDT members, shear connectors should be installed at the interface between steel tubes and concrete. In this study, to investigate the effect of shear connector on the bending behavior of CFDT members, four-point bending tests were carried out on CFDT and CFT members with shear connectors. The locations of the shear connectors were different for each specimen. The shear connectors were found to prevent slip between steel tubes and concrete. The maximum load of CFDT members was improved by shear connectors, particularly when placed on the inner surface of the outer steel tube.
Prestressing bars or prestressing wires are employed as transverse prestressing tendons in cross-beams and concrete slabs of prestressed concrete (PC) bridges. The ducts for inserted tendons are normally grouted. Inadequately grouting, however, may lead to tendon corrosion and breakage. The breakage of tendons sometimes results in the projection of a torn portion from the bridge, causing a serious public safety hazard. The impact Elastic Wave Methods technique was employed to study the effect of voids in post-tensioning grouted ducts of PC bridges in service. Two indices were introduced to evaluate inadequately grouted ducts for transverse prestressing. One is the velocity of the elastic wave propagated in the tendon, and the other is the presence of high frequency components observed at the beginning portion of the received wave. After confirming the applicability of the technique for detecting inadequately grouted ducts for transverse prestressing, it was demonstrated that the technique can also be applied for inspecting restored ducts after re-grouting. Further, it may be possible to improve the reliability of the diagnosis by taking into account the rise time of the received waves.
This paper investigates the shear performance of 3-dimensional RC beam-column joints subjected to bi-directional cyclic loading, based on the analytical results obtained from nonlinear FEM. First, comparing the experimental results for a basic test specimen, the analysis results showed good agreement with the experimental results, demonstrating the applicability of the FEM modeling and analytical method proposed for this study. Next, the study focused on the shear performance of beam-column joints subjected to bi-directional cyclic loading, and significant shear strength deterioration due to the bi-directional loading was verified. In addition, the authors investigated the influence of the reverse beam and the column/beam flexural strength ratio by using different analysis specimens, based on the analytical results, including internal stress and failure mode.
Fatigue strength of steel-making slag concrete (SSC) under compression in submerged condition was experimentally investigated on the assumption that SSC will be applied to marine or harbor structures. The fatigue strength of SSC under compression in submerged condition was found to be slightly smaller than that of ordinary cement concrete (OCC). An investigation focusing on the deformation characteristics of specimens revealed that the incremental strain per cycle under the same stress ratio on SSC is larger than that on OCC, possibly leading to shorter fatigue life for SSC. For the purpose of improving the fatigue strength of SSC, hot-metal pretreatment slag was grinded in the Los Angeles machine to produce fine and coarse aggregate for SSC, and the aggregate thus obtained was used. The resulting finding was that use of grinded hot-metal pretreatment slag enhances the durability against fatigue of SSC in compression under submerged condition.
The purpose of this study was to clarify the inhibitory effect of impregnating mortar with an aqueous solution of hydrogen peroxide enriched with aluminum dihydrogen phosphate (HPS) on the alkali-silica reaction (ASR). The expansion over three years of mortar processed using HPS was measured and the outermost surface of the aggregate was analyzed by X-ray photoelectron spectroscopy (XPS). The results obtained were as follows: (1) Compared with the non-soaked aggregate, the outermost surface (1 to 5 nm depth) of HPS-soaked reactive aggregate was improved with silica featuring a more stable Si-O bond as the result of an increase in the amount of active oxygen content; (2) The expansion of the mortar containing HPS-soaked reactive aggregate was significantly smaller than that of mortar containing non-soaked aggregate after 3 years, as determined using the JIS mortar bar method; (3) The expansion of the HPS-impregnated mortar containing reactive aggregate over the 3-year period following 7 days of moist curing tended to be smaller than that of the lithium nitrite aqueous solution (L) impregnated mortar and non-impregnated mortar. From these results, HPS was recognized to be effective in inhibiting the alkali-silica reaction for long periods.
