The Miyagi Prefecture Earthquake in 1978 was the first earthquake to cause serious damage to railway concrete structures in Japan. This was followed by the South Hyogo Prefecture Earthquake in 1995, which caused shear failure of columns of RC viaducts carrying the Shinkansen (Bullet Train) and old railroad lines predating the 1983 seismic design standard. As the result of various concrete structures sustaining extensive damage never experienced before, the seismic design standard was greatly revised, and the seismic rehabilitation of existing structures was started on a full scale. Basic concept of seismic rehabilitation is to prevent shear failure of columns that led to the collapse of RC viaducts and bridges catastrophically occurred in whole structural frame during the past large earthquakes. To enhance the seismic capacity of the whole structural frame, increase in ductility of each column through seismic rehabilitation capable of absorbing seismic energy is needed. As the space under many railway viaducts is used by stations and shops, negotiations regarding the relocation of businesses, the removal of large obstacles such as heavy machinery, and the development of valid seismic rehabilitation methods to reinforce a large number of massive columns were called for. At present, we are trying to enhance seismic capacity as much as possible for new structures in order to prevent serious damage and enable early restoration.
This paper proposes an economical structural system that reduces seismic damage and needs little or no repair by combining precast prestressed concrete elements and corrugated steel panel dampers. Precast prestressed concrete structures show high self-centering characteristics with negligible damage. However, the lateral displacement response during earthquakes tends to be larger than ordinary reinforced concrete (RC) structures because of their lower hysteretic energy dissipation capability. Corrugated steel panels attached to a moment-resisting frame improve its seismic performance with high energy dissipating capability as a hysteretic damper. Five portal frames with corrugated steel panel dampers were tested to investigate the hysteretic characteristics of the proposed hybrid system. Experimental variables were the type of frame structure and the yield strength of the corrugated steel panels. All precast prestressed concrete frames showed a sufficient amount of energy dissipation, and much smaller residual deformations and damage than the monolithic RC frame. Superposition of the simulated hysteretic loops of the frames with that of the damper agreed well with the experimental results obtained by reversed-cyclic loading tests. Using a simple calculation method to estimate the equivalent viscous damping ratios and residual displacements, a design procedure seeking the optimization of the hybrid system is examined.
This paper presents a nonlinear finite element analysis of substandard reinforced concrete beam-column joint strength-ened by cast in-situ expansion. The nonlinear finite element analysis is based on two-dimensional reinforced concrete planar elements to simulate beam, column, joint panel and joint expansions. The one dimensional discrete joint element is employed to simulate local deformation at the interface between members and expansions. Five specimens, control specimen and four strengthened specimens have been analyzed. The analysis correctly reproduces load-drift ratio relations including hysteretic loops, deformed shape, cracking process and failure mode. The load resistant mechanism of the strengthened specimens is also investigated. The strain profile of steel beam bar is examined to understand the transmission of forces in expanded joints. The principal compressive stress is analytically computed in the joint panel zone and expansion parts. The analysis illustrates the diagonal strut as the main load resistance in the joint panel. For strengthened specimens, the plot of analytical principal stress identifies two load bearing mechanisms. In addition to the main diagonal strut in the joint panel, the inclined strut mechanism also forms along the edge of the expansions. The size of expansion has a direct impact on the capacity of this inclined strut. The finite element analysis is also applied to exterior beam-column joints. Similar conclusions as the interior beam-column joint can be drawn.
The authors have developed a method to evaluate the residual seismic capacity of reinforced concrete structures damaged by earthquakes, which is employed in the Guideline for Post-Earthquake Damage Evaluation and Rehabilitation. Presented in this paper are outlines of the damage rating procedures based on the residual seismic capacity index R, defined as the ratio of residual seismic capacity to original capacity. The procedure was applied to low-rise reinforced concrete buildings damaged in recent earthquakes in Japan including the 1995 Hyogo-Ken-Nambu (Kobe) Earthquake and its validity was discussed. Good agreement between the residual seismic capacity index R and damage levels observed in damaged buildings was found. Moreover, nonlinear seismic response analyses of single-degree-of-freedom (SDF) systems were carried out. It was observed that the residual seismic capacity of a damaged RC building structure can be evaluated conservatively according to the R index employed in the Guideline, and from a practical point of view, the R index is an effective way to identify the safety of damaged structures against aftershocks.
