By David Metcalfe
Accelerated precast construction of culverts and small bridges is the use of innovative scheduling, delivery, design, manufacturing and construction methods to reduce on-site construction time and costs, while improving safety and reducing impacts on road users.
The growing specification of accelerated precast construction (APC) in Canada by transportation planners and designers focuses on identifying efficient ways to build transportation-related structures in a time of increased public demand for infrastructure rehabilitation, expansion and replacement, along with the ongoing demand for skilled labour. The inclusion of engineered structural mesh in precast concrete structural design may provide immediate relief to limited municipal and provincial infrastructure budgets, while contributing to the resiliency of modern highway systems.
Precast reinforced concrete boxes, arches and other three-sided spans are used not only for culverts and small bridges, but also for drainage and retention/detention components of stormwater management plans. Box and pipe culverts are in fact underground bridge structures. Simultaneous construction activity is possible using precast boxes and slabs for bridge deck applications. More than one structure on a project can be assembled at the same time, thereby speeding up project delivery.
This lowers costs for local communities in several ways. Reduced project delivery time minimizes traffic delays, community disruption, and the land required for rights of way, temporary alignments and utility relocation. Shortened installation time reduces impacts on the environment and the possibility of weather-related delays.
Accelerating the repetitive production of standard precast products with engineered structural wire mesh rather than with rebar contributes significantly to reduced project delivery time. Precast elements can be delivered to construction sites within schedules for stock piling or for just-in-time delivery.
Structural welded wire reinforcement (WWR) offers several other benefits over the use of traditional rebar:
- WWR is made from higher yield strength steel, up to 550 MPa (80 Ksi) compared to regular 400 MPa (60 Ksi) rebar.
- Steel content of a precast structure could be reduced by as much as 27%.
- WWR is easier and faster to install, with reduced set-up time and smaller crews than for individual rebar placing and hand tying. Savings can be as much as 50% – 80%.
- Smaller wire mats (more closely welded together and equally spaced) result in improved crack control in precast products.
- WWR ensures proper positioning in mold forms with the added benefit of hassle-free inspections.
- WWR can be bent and rolled to fit a wide range of precast and poured-in-place structures, including pipe and boxes.
- Construction with mesh-reinforced products can be as much as 20% faster than projects that specify standard rebar reinforcement.
- Using welded wire reinforcement makes the placing or assembly processes easier and thereby contributes to a safer workplace. There is a decrease in the risk of work-related accidents.
- Deformed and welded bars create a solid anchor in concrete and there is no reinforcement shifting while concrete is being poured.
- WWR mats are generally available in widths up to 10 feet and lengths up to 39 feet.
Much investment has been made into the technology of precast boxes for culverts in recent years. Not too long ago, the largest precast box was 3.75 m x 3.75 m. Some customers needed larger structures to accelerate construction schedules. Precasters responded with concrete boxes that are 6.25 m wide and 4.4 m high. Engineered structural wire mesh can accelerate installation time because less steel area is required for the same resistance. Full compliance with standards and codes is assured during installation.
The health and safety of the travelling public is of paramount interest to all involved in the construction of highway infrastructure. The speed of accelerated precast construction, facilitated by precast concrete elements produced with engineered structural wire mesh, not only ensures safe transportation systems, but ones that are resilient to natural and man-made disasters.
Changing weather patterns are affecting the hydraulics of critical infrastructure systems, such as storm sewers, culverts and small bridges in many urban areas.
David Metcalfe, C.E.T., is with Numesh Inc. This article appears in ES&E Magazine’s February 2016 issue.
I had no idea there were so many different uses and variations of wire mesh! I heard that there is a wire mesh that is so strong that you have to cut it with angle grinders. I’ve never really thought of using wire mesh for my construction projects, but I bet it would be a really great option, now that I think about it. I think a key point that I would be looking for in a mesh would be durability. If I could find durable mesh that could withstand a lot for a long time, then I would totally go for it. Plus, I bet the combination of mesh and rebar, as mentioned, would be indestructible.
There was a bridge built near the freeway where I live. I was so curious as to what that metal mesh was used for. I had no idea that construction companies used that so that they can form multiple structures at the same time to speed up the process. It is so cool that I now learned what that was used for! Thanks!
Concrete on its own is strong, but with wire, it is stronger. That’s why if I had to do make something out of concrete, I’d do it this way. Now, are there major differences between doing it with or without?
Concrete reinforcement is very interesting to me because of what it allows. I did not know that there is a precast system that contractors can buy. This seems really interesting to me since they are shipped out also.
based on my years of study and research I find blue concrete to be the strongest module of concrete world. Using these would reinforce a 4″ thick concrete to withstand 50 tons of pressure. Please advise that the policy varies from person to person and application.