Dive summary:

• Planning how and where to take concrete cores for strength testing after laboratory cylinders raise an alert can give the in-place material the best chance of showing acceptable qualities and avoiding the expense and delay that could result if the cores show the material is sub-standard and has to be replaced.
• Concrete is usually stronger near the bottom or a pour than near the top and nearer the center than an edge or joint because of how materials settle.
• Strength also can be affected by whether cores are taken by a drill working parallel to the casting direction or perpendicular to it, and it's important to handle cores properly right after drilling to avoid changing the moisture content with either drill water or excessive evaporation.

From the article:

While the process of core testing may seem straightforward, there are many details contractors must consider to achieve accurate results. ...