Issue 43

S. Jing et alii, Frattura ed Integrità Strutturale, 43 (2018) 33-42; DOI: 10.3221/IGF-ESIS.43.02 34 properties of ECC. In concrete structures, the compression performance must be guaranteed for columns, walls and other load-bearing members. Due to the deficiency of coarse aggregate, the increase in strength of ECC will be limited to a certain extent. Therefore, in order to promote the ECC in concrete structures, it is necessary to design an appropriate mix ratio to ensure that it has reliable strength, but also good toughness. The classic mixing ratio of ECC showed that the raw materials made up of cement, fly ash, fine quartz sand, water and monofilament staple fibers. On the one hand, an excessive water-binder ratio could severely reduce the strength of the material because it lacks coarse aggregate; on the other hand, if the water-binder ratio is too small, the fluidity of the mixture may deteriorate, which could hinder the dispersion of fibers. In this experiment, the superplasticizer was used to reduce the amount of water, while ensuring a good fluidity of the mixture. In this paper, four main factors, namely fly ash content, sand-binder ratio, water-binder ratio and plasticizer content were considered, using the compressive strength and ultimate tensile strain of ECC as indexes. The influence of the regularities of various factors on the strength and toughness of high toughness cementitious composites was studied through the orthogonal experiment. E XPERIMENTAL DESIGN Experimental materials he cementitious materials consist of P.O 42.5 ordinary Portland cement and first-grade fly ash. Fine quartz sand with a size of 0.1mm-0.2mm was used as fine aggregate. Ordinary tap water with polycarboxylic superplasticizer was used to mix the dry material. Previous studies showed that polyvinyl alcohol (PVA) and polyethylene (PE) fiber were the primary types of fiber to manufacture ECC. C. Redon adopted different amounts of special oil coated with the surface of PVA fiber, and found those which could dissipate a large amount of energy in the process of gradually pulling out from the cement matrix [6]. In this paper, Kuraray TM PVA fiber (Φ15μm×12mm) was used to manufacture high toughness cementitious composites. Orthogonal design The mix proportion of concrete is commonly tested with either one variable or multiple variables at a time. Each of the two methods has its unique advantages and disadvantages. The one-variable-at-a-time method is not representative enough and limited to a small range, while the multiple-variable-at-a-time method requires repeated tests on the interacting factors. For instance, in the case of three factors and three levels, 27 tests need to be carried out, making the experiment extremely complicated. The merits of the above two methods are combined into the orthogonal test method. Based on mathematical statistics principles, the test solution relies on a set of orthogonal forms to sieve out the most representative samples for comprehensive tests, aiming to obtain the effect of factors on indices and identify the best combinations of influencing factors [7]. In the orthogonal test, the test factors refer to those influencing the object, and the levels denote the conditions of the test factors. In this research, the test factors include fly ash content, sand-binder ratio, water-binder ratio and plasticizer content, each of which has three levels of conditions. The purpose of the orthogonal test is to examine the impact of the test factors on the compressive strength and the ultimate tensile strain of materials. Level of Factor Fly Ash Content Sand-binder Ratio Water-binder Ratio Plasticizer Content (A) (B) (C) (D) 1 0.9 0.30 0.20 0.40% 2 1.2 0.33 0.23 0.70% 3 1.5 0.36 0.26 1.00% Table 1 : Factors and levels of orthogonal test. Mixing ratio This study took fiber with a volume content of 2.0% as an invariant, considered the four factors fly ash content, sand- binder ratio, water-binder ratio and plasticizer content. Each factor includes three levels,as shown in Table 1. Three-level variables of fly ash content adopted the mass ratio of fly ash to cement (m FA/C ), being equal to 0.9, 1.2 and 1.5 which were respectively expressed by A1, A2 and A3. Three-level variables of the sand-cement ratio adopted the mass ratio of sand to cementing materials (m S/B ) for the 0.30, 0.33 and 0.36 and were respectively expressed by B1, B2 and B3. Three-level T