THE EFFECT OF PARTIAL REPLACEMENT OF CEMENT WITH FLY ASH ON THE STRENGTH OF CONCRETE FOR TRANSPORTATION STRUCTURES AND ROAD PAVEMENTS

The effect of replacing part of the cement with fly ash on the strength of concrete for transportation structures and road pavements has been determined. Portland cement CEM II/A-S 500, crushed stone (5–20 mm fraction), quartz sand with fineness modulus of 2.3, the superplasticizer Polyplast SP-1, and fly ash from the Darnytsia Thermal Power Plant were used in concrete production. The properties of three concrete compositions were investigated. Composition No. 1 (without fly ash) served as the control, with 300 kg/m³ of Portland cement used as the binder. In composition No. 2, 10% of the Portland cement was replaced with 75 kg/m³ of fly ash. In composition No. 3, 20% of the Portland cement was replaced with 150 kg/m³ of fly ash. All concrete compositions included 2.4 kg/m³ of superplasticizer. All concrete mixtures exhibited equal workability (S1), with the water/cement ratio (W/C) depending on the composition. For the control composition No. 1, the W/C ratio was 0.390. For composition No. 2, the actual W/C ratio, calculated as the total binder content (cement and fly ash), was 0.333. For composition No. 3, the W/C ratio was 0.308. Thus, as the proportion of fly ash in the binder increased, the W/C ratio of the mixtures decreased. The average density of the control concrete (composition No. 1) and composition No. 2 was approximately equal (2441 kg/m³ and 2446 kg/m³, respectively), while composition No. 3 exhibited a slightly lower density (2423 kg/m³). This can be explained by the fact that replacing part of the cement with a larger mass of fly ash reduces the W/C ratio while simultaneously increasing the spacing of coarse aggregates. Compressive strength was measured at 7 and 28 days. At 7 days, the compressive strength of composition No. 2, where 30 kg/m³ of cement was replaced with 75 kg/m³ of fly ash, was 6.8% lower than that of the control (composition No. 1). However, at 28 days, the compressive strength of composition No. 2 was 3.8% higher than that of the control. For composition No. 3, replacing 60 kg/m³ of cement with 150 kg/m³ of fly ash resulted in a 28.3% decrease in compressive strength at 7 days and a 14.0% decrease at 28 days compared to the control. Thus, concretes containing fly ash demonstrated slower strength gain compared to concrete using only Portland cement as the binder. Replacing 10% of the Portland cement with a rational amount of fly ash produced concrete with strength comparable to that of the control composition. However, replacing 20% of the Portland cement was not fully compensated by the fly ash. Therefore, the use of fly ash in concrete for transportation structures and road pavements is both feasible and effective. The introduction of a rational amount of fly ash reduces binder consumption, which has significant ecological benefits and is economically viable.