By Randona Conrad
Many municipalities, transportation organizations, private companies and other sectors that manage the maintenance of culverts are beginning to see the benefits of trenchless technologies for repairing damaged culverts. Engineers and maintenance professionals are often quite well acquainted with cast-in-place liners, centrifugally cast concrete, slip-lining and other technologies for repairing culverts. But, few have been introduced to the emerging technology of concrete cloth as a tool for culvert remediation.
Trenchless technology is gaining favour not only for reducing project timelines and cost, but also for environmental considerations. Dr. John W. Heavens, a former Technical Secretary with the International Society for Trenchless Technology, states: “When you include social costs such as lost time and accidents, which are frequently twice as high as direct costs, the trenchless method (referring to all types) nearly always has the lowest costs and is the most environmentally responsible choice.”
In a 2005 study by the National Research Council of Canada, researchers found that social costs can add up to 400% of construction costs. This is particularly relevant when culverts are in high traffic areas or buried deeply.
The key to utilizing many trenchless technologies is planning ahead. Study after study has found that completing a culvert inventory and regular condition assessments saves organizations from costly failures. Preventative measures may be overlooked when working with tight budgets. However, it is always less expensive in the long run to address deterioration, rather than let a known condition go on until a failure occurs.
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Concrete cloth allows for truly excavation-free culvert lining. Some trenchless technologies are not completely excavation-free as an insertion pit is needed for pipe bursting or slip lining methods. With up to 200 m2 on a single pallet and the potential to complete work without any heavy equipment, this solution offers an option that requires an extremely small staging area and very minimal environmental impact.
The geosynthetic cementitious composite mat is a flexible cement impregnated fabric that hardens on hydration to form a thin, durable waterproof layer. It is composed of a hydrophilic layer of canvas, an internal fibre matrix that holds a proprietary blend of high early strength concrete with a limited alkaline reserve, and a PVC backing. The PVC backing on one surface ensures that the material is completely waterproof and chemically resistant. Unlike some concretes, it is not classified as an irritant and has even been approved for use in live water courses.
The three-dimensional fibre matrix is a key feature of this material, particularly in culvert applications. It traps the concrete powder to make this material ideal for use in water. It cannot be over hydrated and will initially set after only two hours, curing to 80% strength within 24 hours. Testing, done to indicate the effect of underwater setting, shows that concrete cloth loses only 3% by mass.
This solution is designed for culverts greater than one meter in diameter that have eroded along the invert, but do not suffer from a diminished structural capacity. It provides a smooth interior surface for minimal loss of flow capacity and minimizes the potential for debris build up, which lowers maintenance costs. It is ideal in situations where capacity is an integral part of the design. By lining the culvert with a thin (5 mm-13 mm) layer of geosynthetic cementitious composite mat, capacity is not significantly reduced.
In September of 2015, concrete cloth was used to line a culvert for the City of Tacoma, Washington. The culvert was rusting but was still structurally sound. Removing the culvert for replacement would have been incredibly costly. Concrete cloth can typically be installed and the projects completed for as little as 2% of the cost of replacement. It is approximately 30%-70% of the cost of other trenchless lining solutions.
For environmentally sensitive areas, or difficult to reach sites, concrete cloth does come in a batch roll format which is man-portable and allows construction without heavy machinery. In this case, a bulk roll was held above the culvert, using a small backhoe with a spreader beam attachment. It was pulled into the culvert to cover some 52 metres. The roll was then cut using a utility knife and a second strip was pulled in along the length of the culvert.
The layers of concrete cloth were attached every 200 mm to the side of the culvert using self-tapping screws, with washers for added strength.
The free edge of the concrete cloth was sealed to limit future high water infiltration between it and the pipe. Once the concrete cloth had been fixed to the edges along the length of the culvert, workers used a caulking gun to apply two lines of Skia 1-A sealant to a 100 mm overlap at the seam.
This sealant and the joint ensure that the concrete cloth is waterproof. Self-tapping screws with washers were applied along the joint, securing the centre of the concrete cloth to the culvert. It was hydrated using a 25 mm fire hose with water from a local water line.
One day was used to prepare and stage the project. Installation took a crew of eight workers two eight-hour days.
Randona Conrad is with Nuna Innovations Inc. This article appears in ES&E Magazine’s April 2016 issue.