Single and Double Expansion Shields are used primarily used in brick and block though they do work in solid concrete as well. As the screw is driven into the anchor, it starts to pull down towards the middle resulting in an expanded anchor. These anchors only expand so much. The reason for this is to prevent the brick or block from cracking. The limit of expansion highly reduces, but does not eliminate, the chance of cracking brick or block. Do not use this anchor in overhead applications.