Aggregate is the sand, gravel, and crushed stone that makes up 60 to 75 percent of a concrete mix, giving the finished slab its bulk, stability, and compressive strength.
In depth
Cement paste is the glue, but aggregate is the body of concrete. The size, shape, and gradation of the aggregate affect how strong the slab is, how well it resists wear, and how it behaves through freeze-thaw cycles. Well-graded aggregate packs tightly, needs less water, and produces denser, more durable concrete.
Poorly graded or dirty aggregate weakens the mix and can contribute to surface problems like scaling and spalling years later. Aggregate quality is set the day the slab is poured and cannot be fixed afterward, which is one reason two driveways of the same age can be in very different condition.
How Acme applies it. We do not pour new concrete, we save existing slabs. When we evaluate a sunken or cracked slab, the condition of the original mix, including its aggregate, helps us judge whether the concrete is a good candidate for raising or whether it is too deteriorated to lift.
Related terms: Compressive Strength, Scaling, Spalling, Freeze-Thaw Cycle
Frequently asked questions
Does aggregate affect whether my slab can be raised?
Indirectly, yes. A slab with sound aggregate and no widespread deterioration can be lifted safely. If the concrete itself is crumbling, raising it may not be worth the cost.
What is the difference between aggregate and subbase?
Aggregate is inside the concrete mix itself. Subbase is the layer of compacted gravel or stone placed under the slab before pouring.