Methacrylate ester as well as allylether based polycarboxylates (PCEs) were synthesized to plasticize pastes of cement and silica fume having a water/cement ratio of 0.22. Methacrylate ester copolymers were found to disperse cement well, whereas allylether copolymers are more effective with silica fume. Mechanistic investigations revealed that in cement pore solution, the surface charge of silica fume becomes positive by adsorption of Ca2+ onto negatively charged silanolate groups present on the silica surface. This way, polycarboxylate copolymers adsorb to and disperse silica fume grains. Thus, mixtures of both copolymers were tested in cement-silica fume pastes. These blends provide significantly better dispersion than using only one polymer. Apparently, the surfaces of hydrating cement (here mainly ettringite) and silica fume are quite different with respect to their chemical composition. Therefore, PCEs with different molecular architectures are required to provide maximum coordination with calcium atoms present on these surfaces.
The effect of the properties of silica fume on the fluidity of ultra high strength cement paste (UHCP) using a polycarboxylate based superplasticizer, silica fume, and low heat Portland cement was investigated. As the dosage of superplasticizer was increased, the flow curve of UHCP changed from Bingham flow to pseudo-plastic flow to Newtonian flow. Also, the effect of silica fume on the fluidity of UHCP was large, in particular the effect on the shape of powder particles smaller than 0.3μm. It is suggested that this is due to the agglomeration structure, the packing state, and the ball bearing effect of the paste due to the shape of the silica fume.
This paper reports an investigation on the strength properties, a time dependent property as well as durability characteristics of normal and water reduced high strength concrete (WRHSC), with or without rice husk ash (RHA), designed to produce Grade 60 at 28 days. RHA was ‘added’ or ‘replaced’ at/by 5% - 20% of the cement content. A PCE superplasti-cizer was added to all mixes to provide workability in the range of 150 - 200 mm slump. Based on the initial trial mixes, four high strength concrete mixes were selected for further tests on mechanical properties (compressive strength, tensile splitting strength, flexural strength and static modulus of elasticity), drying shrinkage and durability (water absorption, sorptivity, chemical resistance to MgSO4 solution). Incorporation of RHA increases the strength, reduces the drying shrinkage and improves the durability of concrete compared to conventional OPC concrete.
This research study was carried out to identify the behavior of macro-cell corrosion when the embedded steel of concrete specimens is exposed to the effect of homogeneous and non-homogeneous chloride environments. Macro-cell corrosion currents were periodically measured using a segmented steel bar. Results show that macro-cell corrosion can be observed even when the surrounding chloride environment is homogenous. When the chloride environment is non-homogeneous, higher macro-cell corrosion activity was observed between steel segments exposed to different levels of chloride contents. Macro-cell corrosion activity becomes more significant when the difference in chloride content is increased. It was also observed that the macro-cell anodic and cathodic reactions periodically change their overall electrochemical behavior from anodic to cathodic or cathodic to anodic. It is proposed that the time dependent variation of macro-cell corrosion occurs due to the spatial variation of oxygen and moisture concentrations caused by an ongoing corrosion process that is briefly explained in this paper.
Maintenance of reinforced concrete is a major concern around the world. In particular, special attention is now given to chloride induced corrosion, which is considered as one of the most serious causes of concrete deterioration. Since corrosion is an electrochemical process, the influence of temperature on the deterioration of reinforced concrete should be considered. From these backgrounds, the objective of this study is to investigate the influence of temperature on the deterioration process of chloride induced corrosion in reinforced concrete. The results show that the rates of diffusion of substances and corrosion of steel bars rise with increases in temperature and these phenomena are explained by the Arrhenius theory using the concept of activation energy.
The use of Slurry Infiltrated Fiber Concrete (SIFCON) in reinforced concrete corner connections subjected to opening bending moments has been experimentally investigated. An experimental program has been carried out, in which fifteen specimens have been tested; six reinforced concrete joints, one fiber reinforced concrete joint, and eight SIFCON joints. Different reinforcing bars' details and different volumes of fraction of fibers (Vf) have been investigated. It was found that, in all the RC specimens, the joints failed before reaching the capacity of the connecting members. There was also a significant difference in the different joints' efficiency due to the variety of reinforcement details. The use of SIFCON in the joints increased both the joints capacity and ductility. The enhancement of the joint capacity and ductility could reach as high as 66% and 173%, respectively. This is attributed to the ability of the high volume of fibers to effectively bridge the cracks and retard the compression failure of the diagonal struts in the joints. The increase in the amount of fibers was proven to directly enhance the behavior of the SIFCON joints. In joints with Vf=6% and 8%, the joint capacity exceeded the connecting members' capacity, leading to failure in the members before the joints, which is an advantageous requirement of the design.
In this paper, a tension stiffening model based on the bond-slip relationship is introduced and adopted in a finite multilayered shell element formulation for surface structure analysis. The tension stiffening effect evaluated at the meso-level is taken into account in the constitutive law of reinforcement at the macro level by defining a crack element at the Gauss point. The crack element is iteratively analyzed by means of a step-by-step integration, which allows application of any complicated bond laws. To define the crack element, a crack spacing model considers the crack formation grade. As a relevant factor in this tension stiffening concept, the reinforcement cracking stress may be evaluated by taking the fractile value of the concrete tensile strength. Through several simulations, the validity of the concept is systematically in-vestigated under monotonic and cyclic loading. The analysis under cyclic loading shows the effect of the re-contact of the crack flanks. The numerical examples demonstrate the applicability of the applied reinforced concrete model with a tension stiffening effect.
