High Performance Concrete is a material that was regarded as “academic” for quite a number of years. Now, the profits of this material are becoming to be recognized. The high compressive strength is not the only advantage of this material. The fibers lead to small crack distances and give the material large ductility. The very dense material structure can as well result in high durability. This makes the material suitable for the design of lightweight slender structures with a long service life, as well as surprising architectural structures. On the other hand the material is appropriate for repair of structures, such as bridge decks. First applications show convincingly a large potential. At this moment an international committee (fib Task Group 8.6) works on producing an international recommendation.
Sub-working group 1 of the Japan Concrete Institute (JCI) committee on Utilization of High-Strength Concrete (HSC) and High-Performance Concrete (HPC) (JCI-TC063A) has been working on collecting information from research on HSC and HPC as well as their practical use. The committee published a report in 2006 in Japanese. The objective of the report is to present state-of-the-art information on concrete with strengths in excess of about 60 MPa and high-strength steel reinforcement such as prestressing steel, excluding concrete made with atypical materials, such as fibers, or uncommon techniques. It primarily addresses the mechanical properties of high-strength concrete and high-strength steel reinforcement. A concise digest of the report has already been published as a keynote paper in the proceedings of 8th International Symposium on Utilization of HSC and HPC in October 2008 in Tokyo. This paper is based on the keynote paper with additional information on High Strength Steel, HSS, reinforcing bars in Japan.
Concrete cylinders with a design strength of 100 to 150 N/mm2 were subjected to thermal histories with different maximum temperatures simulating actual structures to examine the development of their mechanical properties. The tendencies of the mechanical properties of concrete subjected to a maximum temperature exceeding 45 to 60° C were found to significantly differ from those of concrete cured at lower temperatures. The authors thus propose a technique for estimating the compressive strength of concrete from the maximum curing temperature and effective age. We also report on the investigation of the applicability of conventional prediction equations to the evaluation of static modulus and splitting tensile strength using compressive strength.
High-rise buildings are subjected during severe earthquakes and strong winds to intensive large axial and lateral loads, particularly at their lower stories, where the exterior columns experience varying high axial loads. Herein, the seismic performance of two precast high-strength reinforced concrete exterior beam-column joints subjected to different varying high-axial levels is investigated while premature shear failure is generally suspected. High-grade steel bars were used for reinforcement. Splice grout-sleeves and mechanical anchors were used in the columns and beams, respectively. The strength of the concrete was 70 MPa. The maximum axial tension level in columns was 90% of the yield strength of the main bars. Under high axial tension, the tested subassemblies exhibited stable response, good damping characteristics, appropriate lateral force resistance and energy absorption capacity. They performed well within the lateral story drift angle of 3% where the successive failure modes (beam/column flexural yielding followed by joint shear failure) confirmed the appropriateness of the design equations in AIJ and ACI regulations. Furthermore, the use of mechanical anchors proved to be very effective and no sign of concrete crushing (pushing toward the exterior face of the joint) was observed within the lateral story drift angle of 2%.
Assembled precast members and related connections should be economically feasible, bring ease of construction, and provide acceptable static properties as well as adequate dynamic characteristics in high seismic zones. The seismic performance of an assembled precast high-strength concrete beam with a simple and innovative lap splice connection in high-rise buildings is discussed. The flexibility variation along the lap splice connection of the beam, which involves a reduced profile, is also investigated. The lap splice connection, located at beam mid-span, was connected by transverse untensioned bolts. The simplicity of the form and developed mechanism are intended to be suitable for construction sites. The experimental test results confirmed the adequacy of such assemblies to satisfy a safe level under different loadings. The beam under reversed cyclic loading proved to be ductile and failure occurred outside the lap splice connection similarly to monolithic ordinary reinforced concrete beams. The flexural stiffness varied along the lap splice connection of the assembled beam and declined at the transition section of the reduced profile under large loading. Therefore, this reduction, within design limits, would not affect the performance of the assembled beam.
