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.

This work investigated the rheological properties of pastes made from magnesia cement modified with superabsorbent polymers (SAP) with the aim of uncovering the mechanisms governing their interactions. The findings highlighted the significant improvement in static yield stress brought about by the addition of SAP, with minimal impact on dynamic flow properties. Furthermore, the incorporation of small quantities of methylcellulose was observed to amplify SAP's absorption capacity during the initial stages of hydration. This study also revealed that SAP's absorption behavior does not impede the rate at which hydration products form. In addition, the value of the critical strain can be used as a rheological indicator of SAP’s degree of absorption. Overall, this investigation indicated that the incorporation of SAP was highly advantageous with respect to enhancing the rheological characteristics of magnesia cement, particularly with respect to facilitating its usage for 3-D printing applications.

This paper presents a novel framework of analysis to assess the resistance of existing reinforced concrete (RC) members experiencing spatial variability of crack patterns and spatial variability of concrete mechanical properties. The spatial variabilities are considered by using digital image processing (DIP) to map crack patterns onto three-dimensional (3-D) nonlinear finite element (NFE) models, where the concrete mechanical properties (compressive strength, tensile strength, damage, and modulus of elasticity) are spatially varied using random fields (RFs) to form random NFE (RNFE) models. The framework was developed and applied to assess a corroded RC beam (to determine the distribution of the resistance) and column (to determine the reliability of the column at the ultimate limit state [ULS]). Research findings indicate improved accuracy in assessing the resistance of the corroded members up to 20%, and the adaptivity of the developed framework for performing reliability analysis of existing RC structures.

This study introduces a new anchorage strategy using ultra-high-performance concrete (UHPC) to attach unbonded post- tensioning (PT) strands to existing foundations. This solution complements a seismic retrofit scheme investigated by the authors, which transforms nonductile cast-in-place reinforced concrete (RC) shear walls into unbonded PT rocking shear walls following concepts of selective weakening and self-centering. In the proposed PT anchorage scheme, mild steel reinforcements are inserted through the shear wall thickness and into the foundation. Subsequently, UHPC is cast around the wall base, forming a vertical extension connected to the foundation, which is used to anchor the unbonded PT strands. The feasibility and performance of the anchorage scheme was investigated through a combination of laboratory testing and numerical simulations. Pullout testing on four scaled-down anchorage specimens was conducted in the laboratory. Hairline cracks were observed in the UHPC during testing. Additionally, three-dimensional (3-D) finite element (FE) models were created, validated, and used to study the performance of the proposed anchorage scheme under lateral loading. The simulation results support the effectiveness of the proposed anchorage strategy.