Often in our meetings and presentations, people want to know, "Do you have problems with cracking in your concrete boardwalks?" It's a fair question-- people see cracks in sidewalks, parking garages, temporary highway barriers, etc. Nobody wants to see big, ugly cracks in the walking surface of a new boardwalk application. I've spent the last 20+ years of my professional life designing precast concrete structures and I wanted to dive into this issue to explain two things:
1. How and why precast concrete cracks
2. Our experience with the PermaTrak boardwalk system as it relates to cracking
Precast Concrete Cracking - Explanation and Design Guides
To start:
- Precast concrete does crack, it is designed to do so. If concrete didn't crack, the steel reinforcing wouldn't engage.
- Cracking does not necessarily mean the structure has failed or has lost its structural capacity.
Why does it happen? From the perspective of a structural engineer, I'll do my best to answer this. 
Precast concrete can handle very large compression loads compared to its relatively small tensile loads. If a concrete member “feels” much tension, cracks are likely. This is where the steel reinforcing comes in to help. Properly sized and positioned reinforcing will handle the tensile loads while the concrete resists the compressive loads. Together, they're a perfect match as a building material.
Structural engineers are trained in the science of sizing the reinforcing and determining its correct position in the concrete member. The reinforcing minimizes the crack width so that it meets allowable crack width criteria. Engineers generally use one of two design guides to design for cracking: ACI 318-11 (Building Code Requirements for Structural Concrete and Commentary) and AASHTO LRFD Bridge Design Specifications.
Our engineering team uses the crack control equation when designing a boardwalk system. A crack width is computed to define allowable crack widths. Allowable crack widths for concrete members in exterior exposures are more severe (smaller widths allowed) than interior exposures (larger widths allowed). Similarly, concrete members in watertight applications (i.e. wastewater treatment tanks) are designed for smaller crack widths then other applications. The American Concrete Institute (ACI) provides engineers with limitations for allowable crack widths based upon the situation.
We design our concrete boardwalks in accordance with this crack control equation, under exterior exposure conditions. The precast concrete components that make up a PermaTrak system are designed to control crack widths when supporting design loads and handling lifting stresses. By controlling the width of the crack at the component's surface, you also control the depth of the crack so it doesn't reach the steel reinforcing.
There are three important considerations regarding crack control during the structural design process of our product:
1. Handling of the PermaTrak components at the production facility
2. Loading and unloading on a flatbed truck
3. Transportation of the components to the jobsite
Our Experience with Cracking on the PermaTrak Boardwalk System
Now that you know a little more about how and why precast concrete cracks, I'll answer the question you probably care most about: "Does PermaTrak have a lot of cracks in our concrete boardwalk system?"
As I mentioned above, some cracking is acceptable as long as it meets ACI criteria. For our structures that acceptable width is 0.016''. To give you an idea, 0.016'' is about half the width of a standard business card. While that gives us a baseline for crack acceptability, that doesn't serve as a standard for PermaTrak. The truth is PermaTrak won't let any cracks get close to that width.
I'll give you a real life example from a boardwalk project in Florida, installed summer of 2011. The installing contractor had unloaded and stored our PermaTrak material on site, before completing the installation. Over the weekend, a landscaper working near the boardwalk needed to move two PermaTrak beams sitting off to the side. Unfortunately, he picked up the two beams from the center, rather than having a wide spread on his forklift. These beams weren't meant to be handled this way and the landscaper had no contact with our team regarding proper handling techniques. Multiple cracks developed near the outside 1/3 of the beams. Instead of using those beams as intended, we simply replaced them with two new beams and shipped them to the jobsite.
Over the last three years, we have designed and supplied 50+ boardwalk structures. We have seen less than 10 tread components with cracks. All of these would meet the ACI criteria for cracking, but we understand it doesn't look attractive. We are focused on a quality product and durable boardwalk structure that pleases our clients. If a concrete tread cracks during loading, unloading or maneuvering, we can simply produce another tread and replace the one in question.
Hopefully that helps explain the science behind precast concrete cracking, as well as our approach on cracking for PermaTrak's boardwalk system. You can always send me a quick email if you have any other specific questions.
Jason Philbin, P.E., M.ASCE
President, PermaTrak North America
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