International Concrete Abstracts Portal

Reinforced concrete (RC) members undergoing shrinkage are susceptible to cracking when restrained; however, studies on this behavior are limited. Thus, the main objective of this paper is to present crack-widths, crack-patterns, and shrinkage strains from an experimental study on three RC walls with aspect ratios of 3.26 and 1.08, and horizontal reinforcement ratios of 0.2% and 0.35%, as well as a rectangular tank with 0.24% reinforcement. A 3-D nonlinear finite element (FE) analysis is conducted, and the results reveal that although the model predicts strains and maximum crack-widths reasonably well, the crack-pattern differs from the experiments. The possible reasons for this difference are discussed, and a parametric study is done to propose design equations to estimate restraint factors along the wall centerline for different aspect ratios. These equations can be used to estimate the cracking potential in the design stage without the need for a nonlinear FE analysis. For L/h above four, horizontal reinforcement has a negligible effect on the restraint, and for L/h above eight, full-height cracks can be expected due to almost uniform restraint. Finally, the design codes are compared, and it is found that ACI 207.2R-07 and CIRIA C766 predict shrinkage-induced crack-widths conservatively and reasonably accurately.

Ultra-high-performance concrete (UHPC) is composed of a high volume fraction of binder and steel fibers, and a very low water content, resulting in enhanced strength and ductility, along with higher cost and environmental impacts. This study develops a UHPC mixture amenable to three-dimensional (3-D) printing, with 30% of cement (by mass) replaced with a combination of replacement materials. The proportioned UHPC mixture with 1.5% fiber volume fraction demonstrates 28-day compressive strengths of > 120 MPa (17.4 kips), and limited anisotropy when tested in the three orthogonal directions. Furthermore, 3-D printed layered composites are developed where UHPC (with and without fiber reinforcement) and conventional concrete layers are synergistically used in appropriate locations of the beam so as to achieve mechanical performance that is comparable to 3-D printed UHPC sections. Such manufacturing flexibility offered by 3-D printing allows conserving resources and attaining desirable economic and environmental outcomes, as is shown using life cycle and techno-economic analyses (LCA/TEA). Experimental and theoretical analysis of load carrying capacity and preliminary LCA/TEA show that >50% of the fiber-reinforced UHPC beam volume (in the compression zone) can be replaced with conventional concrete, resulting in only a <20% reduction in peak load carrying capacity, but >35% reduction in cost and >20% reduction in CO2 emissions. These findings show that targeted layering of different materials through 3-D printing enables the development and construction of 3-D-printed performance-equivalent structural members with lower cost and environmental impacts.

Crack densities obtained from on-site surveys of 74 bridge deck placements containing concrete mixtures with paste contents between 22.8% and 29.4% are evaluated. Twenty of the placements were constructed with a crack-reducing technology (shrinkage-reducing admixtures, internal curing, or fiber reinforcement) and 54 without; three of the decks with fiber reinforcement and nine of the decks without crack-reducing technologies involved poor construction practices. The results indicate that using a concrete mixture with a low paste content is the most effective way to reduce bridge deck cracking. Bridge decks with paste contents exceeding 27.3% had a significantly higher crack density than decks with lower paste contents. Crack-reducing technologies can play a role in reducing cracking in bridge decks, but they must be used in conjunction with a low paste content concrete and good construction practices to achieve minimal cracking in a deck. Failure to follow proper procedures to consolidate, finish, or cure concrete will result in bridge decks that exhibit increased cracking, even when low paste contents are used.

This study investigates the ultimate flexural strength (UFS) of reinforced concrete beams strengthened with CFRP (RCB-SCFRP), focusing on the identification and quantification of flexural overstrength concerning the nominal flexural strength (NFS) as defined by ACI 440.2R. A total of 106 full-scale specimens tested were carefully selected from previous research, varying in concrete strength, reinforcement configurations, and CFRP materials from multiple manufacturers. Results show that ACI 440.2R provisions accurately and conservatively estimate the flexural capacity of CFRP-strengthened beams. Including CFRP transverse reinforcement (TR) resulted in a slight increase in UFS. The type of strengthening, whether preloaded and repaired or strengthened, had little effect on the UFS/NFS ratio. Steel reinforcement ratio (SRR) significantly influenced overstrength, with higher UFS/NFS ratios observed between 0.70% and 1.00% SRR. CFRP axial rigidity notably affected overstrength, with optimal performance between 0.10 and 0.50 GPa·mm. Deflection ductility was mainly affected by the rigidity of CFRP, with a 13% increase noted due to CFRP TR. A log-normal model was developed to estimate UFS for RCB-SCFRP beams based on experimental data and ACI 440.2R guidelines.

This paper presents an experimental study on the residual bond of glass fiber-reinforced polymer (GFRP) rebars embedded in ultra-high-performance concrete (UHPC) subjected to elevated temperatures, including a comparison with ordinary concrete. Based on the range of thermal loading from 25°C (77°F) to 300o°C (572o°F), material and push-out tests are conducted to examine the temperature-dependent properties of the constituents and the behavior of the interface. Also performed are chemical and radiometric analyses. The average specific heat and thermal conductivity of UHPC are 12.1% and 6.1% higher than those of ordinary concrete, respectively. The temperature-induced reduction of density in these mixtures ranges between 5.4% and 6.2% at 300o°C (572o°F). Thermal damage to GFRP, in the context of microcracking, is observed after exposure to 150°C (302°F). Fourier transform infrared spectroscopy reveals prominent wavenumbers at 668 cm^-1 (263 in.^-1) and 2,360 cm^-1 (929 in.^-1), related to the bond between the fibers and resin in the rebars, while spectroradiometry characterizes the thermal degradation of GFRP through diminished reflectivity in conjunction with the peak wavelength positions of 584 nm (2,299×10^-8 in.) and 1,871 nm (7,366×10^-8 in.). The linearly ascending bond-slip response of the interface alters after reaching the maximum shear stresses, leading to gradual and abrupt declines for the ordinary concrete and UHPC, respectively. The failure mode of the ordinary concrete interface is temperature-sensitive; however, spalling in the bonded region is consistently noticed in the UHPC interface. The fracture energy of the interface with UHPC exceeds that of the interface with the ordinary concrete beyond 150o°C (302o°F). Design recommendations are provided for estimating reductions in the residual bond of the GFRP system exposed to elevated temperatures.