This paper presents an experimental investigation to clarify shear cracking behavior of reinforced concrete beams. The effects of the various influential parameters on the spacing between shear cracks and the relationship between shear crack width and stirrup strain at the intersection with shear cracks were carefully investigated. It was found that shear crack width proportionally increases with both the strain of shear reinforcement and with the spacing between shear cracks. Greater diagonal crack spacings were found in larger beams and hence resulted in wider shear crack width. The test results also revealed that shear reinforcement characteristics (side concrete cover to stirrup, stirrup spacing and/or stirrup configuration) and longitudinal reinforcement ratio play a critical role in controlling the diagonal crack spacings and openings. It was illustrated that the distance of shear crack from the crack tip and the intersection with the nearest reinforcement can significantly affect the variation of shear crack width along the same shear crack. Conversely, the loading paths (loading, unloading and reloading paths) show an insignificant effect on shear crack width-stirrup strain relationship. Finally, the experimental results presented are useful information for the development of a rational shear crack displacement prediction method in existing design codes.
Monotonic shear loading tests were conducted on three half-scaled reinforced concrete deep beams with shear span-to-depth ratios of 0.5 to 0.75. The obtained test results were investigated in detail based on the experimental measurements and finite element analysis. From these investigations, a new macro model for deep beams was established. This model is composed of two crooked main struts formed between both beam end sections and branched-off sub struts. The compressive force introduced to main struts balances the flexural compression and the external shear force. The bond stress of the longitudinal reinforcement and the tensile force of the stirrup produce the diagonal compression in the sub strut. Theoretically predicted shear strengths of tested deep beams showed good agreement with experimentally observed shear strengths, where the effective strength of concrete was assumed to be 75% of the cylinder strength.
Strengthening of reinforced concrete beams using steel fiber reinforced concrete (SFRC) overlays has been investigated in the test program described in this paper. Two methods have been used to connect the overlays to the original beams, i.e. chemical and mechanical bonding. In the chemical bonding, 2-component epoxy resin bonding agent has been used. The mechanical bonding was achieved by welding the stirrups in the overlays to the stirrups in the original beams only near each support. In general, the weld bonded strengthened beams have achieved a better structural behavior in terms of load carrying capacity and failure mode compared to the epoxy bonded beams. The epoxy bonded beams have reached same load and ductility levels obtained from an identical monolithically cast control beam which was included in the test program for comparison purposes. However, at failure separation cracks have occurred at the common interface between the overlays and the original beams. On the other hand, the weld bonded strengthened beams have behaved in a flexural ductile manner and achieved higher load carrying capacity compared to the control beam. The interlaminar shear failure did not occur in these beams which have acted as a single unit up to the failure.
To accommodate the ever increasing height of high-rise buildings and required large column-spacing, and assure a high structural performance to supporting elements, use of high-strength materials is sought as a solution. Accordingly, an experimental investigation of the effectiveness of very-high-strength steel bars in improving the performance of ultra-high-strength concrete columns is described. The concrete of 171 MPa strength contained steel fibers. Two grades of high-strength steel bars were used for longitudinal bars in columns. While the SD685-grade is already used in practice, the SD980-grade is still under development. The 1/4 scaled columns were subjected to high levels of compression and tension, and to cyclic lateral loads with an anti-symmetric double curvature bending. The tested columns proved to be ductile and showed good performances. The maximum recorded lateral strength values were at least 30% and 10% greater than those obtained by using ACI and AIJ equations, respectively. The advantage of using steel fibers was apparent by the limited and narrow cracks even at large lateral drifts. It is also suspected to have an impact on preventing buckling of the longitudinal reinforcement. The SD980-grade bars were found very effective in terms of tension axial strength, delay of crack evolution and shear strength degradation, and to be slightly less effective than the SD685 grade bars in terms of shear strength and energy dissipation.
As the construction of marine concrete structures become more common, durability issues are gaining importance. This study introduces a newly developed chloride-inhibiting low-heat cement that is a quaternary blended cement. The primary target application of the newly developed cement is massive marine concrete structures. The adiabatic temperature rise of the newly developed cement was about 30% and 28% lower than for ordinary Portland cement and blast furnace slag cement, respectively, and resistance to chloride ion penetration was much higher compared to blast furnace slag cement. Based on the ASTM C1202 guidelines, the adiabatic temperature rise of the new cement could be considered very low at 28 days, while that of the blast furnace slag cement was in low range. Thus the primary purposes of the newly developed cement can be said to have been successfully attained. In addition, the developed chloride-inhibiting and low-heat cement may satisfy the strength requirement of general-purpose marine concrete applications without significant concerns regarding early-age strength. The developed cement was shown to provide sufficient resistance to freezing-thawing attack as long as proper air content is obtained.