Should I use bagged concrete mix or ready-mix concrete?

Bagged concrete (Quikrete, Sakrete): convenient for small jobs (under 1 cubic yard), no minimum order, no truck access required. Ready-mix: more economical and better quality for large pours (over 1 yard), consistent mix design, no labor of mixing. Breakeven is roughly at 1–2 cubic yards.

How long should concrete cure before putting weight on it?

Foot traffic: 24–48 hours. Vehicle traffic: minimum 7 days (10–14 days preferred). Heavy equipment: 28 days for full strength. These are minimums — cold weather extends required curing time significantly. Don't rush loading concrete slabs.

Concrete Mix Design Guide: Understanding PSI, Slump, and Mix Ratios

Understanding Concrete PSI

PSI (pounds per square inch) is the compressive strength concrete reaches after 28 days of curing. Higher PSI = stronger concrete = more cement in the mix. Common residential specifications:

2,500 PSI: Light-duty — interior floors, non-structural fills. Minimum practical strength.
3,000 PSI: Standard for driveways, patios, sidewalks, residential foundations.
3,500–4,000 PSI: Garage floors, reinforced slabs, driveways with heavy vehicles. Recommended for most residential work in freeze-thaw climates.
4,500–5,000 PSI: Heavy-duty commercial, structural applications.

In freeze-thaw climates, specify concrete with air entrainment (5–7% entrained air) regardless of PSI — it dramatically improves freeze-thaw durability.

Slump: Measuring Workability

Slump measures how far wet concrete settles when a cone form is removed. Low slump (1–3") = stiff, hard to work but stronger. High slump (5–7") = more fluid, easier to pour and spread but weaker. Target 3–5" for most residential pours. Never add extra water to increase slump — it dramatically weakens the concrete.

Water-Cement Ratio: The Key to Strength

The water-cement (W/C) ratio is the most important factor in concrete strength. Less water = stronger concrete. Standard residential concrete: W/C ratio of 0.45–0.50. Adding one extra gallon of water per yard reduces strength by ~200 PSI. Use water-reducing admixtures (plasticizers/superplasticizers) to improve workability without adding water.

Curing: Where Most Concrete Fails

Concrete reaches 70% of 28-day strength by day 7, but proper curing is essential. Keep concrete moist for at least 7 days (cover with plastic or burlap and keep wet, or apply curing compound). Avoid curing in extreme heat (above 90°F) or cold (below 40°F) without protection. Hot weather concrete sets too fast; cold weather prevents proper hydration.