Here is the science behind Concrete.
Most people think concrete "dries." It does not. It cures. Concrete can harden underwater. It is a chemical reaction, not an evaporation process.
1. The Reaction: Hydration (Growing Crystals)
Concrete is a mixture of paste (cement + water) and aggregates (sand + rock). The paste is the glue; the aggregates are the filler.
The Science: Exothermic Crystalline Growth
When water hits Portland cement, it triggers an Exothermic Reaction (it generates heat).
The Gel: As mentioned in soil stabilization, the cement grains dissolve and grow spiky crystals called Calcium-Silicate-Hydrate (C-S-H) gel.
The Interlock: These microscopic spikes grow outward, latching onto the sand and gravel, and interlocking with each other. This turns the liquid soup into a solid rock.
The "Set": The initial "set" happens when these crystals touch for the first time. But the reaction continues for years as long as moisture is present.
2. The Golden Rule: Water-Cement Ratio (w/c)
This is the single most important number in concrete science. It dictates strength and durability.
The Physics: Space and Voids
Water has volume.
Necessary Water: You only need a w/c ratio of about 0.25 to chemically hydrate all the cement.
Convenience Water: We usually mix at 0.45 to 0.50. Why? Because 0.25 mix is dry and unworkable. We add extra water just to make it flow (slump) so workers can place it.
The Cost of Flow: The extra water eventually evaporates. When it leaves, it leaves behind Capillary Pores (microscopic air voids).
The Rule: More Water = More Voids = Weaker Concrete. Every extra gallon of water you add to the truck weakens the final product and increases cracking.
Modern Solution: We use Superplasticizers (Admixtures). These are chemicals that impart an electrical charge to cement grains, causing them to repel each other. This makes the concrete flow like a liquid without adding extra water.
3. Reinforcement: The Perfect Marriage
Concrete has a fatal flaw: It is strong in Compression (being squashed) but terrible in Tension (being pulled apart).
Compressive Strength: ~4,000 psi (Strong).
Tensile Strength: ~400 psi (Weak—only about 10% of compressive strength).
The Science: The Composite Beam
To fix this, we insert steel (rebar).
The Beam Theory: When you load a beam (like a floor slab), the top tries to squash (Compression) and the bottom tries to rip apart (Tension).
The Load Transfer: The concrete handles the crushing force at the top. The steel handles the pulling force at the bottom.
The Miracle of Physics: Steel and Concrete have almost the exact same Coefficient of Thermal Expansion.
If they expanded at different rates when it got hot, the steel would shatter the concrete from the inside. Because they expand together, they act as one material.
4. Curing: The Critical Timeline
"Curing" is the maintenance of adequate moisture and temperature to allow hydration to continue.
The Physics: Plastic Shrinkage
The most dangerous time for concrete is the first 24 hours.
Evaporation vs. Bleed: As concrete settles, water "bleeds" to the surface. If the sun/wind evaporates surface water faster than the bleed water can replace it, the surface dries out and tears. These are Plastic Shrinkage Cracks.
Why we wet cure: By flooding the slab or using curing blankets, we trap the moisture inside. This forces the water to react with the cement rather than evaporating into the atmosphere.
The 7-Day vs. 28-Day Rule:
7 Days: Concrete reaches ~70% of its strength.
28 Days: The industry standard benchmark for 100% design strength .
5. Pre-stressing vs. Post-Tensioning
Sometimes standard rebar isn't enough. We need Active Reinforcement.
The Science: The "Rubber Band" Effect
Imagine a row of books on a shelf. If you lift them, they fall apart. If you squeeze the ends together tight, you can lift the whole row as one unit.
Post-Tensioning (PT): We lay cables (tendons) inside the concrete. Once the concrete cures, we pull the cables tight (tension them) to 33,000 lbs of force and lock them off.
The Physics: This puts the entire slab into a permanent state of Compression. Since concrete is strongest in compression, this makes the slab incredibly strong and prevents cracks from ever opening up (because the cable is constantly squeezing them shut).
Concrete Site Checklist
Use this to spot issues during a pour:
Check The "Why"
"Adding Water" If the crew asks the driver to add water to the truck, stop them. It creates weak spots and void warranties. Use plasticizer instead.
Vibration Concrete must be vibrated to release trapped air pockets (honeycombing). If they don't vibrate, the concrete will have Swiss-cheese holes.
Rebar Chairs Rebar must be suspended in the middle of the slab. If it's laying on the dirt (no chairs), it will rust and fail to provide tensile strength.
Cold Joints If the trucks are late and the first load hardens before the second arrives, the two won't bond. You get a permanent crack called a "Cold Joint."
