This study investigates the synergistic effect quarry dust, as a partial replacement for fine aggregate, and micro-silica, used as a partial replacement for cement, on the compressive and flexural strength of high-strength concrete (HSC), with a target grade of 80 MPa. Twenty mixes were produced using a combined percentage replacement of 0%, 5%, 10%, 15%, and 20%, across four water-binder ratios of 0.20, 0.24, 0.28, and 0.32. Compressive strength was measured at curing ages of 3, 7, 14, and 28 days, while flexural strength (modulus of rupture) was determined experimentally on prisms at 28 days and further compared against values predicted by the Eurocode 2 and ACI 318-11 code equations. Results showed that strength increased with percentage replacement up to an optimum of 10% PR, beyond which it declined, with the highest 28-day compressive strength of 73.40 MPa and highest flexural strength of 5.85 MPa both recorded at 10% PR and a water-binder ratio of 0.20, representing increases of 27.43% and 16.30% respectively over the control mixture. The Eurocode 2 equation consistently overestimated experimental flexural strength by approximately 10%, while ACI 318-11 underestimated it by approximately 17–18%. Result also revealed that combined 10% quarry dust and micro-silica replacement at a water-binder ratio of 0.20 produced the maximum strength performance and is recommended for high-end structural applications.

All-in-laterite concrete, in which lateritic material supplies both aggregate fractions, is a low-cost indigenous alternative to conventional concrete, but reinforced use is resisted because its strength variability makes structural performance unpredictable. This paper tests that objection for one laterite source and beam configuration. A published strength regression for this concrete from Idah, Kogi State, Nigeria (twelve mixes, forty-eight standard-cured cubes) was reproduced and embedded in a probabilistic resistance model, with cube-to-in-place strength transfer carried as a random variable. Strength variance used an assumed bounding model of predictive error, batching scatter and ACI 214R-11 control tiers. Fifteen basic variables followed the JCSS Probabilistic Model Code, with Gaussian-copula dependence. The first-order reliability method, verified by importance sampling, assessed sections designed to EN 1992-1-1 and BS 8110 against four limit states. The material supports strength class C12/15. The reference beam attained flexural reliability indices of 5.018 and 5.149 against a target of 3.8; the code-compliant shear-reinforced member attained 8.763, with links governed by minimum-ratio and maximum-spacing rules rather than strength, and 3.297 for quasi-permanent deflection. Concrete strength carried 0.0022 of the flexural limit-state variances, against 0.6332 for imposed load, and the flexural index differed by only 0.029 from conventional concrete of identical class. A concrete-only shear mechanism, screened for an unlinked member, returned 3.103. The flexural conclusion holds while transferred-resistance model uncertainty stays below a coefficient of variation of about 0.15. Strength variability therefore does not govern flexural reliability here, and raising the concrete partial factor would be an ineffective safeguard.