A method for estimation of the thickness of transition zone is proposed by calculating the coarse pore volume in transition zone from discrepancy between pore size distribution of hardened mortar and concrete and hardened cement paste using specific surface area of aggregate. Thickness of layer of coarse pore in transition zone of mortar and concrete at the age of 28 days is calculated at 14.6μm and 12.3μm, respectively, and average thickness of transition zone in mortar and concrete is estimated at 30μm and 25μm, respectively by the method proposed. These values coincide well with those determined by SEM observation. Transition zone in mortar and concrete is generally formed at 3 days after placing mortar and concrete and increases in its thickness reaching the maximum value at 7 days and gradually decreases in its thickness with age although the change is not remarkable especially in concrete. Estimation of development strength and explanation of the difference of strength development between cement paste, mortar and concrete can be obtained from the time dependency of thickness and pore structure of transition zone.
This report made a test study by aiming to get basic data concerning the choice and mixing proportions for aggregate of ultra high-strength concrete (about 90-130 MPa). The results are shown as follows, Compressive strength (of ultra high-strength concrete) is influenced by compressive strength of rock and volume of 400 kN crushing values for coarse aggregate when fixing strength of mortar to low water-to-cementitious ratio. When 400 kN crushing value of aggregate is 11-23 % and water-to-cementitious ratio is less than 0.25, this strength is concluded for material and mixing proportions of ultra high-strength concrete. Compressive strength of concrete increase with decrease in maximum size of coarse aggregate, but optimum maximum size varies in kinds of coarse aggregate. The slopes of the curves in the region subsequent to maximum stress increase with decrease in compressive strength of rock and increase in grading of coarse aggregate and we can see sudden brittle stress fracture developed in concrete. In order to get compressive strength of ultra high-strength concrete, it is effective to raise compressive strength of rock and crushing strength and grading (maximum size) of coarse aggregate and bonding between coarse aggregate and mortar and at the same time to bring strength and deformation of aggregate close to strength of aggregate and deformation of matrix mortar or increase it.
This paper presents a method to determine values for material factors, load factor and structure factor for designing reinforced concrete members on limit state design method. In the proposed method, structure safety is evaluated with probability of failure using probabilistic treatment on load, concrete strength and steel strength. And furthermore, economy is evaluated with Fuzzy Set in order to determine the range of a variety of reinforced concrete members designed by applying same set of values for safety factors. As every value for the factors shown in this study is near standard values shown in some design codes, this method is considered to have reached the stage for practical use.
In this paper, a mechanical model for the design of plain concrete and fiber reinforced concrete shear key joints is developed. The method makes use of well-known results of fracture mechanics and truss model theory, combined in a simple model. The model is in good agreement with push-off shear test, and a nonlinear FEM analysis results. Furthermore, the involvement of the proposed model in the current design of segmental structures is investigated. A nonlinear FEM analysis is carried out, emphasizing the effects of the load-displacement characteristics of joints on the behavior of segmental structures to demonstrate the specified contribution of the model.
During an earthquake structures are subjected to dynamic loading. The strength and mode of failure of RC members are influenced by high strain rate. However, there is no sufficient data available to fully document the effect. In this study, column specimens were tested during either static or dynamic horizontal loading under the high axial load ; the effect of strain rate on strength and mode of failure of RC members was investigated. The results obtained in this study could be summarized as follows : 1) The strain rate just prior to the maximum strength value was about 2×104 μ/sec during dynamic loading ; the maximum strength and mode of failure were influenced by the strain rate. 2) For flexural failure, the crack patterns during dynamic loading were slightly different from the ones during static loading ; however, the modes of failure were similar. For shear failure, the crack patterns, the processes of failure and the degree of damage during dynamic loading were different from those during static loading. 3) For the effects of strain rates, the values of initial stiffness during the dynamic loading were 10 % greater than those during static loading ; the values of stiffness at the yielding of members were 15 %, the maximum strengths of flexural failure were 10 % and of shear failure were 25 % greater than those during static loading. 4) When material strengths were increased with the increase of the strain rate, the ratios of the experimental values to the calculated values during dynamic loading were similar to those during static loading. 5) For flexural failure, in the plastic zone the strengths were also influenced by strain rates ; the strengths during dynamic loading were 10 % greater than those during static loading.
