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Why do they put sand under concrete?

The simple answer is to prevent the concrete from cracking and breaking. If you provide a good foundation for your concrete, it will last longer, and you won't have to re-pour it in a few years.

angi.com - Can You Use Sand as a Base for Concrete?

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Using sand as a concrete base may create trouble down the road

Get quotes from up to 3 pros ! Enter a zip below and get matched to top-rated pros near you. Find pros Just as every home needs a good foundation, your property’s concrete slab needs a good subbase to function properly. Contractors sometimes use sand as a subbase when pouring concrete, but it all depends on the slab’s function as well as your region’s climate. A good contractor will know your area and help guide you in the right decision, but here are some things to keep in mind.

Why Do You Need a Base?

The simple answer is to prevent the concrete from cracking and breaking. If you provide a good foundation for your concrete, it will last longer, and you won’t have to re-pour it in a few years. If you don’t, any shifting that goes on in the dirt below your slab will cause cracks and blemishes that can be unsightly at best and dangerous in more extreme circumstances.

Subgrade vs. Subbase

Subgrade and subbase are two terms thrown around when talking about pouring concrete, and though they look similar, they refer to different things. The subgrade is soil (usually the soil already on the property) tamped down or compacted to withstand any weight loads. The subbase is a material brought in and laid down on the subgrade to provide more stability for the poured concrete.

Why Doesn’t Sand Work as a Subbase?

Simply put, sand isn’t sturdy enough to work well as a subbase for something like a driveway. With areas like patios, which don’t require a ton of load-bearing, sand as a subbase works fine, though it’s not as reliable as gravel. Sand doesn’t have the same load-bearing capacity, is too mobile even when tamped down, and moves if exposed to excessively wet or extremely dry conditions. It is also difficult to maintain a level sand surface when pouring concrete, and therefore difficult to maintain a uniform thickness of the concrete slab. Using sand as a base also requires more concrete; the softer base beneath the slab means you have to use at least an additional inch of poured concrete.

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Related Article 5 Tips for Preventing Unsightly Concrete Cracks While you can't avoid every concrete crack, you can help keep most at bay

What Materials Work as a Subbase?

When pouring concrete—especially for something like a driveway—most contractors choose gravel as their subbase. The “grade” of the gravel (the size of the pebbles) depends on the project. Most local building departments have some guidelines on the grade of gravel they require for construction projects. The American Asphalt Institute and American Concrete Institute also have recommended driveway gradation sheets on their websites. If all else fails, your local street or highway department usually has standard materials that they use when pouring roads and freeways. Your materials supplier should know what the subbase materials are and be able to supply them to you.

Will Sand Always Cause Cracks in Concrete?

Not necessarily. The tendency of sand to shift and cause cracks depends on several factors, including the environment, conditions when it was laid down, and whether or not it was correctly compacted during installation. It also depends on the size of the loads placed on the slab, such as a patio (people, furniture) versus a driveway (cars). As a general rule, stay away from using sand as a subbase. You’re better off using gravel; your concrete will last longer and have less of a chance of cracking over time.

angi.com - Can You Use Sand as a Base for Concrete?

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