Influência da adição de sílica ativa sobre a resistência mecânica e a tenacidade à fratura de concretos de cimento portland para utilização em pavimentos rígidos rodoviários

摘要

The concrete is a ceramic matrix reinforced with ceramic aggregate compound type,characterized by a quasi-brittle mechanical behavior. Its physical and mechanical propertiesare established by its microstructure, controlled by the water/cement ratio, aggregate size andshape, by the use of modifiers like silica fume, etc. The literature show improvements in themechanical strength, durability and density of cement portland concretes modified with silicafume. The rigid road pavements is shortly used in Brazil, which demand the enlargement ofthe research and development field associated with it, including materials, processing andphysics and mechanics properties. In this work is studied the relationship between theaddition of different amounts of silica fume (a mixture basis, with 0%, and additions of 3, 6, 9and 12%, relative to the cement mass) and mechanical strength (resistance to axialcompression, fc, and tensile strength in bending, fct) and fracture toughness - using specimensof type "short-rod" with Chevron notch, calculated from the maximum load (KICVM), andphysical properties (specific gravity , absorption index and void index) of portland cementconcrete applied in road rigid pavements. The chemical composition of gross materials, theconcrete phases and microstructure are analyzed using the XRF, XRD and SEM techniques.The mechanical tests were performed at 7 and 28 days of concrete age. The slump test valuewas defined by technologic demand of the road operational execution as fixed value of 50 +/-10 mm, obtained by initial value of water/cement ratio equal to 0,47, which suffer a smallchange caused by the increasing in the silica fume content that decrease the specified concreteslum test value. The growth of the silica fume content caused a decrease in the values ofabsorption index (a reduction in the porous volume), flexural tensile strength, and axialcompressive strength and in the values of the fracture toughness. Moreover, in all of theconcrete types, the minimum values of fctMk surpass the technical value required by rigidpavements (fctMk ≥ 4,5 MPa) and the fracture toughness values were coherent with theliteratures data. These improving in the mechanical behavior and in the physical properties ofthe concretes are associated to physical and chemical effects promoted by silica addition inthe concrete structure, characterized by a pores volume reduction and by a strengthening ofmatrix/aggregate interface, as demonstrated by SEM fractography analysis of the differentconcretes. A preliminary production cost of concretes analysis, based on the simplifiedWestergaard Theory to design a single plate of concrete, appointed out the economic viabilityof the use of silica fume as a modifier of concrete of portland cement, up to certain levels, inthe construction of road rigid pavements in the State of Rio Grande do Norte / Brazil.