The mechanism of alkalinity lowering of a High Fly ash Silica fume Cement (HFSC) under liquid/solid ratio conditions where the pH is largely controlled by the soluble alkali components (Region I) has been studied. This mechanism was incorporated in the chemical equilibrium model of HFSC. As a result, it is suggested that the dissolution and precipitation behavior of SO42- partially contributes to alkalinity lowering of HFSC in Region I. A chemical equilibrium model of HFSC incorporating alkali (Na, K) adsorption, which was presumed as another contributing factor of the alkalinity lowering effect, was also developed, and an HFSC immersion experiment was analyzed using the model. The results of the developed model showed good agreement with the experiment results. From the above results, it was concluded that the alkalinity lowering of HFSC in Region I was attributed to both the dissolution and precipitation behavior of SO42- and alkali adsorption, in addition to the absence of Ca(OH)2. A chemical equilibrium model of HFSC incorporating alkali and SO42- adsorption was also proposed.
In this study, thin mortar specimens were immersed in either ion exchanged water or NaCl solution for 360 days for the purpose of evaluating the relationships between changes in mechanical characteristics and physico-chemical properties caused by Ca leaching. Following the immersion test, bending and shear tests were carried out to determine the elastic modulus, flexural strength, tensile strength, fracture energy, tensile softening curve and shear strength. Furthermore, physico-chemical properties such as calcium hydroxide and porosity were also determined by chemical analysis. Based on these results, the relationships between the mechanical characteristics and physico-chemical properties were evaluated. Bending strength, tensile strength, fracture energy and shear strength were found to be strongly related to the amount of cement hydrate and porosity, both for the ion exchanged water immersed and the NaCl solution immersed specimens.
Biodegradable resin concrete made of biodegradable resin and aggregate was examined from many angles in order to apply it for temporary materials such as sheet pile and pile. The rate of deterioration of biodegradable concrete specimens was evaluated from the results of three-point bending tests and unconfined compression tests. The strength of biodegradable concrete was found to gradually decrease and the relationship between bending strength and curing period was similar to the relationship between void ratio and compressive strength of porous concrete. The fact that aggregate breaks away from resin was confirmed on a broken-out section after a three-point bending test. Based on these results, it is concluded that the degradation mechanism is due to the decomposition of biodegradable resin by microorganisms and the bond strength between biodegradable resin and aggregate.
Considering global warming, reducing the carbon dioxide emissions of the cement industry is a pressing issue. One approach is to replace ordinary portland cement with blast-furnace slag cement type A (BA). However, BA offers inferior resistance to ASR and penetrating chloride ion compared with blast-furnace slag cement type B (BB). In the aim to improve the durability, environmental impact and strength of BA, this study proposes a new cement obtained by adding fly ash and limestone powder to BA to achieve the same level of ASR resistance performance as BB. Further, the mechanism of penetration and immobilization of chloride ion in cement hydrated products are studied.
Accelerated and exposure tests were carried out for concrete made of domestic andesite aggregate in its pessimum composition. In both tests, significant ASR expansion was observed under the condition of 2.60 kg/m3 of total alkali, which is less than 3.0 kg/m3, the upper limit to prevent ASR. In the exposure test, the expansion ratio reached 1250×10-6 at the age of 780 days, showing that the traditional total alkali limit is insufficient for the pessimum condition using highly reactive aggregate and this phenomena could be detected also by acceleration test. Even under the pessimum condition, 15mass% fly ash addition was effective to suppress expansion in the short term. However, longer term behavior should be checked and further investigation is required to conclusively establish the effectiveness of this method. RFID strain sensors were found to be very useful to check the difference in expansion at the surface and in the center of large concrete blocks over long periods of at least two years. Their use allowed measurement of the expansion in the center of a reinforced concrete block having a size of 40×40×60 cm, with the finding that the expansion in the center was much less than near the surface.
