International Concrete Abstracts Portal

In this study, uniaxial unconfined compression tests were carried out on four different composites with two types of rocks and shotcretes. The failure process was monitored by digital image correlation (DIC) and acoustic emission (AE) technologies to reveal the damage mechanism of the composites. The experimental results showed that with the load increase, the shotcrete part of the composites first entered the plastic stage, and then the main crack formed and penetrated across the interface to the rock side. The peak strength of the composite material was mainly determined by the shotcrete, and the fiber could effectively improve the ductility of the composites. Besides, the multiple analysis method of AE parameters showed that polypropylene (PP) fiber had a greater influence on the damage mechanism of the composite than the rock types. Finally, based on the rate process theory, a damage model was proposed, and the evaluation criteria of different rock shotcrete composites was given.

Shotcrete (sprayed concrete) differs from ordinary cast concrete through the application technique and the addition of set accelerators that promote immediate stiffening. The bond strength development between shotcrete and rock is an important property that depends on the texture of the rock, the type of accelerator, and application technique. This investigation focuses on the development of the microstructure in the interfacial transition zone (ITZ) and the strength of the bond at the shotcrete-hard rock boundary. The results show that the bond strength is related to the hydration process—that is, the strength gain of the shotcrete—and remains low before the acceleration period of the cement hydration. With a scanning electron microscope (SEM), it is possible to observe changes over time for the early development of the interfacial zone, both before and after proper cement hydration. Results from tests with wet-sprayed concrete on granite rock are presented. The test method—using both bond strength and the SEM to investigate the development of the microstructure at the ITZ—is interesting, but has to be more broadly examined. Different mixtures, accelerators, and rock types have to be used.

Shotcrete used for rock tunnel linings calls for skilled technicians, which is the key aspect to control the rebound. Three-dimensional (3D) concrete printing of tunnel linings has the potential to reduce manual labor for construction workers and to eliminate rebound, especially at overhead positions. In this study, the sag resistance and bond properties of printable concrete for overhead applications were explored. Mixtures with the addition of redispersible polymer powders (RDPs) and cellulose ethers (CE) were formulated. Roughened concrete slabs were used to replace the tunnel wall rock. A tack test with a loading control mode and a stress growth test were performed. To verify the results of the tack test and the stress growth test, a 3D concrete printing test, involving upside-down printing against the lower face of a supported concrete slab, was performed afterward. Also, a pulloff test was performed to measure the bond strength of the printed layers in the hardened stage. The results showed that the sag resistance of printable concrete is related to two aspects: the adhesion at the interface and the shear resistance of the fresh material itself. The adhesion and shear resistance properties determined two different failure modes: adhesion failure and cohesion failure. The results also demonstrated that the tack test results were more consistent with the upside-down printing test results, compared to the stress growth test.

Wet-mix shotcrete has been used more and more for structural applications in the past few decades. Recently, wet-mix shotcrete was successfully used to construct a mass structural wall with congested reinforcement and minimum dimensions of 1.0 m in a sewage treatment plant. A low-heat shotcrete mixture that included up to 40% slag was proposed for shotcrete application. A preconstruction mockup was shot to established proper work procedures for shotcrete application and qualify the shotcrete mixture and shotcrete nozzlemen. Extraction of cores and cut windows from the mockup confirmed proper consolidation around the congested reinforcement. A thermal control plan was developed, which included laboratory and field testing requirements, thermal analysis modeling with a three-dimensional (3-D) finite element program, and thermal control requirements, including installation of cooling pipes and thermal blankets. Shotcrete proved to be an efficient means for mass concrete structural construction. Thermal control for mass shotcrete construction was studied, and the proposed thermal control plan was proved to function properly. The general guidance for mass shotcrete construction is provided.

The development of design criteria intended for reinforced shotcrete structures has become a pressing matter for the industry. This is particularly true when imperfections behind reinforcing bars might be created during spraying operations. Current guidelines categorize imperfections qualitatively and only address the inspection of shotcrete for quality control purposes. Moreover, they disregard the bar-concrete bond strength as a function of the bars’ encapsulation quality limiting their usefulness for structural engineers wanting to rightfully include shotcrete in the design. In this paper, data from bond specimens with varying reinforcing bar encapsulation qualities was used to derive modification factors for the development length equation of straight bars. The modification factors were computed as bar stress ratios between perfectly encapsulated bars and those having imperfections using notions of possibility theory. As such, modification factors can be selected based on the void size and an equal bar stress probability associated to a building’s risk category.