Keitetsu Rokugo

Fracture energy and strain softening of concrete as determined by means of compact tension specimens

Fracture mechanics parameters of concrete are determined by means of the compact tension (CT) test. The effects of ligament length, rate of loading and concrete composition on the specific fracture energy GF and the strain-softening diagram are investigated. As a first approximation of the real softening behaviour of concrete, a bilinear strain softening diagram is used in a finite-element analysis. A parameter study shows that several bilinear diagrams can represent the real behaviour equally well. With the bilinear softening diagram, a good agreement between both calculated and measured load-displacement curves and GF-values is obtained. The determined strain-softening diagrams are transformed into a normalized presentation. For each investigated testing condition, characteristics shapes of this normalized strain-softening diagram are obtained.

Tension softening diagrams and evaluation of properties of steel fiber reinforced concrete

Abstract For a better understanding of the fracture behavior of concrete structures, knowledge of the post-cracking behavior of concrete material is essential. The tension softening diagram can describe the post-cracking behavior of concrete in tension. In this paper, the properties of various kinds of steel fiber reinforced concrete (SFRC) were evaluated by means of the tension softening diagrams, after discussion of the applicability of the fictitious crack model to SFRC. The fracture energy of conventional SFRC was independent of the specimen size. For the evaluation of the properties of SFRC with different matrix strength, the shape of the tension softening diagram and the fracture energy were superior to the flexural strength. The fracture energy of SFRC with high strength matrix was dependent on the tensile strength of the steel fiber.

Application of tension softening diagrams to evaluation of bond properties at concrete interfaces

Abstract The bond properties at joint interfaces in concrete structures were evaluated through tension softening diagrams. For the joint surfaces between old and new concrete, the conditions to obtain good bond properties were clarified. The size effect on the flexural bond strength was well predicted through numerical analysis with tension softening diagrams. For the interfaces between repair materials and substrate concrete, the properties of repair materials and the conditions of interfaces were characterized by the shape of the tension softening diagrams and fracture energy. The shrinking and cracking behavior of repair materials was also investigated through experiments and numerical analysis.

Influence of Rapid Freeze-Thaw Cycling on the Mechanical Properties of Sustainable Strain-Hardening Cement Composite (2SHCC)

This paper provides experimental results to investigate the mechanical properties of sustainable strain-hardening cement composite (2SHCC) for infrastructures after freeze-thaw actions. To improve the sustainability of SHCC materials in this study, high energy-consumptive components—silica sand, cement, and polyvinyl alcohol (PVA) fibers—in the conventional SHCC materials are partially replaced with recycled materials such as recycled sand, fly ash, and polyethylene terephthalate (PET) fibers, respectively. To investigate the mechanical properties of green SHCC that contains recycled materials, the cement, PVA fiber and silica sand were replaced with 10% fly ash, 25% PET fiber, and 10% recycled aggregate based on preliminary experimental results for the development of 2SHCC material, respectively. The dynamic modulus of elasticity and weight for 2SHCC material were measured at every 30 cycles of freeze-thaw. The effects of freeze-thaw cycles on the mechanical properties of sustainable SHCC are evaluated by conducting compressive tests, four-point flexural tests, direct tensile tests and prism splitting tests after 90, 180, and 300 cycles of rapid freeze-thaw. Freeze-thaw testing was conducted according to ASTM C 666 Procedure A. Test results show that after 300 cycles of freezing and thawing actions, the dynamic modulus of elasticity and mass loss of damaged 2SHCC were similar to those of virgin 2SHCC, while the freeze-thaw cycles influence mechanical properties of the 2SHCC material except for compressive behavior.

Steel Ring-Based Restraint of HSC Explosive Spalling in High Temperature Environments

This paper reports on an experimental study regarding the behavior of restrained high-strength concrete in response to the type of extreme heating associated with fire. The study was intended to support estimation of thermal stress from the strain in a restraining steel ring and vapor pressure in restrained concrete under the conditions of a RABT 30 rapid heating curve. The size of the specimens was φ300 X 100 mm, and the results showed that explosive spalling occurred between 4 and 10 minutes in terms of heating time. It was also observed that the thermal stress was greater than the vapor pressure value of 0.1 MPa at a point 10 mm from the heated surface at 5 minutes. The maximum spalling depth was about 61 mm. It was inferred that spalling behavior caused by thermal stress may become predominant under restrained conditions.

