Accurately measuring the working stress of concrete through the stress release method is a crucial foundation for assessing the operational condition of building structures and formulating maintenance and reinforcement strategies. The slotting method, employed within the stress release technique, not only addresses the limitations associated with the core drilling and hole drilling methods but also offers a practical solution for engineering detection. This paper presents a novel multi-step slotting method employing a stress release rate model as its foundation. The fundamental equations governing space-related issues are introduced, and a theoretical model of the stress release rate is derived. By employing a multi-step slotting process instead of the conventional one-step slotting approach, the limitations of the traditional drilling method are overcome. The stress release rate model is calibrated using numerical simulation outcomes, followed by both numerical simulation and experimental verification. With a relative error of 3.5% between theoretical and simulated values, and 9.4% with experimental values after excluding the initial slotting data, it is evident that the stress release rate model demonstrates notable accuracy and applicability. This reaffirms the effectiveness and convenience of the multi-step slotting method for measuring concrete working stress.

To study the damage characteristics and failure mechanism of reinforced concrete damaged beams under cyclic load, the load-strain curve and stiffness degradation curve of reinforced concrete (RC) beams strengthened by adding stirrup, longitudinal reinforcement, and high ductile concrete (HDC) under repeated load were compared, as well as the flexural ability before and after strengthened. The results show that: compared with the original beam, the strengthened method with longitudinal strengthened at the bottom of the beam has the most obvious improvement in the flexural capacity of the beam. When the longitudinal strengthened is added, the flexural capacity can be increased by 86.25%. According to the actual failure mode of the reinforced beam, the stress reduction coefficient and height reduction coefficient are theoretically derived, and the bending capacity of the reinforced beam under each strengthened method is calculated. The theoretical value is in good agreement with the test value.

In this study, a procedure for interpreting impact-echo data in an automated, simple manner for detecting defects in concrete bridge decks is presented. Such a procedure is needed because it can be challenging for inexperienced impact-echo users to correctly distinguish between sound and defective concrete. This data interpretation procedure was developed considering the statistical nature of impact-echo data in a manner to allow impact-echo users of all skill levels to understand and implement the procedure. The developed method predominantly relies on conducting segmented linear regression analysis of the cumulative probabilities of an impact-echo data set to identify frequency thresholds distinguishing sound concrete from defective concrete. The accuracy of this method was validated using two case studies of five slab specimens and a full-scale bridge deck, each containing various typical defects. The developed procedure was found to be able to predict the condition of the slab specimens containing shallow delaminations without human assistance within 3.1 percentage points of the maximum attainable accuracy. It was also able to correctly predict the condition of the full-scale bridge deck containing delaminations, voids, corrosion damage, concrete deterioration, and poorly constructed concrete within 3.5 percentage points of the maximum attainable accuracy.

The use of recycled concrete aggregates (RCAs) in lieu of natural aggregates improves the sustainability of the built environment. Barriers to the use of RCA include its variable composition, including the residual mortar content (RMC), chemical composition, and its potential to contain contaminants, which can negatively affect the properties of concrete or present environmental concerns. In this study, a rapid, economical method to estimate the RMC and provide the chemical characterization of RCA was developed using a portable handheld X-ray fluorescence (PHXRF) device. Models were developed using reference tests (RMC test based on the thermal shock method and chemical composition from whole-rock analysis) to correlate PHXRF results to measured values. The PHXRF shows strong potential for estimating the RMC and chemical composition of RCA. Paired with locally calibrated reference samples, the test method could be used in laboratory or field applications to characterize RCA and increase its use in bound and unbound applications.

The use of three-dimensional (3-D) printing with cementitious materials is increasing in the construction industry. Limited information exists on the freezing-and-thawing (FT) performance of the 3-D-printed elements. A few studies have used standard FT testing procedures (ASTM C666) to assess the FT response; however, ASTM C666 is insensitive to anisotropy caused by printing directionality. This paper investigates the FT response of 3-D-printed cement paste elements using thermomechanical analysis (TMA) to examine the influence of directionality in comparison to cast counterparts. Cement paste with a water-cement ratio (w/c) of 0.275 was used. The critical degree of saturation (DOSCR) as well as the coefficient of thermal expansion (COTE) were determined for specimens with varying degrees of saturation (DOS). Micro-computed tomography (micro-CT) was conducted to quantitatively understand the heterogeneities in the pore microstructure of 3-D-printed materials. For the specimens fabricated in this study, the COTE and DOSCR are independent of the 3-D-printing directionality and were comparable to conventionally cast specimens. For samples at 100% saturation, the FT damage was higher in the 3-D-printed samples as compared to the cast samples. The use of a low w/c in the 3-D-printed materials, desired from a buildability perspective, led to low capillary porosity, which thus decreased the amount of freezable pore solution and increased the FT resistance of the 3-D-printed materials. Micro-CT analysis demonstrated a significant 4.6 times higher average porosity in the interfacial regions compared to the filament cores.