Experimental investigation and numerical simulation on effect of drilled cores on axial capacity of columns

Abstract

Drilling out of core from columns now a days is one of the most established and reliable practice to assess the in-situ concrete strength of structural members, especially columns, which is one of the most important structural member of any RCC structure. The tragic collapse of Rana Plaza in 2013 and some other structural failures due to poor construction has triggered structural integrity assessment of existing RCC structures in Bangladesh. Besides, it is quite common in this country, a significant number of structures have been constructed without following any code provision. In addition, recent earthquakes are also causing concerns among engineers to check the adequacy of the existing structures. Besides, this test is performed when doubt exists about the in-place concrete quality due to either low strength test results during construction or signs of distress in the structure and also used to assess strength information on older structures. In this regards, it is now quite common that buildings are being recommended for evaluation of concrete in-built strength through core extraction. There are codes that provide guideline regarding the procedure of core drilling. However, core drilling has the potential to impose a negative effect on the structural capacity of the element being drilled for cores. The relevant ASTM standard also advises that concerned engineers must be aware of the associated impact of core drilling. Despite such potential negative impact, there have been few studies available on the effect of core extraction on the structural capacity of RC columns. In these circumstances, a comprehensive study has been undertaken to investigate the effect of core extraction on capacity of RC columns. The effect of compressive strength of concrete, column size, lateral tie spacing, core size and locations were investigated on structural capacity of column after core removal.

The study was divided into two phases. In the first phase, sixteen lab-scale columns were made and their ultimate load capacity and crack failure pattern have been observed for two core locations and compared with columns with no cores drilled. The experimental columns were made with stone aggregate for target strength of 27.6 MPa to represent a ubiquitous concrete mix. In the second phase, finite element analysis (FEA) of tested column specimens has been conducted using ABAQUS environment and compared with the experimental outcomes for validation. After satisfactory validation of the FEA models, further FEA have been performed on columns having real scale dimensions for different core sizes, compressive strengths, column sizes and different lateral confinement in order to develop a quantitative guideline for safe core extraction.

It was observed that core drilled from one third height from support of the column resulted in significantly higher reduction in column capacity than that of column with core at mid height. The behavior of core drilled columns shows a significant dependency on core diameter particularly when column size is small. The effect of core drilling was found to be more pronounced for low strength columns and higher tie spacing. A noteworthy observation was obtained from the study that columns having dimension smaller than 300mm x 450mm would require special consideration for core extraction regardless of tie bar spacing, core size and concrete strength. Finally, graphical guidelines have been prepared based on combined interrelation of all the parameters for the entire range of column dimensions, tie bar spacing and compressive strength. The parameters required to utilize the graphs can be obtained from design drawings or from on-site investigations.