We studied the relationship between the strength development and hydration reaction in concrete containing silica fume and having an actual strength of 150N/mm2 or more. By the age of 91 days, concrete that had been exposed to a high-temperature history at an early age by a simple adiabatic curing method was some 20 to 30N/mm2 stronger than concrete that had been cured by the standard method. This outcome was the opposite to that exhibited by concrete without silica fume and having a compressive strength in the range of 60 to 80N/mm2. In concrete with such a low water-binder ratio, the water content required for hydration was so small that it took one year or more by the standard curing method to develop the strength according to decrease in water-binder ratio. On the other hand, we considered that when structural concrete is exposed to a high-temperature history at an early age, the reaction of the silica fume increases remarkably, to form an extremely dense, hardened composite at an early age, thereby even at the age of 91 days, further significant difference in strength between this concrete and concrete cured by the standard method still remained. The reaction ratio of the silica fume in concrete cured by the simple adiabatic method for 7 days was higher than in that cured by the standard method for 1 year, which corresponds to the trend in strength development.
Almost all plastic spacers for reinforcing bar using at construction sites are wheels in shape and play an important part in reinforced concrete structures. As the plastic is lower than the concrete in the strength and elastic modulus, and divide concrete partially, plastic spacers are weak points of reinforced concrete structures. What is worse, one column of thermal expansion coefficient is big and thereby they produce cracks in concrete. We devised the new star-shaped plastic spacers and inspected them by experiments. As a result, what we understood are as follows. (1) Cracks by thermal stress of the new star-shaped plastic spacers are reduced to half in comparison with that of wheel-shaped plastic spacers. (2) It may be said that wheel-shaped plastic spacers must be installed perpendicularly. However, in a good thing the new star-shaped plastic spacers can be installed horizontally.
As the Ultra High Strength Fiber Reinforced Concrete (UFC) does not need to be reinforced by bars, thin plates can be produced and applied for the concrete structures. The UFC plates can effectively extend the applications to the shell structures as well as the panel members and the lining members for tunnel. In this study, the flexure and shear loading experiments have been carried out under the various support conditions to clarify the load-bearing capacity and the fracture behavior of the UFC thin plates. The load bearing mechanism of the UFC plates was examined through various simple calculations and the non-linear FEM considering the tension softening behavior. The behavior and applicability of UFC pre-cast panels joined by different methods were also compared experimentally.
A simplified 1-mass 3-degrees-of-freedom model, named Equivalent Single-Story Model, is proposed for multi-story unsymmetric buildings with regular elevation and brace-type steel seismic control devices in this paper. In the examples, the nonlinear response of four and seven story unsymmetric frame building models with ductile structural walls rehabilitated by brace-type steel seismic control devices obtained from three-dimensional frame building model are compared with those of proposed models. The results show that the proposed simplified model can satisfactorily estimate the maximum story drift and the cumulative plastic ductility ratio of seismic control devices.
In our previous study, an investigation on the feasibility of using BOTDR-based distributed fiber optic strain sensors with a minimum spatial resolution of 1 m on crack monitoring of RC structures was reported. In the present paper, we examine the effectiveness of PPP-BOTDA (pulse-prepump Brillouin Optical Time Domain Analysis) based distributed fiber optic sensors with a higher spatial resolution of 100mm level, which has been developed newly. Through experimental investigations on the crack initiation, propagation and distribution of two PC beams under static loading and accelerated corrosive action on PC reinforcements by using the PPP-BOTDA based sensors, it is confirmed that the locations of different cracks with about 100mm spacing can be identified individually. And the small strain changes during the occurrence of cracks can be also monitored. Moreover, it is also realized that the overall bonded optical fiber on concrete surface still can not be used to measure quantitatively the cracks even the spatial resolution of PPP-BOTDA has been improved to the level of 100mm.
There is a construction method which uses precast concrete members with very high strength in the 150 N/ mm2 class and connects each member by the joint mortar. The purpose of this investigation is to develop mortar which is injected into the joint. It is needed for the mortar to have several capacities, such as fluidity and strength almost equal to the strength of precast concrete members without special curing. By the experiment that changed the water cement ratio or the sand cement ratio, compressive strength reached until 182-day up 200 N/mm2. A satisfactory filling for the joint, a narrow section with a 3 cm opening for instance, was ensured without detecting any flaw or unfilled part when the fluidity of the mortar was controlled with the flow time of J14 funnel.
To find out the re-deterioration mechanism by the drying shrinkage at patch repair region, moisture diffusion coefficient of repair materials which is on the drying temperature and polymer-cement ratio was measured by the Bolzmann-Matano analysis method. Re-emulsification type polymer was adopted in the polymer modified cement mortar (PCM) used as repair materials. The Two-dimensional coupled analysis (FEM) of the moisture diffusion, volumn change by moisture, and mechanical properties was attempted to estimate qualitatively the stress increment occuring at the patch repair region such as repair materials, substrate and interface. The boundary condition is real environmental data of Sapporo, Tokyo and Naha. The cracking mechanism after repair and the selection method of repair materials were discussed.
To find out the re-deterioration mechanism by the water absorption and the constraint stress evolution at patch repair region, water absorption coefficient of repair materials which is on the relative moisture content condition was measured by the Bolzmann-Matano analysis method. Re-emulsification type polymer resin was adopted in the polymer modified cement mortar (PCM) used as repair materials. The Two-dimensional coupled analysis (FEM) of the water absorption, volume change by moisture, and mechanical properties was attempted to estimate qualitatively the stress increment occurring at the patch repair region such as repair materials, substrate and interface. The cracking mechanism after repair and the selection method of repair materials were discussed.
Ultrasonic testing of a reinforced concrete member is affected by the rebars embedded in the concrete. With the surface transmission of the pulse, there is some uncertainty regarding the exact length of the transmission path. An investigation of the pulse velocity of the rebar, the effects of the rebar and the exact transmission path length of the pulse has been made, using the bar diameter and the water/cement ratio as the variables. It is found that the pulse velocity of the rebar is under the diameter of the rebar, and the rebar appears to act compositely with surrounding concrete in transmitting ultrasonic pulse. If there is no effect of the rebar on the transit time of the pulse, the slope of the time-distance line is the mean pulse velocity. A method that examines the presence of an effect of the rebar is proposed.
The influence of limestone powder (LSP) addition on the optimum gypsum (SO3) content for Portland cement with different Al2O3 content was investigated. When setting time, compressive strength and size stability were considered, the optimum SO3 content for cement having Al2O3 content of 3.5-7.2 mass% was varied from approximately 3 to 5 mass% positively depending on the Al2O3 content of cement. The influence of LSP addition on the optimum SO3 content was negligible. By the analysis of combined water, porosity, and phase composition by XRD/Rietveld method, at less SO3 content than the optimum content, with the increase of SO3 content, the cement hydration was promoted mainly in Ca aluminate and the compressive strength increased. At the more SO3 content, excess formation of expansive ettringite introduced more pores and compressive strength decreased. By the addition of LSP, the maximum compressive strength was obtained at lower SO3 content. This is attributed to the hydration acceleration for silicate minerals and increase of carbo-aluminate hydrates content. This effect was significant for cement with high C3A and C4AF content.