When filling a large crack or joint in concrete or asphalt it is best to use a backer rod in the gap first.
Backer Rod is a round polyethylene foam joint-filler and backing material used for filling expansion joints and large or deep joints before installing elastomeric sealants.
It is used mostly in the construction trades for filling expansion joints in concrete, wood and asphalt to allow for the joint to move, while allow for the use of less sealant and to keep the sealant from sinking into the joint.
Backer rod can also be used in concrete and asphalt repairs to fill deep or large gaps before adding sealer. Some contractors will use fine sand to fill the gap first, but if the gap gets wider or deeper the sand will fall away and leave the sealant with no backing.