For common detailing of footings in steel or precast concrete structures, longitudinal reinforcement of foundation beams is bent horizontally and spliced with reinforcement of cast-in-place footings to insure an adequate juncture for load transfer. In this study, instead of bending longitudinal reinforcement bars of both, beams, and footings, headed reinforcement bars are adopted. By doing so, a discontinuity region is created where longitudinal bars of footings become eccentric to those of beams that are embedded in the footings. To allow developing forces in longitudinal bars of beams flow to longitudinal bars of footings, a set of reinforcing ties is provided between them. As such setting of headed reinforcement bars is not common, thorough investigations have been carried out. In this paper, the pull-out performance of eccentrically spliced longitudinal headed bars with different detailing of transverse reinforcement, proposed for precast beam-cast-in-place footing connection, is discussed based on an experimental investigation. A method, based on the friction shear theory, for the strength evaluation of such arrangement is suggested.
Unlike plain concrete, frost damaged reinforced concrete (RC) exhibits anisotropy because of the presence of reinforcing bars. The resultant mechanical responses are influenced strongly by the loading direction. Therefore, to ascertain the mechanical behavior up to failure of RC beams subjected to freeze-thaw action and subsequent mechanical load-ing, anisotropic damage models of frost-damaged RC elements were assessed in this study using three-dimensional nonlinear finite element analysis (3D-NLFEA), which revealed that the anisotropic damage affected load-deflection responses and played a key role in failure modes that differed from those of an undamaged RC beam. This paper is the English translation from the authors’ previous work [Kanazawa, T., Sato, Y. and Takahashi, R., (2019). “Frost damage of reinforced concrete beams and analytical evaluation of its static failure behavior.” Journal of Japan Society of Civil Engineers. Ser. E2 (Materials and Concrete Structures), 75(4), 293-307. (in Japanese)].
In this work, the effects of the individual or hybrid addition of superabsorbent polymers (SAP) with varying dosages (0.1%, 0.2%, 0.3%, and 0.6%) and the lime-type expansive agent (KEA) on the length and mass change, compressive strength, and pore structures (MIP) of mortars were investigated. The results showed that the incorporation of SAP can effectively mitigate its autogenous shrinkage and the length change value of the mortar with SAP smaller than reference until 49 days, regardless of the presence of KEA. The hybrid addition of SAP and KEA increases the initial expansion of the specimens as compared with individual addition of SAP, which is a beneficial effect on compensating for the shrinkage of the mortar under drying conditions. Moreover, the addition of SAP seems to delay cement hydration and increase the volume of macropores (greater than 100 nm), thereby reducing the compressive strength of the mortars. The introduction of KEA slightly promoted the formation of micropores, resulting in a slight increase in compressive strength compared with the samples without KEA. Furthermore, in our view, it promotes pore refinement, so as to reduce moisture evaporation.
In the study herein presented three types of nanoparticles, with different dosages, were used, namely nano-SiO2, nano-Al2O3 and nano-ZnO, which were combined with different concrete formulations. The main goal is to improve the eco-efficiency of concrete, using simultaneously a partial replacement of Portland cement by industrial by-products and reduced amounts of nanoparticles, aiming to avoid the reduction, or even enhance, both mechanical and durability properties of the final mixtures. The interaction between the steel fibers added to the matrixes and the nanoparticles is also addressed. To analyze the effects of nanoparticles additions in the properties of both mortar and concrete mixtures, several tests were performed in fresh and hardened states. Durability indicators were also evaluated, namely, capillary absorption, immersion water absorption and carbonation depth. It was concluded that an increase of both flexural and compressive strengths can be obtained with nanoparticles additions, but that effect depends on the powder dosages used in the binder matrixes and on the type and dosages of nanoparticles. Regarding the matrixes with steel fibers, no additional gains were obtained combining simultaneously nanoparticles with those fibers. It was also concluded that the nano-ZnO addition significantly delay the concrete hardening and show a negative effect when combined with the steel fibers.
