New building materials in structural engineering: “Structural concretes made with Coarse and Fine recycled aggregates”

With more than three tons per head, per year, concrete is the most important and widespread material used in construction worldwide. The Italian code allows the use of waste produced by construction and demolition (C&D) operations to produce recycled aggregates. The great interest, both technical-economic and environmental aroused by this subject has in recent years and all over the world led to a noteworthy increase in experimental and theoretical studies on recycled materials resulting from the construction sector and, in particular, on recycled aggregates. The possibility of utilizing recycled aggregate is a very good solution to the problem of C&D waste and at the same time it reduces quarrying operations and limits the use of natural aggregates. The Italian ministerial decree of 14 January 2008 containing technical regulations for construction works, together with UNI EN 12620 and UNI 8520-2 standards concerning structural materials now allows a limited replacement percentage of only coarse recycled aggregates (sizes above 4 mm), to produce structural recycled concrete. In this scenario, the research activities described herein were developed with the final purposes of: Characterizing “real” coarse and fine recycled aggregates derived from construction and demolition waste by only concrete, randomly taken from an authorised class A storage site. This characterization was performed to determine their performance, compliance with the Italian code and the best experimental practice for the production and use in structural concrete. Furthermore the characterization of recycled aggregates by means of their shape, sizes, density, structure, strength, permeability and resistance to freezing and thawing cycles, directly leads to CE+2 certification, which is not present in Sardinia at the moment. Analysing different concrete mixtures made with different replacement percentages of fine and coarse and only coarse recycled aggregates in place of the natural ones to create a product having good properties during production, transport and implementation, with good compatibility with all devices and machines employed in concrete plants. The intention is to use recycled aggregates produced exclusively by concrete, coming from authorized class A storage sites, immediately after their release from the crusher and to optimize the mix design of the recycled concrete and the relative packaging procedure. Determining the mechanical properties of recycled concrete made with different replacement percentages of coarse and fine and only coarse recycled aggregates and comparing them with ordinary concrete to measure the gap in performance and evaluating their use in structural concrete. Reviewing and examining the scientific scenario in the determination of mechanical properties of the transition zone (ITZ). The results obtained from characterization of fine and coarse recycled aggregates, show a variability in line with what usually occurs in the characterization of natural aggregates, especially for coarse aggregates. The CE marking of recycled aggregates from concrete only, now completely absent in Sardinia, appears to be a feasible goal. However, a complete reorganization of demolition companies with selective demolition and separation of C&D waste is essential, together with a reorganization of authorized storage sites. Experimental results show a generally good behaviour of fresh and hardened recycled concrete. In concrete made with only coarse recycled aggregates, for very high replacement percentages of 50% and 80%, the differences with the strength properties of ordinary concrete are minimal, and sometimes irrelevant. In concrete made with fine and coarse recycled aggregates a reduction in strength was found, but was contained for replacement percentages up to 50%. Fine recycled aggregates present more problems compared to coarse recycled aggregates, especially as regards water absorption and particle size distribution. Their use in practice is possible if the dosage of water, cement and additives to be included in the mix is studied in advance. The study of the mix in producing concrete, and in particular for recycled concretes, plays a role of fundamental importance. The excellent results obtained in this experimental work, in terms of workability and strength are probably for the most part to be attributed to the choices made in this stage, mostly as concerns the choice of the additive and the amount of compensating water added to the mixtures. An important result of the experiments performed concerns the possibility of producing structural concrete using real coarse and fine recycled aggregates coming exclusively from the waste crushed concrete, immediately as it comes out of the crusher, without the need to optimise the grain size curve. However, an optimal mix design must be arrived at, especially as concerns the W/C ratio and the quality and quantity of additive used.