The effects of superplasticizer and coarse aggregate sizes on the mechanical properties of concrete and its drying shrinkage

This study presents the effects of superplasticizer and varying aggregate size on the mechanical properties of concrete. Six concrete sample groups were prepared using a prescribed mix ratio of 1:1½:3, with three aggregate sizes of 12.5 mm, 16 mm, and 19 mm, with and without a superplasticizer (Conplast 430). The compressive strength was determined on cubes of 150 × 150 × 150 mm, with samples cured for 7, 21, and 28 days, while the flexural strength was determined on unreinforced beams of 150 × 150 × 750 mm, with samples cured for 28 and 56 days. To evaluate drying shrinkage, 100 × 100 × 500 mm short concrete columns were cast and investigated for 90 days. The compressive strength results showed that samples made with 12.5 mm coarse aggregate and superplasticizer developed a maximum compressive strength (34.85 N/mm²) at 28 days. The concrete samples made with 12.5 mm, when compared with 16 mm and 19 mm, showed average strength increases of 6.5% and 9.7%, respectively. The flexural strength results showed that samples made with 12.5 mm coarse aggregate and superplasticizer developed a maximum flexural strength of 4.72 N/mm² at 56 days of curing. The drying shrinkage results showed that the shrinkage strain of concrete made with a smaller-size coarse aggregate (12.5 mm) without superplasticizer increased by 14.5% and 20.3% for 16 mm and 19 mm coarse aggregate sizes, respectively, whereas for concrete made with superplasticizer, the 12.5 mm coarse aggregate showed 7.77% and 13.04% increases when compared with 16 mm and 19 mm coarse aggregate sizes, respectively.