Fracture energy has been used as a material parameter to describe the crack resistance of concrete. It is known, however, that the fracture energy depends on specimen size even if the mix proportion of concrete is the same. In this paper, the relation between the fracture properties of concrete and the micro structure, and the size effect are discussed. Three different mix proportions of concrete and three different specimen sizes are used, where the quality and the volume of coarse aggregates are kept constant. Furthermore, the applicabilities of 'size effect law' and 'two parameter fracture model' which have been noticed as potential approaches to predict the size effect of concrete structures, are discussed.
It is necessary to clarify how concrete will be degraded for super long term, when thinking of the use of concrete for construction of nuclear waste repositories. In order to develop an acceleration test method for predicting deterioration due to dissolution of extremely small amount of cement hydrate, the authors examined the amount of Ca2+ dissolved in water and the deterioration of the hydrated cement structure under electrical potential gradient conditions. The results were as follows. (1) Application of potential gradients to the specimen significantly accelerated the dissolution of Ca2+ compared with the result in case of no potential gradient. The amount of Ca2+ dissolved in water was almost proportionate to the amount of potential gradient. (2) As Ca2+ is gradually dissolved, deteriorated region enlarged toward to the interior of the specimen. This deterioration rate is also in proportion to potential gradient. (3) Porosities of the hydrated cement structures increase with above deterioration.
Bending tests were performed on concrete beams reinforced with large-sized deformed bars, designated as D 64, having nominal diameter of 64 mm and screw-type rolled deformation. Welded deformed bar mats were arranged within the concrete cover of the beams in order to control crack widths. It was showed that the additional arrangement of welded deformed bar mat near the concrete surface was effective for crack control of the beams. At the tensile stress of 2000 kgf/cm2 of D 64, the number of crack in the side surfaces of the beams increased by 1.4-2.7 times and the maximum bottom-surface crack width of beams decreased by 0.41-0.66 times those of the beams without the bar mat. These maximum surface crack widths can be estimated approximately by derived equations based on Model Code for Concrete Structures of CEB-FIP (1978).
Steel-concrete composite members are widely installed in structures because of their excellent mechanical characteristics. Under marine environments, however, corrosion of steel plates will occur and may result in initiating cracks in concrete. This paper presents the results of exposure tests under marine environments and their analyses to make clear the corrosion characteristics of the steel plates in composite structures. Corrosion of steel plates was a principal cause to deteriorate composite members and was affected by ambient conditions as splash or tidal zones. Furthermore, it was made clear that corrosion of steel plates occurred by the micro cell on the plate surface. On the basis of the test results, corrosion rates of steel plates are also investigated.
To investigate the bond splitting strength of reinforced concrete members with FRP bars, an experimental program consisting two series of tests was conducted. One is a simple bond test and the other is a cantilever type bond test. Discussions on the bond splitting strength are presented as the strength in case of no lateral reinforcement (τco) and the increment caused by lateral reinforcement (τst), including the results of previous studies. The result show that Young's modulus of longitudinal bar influences bond splitting strength in the both cases. Therefore, a new formula to predict the bond splitting strength for FRP bars is proposed. The result of the previous study on actual beams is verified using the new formula. The observed maximum strengths of the beams show a good correlation with the calculated strengths.
For reutilizing the cement sludge as a concrete waterproofer, the sludge was treated with stearic acid by chemical method. And we compared the performances of the new waterproofers with those of two others which have been used in demostic area. The particles of the new waterproofer are finer than those of standard sand but larger than those of cement. And the surfaces of the particles are coated with metallic soaps which have strong water repellency. Also the exess stearic acid and metallic soaps coated on the sludge react with the Ca++ or Al+++ when mixed with cement mortar. Therefore, the watertightness and the physical properties of the new waterproofers in cement mortar were superior to those of the others.