Abstract:
Proper estimation of structural adequacy of an existing RC building requires a reasonable estimation of the load-carrying capacity of columns. This is generally done by collecting and testing core samples and utilizing provisions of ACI 214.4 and ACI 562. However, the applicability of these provisions for concrete made of burnt clay brick chip aggregate is yet to be established since these code provisions are generally developed based on the characteristics of crushed natural stone concrete.
A detailed experimental study has been conducted to investigate the applicability of the provisions of ACI 562 and ACI 214.4 in predicting the in-situ concrete strength of columns made from both stone and brick aggregate concrete. A total of thirty-two reinforced concrete column samples with monolithic flared ends at the top and bottom were cast and tested under pure axial load. Of these, sixteen samples were made of crushed natural stone aggregate concrete, and the remaining sixteen samples were made of crushed burnt clay brick aggregate concrete. For each column specimen, nine standard cylinder samples were also cast for testing at 7 - day, 28-day, and on-column test day – thus a total of 288 cylinder samples were tested. Within each group of sixteen column samples, four variations in design concrete compressive strength were considered. Each column sample was tested in a universal testing machine until failure under pure axial load. After that, eight numbers of Ø94 mm dia. and six numbers of Ø69 mm dia. core samples were collected from the flared ends of the column samples – thus a total of 448 core samples were collected and tested. Then, the provisions of ACI 562 and ACI 214.4 were used to estimate the in-situ concrete strength. The actual concrete strength of column test results was also evaluated. Concrete compressive strength, thus obtained from different methods and procedures are then compared. A statistical comparative analysis of the results was conducted.
Based on the findings of the study, it is observed that sample screening of 9% of avg. is more reliable than the outlier identification method. The Ø94 mm dia core sample is more reliable than the Ø69 mm dia core sample. The tolerance factor method of ACI 214.4 gives relatively reliable results for 9% of avg. screening method than the outlier identification method. Impact rebound hammer procedure gives more reliable strength for stone aggregate concrete compared to brick aggregate concrete. In all methods of ACI 214.4 and 562, predicted concrete in-situ strength is closer to concrete strength from column test in cases of brick aggregate concrete than from stone aggregate samples. If all the core specimens comply with both the outlier screening method and 9% of the average screening method, the equivalent strength of the core specimens is very close to the 28-day cylinder strength and column test strength in all methods. This is true for both stone and brick aggregate concrete and both Ø69 mm and Ø94 mm dia core samples. In general, the tolerance factor method of ACI 214.4 gives the too conservative result. ACI 562 and the alternate method of ACI 214.4, both are reasonably accurate for stone concrete as well as brick concrete to find the equivalent strength of concrete cores.
