Precast Inc. - November/December 2008 - (Page 25) along the direction of the flow. Fibers oriented in the direction of the principal tensile stresses help to maximize structural performance. By virtue of these advantages, selfconsolidated, steel fiber-reinforced concrete (SCSFRC) has proved to be highly suitable for precast concrete structural applications as confirmed by the most relevant and recent case studies. Case studies using SCSFRC include: slabs on grade and overlays; precast, prestressed beams and roof elements; precast roof slabs; sheet piles; tunnel segments; precast, post-tensioned girders for slope stabilization; thin-wall housing units; and facade panels. Manufactured material quality and consistency in the fresh and hardened state are the most important requirements in capitalizing on the material advantages of SCSFRC in full load-bearing structural applications. Proper mix design and quality control procedures are critical elements for durability and long service life in structures produced with SCSFRC. selection of raw materials for cement paste: cement, cement substitutes, superplasticizers, admixtures selection of fine and coarse aggregates and fibers (type and dosage) optimal grading of solid skeleton (aggregates + fibers - deq-fibers) model for rheological behavior of cement paste: mini-cone flow rest, rheometer test (Marsh cone test) deq-fibers = 3 lf 1+2 lf γ aggregate γ fiber df average diameter of particles: dav measurement of void ration Vvoid assess paste volume ratio Vn dss dss = dav 3 assess solid volume ratio Vpaste – Vvoid Vconcrete – Vpaste 1+ –1 average spacing of solid particles 1600 paste mini-cone flow diameter/viscosity Pas/kg/dm3 + 1200 Mix designs for SCSFRC The first step in developing and promoting a new concrete-based material technology is the establishment of a quality mix design methodology. Depending on the specific structural performance required, specific SCSFRC mix designs must consider these parameters: • Minimum cement or cementitious materials (silica fume, fly ashes, slag); • Mineral filler content; • Air content; • Slump flow; • Nominal maximum size of aggregate (one-fifth to one-third of the fiber length); • Type and dosage of fibers; • Compressive strength; • Tensile strength; • First cracking strength; • Post-cracking residual strength (either in tension or in flexure); • Type of placement; • Admixtures; and • Special types of cement and aggregates. SEGREGATION ZONE maximum flow/viscosity ratio for fludity and segregation resistance (model line – R2 = .0996) + + + ALLOWABLE ZONE 800 400 minimum flow/viscosity ratio for concrete deformability (model line – R2 = .0962) POOR DEFORMABILITY ZONE 0 0.27 0.36 0.46 dss(mm) identify allowable values of dss for self compactability select paste/solid volume ratio identify optimum rheological properties of cement paste and select its composition optimally graded solid skeleton for the given paste/solid ratio MIX-DESIGN of SCSFRC FIGURE 1: MIX-DESIGN FLOW CHART FOR SCSFRC NOVEMBER/DECEMBER 2008 | WWW.PRECAST.ORG M + M M assess correlation between cement paste rheology, solid skeleton grading and paste/solid volume ratio + 25 http://www.precast.org
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.