Many say it’s impossible to build a concrete structure or slab that doesn’t crack, but our experience shows otherwise. It all comes down to the right materials, the right procedures, and the right builder.
Eliminating or minimizing cracks in concrete is something we strive for in every job, and occasionally it’s an absolute requirement. Several years ago, we built a new parking structure for a prestigious museum in Los Angeles. The client wanted the parking structure to match the flawless aesthetics of the museum. 30 years later, the quality of the concrete still shows. In other cases, it may be an environmental requirement, such as a waste holding tank that must never leak or leech its contents. While you could use epoxy coatings, they are difficult to maintain and require expensive re-coating over time. It’s better to do it right the first time.
What causes concrete to crack in the first place? Typically, concrete cracks when the ground underneath shifts or settles, when the concrete shrinks during the drying process, or due to inconsistencies in the concrete, whether from the way it was placed, or in the material itself. In each case, the risk of cracking can be mitigated with the right process and materials.
Preparing the soil: When it comes to soil, the key is making sure that the existing soil, and any backfill, has been properly compacted. Before we start any concrete work, we verify compaction and soil conditions.
Mix design: You can minimize cracking by designing the right concrete mix. Aggregate types and sizes, chemistry and additives, and water content are all factors in minimizing cracking.
Reinforcing bar: While the main purpose of rebar is to add tensile strength (concrete has a lot of compression strength but relatively little tensile strength), that tensile strength helps minimize cracking. The type, amount, and layout of reinforcement all make a difference.
Placement: You can do everything else right, and have the best mix design, but if the concrete isn’t placed properly you’ll eventually have cracks. Whether you use a boom truck, a pump, or place from the chute all have an effect on uniformity and cracking resistance, and the right tool varies depending on the size and shape of the pour. We also use vibrators to help the concrete flow and reduce air pockets. Be aware that the drivers and operators may add water on-site to help the concrete flow, but adding water beyond what’s specified in the mix increases shrinkage and decreases the overall strength. Watch the process carefully, and do a slump test on-site to ensure the mix meets specifications.
Curing: The final key to minimizing or eliminating cracks is the curing process. You want to help the concrete cure evenly by carefully wetting the structure or slab in the days following the pour. There are also chemical curing compounds that can help.
Crack-proof concrete involves a combination of experience, materials and care. It may sound like it adds expense, but it’s a small and incremental amount that pays dividends in the long run. Having to mitigate cracking once the building is complete, foundations are backfilled, and the finishes are in place gets very expensive.
If you’re about to construct a concrete building, slab, or other structure and you want to consult with an expert, give us a call at (805) 910-8300 or email us at info@andercon.net. We’re always happy to chat.
Sincerely,
The ANDERSON Team