A prestressed concrete brace system was first proposed in 2001 to dramatically simplify seismic strengthening procedures. Since the proposed system does not necessitate any steel bar anchorage, the construction period is shorter and the construction cost is lower. The system has been revised in terms of its aesthetics, materials, and construction methods since the first proposal of the system. This paper introduces a series of prestressed concrete brace system and discusses their interesting features. Design procedures and some examples are also introduced with experimental results.
The fluidity and flow mechanism of suspensions containing low-heat Portland cement, limestone powders with different characteristics, and a polycarboxylate-based superplasticizer have been studied. Limestone powders with six different degrees of grinding were prepared using a ball mill and a jet mill. The circularity of the limestone powder particles was determined by analyzing scanning electron microscope (SEM) photographs of the particles using an image analyzer and the point counting method. A computer program for determining the packing fraction of powder particles directly from the particle size distribution was written and the packing fraction was calculated. It was made clear quantitatively that the packing fraction and circularity of limestone powder particles significantly affected the fluidity of the suspensions. The addition of finely ground limestone powders prepared by jet mill increased the fluidity of suspensions. The fluidity of low-heat Portland cement paste with finely ground limestone powders prepared by jet mill at a water-to-powder ratio of 0.225 is as same as the fluidity of low-heat Portland cement paste at a water-to-powder ratio of 0.30. The fluidity of a suspension could be approximated with accuracy using the packing fraction and the circularity of powders. Particles with a circular shape and a high packing fraction result in increased fluidity. The ratio of the contribution made by the packing fraction to this improvement in fluidity to that of shape ranged from 1:1 to about 1:2.
In the present study, early-age heat evolution of over 120 mortar mixes was monitored for 24 hours after mixing using a simple isothermal calorimetry device. This paper describes the processes of calorimetric characterization and performance prediction of these mortar materials. The features of the heat evaluation curves obtained from the calorimetry tests and the effects of the mortar materials, mix proportions, and curing conditions on the heat evaluation curves were studied. A derivative method was employed for exemplifying the physical changes in the tested materials from the heat evolution curves, and the results were compared with set times from ASTM C403 tests. The study indicates that the simple calorimetry technique can be used to flag changes in cementitious materials, pre-screen concrete materials and/or mix design, identify incompatibility of cementitious materials, and forecast setting time and early-age strength.
As the use of industrial waste as a raw material for the production of cement is expanded, it will become increasingly important to control the adiabatic temperature rise of cement. The evaluation of adiabatic temperature rise in concrete is not adequate for daily quality control because it requires considerable labor. In order to establish a convenient method for evaluating adiabatic temperature rise using only about 30ml of mortar sample, we produced an experimental adiabatic calorimeter. This paper investigates the accuracy and the applicability of this equipment. In addition, the adiabatic temperature rises of cement samples with different C3A content were investigated for future utilization of industrial waste. Values of adiabatic temperature rise measured using this equipment were found to correspond to those obtained using existing equipment, with correction for the heat capacity of the sample container. Clear differences in adiabatic temperature rise were observed among mortar samples with different blending conditions. Changes in adiabatic temperature rise were negligible even when the aluminate phase content of the studied cement was increased from 9 mass% to 12 mass%. These results suggest that quality control of cement and preparatory experiments could be practiced reasonably by using this adiabatic calorimeter.
Concrete undergoes deterioration over time, and in extreme cases, this can lead to degradation of the structure. The collection of samples of actual structures that have remained sound over long periods of time and the detailed evaluation of their properties are extremely important in terms of providing empirical data regarding deterioration over time. In the present study, concrete collected from two structures completed around 1940 underwent naked eye observation, estimation of mix proportions, evaluation of mechanical properties, EPMA mapping analysis, measurement of pore size distribution, etc. Both concretes were considered to have been worked carefully and exhibited extremely little deterioration.
A modification of the testing procedure was adopted to improve the PERMIT ion migration test method, a non-destructive method to evaluate the quality of concrete in real structures. The steady state migration test procedure was reanalyzed from the viewpoint of electrolysis. The peak current obtained in the PERMIT ion migration test correlated well with the chloride diffusivity, the theoretical relationship between these was discussed and an equation for calculating the effective chloride diffusivity in concrete from the peak current was determined. The new process was found to be quicker and more effective than the typical process of the PERMIT ion migration test and to simplify the test procedure. As an application example, the effect of different controlled permeability formworks for improving the resistance of cover concrete to the penetration of chloride ions was investigated using the modified PERMIT ion migration test.
