Feasibility of Reducing the Fiber Content in Ultra-High-Performance Fiber-Reinforced Concrete under Flexure
In this study, the flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) is examined as a function of fiber length and volume fraction. Straight steel fiber with three different lengths (lf) of 13, 19.5, and 30 mm and four different volume fractions (vf) of 0.5%, 1.0%, 1.5%, and 2.0% are considered. Test results show that post-cracking flexural properties of UHPFRC, such as flexural strength, deflection capacity, toughness, and cracking behavior, improve with increasing fiber length and volume fraction, while first-cracking properties are not significantly influenced by fiber length and volume fraction. A 0.5 vol % reduction of steel fiber content relative to commercial UHPFRC can be achieved without deterioration of flexural performance by replacing short fibers (lf of 13 mm) with longer fibers (lf of 19.5 mm and 30 mm).
Effect of very severe sulfate environment on bonded composite concrete system
Abstract This paper deals with the physical, mechanical and microstructural performance of composite concrete system (CCS) in a very severe sulfate environment. The CCS was prepared with normal concrete (NC) layer to be tested in compression and ultra-high-performance fiber reinforced concrete (UHPFRC) with 0%, 1%, 1.5%, and 2% fiber volume contents to be tested in tension. The objective was to inspect the degradation level of the bonding area when assuring the absence of any form of debonding under repetitive cycles of immersion in a concentrated sodium sulfate solution and drying in an oven at high temperature (80 °C). Flexural properties, compressive strength and change in mass of the composite system were considered at different immersion-drying cycles. The form and concentration of the corrosion products were investigated at the microstructural level using the sulfate profile and the phase assemblage methods with SEM-EDS and backscattered electron imaging. The results show that, the compressive and flexural strengths of CCS decreased considerably after the long term exposure to very severe sulfate attack. However, the physical and mechanical resistance of CCS improved with increased fiber content, and the bonding area was maintained without mechanical degradation. The microstructural investigations confirmed the presence of high amount of reaction products in NC layer and a limited zone of UHPFRC, mainly around fibers close to the bond area.
A New Vacuum Pressure Infiltration CFRP Method and Preparation Experimental Study of Composite
In order to prepare a carbon-fiber-reinforced polymer composite (CFRP) with ideal microstructure and properties, a new vacuum pressure infiltration CFRP method is proposed based on an analysis of existing CFRP preparation process methods. Research on composite material preparation systems was carried out by using this new method principle. The system mainly includes a fiber pre-forming module, a vacuum heating infiltration module, a hot-press curing molding module, and a data acquisition control module. Under the conditions of natural curing at 0 MPa + 6 h + 25 °C, vacuum heating curing at –0.05 MPa + 30 min + 80 °C, and hot-press curing at 0.7 MPa + 5 min + 50 °C, a two-dimensional (2D) CFRP with excellent microstructure and properties was successfully prepared. Observing the microstructure of the prepared composite material, it can be found that the inside of the composite material was sufficiently and uniformly infiltrated, and common preparation defects such as holes and delamination were effectively controlled. Through the performance test, the bending strength of the material reached 790 MPa.