Rykhlitska O. Concretes with enhanced performance properties based on recycled concrete aggregates

The dissertation is devoted to solving the problem of obtaining concretes with increased operational properties based on recycled concrete aggregates by optimizing the granulometric composition of a mixture of aggregates of different types to form a dense macro- and mesostructure of artificial stone and introducing pozzolanic additives in combination with polycarboxylate superplasticizers to increase the strength of the contact zone at the interface of "recycled concrete aggregate - cement paste", which provides increased strength, water resistance and durability. The quality of recycled concrete aggregates was assessed by physical and mechanical indicators. Rapid-hardening modified fine-grained concretes based on Portland cement with limestone and fine aggregate of optimized grain composition were developed. It has been experimentally confirmed that modification of the concrete structure at the micro level with an active pozzolanic additive (APA) “fly ash – microsilica” in combination with RSE provides an increase in strength after 2 days to 82.7 MPa, and after 28 days – to 125.7 MPa. The regularities of the formation of the phase composition and microstructure of cement stone based on Portland cement with limestone, modified by PCE–APA, have been revealed. The method of X-ray phase analysis has established that the complex combination of PCE and highly dispersed APA “fly ash – microsilica” provides accelerated binding of calcium hydroxide after 1 day of hydration, the amount of which decreases by 1.6 times compared to cement stone without additives. It was established that in the cementing matrix, due to the phenomenon of adsorption modification, fine-dispersed crystals of hydrated phases are mainly formed, which indicates the creation of a homogeneous, dense fine-dispersed microstructure, which contributes to increasing the strength of concretes. The directed regulation of the processes of early structure formation of modified fine-grained concretes determines their improved construction and technical properties. In the fourth section, eco-efficient concrete compositions are designed taking into account the influence of cement activity, W/C ratio and aggregate quality on the strength of concretes in accordance with the Bolomey formula. Based on the results of the strength of control concrete compositions (for ordinary concrete with W/C more than 0.4), it was established that the quality coefficient of granite crushed stone is A=0.60 (ordinary), and for aggregates of recycled concrete it decreases to A=0.55 (aggregates of reduced quality). The granulometric composition of aggregates of eco-efficient concrete was optimized. Based on the results obtained, optimal compositions of concrete based on Portland cement with limestone (C=350 kg/m3) were designed using experimental and statistical modeling. It was established that when replacing 50% of granite aggregate in the composition of the concrete mixture with recycled concrete aggregate and introducing 1.5 wt.% of polycarboxylate superplasticizer, the strength of concrete after 28 days is 59.3 MPa, which corresponds to the compressive strength class C35/45. It is shown that when introducing 3.0 wt. % RSE due to the water-reducing effect ΔВ/Ц=45%, the compressive strength class C50/60 is achieved, which allows such concretes to be classified as high-strength and clinker-efficient. Studies of the structure of concrete based on polyfractional aggregates at the macro-, meso- and micro-levels have established that the weakest part in the concrete structure is the contact zone "aggregate - cement hydration products". It has been established that modification of PCE and pozzolanic additive "fly ash - microsilica" of clinker-efficient concretes based on recycled aggregates of optimized composition at the meso- and micro-levels provides after 1; 2 and 28 days to increase the strength to 24.7; 39.6 and 78.7 MPa, which meets the requirements for high-strength (strength class C50/60) and rapid-hardening 0.50 concretes. The developed fast-hardening eco-efficient concretes based on recycled concrete aggregates are characterized by increased construction and technical properties: elastic modulus Eсm=46.5 GPa, fracture toughness Kі=0.95 MPa·m1/2 , water resistance W12, frost resistance F300. The results of experimental and industrial testing of eco-efficient concretes with increased operational properties are presented. Rapid-hardening ready-mix concretes based on recycled concrete aggregates have been developed, manufactured by Betonix LLC, and implemented by Viking Construction Company in the construction of a monolithic reinforced concrete floor on the third floor of a residential apartment building (Lviv). The technical and economic efficiency of rapid-hardening eco-efficient concretes has been presented. The results of experimental research and industrial implementation have been used in the training of students of specialty 192 Construction and Civil Engineering.