Laboratory mix design experiment to achieve high-performance cement concrete (CLASS C50) for Pagada bridge deck using locally sourced materials.

Abstract

The current state of cement concrete practice in Uganda is limited to low (< 20 MPa) and medium (20 – 40 MPa) classes of concrete for small to medium structures such as those on district roads. This study centred on determining the optimal mix proportions of the constituent cement concrete materials that would achieve a concrete strength class of 50MPa based on locally sourced construction materials. The study was conducted at Pagada Bridge site along Palabek Kal – Pangira road link in Ayuu Alali parish, Lamwo district. The objectives of the study were to characterise the proposed materials based on their physical and mechanical properties, to determine the optimal material proportions in the mix to achieve class C50 concrete, and lastly, to conduct laboratory trial mixing based on the optimised mixes to produce concrete specimens meeting the specified class of concrete. The materials used in this study were Ordinary Portland and Pozzolana Portland Cements of grades 42.5N from Tororo Cement Factory, sand from Kalongo quarry, coarse aggregate from Ngetta quarry, water from Pagada stream, and superplasticizer (Conplast SP431R) manufactured by Fosroc (Kenya) Limited. Samples from the cements, sand, water, and coarse aggregate were drawn and taken to the laboratory to determine their physical, mechanical, and chemical properties. The results obtained in all cases showed that these materials were meeting the minimum requirements for use in high performance concrete works. These material properties were then used as inputs into the ACI 211.4R to obtain material mix proportions for use in the trial mixing operation. During trial mixing, the fresh and hardened properties of concrete were investigated by varying the percentage of fine aggregate in total aggregate were set as 25%, 30%, 35%, 40% and 45%, admixture dosage; 0%, 0.5%, 1.0%, 1,5%, and 2.0%, and for each cement type (Ordinary Portland Cement and Pozzolana Portland Cement). The aim was to obtain a combination that would achieve an optimal performance characteristic in fresh and hardened state. The study indicated that a combination of 40% fine aggregate content, 1% admixture dosage, and Ordinary Portland Cement gave the best (optimal) results concrete having good workability and a 28-day cube compressive target mean strength of 61.9MPa. The study, therefore, recommended 40% fine aggregate content, 1% admixture dosage, and Ordinary Portland Cement, grade 42,5N, to be used for the construction of the Pagada Bridge deck.