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DOSAGE ADJUSTMENT DUE TO THE USE OF RECYCLED AGGREGATE IN THE MANUFACTURE OF CONCRETE
Zoraida Sánchez Roldán, María Martín Morales, Ignacio Valverde Espinosa, Ignacio Valverde Palacios, Montserrat Zamorano
Keywords: construction and demolition waste, recycled aggregate, concrete, dosage adjustment, properties 1. Introduction
The use of recycled aggregates (RA) from recovering and recycling of construction and demolition waste in the manufacture of concrete has shown the highest impact in their properties, both in fresh and hardened state, mainly due to the rougher and porous nature of the mortar adhered to the surface of the RA (Agrela et al., 2011). Usually, the same dosing methods that for conventional concrete manufacture have been used. Nevertheless, it is necessary to make some adjustments in the dosages due to the higher water absorption of RA, which can cause a loss of fluidity (Poon et al., 2002).
Therefore, the production of recycled concrete (RAC) requires some procedure modifications with respect to the conventional concrete manufacture (NAC), such as: (i) adding an additional amount of water; (ii) pre-wetting the RA; (iii) adding an extra amount of superplasticizer additive; or (iv) using mineral additives. For all the above, the objective of this research has been to get a dosage that allows to obtain a recycled concrete with similar characteristics to those of NAC. The use of higher quantities of RA will be guaranteed, in addition to the reduction in the NA consumption and the volume of waste in landfills, in the framework of the circular economy (COM 614/2015).
1. Materials and methods
The granular materials used in the study were NA and RA, both in 0/4 mm and 4/16 mm fractions, a white cement (BL II / AL 42.5R UNE 80305) and a water-reducing admixture (Chryso ® Fluid Optima 227). The RA used was mainly composed of crushed concrete and natural stone (98%), with a low content of ceramic material (0.2%). The water absorption (5.2 wt% in fine fraction and 2.1 wt% in coarse fraction), as well as the quantity (2.1 wt%) and quality of fines (100) of RA fulfilled the Spanish Concrete Code requirements (EHE-08, 2008).
As usual, due to the greater water demand of RA and in order to achieve a RAC soft-type consistence, the pre-soaking treatment of RA was carried out, since it reduces the exchange of water between RA and cement paste. The studies consulted showed no consensus regarding to the most suitable pre-soaking method. For that, a comparative study of different pre-soaking methods were proposed (Sánchez-Roldán, et al., 2016). In addition to a NAC used as a reference, six types of RAC in which NA coarse fraction was replaced by RA, were manufactured. In five of them RA were pre-soaked according to different methods, and in the other one RA was not pre-soaked. The dosage provided by a concrete manufacturer in weight per m3 for a concrete of compressive strength 35MPa was: 50% of coarse aggregate, 33% of fine aggregate, 17% of cement (400 kg/dm3), 0.8% of admixture (on weight in cement) and an effective water/cement ratio (a/c) of 0.48.
2. Results and conclusions
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New Trends in Green Construction