The time that the concrete must keep its properties, known as service life, it is an important design parameter for the performance of concrete structures. A porous concrete has a higher degree of water and aggressive permeability agents, which can cause deterioration of the reinforcement for mechanisms such as corrosion, besides not ensuring the watertightness, for example, on slabs under negative hydrostatic pressure (anti-flotation slabs). This type of slab has been used for the purpose of enabling or facilitating constructive actions associated with structures located beneath the groundwater table. There are several advantages of this type of construction: reduction of environmental impacts as it prevents the installation of permanent submersible pump systems, which reduces the use of electricity; nonoccurrence of settlement caused by the lowering of the groundwater table, which extends to the nearby areas of the construction site; and less impact to the groundwater table, which has serious consequences on the availability of this resource in urban areas. This paper presents a case study on the concreting of an anti-flotation slab, 30 cm thick and volume of 750 m 3 , of reinforced concrete, in a mixed use residential/commercial building, located in Northwest Sector, Brasilia, Brazil. To ensure the watertightness of this anti-flotation slab, a permeabilityreducing admixture was used to prevent hydrostatic condition, known for its ability to promote the sealing of cracks up to 0.4 mm by self-healing phenomenon. The results showed that both the composition of the concrete (containing permeability-reducing admixture) and setting the appropriate constructive strategies were essential to ensure an intact and tight structure.
The autogenous shrinkage is a phenomenon that occurs due to the appearance of tensile stresses in capillary pores. Inherent in the hydration process, not depending on external interference. In high strength concrete, autogenous shrinkage is most pronounced, due to the refinement of microstructure, the high cement content, presence of mineral addictions and low water/cement ratio, which interfere in the transport of water curing. In this context, Superabsorbent polymers (SAP) stand out for their ability to reduce or eliminate the autogenous shrinkage, because they absorb large amounts of water, which is subsequently released into the matrix, avoiding self-desiccation and the onset of tensile stresses (internal curing). However, studies show that SAP can reduce the mechanical strength, which can be compensated by addiction of Nano-silica (NS). In this paper, the effect of these additions in Time Zero (T 0 ) was evaluated. T 0 determines the time at which the material starts to behave as a solid, with the development of a rigid mineral skeleton to oppose the volumetric variations that occurs in the paste, which can lead to cracking, compromising esthetics, durability, and structures safety. The ultrasonic pulse velocity test performed the T 0 , which measures the velocity of propagation of an ultrasonic wave through the material and is taken as the time when a sudden change occurs at this velocity. Nine blends were made containing SAP and NS, where it was found that SAP addition increases the T 0 (up to 11%), while NS decrease its value (in 55%).
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