Double expansion anchors are designed for anchoring into concrete, brick or block base material. Inserting a machine-treaded bolt into the anchor expands the Double Expansion Anchor against the concrete at two points. It is made from Zamac, a non-rusting material, and is available in diameters ranging from 1/4" to 3/4". The double expansion anchor is set flush with the base material and is considered a light to medium-duty fastener. Double expansion allows for better holding values.