Flexural Strength Enhancement of Graded Pervious Concrete with Admixtures: The Role of Cement Mortar Properties and Optimum Apparent Density

Enhancing the flexural strength of Portland cement pervious concrete (PCPC) is considerably more difficult than improving its compressive strength, and this parameter critically limits the application of PCPC to highway pavements. This study investigated how various admixtures modify the flexural strength of PCPC under a fixed aggregate gradation, with the analysis grounded in the properties of the corresponding cement mortars and in the apparent density of the PCPC itself. The mechanical behaviour of cement pastes at five water-to-cement (W/C) ratios and of cement mortars incorporating single or multiple admixtures was characterized first; the flexural and compressive strengths of sixteen PCPC mixtures were then related to their connected porosity and apparent density. The results show that (1) regulating the cement hydration process raised the flexural strength of the cement paste by up to 43.7% while reducing its compressive strength by less than 2%; (2) a trade-off between flexural and compressive strength exists for both the cement mortars and the PCPC; (3) under the fixed gradation, the apparent density—used here as an indirect indicator of the compaction state—governed the flexural-strength enhancement, and the highest flexural strength of 4.05 MPa was achieved by the combined admixture RA (Reinforcing Agent) + SF (Silica fume) + BF (Basalt fibres), corresponding to a 12.5% improvement over the control mixture (3.60 MPa); and (4) the modification effect was most pronounced when the apparent density remained below 2.28 g/cm3, with an optimum apparent-density band of approximately 2.28–2.35 g/cm3 identified for maximizing the flexural strength. Collectively, these findings establish a quantitative basis—including a novel threshold apparent density of ≈2.28 g/cm3—for selecting admixtures and controlling compaction to maximize the flexural strength of pervious concrete pavements.