What Segregation of Concrete Is and How to Fix It

Concrete is one of the strongest and most durable building materials we use in construction, but the segregation of concrete can significantly weaken the material, potentially causing structural damage. Whether you’re working with concrete yourself or relying on a professional for help, understanding the risk of concrete segregation can help avoid serious problems down the road. Below, you’ll learn what concrete segregation is, what causes it, the risks involved, and how to fix it.

What Is Segregation of Concrete?

Concrete is a mixture of cement, water, and aggregates ranging in size from sand to gravel. Concrete segregation is when the larger particles fall out of suspension and accumulate in one area in the concrete as it cures. Once the concrete cures, you might see a honeycomb-like pattern on the face of the concrete where the segregation occurred.

What Causes Concrete Segregation?

A few things can lead to concrete segregation, although they all have to do with larger aggregates like gravel settling at the bottom of the poured slab.

Watery Concrete

Making concrete involves adding water to a concrete mix to yield a thick consistency or “concrete slump,” but adding too much water can lead to concrete segregation as it cures. If there’s too much liquid, smaller aggregates can float to the top, which allows larger particles like gravel to sink to the bottom. This is especially an issue if the person pouring the concrete introduces excessive agitation.

Too Much Agitation

Agitating newly-poured concrete is necessary to remove air pockets and create a stronger, more uniform piece of concrete. Too much agitation can be detrimental, though, as it will cause larger, denser particles to fall to the bottom of the mixture, especially if it’s too watery.

Improper Pouring Form

Any agitation of the concrete before it cures can lead to segregation, including while pouring. If you pour concrete from too high above the concrete form, the larger, heavier particles will drop to the bottom as the mixture settles.

Insufficient Mixing

Finally, you might discover segregation in your concrete if you don’t mix it thoroughly before pouring. As concrete sits, the smaller particles will naturally rise to the top, and heavier gravel pieces will sink to the bottom. If you don’t mix the concrete well before pouring, those larger pieces of aggregate may remain at the bottom and not be fully incorporated into the cured concrete.

Are There Risks Associated With Segregation of Concrete?

Concrete segregation comes with some risks, so it’s important to hire a concrete company near you to tackle complicated concrete projects. This holds especially true if you don’t have experience working with concrete or if it will provide any structural support.

Decreased Strength

The biggest issue with segregated concrete is decreased strength. Concrete is extremely strong when the cement acts as an adhesive to “glue” together the small and large aggregates, but some of that strength gets lost if the mixture isn’t uniform.

Loss of strength can be a problem for any concrete structure, but it can be especially dangerous if the concrete slab serves as a foundation for a structure. For this reason, your local building code likely has guidelines for mixing and pouring concrete in a way that avoids the segregation of larger aggregates.

Limited Durability

Segregation of concrete not only reduces the strength of the concrete as a whole but also increases the risk of surface damage, including cracks. You might see higher concrete repair costs or maintenance costs as a result.

Poor Aesthetic

Finally, concrete segregation doesn’t look very appealing and will leave patches of honeycomb-like patterns on the surface of your concrete. For decorative concrete pieces, like walkways, sidewalks, and patios, most homeowners will opt to pay for concrete repair to cover the affected areas. Even then, patches of different colors will likely be visible.

How to Avoid and Fix Segregated Concrete

Before your concrete cures, there are a few things you can do to avoid segregation of the concrete:

• Get the proportions right: If you’re mixing your own concrete, make sure you get the proportions of the mix to the water right. Too little water will reduce the bonding within the concrete, while too much can lead to segregation and decreased strength.

• Use a concrete mixer: A mechanical mixer can mean the difference between properly mixed concrete and segregation. You can often rent mixers from hardware stores.

• Pour from a low height: If you’re pouring concrete from a mixer, keep the spout as close to your form as possible to reduce the risk of the heavier particles settling to the bottom of the form.

• Agitate just enough: Agitating the concrete after pouring it will remove air pockets, but too much agitation can cause the larger aggregates to settle at the bottom.

Once your concrete cures, there are two options you have to fix it:

• Resurface the concrete: If the concrete slab isn’t bearing weight from any structure or vehicle, you might be able to get away with just paying for concrete resurfacing costs. Resurfacing involves removing any loose particles, cleaning the affected area, and pouring in a flowable, self-leveling mortar to fill in the gaps around the large aggregates.

• Remove and re-pour: If your concrete will provide any kind of structural support or serve as vehicle storage, you’ll either need to remove the concrete and start over or cut the concrete out around the affected area and re-pour. You should speak with a structural engineer or a professional who repairs concrete to help you decide which is the right option for you.