Reinforced concrete pile caps have been traditionally designed with sectional methods in codes and specifications. Since the 1990s, the two-dimensional strut-and-tie model (2D STM) methods, introduced in codes and specifications (Ref. ACI 318-14 and the 8th Edition of the AASHTO LRFD Specifications), have become the design method of choice for pile cap designs. However, sectional methods and 2D STM provisions in current design codes have serious limitations when applied to the design of members with behavior more appropriately captured using three-dimensional (3D) approaches. In this paper, a refined 3D STM method for analysis and design of pile caps is illustrated. In the proposed method, a statically indeterminate 3D STM with diagonal ties is employed to account for the load-carrying capacity in tension of some regions in four-pile caps. The effective strengths of 3D concrete struts and nodal zones are determined by reflecting the effects of the 3D stress states and the degree of concrete confinement provided by reinforcement. The load-carrying capacities of struts and ties are determined using an iterative technique with the axial stresses of concrete struts equal to effective strengths. An extensive comparison between current and proposed methods is conducted using experimental data from 115 reinforced concrete pile caps tested to failure. Besides, several pile caps are designed using the current and proposed methods, and a comparison of the results from the design is conducted.
Accelerated Bridge Construction (ABC) uses prefabricated elements that are made continuous using cast-in-place joints. Deck joints are normally referred to as “Closure Joints.” There have been concerns about long-term durability of these joints that are expected to become rapidly serviceable. Normally, they contain reinforcing bars and enclosures of various shapes that in some cases create congestion within the joint. The specific nature of the joint application, in-situ casting, curing, material incompatibility, cold joints, cavities and steel congestion contribute to creating the potential for leaving defects and anomalies in the closure joints. This, in turn, results in a higher potential for exposure and other detrimental effects with possible degradation in time, and therefore reducing the strength and serviceability of the joint, hence creating a weak link for the structure. The long-term deflections and environmental loading will only exacerbate the situation. Hence, evaluation and health monitoring of the closure joints becomes inevitable. Despite the wide use of non-destructive testing (NDT) methods for bridge structures in general, a concerted attempt for categorization of these methods, comparison of capabilities, and selection of methods most applicable to closure joints is lacking. To address this, a research project was carried out as part of activities in the Accelerated Bridge Construction University Transportation Center (ABC-UTC) of Florida International University. This study included a comprehensive literature review with a focus on NDT methods applicable to health monitoring of ABC closure joints. The study focused on joint types relevant to precast concrete decks commonly used for ABC bridges, therefore, FRP (fiber reinforced plastic), timber (wood), and steel of any shape were excluded for the time being. The study resulted in categorizing the most common closure joints in five general groups based on their features affecting the application of the NDT methods. Accordingly, the most promising NDT methods were identified taking into account the distinctive defects and anomalies associated with closure joints. These methods were evaluated for their efficacy, ease of use and other characteristic influencing their use as preferred methods for each type of joint. A flowchart was introduced to assist in selection of the most applicable NDT method to each type of defect in closure joints. This paper summarizes the results of this study.
Water permeability is a key property for the serviceability and durability of concrete structures, which governs the transport of fluid through the pore network in the cementitious material. A microstructure-based numerical test method is proposed and employed to predict the permeability of hydrating cement paste. Numerical samples characterizing the evolution of the microstructure of cement paste are generated using the computational code HYMOSTRUC3D. Based on the three-dimensional (3D) finite element method (FEM), the pore-scale flow of water induced by pressure-gradient through the sample is simulated and the corresponding permeability is estimated. Water flow characteristics in the hydrating cement paste and the evolution of the permeability against different water-to-cement ratio (w/c), porosity, curing age and degree of hydration are investigated by numerical simulations. The simulated results are verified in comparison with available theoretical solutions, experimental data and numerical predictions obtained from the literature. Due to the dilution and tortuosity effects, the permeability decreases with the increase of cement hydration and the decrease of w/c. The connectivity of the pore throat plays an important role in affecting water movement in hydrating cement paste. The developed modeling approach is capable to investigate the transport properties of cement paste, which may provide basic parameters for multiscale modeling of concrete performance and strongly support the coupled multiphysics analysis in concrete engineering.
Explosive spalling of concrete leads to premature failure of concrete structures under fire. Thus it is important to assess explosive spalling risk of concrete. Thermo-hygral process in concrete is believed to be responsible for explosive spalling of concrete. Currently, simple yet reliable models to predict thermo-hygral behaviour are still desirable. In this study, a one-dimensional numerical model is established to predict moisture migration, pore pressure and explosive spalling in concrete exposed to heating. The effectiveness of the model is validated against five sets of experimental data. The experiments covered (a) moisture migration inside normal strength concrete (NSC); (b) pore pressure buildup inside NSC; (c) pore pressure buildup inside high-performance concrete (HPC) containment vessel; (d) pore pressure buildup inside HPC slab; and (e) one-time explosive spalling of concrete. The results predicted by the model match well the experimental results. The model developed provides an effective and economical tool to assess explosive spalling of concrete in addition to experimental methods.
This article creatively established a four-stage methodology to evaluate the macroscopic elastic modulus of the cement mortar corroded in brine solution. At first, the nanoindentation technique was utilized to measure the microscopic elastic moduli of a 10 × 10 lattice locating on a region containing sand aggregates, cement paste and interface transition zone (ITZ). The compositions and contents of each point were determined by XRD combined with SEM-EDS. Secondly, three different thicknesses (5 µm, 10 µm and 15 µm) of the ITZ were assumed and the corresponding microscopic elastic moduli were calculated based on porosities from imaging process, multi-scale homogenization method and the Voigt parallel mode. The thickness of the corroded ITZ was then determined. Moreover, the Lu & Torquato model was used to calculate the volume fractions of cement matrix, aggregates and ITZ of the mortar, treated as important parameters to predict the macroscopic elastic moduli of corroded and non-corroded mortar with homogenization method. Finally, a Reuss-Voigt equivalent approach was proposed to calculate the macroscopic elastic modulus of the mortar, which showed good agreement with the measured macroscopic elastic modulus from uniaxial compression test. Therefore, the methodology utilized to evaluate the macroscopic elastic modulus of the corroded cement mortar is effective.