Roxana Dana Bucur

Statistical Analysis of the Tensile Strength of Coal Fly Ash Concrete with Fibers Using Central Composite Design

The influence of coal fly ash and glass fiber waste on the tensile strength of cement concrete was studied using central composite design. Coal fly ash was used to replace 10% of the cement in the concrete mix. Glass fiber was added to improve the tensile properties of the concrete in different dosages and lengths. In total, 14 mixes were investigated, one only with 10% coal fly ash replacement of cement and the other thirteen were determined by the experimental design. Using analysis of variance, the order of importance of the variables was established for each property (flexural strength and split tensile strength). From the nonlinear response surfaces, it was found that higher values of flexural strength were obtained for fibers longer than 12 mm and at a dosage of 1-2%. For split tensile strength, higher values were obtained for fibers with a length of 19–28 mm and at a dosage of 1–1.5%.

Studies for understanding effects of additions on the strength of cement concrete

The paper analyzes the effects of different types of additions introduced in concrete mix on the compressive strength. The current studies show that additions contribute to improve some characteristics of concrete and to reduce the cement dosage, so it can obtain concretes which are cheaper and friendlier with environment. In the experimental mixes were introduced: crushed natural aggregates, slag aggregates, silica fume, fly ash, chopped tire, polystyrene granule, glass fibers and metallic fibers. The experimental values of compressive strengths were compared for two concrete grades (C20/25 and C25/30). The study shown that near the well-known possibilities of improving mechanical strengths of cement concrete by increasing cement dosage and strength, by using crushed aggregates and by reducing the water/cement ratio, there are other methods in which less cement is used by replacing it with different wastes or by adding fiber.

Assessment of thermal bridges effect on energy performance and condensation risk in buildings using DoE and RSM methods

Energy performance of buildings, assessed along design stage or rehabilitation phases during service life, is directly influenced by the manner thermal bridges are approached, being that their unavoidable presence in structures affects the risk of surface condensation and the level of thermal comfort. A realistic evaluation of these responses depends on the accuracy of estimate for thermal bridges share in heat loss. The assessment method recommended by European regulations and implemented through national regulations in Romania, uses linear and point thermal transmittances and temperature factors. The broad variety of building details leads to a large typology of thermal bridges, with unattainable coverage through catalogues with precalculated values. This paper contributes to the efforts of expressing mathematical relations for calculation of linear thermal transmittances and temperature factors, for the main types of thermal bridges. For this purpose, a method applied previously to similar problems has been adapted in order to formulate the relationships between the aforementioned parameters and their influencing factors over their variance range. Values of response parameters are determined with polynomial equations, coefficients of which are obtained through statistical data processing with DoE and RSM method. The method validation has been analysed by comparing the results with other sets of values obtained through numerical simulation. For the mathematical models investigated and the variability of factors, there have been made remarks over the attained precision and its adequacy to the submitted aim.