Inspection of Concrete Structures at DamsThe information provided in this fact sheet pertains entirely to the inspection of concrete structures used at dams. The intention is to help dam owners become more aware of common problems that are typically encountered with concrete so that they can more readily assess the seriousness of a particular condition whenever it arises.
Concrete surfaces should be visually examined for spalling and deterioration due to weathering, unusual or extreme stresses, alkali or other chemical attack, erosion, cavitation, vandalism, and other destructive forces. Structural problems may be indicated by cracking, exposure of reinforcing bars, large areas of broken-out concrete, misalignment at joints, undermining and settlement in the structure. Rust stains that are noted on the concrete may indicate that internal corrosion and deterioration of reinforcement steel is occurring. Spillway floor slabs and upstream slope protection slabs should be checked for erosion of underlying base material otherwise known as undermining. Concrete walls and tower structures should be examined to determine if settlement and misalignment of construction joints has occurred.
What to Look For:
- Concrete structures can exhibit many different types of cracking.
Deep, wide cracking is due to stresses which are primarily caused by shrinkage and structural loads. Minor or hairline surface cracking is caused by weathering and the quality of the concrete that was applied. The results of this minor cracking can be the eventual loss of concrete, which exposes reinforcing steel and accelerates deterioration. Generally, minor surface cracking does not affect the structural integrity and performance of the concrete structure in the short term.
- Cracks through concrete surfaces exposed to flowing water may lead to the erosion of embankment or foundation soils from around and/or under the concrete structure.
In this case, the cracks are not the result of a problem but are the detrimental condition which leads to erosion. Proper underdrainage for open channel spillways with structural concrete floors is necessary to control this leakage. Flows from underdrain outlets and pressure relief holes should also be observed and measured. Cloudy flows may indicate that soil erosion is occurring beneath or adjacent to the concrete structure. This could be detrimental to the foundation support.
- Concrete surfaces adjacent to contraction joints and subject to flowing water are of special concern especially in chute slabs.
The adjacent slabs must be flush or the downstream one slightly lower, to prevent erosion of the concrete and to prevent water from being directed into the joint during high velocity flow. All weep holes should be checked for the accumulation of silt and granular deposits at their outlets. These deposits may obstruct flow or indicate loss of support material behind the concrete surfaces. Tapping the concrete surface with a hammer or some other device will help locate voids if they are present as well as give an indication of the condition and soundness of the concrete. Weep holes in the concrete are used to allow free drainage and relieve excessive hydrostatic pressures from building up behind the structure. Excessive hydrostatic pressures behind the concrete could cause it to heave or crack which increases the potential for accelerated deterioration and undermining. Periodic monitoring of weep hole drains should be performed and documented on a regular and routine basis to ensure that they are functioning as designed.
- Structural cracking of concrete is usually identified by long, single or multiple diagonal cracks with accompanying displacements and misalignment.
Cracks extending across concrete slabs which line open channel spillways or provide upstream slope wave protection can indicate a loss of foundation support resulting from settlement, undermining, or erosion of foundation soils. Erosion of foundation soils is the result of inadequate underdrainage and/or cutoff walls. Items to consider when evaluating a suspected structural crack are the concrete thickness, the size and location of the reinforcing steel, the type of foundation, and the drainage provision for the structure. Because many features are hidden beneath the concrete, an as-built drawing may be critical for a proper evaluation.
- Inspection of intake structures, trashracks, upstream conduits, and stilling basin concrete surfaces that are below the water surface are not readily accessible during a regularly scheduled inspection.
Typically, stilling basins and downstream toes require the most regular monitoring and major maintenance. Stilling basins are holding ponds for rock and debris, which can cause extensive damage to the concrete surfaces during the dissipation of flowing water. The downstream toe of the dam and the stilling basin are also susceptible to undermining. Undermining of the downstream toe could affect the structural integrity and performance of the dam. Therefore, special inspections of these features should be performed at least once every five years by dewatering the structure. Investigation of these features using experienced divers may be needed.
For Questions or Comments Contact:
New York State Department of Environmental Conservation
Bureau of Flood Protection and Dam Safety, Dam Safety Section
625 Broadway, 4th Floor
Albany, NY 12233-3504
Telephone: (518) 402-8185
NYSDEC publication, "Guidance For Developing An Emergency Action Plan" (PDF,1.2 MB)
NYSDEC publication entitled, "Owners Guidance Manual for the Inspection and Maintenance of Dams in NYS"