PSI - Issue 28

Chbani Hamza et al. / Procedia Structural Integrity 28 (2020) 430–439 Author name / Structural Integrity Procedia 00 (2019) 000–000

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3.2. Water/Cement ratio Determining the water/cement ratio requires knowledge of the target compressive strength of the concrete, the true class of used cement, the quality and the maximum size of the aggregates. We define, first of all, the average resistance in compression which is only the characteristic resistance aimed at increased by 15%. In our case, the target resistance is 25MPa. The distribution of the compressive strengths of concrete follows a normal law, we admit for this strength an average coefficient of variation (ratio between the standard deviation and the mean) of about 15% Bernier (2004) such as : 1,15 32,2 28    c cm f f MPa (7) The water/cement ratio is given by Bolomey's formula Bolomey (1935) 0,5    CE cm G F f E C (8) The granular coefficient G is related to the quality and to the maximum size of the aggregates. the concrete guide Dreux and Festa (1998) define the different G values, in this work, the used aggregates are of good quality and have a maximum diameter of 16 mm, which gives a granular coefficient of 0,45. 3.3. Cement and water dosing The determination of the consistency class of concrete is essential in the procedure of its formulation, the Abrams cone slump test provides reliable measurements, it is carried out according to the NF P 18-451 standard (2018), currently, it is the most adopted test worldwide. Depending on the slump obtained, we can define the workability of the concrete: plastic, fluid or closed. The cement and water dosage can be easily deduced from the C/E graph as a function of the slump Dreux and Festa (1998). The majority of the structures are built with ordinary plastic concrete of class S2 with a consistency between 50 and 90 mm, this concrete is very practical for solid slabs and engineering structures, for this study we chose a slump of 80mm which leads to a cement dosage of C = 275 kg/m 3 . From the C/E ratio and the cement dosage found, we can easily deduce as a first estimate the amount of water E required for the concrete formulation. This quantity must be corrected taking into account the influence of the specific surface of the aggregates and the size of the largest aggregates D max (equation (9)). 3 / 192(1 4%) kg m E   (9) 3.4. Aggregate dosing The modulus of fineness (equation (10)) is a coefficient allowing characterizing the importance of the fine elements in an aggregate, we calculate this modulus according to the standard NM 10.1.703 (2008). The fineness modulus is smaller the richer the aggregate is in fine elements. (10) The modulus of fineness of the sand found complies with the requirements of standard NM 10.1.703 (2008)which sets this modulus between 1,8 and 3,2. However, this value corresponds to slightly coarse sand. It will give resistant concretes and acceptable workability. We plot a reference granular composition OAB on the graph of granulometric analysis (Fig. 4) by respecting the standard NF-EN 933-1 (2012) such as : -Point O: is the origin of the coordinate graph (0,0) ; -Point B: corresponds to the coordinates (D max ,100%), with D max = the maximum dimension of the aggregates.   sand refusal cumulative M FG (%)