International Concrete Abstracts Portal

Slender reinforced concrete columns have been employed as components of telecommunication and internet infrastructure since the deployment of the system more than 30 years ago. The assessment of these structures must consider the time dependent behavior of concrete. In this context, a numerical investigation is conducted to determine the critical buckling load and the stress distribution in sections subject to creep and shrinkage of concrete. The guidelines used are those from the American Concrete Institute. It is concluded that the maximum stress induced in the reinforcement is 1.14% of the steel yield stress. Therefore, no yielding of the reinforcement is registered to the examined case which ensures safety against permanent deformation. During the elapsed time of 7500 days, the modulus of elasticity of concrete decreased by 53% and the critical buckling load 40%. The results obtained can be applied to similar cases through the slenderness index and the reinforcement ratio.

Controlling deflections in reinforced concrete (RC) flexural members under service loads is a serviceability requirement prescribed by design codes, such as the ACI CODE-318. Serviceability requirements are challenged by productivity requirements, such as faster construction and longer span demands, among others. This paper summarizes a parametric analysis conducted to estimate long-term deflections of one-way RC slabs. The objective of this study is to assess the effect of geometrical, concrete, and construction parameters on the long-term deflections of one-way RC slabs. The effect of these parameters on immediate deflections is also analyzed. Results of this study show that increasing the slab thickness and the area of tension reinforcement proved to be the most effective strategies for reducing both immediate and long-term deflections of one-way RC slabs. Additionally, the results of the parametric study highlight the relative influence of each studied parameter in controlling deflections.

To address the autogenous shrinkage issue of ultra-high-performance concrete (UHPC), internal curing technology has shown great potential in resolving this challenge by providing additional moisture. To further improve its curing efficiency, this study proposes an innovative internal curing technology that can significantly reduce autogenous shrinkage without increasing the amount of internal curing water or compromising mechanical strength. This approach utilizes perforated cenospheres (PCs) as internal curing agents while substituting internal curing water with urea solutions. In addition to replenishing water, urea solutions, once released into the cement paste, can react with portlandite. This reaction generates CaCO₃; owing to the intrinsic properties of CaCO₃, it has a larger macroscopic volume and a much higher elastic modulus than portlandite. This approach effectively reduces chemical shrinkage while concurrently increasing the stiffness of the cement paste, thereby achieving a significant reduction in autogenous shrinkage. As a result, replacing water with 3% urea solution in PCs enhances the autogenous shrinkage of UHPC, reducing it from less than 50% to over 90%.

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.

Several approaches are currently used to proportion recycled aggregate concrete (RAC), each having limitations. An effective and universal way to proportion RAC is not only an important tool for developing high-quality concrete but also a critical milestone for promoting the wider use of recycled concrete aggregate (RCA) in concrete. A mixture design method based on particle packing and excess paste theory is proposed in this study. Given the focus on pavement concrete, the modified Box Test was used to quantify RAC workability. RAC mixtures with five different RCAs of varying quality, developed using the proposed method, showed excellent workability (Box Test Rating E1-S1), whereas mixtures developed with conventional mixture design methods failed to achieve adequate workability. Mechanical properties of optimized RACs were either comparable or improved. The adverse effect of RCA on concrete resistivity and shrinkage appeared negligible and was mitigated by the mixture design approach developed in this study. Compared with conventional Direct Weight Replacement (DWR)/Direct Volume Replacement (DVR) mixtures, the proposed design achieved a reduction of surface voids by more than 80%, up to 25% higher compressive strength, and 20% lower shrinkage at 28 days, while maintaining comparable resistivity.