In this study, the explosive resistance of fiber reinforced cement composite was examined by conducting contact explosion tests. The efficiency of spall resistance was improved compared to plain specimens by mixing 2.0 vol.% of PVA, PE, PP and steel fiber, respectively. Explosive resistance was also found to be affected by direct tensile performance and bending performance rather than compressive strength. The protective thickness for spalling and perforation proposed by Morishita et al. was found to be lower in the fiber-reinforced specimens. It was also revealed that changes in cavity thickness between 0, 10, 30 and 50 mm affect explosive resistance. In addition, concrete specimens made with cavity thickness larger than 30 mm and PVA+S panels were found to offer better spall protection.
TSA (thaumasite form of sulfate attack) is known as one of the forms of sulfate attack that can occur in a cold environment, weakening the hard tissue of concrete. Practically, thaumasite is generated with ettringite in concrete upon penetration of sulfate ion, which is a component of both. Therefore, an investigation of TSA requires detection of thaumasite distinctly from ettringite. Thaumasite being a member of the ettringite group and thus showing diffraction lines under X-ray diffraction (XRD) quite similar to those of ettringite, distinguishing thaumasite from ettringite is rather difficult. In this study, changing of XRD patterns of thaumasite and ettringite by heat treatment and vacuum treatment was examined, focusing on their crystal water. Although the crystal water of ettringite was easily lost under vacuum treatment, the crystal water of thaumasite remained stable as attested by the observed X-ray diffraction lines. The applicability of this method was examined by using thaumasite produced using cement materials and concrete specimens exposed to a low-temperature, sulfate-rich environment.
By analysis of the data of periodic inspections of national highway bridges in Japan, the actual condition of concrete members and the characteristic of degradation were clarified. Many concrete girders have a cracked skin, which is conjectured to have occurred soon after the completion of construction. Moreover, the data showed that the kind of cracked skin that tends to occur differs for pre-tensioning girders and post-tensioning girders. Furthermore, it became clear that degradation speed differs for differing forms of the component section. Multiple linear regression analysis was conducted using many form-related items, such as the environment conditions, the bridge types and the applied design criteria. Notwithstanding, the results did not allow identification of the dominant factor that influences the degradation characteristics.
This paper presents an experimental study examining the modification effect and penetration mechanism of silicate-based surface penetrants. The penetration mechanism was evaluated through observation of silicate-based surface penetrants absorption under a vacuum condition. The modification conditions were evaluated by the accelerated carbonation test. Based on the results, it is thought that the penetration mechanism of silicate-based surface penetrants is governed by the concentration diffusion. Further, the modification effect of silicate-based surface penetrants appears to be governed by the quantity of calcium hydroxide because the modification effect changed with the water to cement ratio, cement admixture, and the use of auxiliary materials.
This paper presents elasto-plastic and failure analysis of RC structure members. Large-scale solid element FEM is used for elasto-plastic and failure analysis of RC structures. Shear failure problems of concrete structures was run as numerical examples to validate the feasibility of this method. It was shown that this method can reproduce other experiments when the parameters for the failure criteria are determined. The feasibility of elasto-plastic and failure analysis using large-scale solid element FEM is discussed.
In order to elucidate the mechanisms of ASR surface cracking in massive concrete bodies, relative humidity and strains within a massive concrete cylinder in the drying and re-saturating process were measured, observing the formation of surface cracks. Mortar bar expansion tests were conducted for revealing the expansion behavior of mortars in dry atmospheres with various R.H. values and the re-saturating process. Non- or less-expansive layers were formed in near-surface regions in the concrete cylinder in the drying process, but ASR expansions actively progressed in inner portions. As a result, tensile stresses were induced in the non- or less-expansive layers in near-surface regions. When tensile stresses exceeded the tensile strength, surface cracks were produced. After re-saturating, R.H. values of near-surface regions rapidly increased because water intruded into the concrete through surface cracks formed during the drying process. In the re-saturating process, expansions in non- or less-expansive layers were very small, but in the middle portions, whose R.H. values were maintained in the 80% to 90% R.H. range in the drying process, expansion actively progressed, resulting in further development of surface cracks.