Chloride Proofing Performance and Rebar Corrosion Protection Performance of SHCC Having Bending Cracks

The chloride penetration depth and the corroded area on rebar were investigated for SHCC test specimens prepared with Portland cement on its own, and with 25% replacement with limestone powder. After being given bending cracks, the specimens were subjected to wetting and drying cycles of chloride solution by a ponding test. It was found that maximum chloride penetration depth was affected by the cover depth and the deformation applied to the test specimen. The corroded area on the rebar was correlated to the number of cracks in the SHCC and the total of the crack widths. For the same number of cracks and total width of the cracks, the corrosion area was greater in the mix with limestone powder.

Application of SHCC

Utilizing concept for SHCC as a new material, SHCC material evaluation for application, and actual application examples are presented in this chapter. SHCC has been applied to newly constructed structures, making use of its excellent mechanical performance. It has also been used for surface repair of existing concrete structures making use of its finely distributed cracking behavior. Appropriate use of the tensile performance can work out a structural component excellent in both durability and mechanical performance.

Shock-Absorbing Blocks Made of HPFRCC for Better Girder-End Structures

The authors intend to propose shock-absorbing blocks to be placed at the girder-ends of bridges with the aim of reducing the joint gaps. These blocks are made to have an effect of alleviating possible colliding forces between main girders and the parapet walls of abutments during an earthquake, as well as an effect of restraining the displacement of superstructures. The post-earthquake restorability of these blocks is also considered. Static loading tests in horizontal and vertical directions were conducted on these blocks to investigate their properties. These shock-absorbing blocks showed excellent deformability and were found to be capable of reducing the girder-end joint gaps.

Securing Bridge Girder-End Gap during Earthquake Using Large-Deformation Blocks

The girder-end gap of a bridge should preferably be narrowed from the standpoint of the durability of the expansion joint and noise reduction but is required to be widened, when seismic isolation supports are used, from the aspect of preventing collision. This study began with research into noise generated from expansion joints and the load-bearing capacity of the parapet walls of bridges. Special blocks were then fabricated using a HPFRCC and expanded polystyrene. These are capable of bearing vertical loads including wheel loads on a normal basis, readily having large deformation in the bridge longitudinal direction during an earthquake to follow the sway and slide of the girders, and being easily exchangeable after an earthquake to make the bridge highly restorable. The blocks, which are highly anisotropic in terms of mechanics, were found to provide the expected functions, being deformable in the bridge-axis direction by more than 20%. The inclusion of fibers was effective in preventing scaling from fractured blocks, and the use of expanded polystyrene enabled the blocks to recover more than 50% of their ultimate deformation.

Applications of SHCC in Japan – Tools and Tips for Promoting its Use

This paper reviews the progress of SHCC applications in Japan and describes suggestions for spreading the use of SHCC. An example of the steps of research activities to be taken for utilizing a new cementitious material is described. As the first step, fundamental and reliable research data should be accumulated to establish the production and construction methods. As the second step, assessment test methods for characteristic performances should be established, while formulating draft recommendations for design and construction incorporating the performances. As the third step, ideas for application making use of the performances should be proposed, and application should be carefully commenced, beginning with small-scale projects. The results of a hearing survey are reported regarding obstacles to the application of new maintenance techniques. Major obstacles for engineers of the ordering parties include the troublesome task of explaining the necessity of adopting the new technique and the issue of responsibility when troubles occur. When developing a cementitious material with high performance, the following conditions should be met: (a) the material is clearly competitive in performance and (future) cost in comparison with conventional materials; (b) its characteristic material performance can be simply quantified and incorporated in the design; (c) a construction method to ensure the effect of its characteristic material performance has been established; and (d) it is an effective material to cope with a serious problem that is present.