Concrete is a mixture of cement, sand, coarse aggregate (gravel), and water. The ratio of these ingredients determines the compressive strength, workability, and durability of the finished product. Getting the mix ratio right is the most important factor in concrete quality — too much water weakens it, too little makes it unworkable, and wrong aggregate ratios affect long-term strength.

Understanding mix ratios. Concrete mix ratios are expressed as cement:sand:aggregate by volume. A 1:2:3 mix means 1 part cement, 2 parts sand, and 3 parts gravel. Higher cement content (like 1:1.5:2.5) produces stronger concrete but costs more and increases shrinkage risk. Lower cement content (1:3:6) is used for lean concrete in non-structural applications like sub-base layers.

PSI strength requirements by application. Residential driveways and patios require 3,000 to 4,000 PSI. Structural footings and foundations typically need 3,000 to 3,500 PSI minimum. Columns, beams, and high-load slabs use 4,000 to 5,000 PSI. Sidewalks and non-structural flatwork can use 2,500 to 3,000 PSI. Always check local building codes — they specify minimum PSI for permitted work.

The water-cement ratio. The water-to-cement (w/c) ratio is the single biggest determinant of concrete strength. A ratio of 0.45 or below produces high-strength, durable concrete. A ratio above 0.60 significantly reduces strength and increases permeability. Never add extra water to make mixing easier — use a plasticizer instead if workability is an issue. Each additional gallon of water per cubic yard can reduce compressive strength by up to 200 PSI.

When to use ready-mix vs. bags. Bagged premix concrete is practical for small pours under 0.5 cubic yards — fence posts, small slabs, repairs. For larger pours, ready-mix concrete from a truck is almost always more economical, more consistent, and stronger than field-mixed bagged material. Order 10% more than your calculated volume to account for spillage, forms that aren't perfectly level, and surface irregularities.

Curing is as important as mixing. Concrete gains strength over time through hydration — a chemical reaction that requires moisture. Fresh concrete should be kept moist for at least 7 days to reach adequate strength (about 70% of 28-day strength). At 28 days, properly cured concrete reaches its design strength. Covering with plastic sheeting or burlap and keeping it wet prevents premature drying. Never let concrete dry too fast — hot, windy conditions require accelerated curing measures.