Core location and orientation
Strengths are affected by the location and drilling orientation of cores relative to the structural element. In general, concrete at the bottom of an element is stronger than concrete near the top of an element or near the top of a lift because of the effects of bleeding and settlement of the coarse aggregates. Bleed water decreases strength in the upper portions of walls, columns, beams and slabs by increasing the water to cementitious materials (w/cm) ratio. Figure 2 illustrates the top-to-bottom strength variation for a wall and also shows concrete is typically weaker along edges of a unit of deposit or formed joints.
As shown in Figure 3, bleeding creates a weak cement-aggregate bond, or planes of weakness, under coarse aggregate particles. Planes of weakness are always horizontal in concrete. When the planes of weakness are located parallel to the applied load (P) of the testing machine as shown for Core B (drilled perpendicular to the casting direction) strength can be reduced 10 percent or more as compared to Core A (drilled parallel to the casting direction). ASTM C42 requires these factors to be considered when planning a core test program.
Always exercise care when drilling and handling cores, especially to preserve the moisture content of cores. The amount and distribution of moisture in cores will affect strengths. ASTM C42 requires special handling with regard to moisture preservation and conditioning both in the field and in the lab before testing. In the field, wipe off drill water from the cores upon extraction and allow the surface moisture to evaporate (up to one hour). Then place cores into separate plastic bags and seal them to prevent moisture loss. In the lab, ASTM C42 requires a testing facility worker to store cores in sealed plastic bags for at least five days after end preparation to reduce moisture gradients.
As with most construction activities, advanced planning is required for a successful coring operation and low-strength investigation. When possible, minimize the number of cores removed from the structure to avoid the Swiss cheese appearance and remember that proper repair of the core holes is part of the investigation too.
1. ACI 318 Building Code Requirements for Structural Concrete, American Concrete Institute, www.concrete.org
2. NRMCA Publication No. 185 “Understanding Concrete Core Testing,” Bruce Suprenant, 1994, National Ready Mixed Concrete Association, www.nrmca.org