Controlled Low Strength Material (CLSM--also known as flowable fill) is a high-slump product which usually contains portland cement, fly ash, fine aggregate, and water. Since 1990, it has been used by the Colorado Department of Transportation (CDOT) for backfilling around pipes or box culverts under existing substandard bridges to convert the structure from a bridge to an on-grade highway.
For an existing bridge to be converted to a culvert crossing using CLSM, two conditions must be met: 1) the new culverts must have the appropriate capacity and 2) the new culverts must fit within the limits of the existing bridge structure and flow line of the channel. If either of these conditions is not met, the bridge must be reconstructed using conventional methods. Because CLSM is more expensive than normal backfill material, it is used only around the culvert at locations where conventional backfill would be difficult or impossible to compact. It can be used in these locations because it is able to fill small void spaces, can be placed with minimal labor, it eliminates compaction and associated equipment and it is able to resist mild erosion.
CDOT has saved more than 50% of the cost of total bridge replacement by using CLSM. CLSM repair-work does not require additional right-of-way for detours, requires about one-tenth the traffic control, and saves about two months of construction time, as opposed to total bridge replacement.