Evaluation of Creep of Lightweight Concrete Containing Tuff Aggregates

Abstract

Shrinkage and creep of concrete are major serviceability problems for concrete structures and thus its evaluation is key in providing insight to a better understating. The study's problem statement was that serviceability factors (creep and shrinkage of concrete) interact. The existing prediction models vary, and not easy to compare one to another with an ± 20 % range of accuracies. The justification for the study was that Tuff Aggregates are naturally found, unlike Artificial Light Aggregates (ALA), which are produced mostly through sintering. The Normal Weight Concrete (NWC) is lighter, thus resulting in lesser sustained loads, thus reducing creep and reduces depletion of conventional aggregate/NWA due to increased construction activities. The main objective of the study was the evaluation of creep of lightweight concrete containing tuff aggregates, while the specific objectives were; to characterize the constituents of concrete that were to be used for NWC and LWC, determine the plastic and hardened properties of concrete containing tuff aggregate, and compare with that of NWC, to determine creep and shrinkage development of concrete containing tuff aggregates. The significance of the study was that it aided in quantifying the E values, creep, and shrinkage of tuff aggregate concrete, which were to be replicated for design and analysis. The scope of the study was limited to normal strength classes of C25, C30, C35, C40, and C45. The literature reviewed was tuff aggregates, cement hydration, stress-strain behaviour of concrete, Modulus of Elasticity, Creep (mechanisms, factors, and prediction models), and Cracking (Flexural, Shrinkage). The methods chosen are; concrete mix design to ACI 211.2 and while the replacement percentages being 0, 12.5, 25, 50, 75, and 100 %, plastic and hardened tests of concrete and cement, modulus of elasticity, creep and shrinkage testing of beam specimens. The test results showed that Tuff Aggregate (TA) had a Specific Gravity of 2.094, water absorption of 11.1%, and Aggregate Crushing Value (ACV) of 17.9%, while Normal Aggregate (NA) has 2.552, 1.7 %, 14.9 % respectively. The TA chemical composition is predominantly of SiO2, Al2O3, K2O, and Fe of different proportions. In classes, C25, C30, C35, C40, and C45, the compressive strength and bulk density decreased with the % of the replacement of TA for 28 days and 56 days after casting and curing. Iterative mathematical steps were developed to predict the reduction in compressive strength with an increase in % of replacement of TA. Modulus of Elasticity (E) decreased with an increase in % of replacement, and coefficients were developed for the 12.5 %, 25 %, 50 %, 75 %, and 100 % of replacement of NA with TA. Sixty days of shrinkage strain measurement for unsealed beam specimens was done. The maximum value of 0.00317 mm/m and 0.000236 mm/m of the 0 % and 100 % replacements were measured. However, there was

an increase in shrinkage with time. Shear strain increased with time, whereby NWC produced lower values than LWC. The strain of reinforcements for the NWC and LWC was measured to be 0.000473 mm/m and 0.000427 mm/m, respectively. LWC experienced higher total bottom strains (creep + shrinkage included) than NWC at 694 and 360 micro strains, respectively. LWC produced higher creep coefficient values of 1.63 while NWC was 0.6. ACI and BS EN models produced values of 0.64 and 0.597, respectively. Thus, it is concluded that LWC has lower compressive strength, modulus of elasticity, and creep values than NWC. NWC experiences higher shrinkage than LWC owing to continued internal curing that reduces autogenous shrinkage. This study developed two mathematical models and recommended them for structural analysis and design.