Modern concrete used in precast manhole structures is strong, dense and durable. However, in aggressive environments, additional protection – such as impervious liners or coatings – may be needed. When these protective measures are included, they can increase the service life of the manhole.
Prior to adding a coating, the customer should take cost and application into consideration. Using a coating to waterproof or damp proof the wall of a precast concrete manhole manufactured in accordance with ASTM C478, “Standard Specification for Circular Precast Reinforced Concrete Manhole Sections,” adds substantial cost to the structure.
Concrete mixes used in today’s precast structures must provide a minimum compressive strength of 4,000 psi. In practice, many precast products as installed on the job site far exceed this minimum. These structures also tend to have a much lower water-cement ratio due to the use of water-reducing admixtures or dry-cast processes.
Various conditions can lead to damp walls in such concrete structures, including even simple atmospheric condensation from a temperature differential between the air and wall. In many cases, the damp walls are resolved by applying a bitumen coating to the outside of the structure. But is such a coating necessary in every case?
Infiltration through the wall has rarely been a point of concern. Infiltration through the barrel portion of a manhole wall is remedied in today’s concrete precast products by using the NPCA Quality Control Manual for Precast Concrete Plants. Quality issues can also be identified via vacuum testing in the plant or field per ASTM C1244, “Standard Test Method for Concrete Sewer Manholes by the Negative Air Pressure (Vacuum) Test Prior to Backfill.”
A good trade-out to consider in your municipal sanitary sewer manhole standard or specification is to eliminate the exterior coating and replace it with NPCA Plant Certification along with vacuum testing requirements in accordance with ASTM C1244.
The best solution for the demanding job of conveyance of sanitary or stormwater – one that combines durability, quality and structural integrity – has been and remains precast concrete. To learn more about watertightness, please read 5 Rules of Watertightness or contact Eric Carleton, P.E., director of codes and standards for NPCA.