Concrete has a good reputation for fire resistance because it has low thermal conductivity and is non-combustible. However, concrete loses strength when exposed to elevated temperatures as a result of damage to the pore structure and chemical degradation of the calcium silicate hydrate. Reports on strength loss due to fire exposure are ubiquitous in the literature. However, there have been limited reports on the changes in the pore structure, which greatly affects the durability of the concrete. In cases where the strength is sufficient for the structural element to remain in service, other considerations, such as the durability of the structural element comes into play. The ingress of aggressive agents is typically through means, such as water, which leads to sorptivity being a particular important transport property of the degraded concrete. The sorptivity of the concrete will depend on the age of the sample at the time of damage, the cement content, w/c of the original mix design, as well as the length of time the damaged concrete has been re-exposed to water. These properties are reported along with mechanical properties to better demonstrate the complexity in the relationship between transport properties and strength. Furthermore, sorptivity can become crucial to predicting long term durability as well as identifying potential repair mechanisms.
The advanced transient concrete model (ATCM) is an extended model for concrete in compression at elevated temperature that incorporates elastic, plastic and creep strain as a function of temperature and stress history. The ATCM is applied with the material model of the thermal induced strain model. The non-linear model comprises thermal strain, elastic strain, plastic strain and transient temperature strains and load history modelling of restraint concrete structures subjected to fire. The mechanical strain is calculated as a function of elastic strain, plastic strain and thermal induced strain. The thermal induced strain is relative independent compared to dependence of Young's modulus by load history. Actually the term comprises elastic, plastic and (pure) transient creep strains as we will show. A comparison is given between experimental results with cylindrical specimens and calculated results. The equations of the ATCM consider a lot of capabilities, especially for considering the irreversible effects of temperature on some material properties. By considering the load history during heating up, an increasing load bearing capacity due to a higher stiffness of concrete may be obtained. With this model it is possible to apply the thermal-physical behaviour of material laws for calculation of structures under extreme temperature conditions. The effect of load history in highly loaded structures under fire load will be investigated. The theoretical basis is given in this paper.
Bridge decks damaged by both internal actions and environmental actions need to be repaired in order to restore the initial safety level. The usual repair procedure requires removing the damaged concrete, introducing additional reinforcement and shear connectors, and final concreting of a new layer. The efficiency of repair with the above procedure is investigated in the present paper by means of an experimental program on seven repaired real-scale slabs, on which both static and fatigue tests were performed. The specimens are representative of a typical transversal strip of bridge deck. The numerical interpretation of the experimental results confirms the good overall behaviour of the repaired beams, putting in evidence the possibility to reduce the shear connectors by use of self compacting concrete for the new casting.
Stress analysis of RC structures that have been retrofitted by an externally bonded layer of Polymer Cement Mortar (PCM) based materials requires the development of a constitutive model of the PCM-concrete interface under static and fatigue loading. In this paper, the bending fatigue and shear fatigue tests were conducted to determine the interface fatigue degradation properties by using the J-integral method based on the energy equivalence. Substrate concretes with four different compressive strengths and interface roughness were applied to understand the effects of substrate concrete and interface roughness on the fatigue degradation properties. It was found that the failure characteristics of the PCM-concrete interface is similar to those of cementitious material and the failure modes of PCM-concrete interface has an important effect on the interface tensile and shear degradation relation and S-N relation under fatigue loading. The degradation relation is similar in the case that the failure mode is identical, regardless of the type of substrate concrete. The fatigue resistance of the PCM-concrete interface is inferior to that in the concrete cohesion layer.
Highly inelastic nonlinear interaction of liquefied soil and underground RC ducts is computationally investigated in view of the structural damage. The required ductility is expected to be insensitive to the risk of liquefaction for normally deposited layers of soil, and the lesser ductility is acceptable for the case of highly liquefiable foundation similar to the seismic isolation. Although the structural nonlinearity has a fewer effect on the uplift of the underground ducts, the amount of main reinforcement may control the structural damage with the same efficiency for both drained and undrained soil deposits.