Performance-Based Specifications of Fiber-Reinforced Concrete with Adapted Rheology to Enhance Performance and Reduce Steel-Reinforcement in Structural Members

The research plan proposed here is a logical step for further enhancement of the Missouri Department of Transportation (MoDOT)/RE-CAST project 1-A entitled “Economical and Crack-Free High-Performance Concrete for Pavement and Transportation Infrastructure Construction” and project 3-A entitled “Performance of Fiber-Reinforced Self-Consolidating Concrete (FR-SCC) for Repair of Bridge Sub-Structures and Fiber-Reinforced Super-Workable Concrete (FR-SWC) for Infrastructure Construction”. Both projects involved the optimization of fiber-reinforced concrete to reduce shrinkage/cracking and increase tensile strength and ductility to enhance the resilience of structural members. The outcome of these research projects was applied into an implementation project for a two-span concrete bridge replacement at Route M/J over Route 50 near Taos, Missouri (Figure 1). The optimized flowable fiber-reinforced concrete (FRC) incorporated 0.5 vol. % of micro-macro steel fibers and 5 wt. % CaO based expansive agent (EA) to mitigate the risk of cracking and extend the service life of the bridge. The air-entrained concrete also incorporated a polycarboxylate-based high-range water-reducing admixture (HRWRA) and a polysaccharide-based viscosity-modifying admixture (VMA) to ensure high workability and adequate stability of the fresh mixture. The use of an EA proved necessary to mitigate shrinkage and early-age cracking of the concrete. In addition, the incorporation of EA in FRC can develop internal compressive pre-stressing effect, so called “chemically pre-stressing”, in the matrix, which can enhance mechanical properties. However, the effectiveness of using an EA to compensate for shrinkage is significantly influenced by the availability of water necessary for the chemical reaction leading to the expansion. Such water is provided through the mixture design and/or external source (i.e., moist curing). Therefore, in the first phase of the proposed study, the system of EA-fiber characteristics-moist curing with enhanced shrinkage cracking resistance, improved mechanical properties (due to the chemically-pre-stressing condition) and transport properties (due to lower crack potential and crack width) will be systematically optimized. Such optimization can also lead to the reduction of the EA/fiber content that can lead to cost savings. It has also been recognized that the FR-SWC can be produced using EA and various types of fibers. Proper use of fibers was shown to increase flexural strength and flexural toughness in monolith beams cast using FRC vs. those cast using regular concrete. As such, the incorporation of fibers can replace a portion of the steel reinforcement bars and obtaining same flexural strength, and even associated improve in toughness and crack resistance for the enhancement of resilience. The incorporation of fibers can also enable partially or complete substituting of welded wire mesh reinforcement (such as shear reinforcement in beams and roof elements). This reduces the need for manufacturing, detailing, and placing of reinforcement cages and leads to improvement of construction efficiency. Furthermore, the element thickness and the structure self-weight can be reduced since minimum cover requirements do not hold any more. Therefore, in the second phase of this study, steel reinforcement in structural members will be partially replaced by means of steel fibers, a careful design of the rheological properties of the fluid fibrous mixture which can lead to efficient alignment of the fibers along the casting-flow direction will be conducted to make the best use of fibers in the structural elements. It is important to note that the majority of the test methods developed to evaluate corrosion resistance of concrete such as bulk resistivity, surface resistivity, and rapid chloride-ion permeability cannot provide direct information about the corrosion resistance of the concrete in the presence of steel fibers. Therefore, special attempt will also be made to properly evaluate the corrosion resistance of the optimized concrete containing steel fibers. In addition, the effect of the lightweight aggregate (LWA) as internal curing on shrinkage and cracking potential was also investigated. The use of lightweight aggregate has significant benefit to provide internal curing and enhance cement hydration along with forming new hydration products via pozzolanic reaction of the SCMs. The proposed project seeks to optimize the fiber characteristics-EA-LWS-moist curing system to enhance restrained shrinkage cracking and transport properties of Eco-Bridge and FR-SWC mixtures developed in projects 1-A and 3-A, respectively. The project will also seek to partially replace steel reinforcement in structural members by means of steel fibers. The project aims at evaluating the combined effect of calcium oxide-based EA (CaO-based), LWA, and fiber content under different moist-curing regimes on restrained shrinkage, mechanical properties, frost durability, transport properties, and corrosion resistance of Eco high-performance concrete (Eco-HPC) targeted for bridge applications (Eco-Bridge-Crete). It is important to note that the majority of the test methods developed to evaluate corrosion resistance of concrete such as bulk resistivity, surface resistivity, and rapid chloride-ion permeability cannot provide direct information about the corrosion resistance of the concrete in the presence of steel fibers. Therefore, special attempt will be made to properly evaluate the corrosion resistance of optimized FRC. Research objectives are as follows: (1) Optimization of fiber characteristics-EA-LWS-moist curing system to develop high resistance to cracking and enhanced mechanical properties and corrosion resistance for Eco-Bridge-Crete and FR-SWC; (2) Development of a prediction model to predict the performance of FR-SCC and FR-SWC (two separate models) given the EA, LWS and fiber contents and curing conditions; (3) Quantification of the amount of steel reinforcement that can be reduced by fibers in FR-SWC; and (4) Evaluation of the enhancement in flexural toughness and crack resistance due to partial replacement of steel reinforcement with fibers.