This paper analyzes field experimental data obtained on about 30 concrete structures, both new (age up to 1 year) and old (age up to 60 years). The data include in situ non-destructive tests (NDT) of air-permeability kT, electrical resistivity ρ and surface moisture m, as well as tests conducted on drilled cores: O2-permeability kO, water sorptivity a24, MIP, carbonation rate Kc and chloride content Cl at 10 to 20 mm depth. The main conclusions are that in situ kT of new structures is a good indicator of both kO and a24. Regarding old structures, high values of kT and kO are accompanied by low a24 values and by tight MIP pore structure. This phenomenon is attributed to microcracks, with strong effect on permeation but not so much on capillary suction. Similarly, high values of kT are not always accompanied by deep carbonation depths. The chloride content did not show correlation with either kT or ρ. In situ measurements of ρ, under the testing conditions, did not correlate with any other durability test. Finally, the spread of kT values for old structures is significantly wider than for young structures, suggesting that age improves durable concrete but weathering and damage impair non-durable concrete.
Deferred strain and cracking under sustained loading can be more prominent in self-consolidating concrete (SCC) used in repair applications than conventional concrete given its higher paste content. Flexural creep and subsequent creep recovery were monitored over 19 months tests for SCC, fiber-reinforced SCC (FR-SCC), fiber-reinforced conventional vibrated concrete (FR-CVC), and fiber-reinforced self-consolidating mortar (FR-SCM). Synthetic and steel fibers were used. Expansive agent (EA) was employed in FR-SCC with synthetic fibers. Fiber volumes of 0.5% and 0.8% were used in FR-CVC/FR-SCC and FR-SCM, respectively. Restrained shrinkage was also determined. The overall creep performance was based on the control of deferred deflection, crack opening, and strain in steel and concrete. The use of fibers enhanced creep performance by 5 to 7 times compared to SCC. FR-SCC with steel fibers provided 45% higher creep performance than FR-SCC with synthetic fibers. The incorporation of EA in FR-SCC enabled 80% additional enhancement of creep performance. The FR-SCC and FR-SCM mixtures exhibited crack widths lower than 0.2 mm at service loads as high as 70% of nominal load. The creep recovery of the FR-SCC was on the order of 20% to 70%, regardless of mixture type. Flexural creep and restrained shrinkage tests indicated similar tendencies of concrete performance. The best performance was obtained for the FR-SCC made with EA, followed by FR-SCC, then SCC and FR-SCM.
Various types of shear reinforcement are used for example general closed stirrup and reinforcement bar with mechanical anchor. However, most standards and specifications take only the cross-sectional area of the vertical components of the reinforcement components into account when determining their effect, such as on shear crack development and shear strength. Consequently, the full effect of different shear reinforcement shapes on the shear failure behavior of reinforced concrete (RC) beams is not clear. In this study, differences in shear failure behavior of RC beams using three types of shear reinforcement (closed stirrups, U-shaped stirrups, and rod-shaped reinforcements with mechanical anchor) were investigated by carrying out loading experiments. The three-dimensional displacement distribution on the side faces of each beam and the internal crack patterns were obtained. It was clarified that there is a clear difference in internal crack pattern and spreading deformation behavior according to shear reinforcement shape, and this influences the shear strength of the RC beam.
Cementitious materials are commonly used in nuclear repository sites to immobilize intermediate-level radioactive wastes. This is due to the large surface area of the calcium silicate hydrate (C-S-H) gel, the main hydration product of ordinary Portland cement, which provides many sorption sites in which the contaminants can be adsorbed. The retention capacity of these materials is strongly dependent on the composition, the water content, the pH or the presence of additives. Likewise, it is also known that the durability and performance of cement and concrete are adversely affected in chloride and/or sulfate environments. In this work, atomistic simulations have been employed to analyze the effect of the presence of chlorides and sulfates in the retention and transport of 137Cs, one of the most hazardous radioisotopes, in calcium silicate hydrate. The simulations suggest that the presence of a moderate amount of chlorides does not alter significantly the Cs uptake in C-S-H gel, while a moderate content of sulfates enhances substantially the retention of Cs ions and reduces their migration throughout the pore. This behavior is attributed to the ability of the sulfates to pull Ca out the high-affinity sites from the C-S-H surface, allowing Cs ions to occupy them.