PSI - Issue 17

Piotr Smarzewski / Procedia Structural Integrity 17 (2019) 5–12 Piotr Smarzewski / Structural Integrity Procedia 00 (2019) 000 – 000

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reducing the quantities of steel reinforcement. Although, HPC is stronger and stiffer than normal strength concrete it is also more brittle. The increased brittleness can make that HPC will be more vulnerable to the cracks propagation as well as to brittle failure (Kjellsen et al., 2000). High material ductility of the HPC may be achieved by appropriate selection of the type and size of the aggregate (Zhou et al., 1995; Tasdemir et al., 1996) or the use of fibres (Köksal et al., 2008; Smarzewski, 2019b). In last decade ultra-high performance concrete has drawn much attention due to its great mechanical properties and long durability (Rong et al., 2015; Gesoglu et al., 2016; Smarzewski, 2019a). To reduce the costs and environmental impact, mineral admixtures are preferred to substitute high volume of high-grade cement (Peng et al., 2015). Sabir (1995) reported that SF influences the HPC in twice major ways. Silica fume plays a part to the hydration reaction between cement and water by reacting with the calcium hydroxide to create calcium silicate hydrate (CSH) gel, which is the binder between the aggregates and provides HPC its strength. Secondly, the ultra-fineness particles of SF (< 1  m) take up the voids between the cement grains, as a result acting as a micro-filler, reducing the porosity of the bulk cement matrix. This affects the less permeability and greater durability of HPC. The mechanical strengths of HPC are also increased because of having a pozzolanic activity of SF. Filling influence of SF is more dominant than its pozzolanic effect. The most recognized influence of SF on HPC is the improvement on aggregate – cement paste interface (Toutanji et al., 1999; Atis et al., 2005). SF increases the water demand of HPC because of its addition directly and ultra-fineness. For that reason, SF is utilized together with a superplasticizer to control the workability. Nevertheless, researchers mention some disadvantages of using SF in HPC such as the decrease of plasticity during the concrete production and the increase of shrinkage during the concrete curing. A study of the microstructure and microchemistry of the concrete mixes was reported by Kjellsen et al., 1998. In general, the references such as Wedding and Swamy (1986) and John and Shah (1989) have indicated that the empirical relationships between compressive strength and other characteristics such as tensile strength, flexural strength and modulus of elasticity adopted for normal strength concrete should not be applied for high-performance concrete. Caution should be worked out in extrapolating data from normal to high-strength concrete. Generally, Ahmad and Shah (1995) reported that cracking is more localized and approaches homogenous material behaviour as well as HPC is more brittle than conventional concrete. Researchers disagree about the optimal content of SF (Duval and Kadri, 1998; Köksal et al., 2008). To some of these the content is about 15%, whereas for others the increase in mechanical properties may reach for 30% of cement replacement by SF. In this study the results of the effect of SF on the mechanical and fracture properties of HPC with SF content in the range 5%-25% are presented. The water/binder ratio was held at 0.25. The HPC specimens were tested for compressive strength, tensile splitting strength, elastic modulus, and fracture energy. Testing was conducted at 28 days. The fracture parameters and characteristic length were calculated. The Portland cement was CEM I 52.5R complying with PN-EN 197-1:2012 and PN-B-19707:2013 standards. The physical properties of the condensed cement and silica fume are given in Table 1. The coarse aggregate was 0.5/8 mm size crushed granodiorite. The fine aggregate was 0.125/0.5 mm size quartz sand. The particle size distribution of quartz sand and granodiorite coarse aggregate was determined in accordance with PN-EN 933-1:2012. A polycarboxylate ethers-based light yellow liquid superplasticizer was employed in the HPC mixtures, expressed as a percentage of the mass of cement + SF. The material has a specific gravity of 1.07 at 20 ºC, contains chlorides < 0.1%, alkali < 1.5% and does not induce air entrainment. In all the mixtures containing SF, the water to cement ratio is based on the total cementitious materials (cement + silica fume). Content of SF expressed as a percentage, is defined as the mass ratio of SF solids to the cement employed in the reference mixture. 2. Experimental details 2.1. Materials and definitions