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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.

Cross-tensioned prestressed concrete pavement (CTPCP) has superior mechanical and durable performance over ordinary concrete pavement. An approximate model to predict stresses and displacement of CTPCP under temperature loading is developed. Elasticplastic model is adopted to describe the performance of sliding layer between CTPCP and subgrade. The stresses in concrete are divided into friction introduced, curling, and prestressed components. Friction introduced component is obtained with the equivalent equation of CTPCP and curling component is obtained with Westergaard solution for concrete pavement with infinite length but finite width. Furthermore, influences of parameters, including length and thickness of slab, elastic modulus of concrete, frictional coefficient, space, angle and position of prestressed strands and reaction modulus of subgrade, on stresses and displacements are discussed. Results show that decreasing length and thickness of pavement, frictional coefficient, and elastic modulus of concrete are effective ways to reduce stress under temperature loading. Furthermore, decreasing space but increasing diameter of prestressed strands is another way to prevent too large tensile stress in CTPCP. Additionally, it seems to be more concise that the perfect plastic model is adopted to predict friction introduced stress in engineering application after comparative analysis of difference between to bilinear model and plastic model.

Large-scale load testing was completed on both plain and fiberreinforced concrete slabs-on-ground. The fiber-reinforced concrete used a new synthetic macrofiber. Although the synthetic fibers did not alter the tensile cracking load of the plain concrete slab, the flexural cracking load of the plain concrete slab was increased by 25 and 32% with synthetic fiber addition of 0.32 and 0.48% by volume, respectively, for the center loading configuration. Similarly, synthetic fibers at 0.48% volume fraction increased the flexural cracking load of plain concrete slab under edge loading by 28%. The ultimate load capacity of the plain concrete slab under center loading was increased by 20 and 34% with the addition of 0.32 and 0.48% synthetic fibers, respectively. Embedded strain gauges in the concrete slabs and deflection profile measurements indicated the fibers effectively distributed the load throughout the slab volume as cracking progressed, resulting in the increased concrete slab flexural and ultimate capacities.

Conductive concrete is a category of concrete containing electrically conductive components to attain stable and high electrical conductivity. Due to its electrical resistance and impedance, a thin conductive concrete overlay can generate enough heat to prevent ice formation on a bridge deck when connected to a power source. Steel fibers and steel shavings were used for the conductive materials in this study. A conventional concrete slab, 1.2 x 3.6 m (4 x 12 ft), has been constructed with a 9 cm (3.5 in.) conductive concrete overlay for conducting deicing experiments in the natural environment. The conductive concrete mixture was developed at the University of Nebraska-Lincoln specifically for bridge deck deicing. Anti-icing and deicing experiments were conducted in five snowstorms. The average power density of approximately 590 W/m2 (55 W/ft2) was delivered to the conductive concrete overlay to prevent snow accumulation and ice formation. The experiment setup, energy consumption, and costs during the winter storms of 1998 are presented. A coupled thermal-electric finite element analysis was conducted to study the joule heating of the conductive concrete overlay. The numerical results showed that the model served as a useful tool for predicting the heating performance of the conductive concrete overlay.

The objective of the present study is to identify a simple, reliable, and rational method for evaluating chloride ion diffusion coefficients for civil engineering applications. To make the conclusions of the study relevant to field concrete structures, the chloride penetration data used to estimate the diffusion coefficients were generated using fairly large-sized reinforced concrete slabs subjected to long-term cyclic exposure to a chloride environment. Furthermore, to make the study comprehensive, the parameters influencing the microstructure of the concrete such as water-to-binder ratio (w/b) and supplementary mineral admixtures were included. The simplified linear error-function-based method (SLEM) and Newton-Raphson method estimated almost identical values of diffusion coefficients irrespective of the w/b and the type of mineral admixture in the mixture, while the least square fit method estimated consistently lower diffusion coefficients. On the other hand, the values of diffusion coefficients estimated by the graphical method showed a mixed trend of higher and lower values compared with those estimated by the other three methods. Nevertheless, all four methods employed to evaluate the chloride ion diffusion were unanimous in estimating lower diffusion coefficients for the concrete slabs having mineral admixtures compared to the control concrete slab, and the slab cast with concrete of w/ b = 0.45.