Analysis of concrete applied after prolonged mixing time

2019 Australian Journal of Basic and Applied Sciences  
INTRODUCTION The mixture called concrete forms one of the most used materials in the world. This is due to its ease in preparation, use and the achievement of excellent performance after its application, although it is not found ready in nature. Concrete may be easily fabricated by mixing materials such as cement, sand, gravel and water, which may still get additions and/or additives as required improvement or enhancement in some property. After being prepared, in the fresh state, it may be
more » ... tate, it may be cast into molds to be shaped by the user and, as it hardens, it gets the desired strength and form, at which time should be demolded. According to Urban and Sicakova (2018) , mixing and delivery can negatively influence the homogeneity and uniformity of concrete mixtures, leading to effects on technical parameters such as workability, longer-term proceedings and performance characteristics of concrete. At construction sites, due to lack of free space for large materials storages and the need to maximize yields and promote waste reduction, an increase of the use of concrete produced in concrete batching plants has been noticed daily. The ready-mixed concrete, also known as concrete dosed in batching plants, is a type of material produced with higher technological quality control, in which production is carried out with greater precision, monitoring the number of constituent materials of the mixture to meet the most varied types of works. They can also be produced on a larger scale, thus favouring the increase in productivity of worksites and providing conditions to achieve high resistance with the use of additives and additions. The ready mixed concrete has the great advantage of being guaranteed by the supplier concerning security and the attainment of the specified characteristic strength (fck) for elements of the work. This is due to greater technology control in product ion, being expected that the quality of concrete is greater than mixed on site, but some care should be taken in receiving concrete: at Abstract The Brazilian Standard NBR -"Norma Brasileira" 7212 (ABNT -"Associação Brasileira de Normas Técnicas", 2012) recommends that ready mixed concrete, transported by mixer truck for longer than 1.5 hours, must be refused by the user. It also advises that concrete shall not be released and thickened in a period exceeding 2.5 hours after the addition of water to the mixture, considering such concrete as overdue. It is common to see delays in concrete operations, so it is important to know the main properties of concrete used after prolonged mixing time. Further more, material waste is a worldwide problem and concrete is significantly wasted in construction activities. This study tests the hypothesis of using concrete after prolonged mixing time and aims to evaluate changes in key properties of a concrete dosed with Portland cement Type III (ASTM -American Society for Testing and Materials) , mixed during 4 hours without the use of setting time retardant, hourly assessing changes in workability, stiffness, compressive strength and corrosion potential. Resonant frequency, electrical volumetric resistivity and compressive strength tests were performed. The results showed that even with the use of superplasticizer, there was great loss of concrete slump. It was also noted an increasing tendency of the dynamic modulus of elasticity and a decreasing tendency of the electrical resistivity of the concrete, but there was no significant loss in strength during the assessment time. Even though resistivity shows that the higher the mixing time, the greater will be the propensity of concrete not to protect the reinforcement bars, it is concluded that it is still possible to use concrete after the mixing time recommended by NBR 7212: 2012 and this way reduce material waste in civil construction. Citation: Charles Ferreira de Oliveira, et al.,2019. Analysis of concrete applied after prolonged mixing time. Australian Journal of Basic and Applied Sciences, 13(10): 41-50.
doi:10.22587/ajbas.2019.13.11.6 fatcat:f6o6xmj2jbaztc3bofbh5ssejm