What Is Concrete? The Complete Breakdown

The Four Ingredients of Concrete

Every batch of concrete contains four things. Get the ratios wrong and you'll have problems. Here's what goes in:

1. Portland Cement (The Glue)

Cement is NOT concrete—it's the ingredient that makes concrete work. Portland cement is a fine gray powder made by heating limestone and clay to 2,700°F in a kiln. When water hits it, cement undergoes hydration: calcium silicates react with water to form crystite bonds.

Cement makes up 10-15% of the mix by volume. Too much cement makes concrete prone to cracking. Too little makes it weak. The ratio matters.

2. Water (The Catalyst)

Water triggers the chemical reaction. The water-to-cement ratio is the single most important factor in concrete strength. Standard ratio: 0.45-0.55 (45-55 pounds of water per 100 pounds of cement).

More water = easier to work with but weaker concrete. Less water = stronger but harder to pour. Never add extra water to make concrete easier to spread—you're destroying its strength.

3. Fine Aggregate (Sand)

Sand fills the gaps between larger stones and gives concrete workability. It makes up about 25-30% of the mix. The sand should be clean—no dirt, clay, or organic matter. Beach sand doesn't work because salt causes corrosion and the grains are too rounded.

4. Coarse Aggregate (Gravel or Crushed Stone)

This is the backbone of concrete—the rocks that give it mass and strength. Coarse aggregate makes up 30-45% of the mix. Size matters: typical driveways use 3/4-inch stone, while structural concrete might use larger aggregate.

How Concrete Actually Works

Concrete doesn't "dry"—it cures. This is the most misunderstood thing about concrete.

When you mix cement and water, a chemical reaction called hydration begins. Calcium silicate compounds in the cement react with water to form calcium silicate hydrate (C-S-H) crystals. These crystals interlock and bind the aggregate together.

The reaction is exothermic—it generates heat. This is why massive concrete pours can actually get hot enough to cause problems. It's also why concrete can cure underwater (and actually cures better when kept moist).

The Curing Timeline

Time	Strength	What's Happening
24 hours	~15%	Initial set, can walk on it
7 days	~65%	Light vehicle traffic OK
28 days	~100%	Design strength reached
Years	105-110%	Continues to strengthen slowly

Concrete vs. Cement: The Difference

People use these words interchangeably. They're not the same thing.

Cement is an ingredient—the powder that acts as a binder. You can't build anything with cement alone.

Concrete is the finished product—cement plus water plus aggregates, mixed and cured.

Saying "cement driveway" is like saying "flour cake." Technically wrong, but everyone knows what you mean.

Types of Concrete Mixes

Not all concrete is the same. Different jobs need different strengths.

By PSI Rating

• 2,500 PSI: Sidewalks, patios, light residential. The minimum.
• 3,000 PSI: Standard residential driveways, garage floors.
• 4,000 PSI: Heavy-duty driveways, commercial floors. What pros use.
• 5,000+ PSI: Industrial applications, heavy equipment pads.

By Mix Type

• Ready-mix: Delivered by truck, mixed at a plant. Most common.
• Bagged mix: DIY bags from hardware stores. Good for small projects.
• High-early: Gains strength faster. Costs more.
• Fiber-reinforced: Contains synthetic fibers for crack resistance.
• Air-entrained: Has tiny air bubbles for freeze-thaw resistance.

Why Concrete Cracks

All concrete cracks. The question is whether you control where it cracks.

Concrete shrinks as it cures—about 1/16 inch per 10 feet. If you don't give it a place to crack (control joints), it will crack randomly. Control joints are the lines cut into sidewalks and driveways. They create weak points so the concrete cracks there instead of across the middle of your slab.

Other causes of cracking:

• Too much water in the mix (most common cause)
• Curing too fast (hot, dry, windy conditions)
• Poor subgrade preparation (soft spots underneath)
• Missing reinforcement (no rebar or wire mesh)
• Freeze-thaw cycles (water gets in, expands, breaks apart)

What Is PSI and Why Does It Matter?

PSI stands for pounds per square inch—the compressive strength of concrete. A 4,000 PSI mix can handle 4,000 pounds of pressure on every square inch before crushing. For driveways, 3,500-4,000 PSI is the sweet spot. Sidewalks can get away with 3,000 PSI. Anything structural needs an engineer's specification.

Can Concrete Be Recycled?

Yes. Old concrete gets crushed into recycled concrete aggregate (RCA) and used as base material for new construction. It's common in road building. Some new concrete mixes incorporate RCA as a portion of the aggregate, though it's not as strong as virgin material.

How Long Does Concrete Last?

Properly installed residential concrete lasts 25-50 years. Commercial and infrastructure concrete is designed for 50-100+ years. The Romans built concrete structures 2,000 years ago that still stand. Longevity depends on the mix, installation quality, and environmental exposure.

Is Concrete Waterproof?

No. Concrete is porous—water can pass through it slowly. That's why basements can get damp and why sealers exist. Waterproofing concrete requires additives or coatings. Some specialized mixes are more water-resistant, but no concrete is truly waterproof without treatment.

What's the Difference Between Concrete and Asphalt?

Concrete uses cement as a binder. Asphalt uses bitumen (petroleum-based tar). Concrete is rigid, lasts longer, and handles heat better. Asphalt is flexible, cheaper upfront, but needs more maintenance. For driveways, concrete costs more initially but often wins on lifetime cost.

Key Takeaways

• Concrete = cement + water + sand + gravel (cement is just one ingredient)
• The water-to-cement ratio determines strength—less water means stronger concrete
• Concrete cures through hydration, not drying—keep it moist for best results
• Standard driveway concrete is 3,500-4,000 PSI
• Full strength takes 28 days; the concrete keeps getting stronger for years
• All concrete cracks—control joints tell it where to crack
• Properly installed concrete lasts 25-50+ years

Need help with your concrete project? Get a free quote.