In order to investigate the self-healing capability of fibre reinforced cementitious composites (FRCC), mechanical properties and surface morphology of crack in FRCC were studied. Three types of FRCC specimens containing (1) polyethylene (PE) fibre, (2) steel cord (SC) fibre, and (3) hybrid fibres composite (both of PE and SC) were prepared. These specimens, in which cracks were introduced by tension test, were retained in water for 28 days. The self-healing capability of the specimens was investigated by means of microscope observation, water permeability test, tension test and backscattered electron image analysis. It was found that many very fine fibres of PE were bridging over the crack and crystallization products became easy to be attached to a large number of PE fibres. As a result, water permeability coefficient decreased and tensile strength was improved significantly. Therefore amount of the PE fibre per volume was indicated to have a great influence on self-healing. Furthermore, by means of backscattered electron image analysis, it was also shown that the difference of hydration degree in each FRCC has only little influence on the self-healing capability in case of the employed test series.
The wedge splitting method according Tschegg has been used to determine the fracture-mechanical values for cubes (e.g. 150 × 150 × 150 mm3) under uniaxial (tension) and biaxial (compression + orthogonal tension) loading. The steel-fibre-reinforced concrete had fibre lengths of 15 to 60 mm and a volume content of 0.5 to 1%. The fracture behaviour and the influence of the fibre length and the fibre content on the fracture parameters can be explained qualitatively using a very simple model. Finally, some aspects relating to the biaxial fracture testing in steel-fibre-reinforced concrete which are of interest to engineers are discussed.
A study of the axial response of plain concrete under varying confining pressure was conducted to examine the sensitivity of confining pressure on the compressive strength and deformability of concrete. The concrete was examined through the use of a novel triaxial cell with pressure controlled through dilation strain feedback. The study examined realistic levels of constant, linear elastic, elastic-perfectly plastic, and bilinear varying confinement typical of fiber reinforced polymer and steel jacketing applications. It was found that the axial stress-strain response of confined concrete is dependent on the variation of confining pressure applied. The assumption of constant confinement is not appropriate for concrete systems jacketed with passive confinement such as steel or FRP materials. These systems must be comprehensively examined under varying confinement using full-scale jacketed specimens or triaxial studies. The use of a triaxial cell was shown to provide an efficient and cost-effective means of evaluating confinement performance.
A unique model for estimating the axial response of concrete under varying or constant levels of confinement is developed. The proposed model is empirically developed from a companion experimental study of concrete tested under linear, elastic perfectly plastic (EPP), elasto-plastic with strain hardening, and constant triaxial confinement. The axial stress-strain response is divided into three regions separated by two characteristic response levels. The model is based on the variation of the confining pressure due to the lateral dilation of the concrete. An estimate of dilation with respect to axial strain is also developed. The model is shown to provide an accurate prediction of historical experimental studies of constant confinement tests on cylinders, fiber reinforced polymer (FRP) jacketed specimens, and columns confined with conventional lateral reinforcement or FRP jacket.
This paper presents an experimental investigation of bond-strengthening hooks as a new method to increase bond strength along flexural reinforcing bars in reinforced concrete (RC) beams. The proposed method attempts to increase confining stiffness around the flexural bars by placing U-shaped hooks and to prevent premature bond splitting failure. Ten specimens with different numbers and sizes of hooks were prepared to verify the strengthening effectiveness under monotonic four-point loading. The test results indicated that the hooks increased the bond strength along flexural bars although the strengthening effectiveness was limited by the effective number of anchors of hooks Nbe. This limit is determined by the size of the stress-transmitting zones of the concrete around the anchors of the hooks. The bond-strengthening effectiveness of hooks was found to be equivalent to that of conventional internal ties.
The effect of an aggressive chemical environment on concrete prepared with ordinary Portland cement and silica fume, either as a binary combination or a ternary combination with fly ash, is investigated in the present study. The adverse environmental conditions are simulated by using either 1% sulfuric acid, 1% hydrochloric acid or 1% nitric acid. The corrosion process was monitored by measuring the mass loss and compressive strength for a period of one year. It was found that the course of action of acid attack is dependent on the type of acid and solubility of the calcium salt formed. The presence of mineral admixtures was found to lower the detrimental effect of all types of acids on concrete. Ternary mixes with OPC, silica fume and fly ash performed better than binary mixes containing only silica fume as supplementary cementitious material.