International Concrete Abstracts Portal

Portland limestone cement (PLC) is being more and more specified and used in wet-mix shotcrete construction for ground support in tunnels and mines across the USA and Canada. The most widely used cement of Type GU (general use) in Canada and Type I in the USA is being replaced by Type GUL (general use limestone) in Canada and Type IL in the USA. There is no significant research being conducted on the performance of shotcrete made using PLC, including plastic properties, early age strength development, compressive strength development and, when fibers are added, flexural toughness and residual tensile strength development. This paper presents studies on the properties of wet-mix shotcrete produced with PLC. Results show that PLC requires a higher dosage of alkali-free accelerator (AFA) to achieve similar development of early age compressive strength compared to shotcrete made with PC. Development of compressive strength at 7 and 28 days for shotcrete made with PLC is similar to shotcrete made with PC. When both steel fibers and macrosynthetic fibers are used in wet-mix shotcrete made with PLC, development of residual tensile strength with notched beams and flexural toughness for round determinate panels is also similar to that for wet-mix shotcrete produced with PC. Future research on wet-mix shotcrete with Type GUL cement is also discussed.

This study focuses on enhancing the durability of two-component grouting materials by incorporating ground-granulated blast- furnace slag (GGBFS) and replacing cement with industrial waste to reduce environmental pollution. A ternary cementitious system was developed using 30% GGBFS and 10% carbide slag (CS) as partial cement replacements. The research investigates the effects of different water-bentonite ratios, water-binder ratios (w/b), and A/B component volume ratios on the physical and mechanical properties of the grout, including density, fluidity, bleeding rate, setting time, and strength performance. The microstructural evolution and hydration products were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), and thermogravimetric analysis (TGA). The findings provide insights for optimizing the mixture design of grouting materials in shield-tunneling applications, with a focus on improving performance and sustainability.

Desulfurization gypsum – alkali-activated slag-based tunnel fireproof mortar (GAM) was a lightweight cement-based fireproof material. A series of tests were conducted to investigate the effect of varying Na2O equivalents (2%, 4%, 6% and 8%) on the wind and vibration resistance of GAM, along with its bond strength, etc. Through XRD, TG-DTG, SEM and MIP analyses, the underlying mechanisms were elucidated. The study indicates as the Na2O equivalent increases, the weight loss of C-(A)-S-H gel and AFt phase exhibits an initial increase followed by a decrease. The weight loss of CaSO4·2H2O crystals, the crystal-to-gel ratio and total porosity first decrease and then increase. Therefore, the fire resistance of GAM first decreases and then increases, while the dry density, compressive strength, bond strength, and wind and vibration resistance of GAM initially increase and subsequently decrease with increasing Na2O equivalent. GAM with a Na2O equivalent of 4% demonstrates optimal wind and vibration resistance.

Inner surface reinforcement is one of the most widely adopted techniques for upgrading or strengthening shield tunnels. An important failure mode in this method is the debonding of the thin plates from the segments, resulting in less reinforcement effect than expected. A shield tunnel lining is a discontinuous curved structure formed by connecting segments with bolts, and its structural form and internal force state are essentially different from reinforced concrete beams. However, there are few reports on the evolution process of debonding failure of similar structures. Therefore, to develop a thorough understanding of the debonding failure, a three-dimensional refined numerical model for a shield tunnel strengthened by a thin plate at the inner surface based on the mixed-mode cohesive method was proposed. The validity and rationality of the model were corroborated by a full-scale experiment. Then, the model was applied to other inner surface reinforcement schemes commonly used in practice to explore the debonding mechanism of the adhesive layer. Finally, anti-debonding measures were proposed, and their effectiveness was elucidated by numerical analysis. The results show that the proposed numerical model can accurately predict the failure process of the adhesive interface of the shield tunnel strengthened by a thin plate. There are obvious interfacial stress concentrations at the joints and the plate ends, which are the essential reasons for the debonding failure initiating from those places. Anchoring the thin plate only at the plate ends and joints can significantly and sufficiently increase the debonding load. Therefore, it is not necessary to add anchoring measures elsewhere.

The geometry of arched (vertically curved) reinforced concrete (RC) members contributes to the development of additional stresses, affecting their flexural and shear strengths. This aspect of curvilinear RC members reinforced with glass fiber-reinforced polymer (GFRP) bars has not been reported in the literature. In addition, no specific design recommendations consider the effect of curvilinearity on the flexural and shear strengths of curved GFRP-RC members. This study has performed pioneering work in developing models to predict the flexural and shear strengths of curvilinear GFRP-RC members, with a focus on precast concrete tunnel lining segments. Eleven full-scale curvilinear GFRPreinforced tunnel segment specimens were tested under bending load as the experimental database. Then, a model was developed for predicting the flexural strength of curvilinear GFRP-RC members. This was followed by the development of two shear-strength prediction models based on the Modified Compression Field Theory (MCFT) and critical shear crack theory (CSCT). After comparing the experimental and analytical results, a parametric study was performed to evaluate the effect of different parameters on the flexural and shear strengths of curvilinear GFRP-reinforced members. The results indicate that neglecting the curvilinearity effect led to a 17% overestimation of the flexural strength, while the proposed models could predict the flexural strength of the specimens accurately. The proposed models based on the MCFT—referred to as the semi-simplified Modified Compression Field Theory (SSMCFT) and the improved simplified Modified Compression Field Theory (ISMCFT)—predicted the shear strength of the specimens with 28% conservativeness. In addition, the modified critical shear crack theory (MCSCT) model was 10% conservative in predicting the shear strength of curvilinear GFRP-RC members.