Concrete Glossary — Cement and Concrete Terms Defined

Admixture: A chemical added to a concrete mix in small amounts to change how it behaves — accelerators speed setting, retarders slow it, water reducers keep it workable with less water, and air-entraining agents add freeze-thaw resistance. Read the guide →
Aggregate: The sand (fine aggregate) and gravel or crushed stone (coarse aggregate) that make up most of concrete's volume and strength. Cement paste glues the aggregate together. Read the guide →
Air entrainment: Tiny air bubbles deliberately mixed into concrete so that water freezing inside has room to expand, preventing surface scaling in cold climates. Read the guide →
Bleed water: Water that rises to the surface of fresh concrete after it is placed. You must wait for the bleed-water sheen to evaporate before troweling, or the surface will be weak and dusty. Read the guide →
Broom finish: A textured surface made by dragging a broom across floated concrete. The texture gives traction when wet, which is why driveways, sidewalks, and patios get one. Read the guide →
Cement (portland): The fine gray powder, made from kiln-fired limestone and clay, that acts as the binder in concrete. Cement is not concrete — it is one ingredient. Read the guide →
Concrete: The finished building material made by mixing portland cement, water, sand, and gravel. The cement and water form a paste that hardens and binds the aggregate into rock. Read the guide →
Control joint: A planned, tooled or sawn groove that pre-weakens a slab in a straight line so shrinkage cracking happens there, out of sight, instead of wandering. Space joints, in feet, about 2 to 3 times the slab thickness in inches. Read the guide →
Cubic yard: The unit ready-mix concrete is sold in — 27 cubic feet. A cubic yard covers about 81 square feet at 4 inches thick. Open the calculator →
Curing: Keeping fresh concrete moist (and at a workable temperature) so it can gain strength. Most strength develops in the first week; full design strength takes about 28 days. Read the guide →
Dry pour: Placing bagged concrete mix dry into forms and wetting it afterward, instead of mixing first. Workable for small, non-structural pads but weaker and less reliable than mixed concrete. Read the guide →
Efflorescence: The white, powdery salt deposit that can appear on cured concrete as moisture moves through it and evaporates. Cosmetic, not structural; it can be cleaned and sealed against. Read the guide →
Expansion joint: A full-depth gap with a compressible filler placed where a slab meets a fixed object — a wall, footing, column, or existing slab — so the two can move independently without cracking each other. Also called an isolation joint. Read the guide →
Finishing: The sequence of working a concrete surface after placing — screeding, floating, edging, jointing, and troweling or brooming — each step done at the right moment. Read the guide →
Float: A flat tool (and the step of using it) that smooths and levels fresh concrete and pushes the stone down, bringing a workable paste to the surface before final finishing. Read the guide →
Footing: A widened concrete base below a wall, column, or post that spreads its load onto the soil. Footings are usually placed below the local frost line. Read the guide →
Hydration: The chemical reaction between cement and water that hardens concrete. It is hydration, not drying, that gives concrete its strength — which is why curing keeps it moist. Read the guide →
Mix ratio: The proportion of cement to sand to coarse aggregate in concrete. The classic all-purpose mix is 1:2:3 (1 part cement, 2 sand, 3 gravel), which yields about 3,000 psi. Read the guide →
PSI: Pounds per square inch — concrete's compressive strength. Common grades run from 2,500 psi for light slabs to 4,000 psi for driveways and structural work. Read the guide →
Ready-mix: Concrete batched at a plant and delivered by truck, sold by the cubic yard. Past about a yard of material, ready-mix usually beats hand-mixing on cost and consistency. Open the calculator →
Rebar: Reinforcing steel bar embedded in concrete to hold cracks tight and let the slab act as one piece. Residential work commonly uses #3 or #4 bar in a 12-to-18-inch grid. Read the guide →
Reinforcement: Steel or fiber added to concrete to carry tension — rebar, welded wire mesh, or fibers. Structural and load-bearing slabs need it; thin non-structural pads often do not. Read the guide →
Retarder: An admixture that slows concrete's set, buying working time in hot weather or on large pours. Read the guide →
Screed: A straightedge — and the step of using it — to strike off and level fresh concrete to the top of the forms right after placing. Read the guide →
Sealer: A coating or penetrating treatment that protects cured concrete from water, stains, and de-icing salt. New concrete is usually sealed after about 28 days of curing. Read the guide →
Slab: A flat, horizontal pour of concrete — a floor, patio, driveway, or pad. Most residential slabs are 4 inches thick over a compacted base. Open the calculator →
Slump: A measure of fresh concrete's wetness and workability, from a standard cone test. Higher slump is wetter and easier to place but, if from added water, weaker. Read the guide →
Spalling: Flaking or chipping of a concrete surface, usually from freeze-thaw cycles or de-icing salt attacking porous or poorly cured concrete. Sealing helps prevent it. Read the guide →
Subgrade and subbase: The compacted soil (subgrade) and the layer of compacted crushed stone (subbase) beneath a slab. A good base is the single best defense against settlement cracking — about 4 inches of compacted stone is standard. Read the guide →
Trowel finish: A smooth, dense surface made by working floated concrete with a steel trowel. Ideal for interior floors; too slick for outdoor flatwork, which gets a broom finish instead. Read the guide →
Vapor barrier: A polyethylene sheet (6-mil or thicker) laid over the base under an interior slab to stop ground moisture from wicking up through the concrete. Read the guide →
Waste allowance: Extra concrete ordered beyond the exact calculated volume — typically 5 to 10 percent — to cover spillage, uneven subgrade, and over-excavation so you don't come up short. Open the calculator →
Water-cement ratio: The weight of water divided by the weight of cement in a mix — the single biggest lever on strength. Lower (around 0.45 to 0.55) means stronger, more durable concrete. Read the guide →
Welded wire mesh: Sheets or rolls of welded steel wire used as light reinforcement in slabs, an alternative to a rebar grid for non-structural flatwork. Read the guide →
Workability: How easily fresh concrete can be placed, consolidated, and finished. More water improves workability but lowers strength, so pros use the stiffest mix they can place well. Read the guide →
Yield: How much volume a bag of concrete mix produces. An 80-lb bag yields about 0.6 cubic feet, a 60-lb about 0.45, and a 40-lb about 0.30. Open the calculator →