Everyone in San Marco has seen the same failure: a driveway with one panel that has dropped a half inch below the rest, usually near where the old sewer lateral was replaced. The concrete did not fail. Something under it moved, and nobody checked before the pour.
Sand is not automatically a good base
Northeast Florida sand drains beautifully, which is why people assume it needs no preparation. Clean uniform sand actually compacts poorly on its own because there is no range of particle sizes to lock together. It densifies under vibration and then loses that density if it is disturbed, saturated, or loaded unevenly.
Then there is what is under the sand. Near the river you get organic silts and soft clay layers at depth. A slab spanning a soft pocket is what cracks, and the pocket is invisible from the surface.
What a real base build looks like
- Strip organics. Every root, every bit of topsoil. Organic material rots and leaves a void.
- Place in lifts. Six to eight inches loose, compacted before the next lift goes on. Dumping eighteen inches and running a plate over it compacts the top four and nothing else.
- Get the moisture right. Sand compacts best slightly damp. Bone-dry sand shifts under the plate; saturated sand pumps. If you can squeeze a handful into a shape that just holds, you are close.
- Use a graded base over the sand where the loading justifies it. Crushed limerock at four to six inches gives you the particle-size range that clean sand lacks.
- Proof roll before you form. Drive a loaded truck across it slowly and watch for rutting or springing. Anything that moves under the wheel gets excavated and replaced. This takes twenty minutes and finds the problems that cost thousands.
The three defects I keep finding
Utility trench backfill. A trench backfilled with spoil and tamped by a shovel handle will settle for years. Any slab crossing a recent trench needs that trench excavated and rebuilt in compacted lifts, or the slab needs to be reinforced to bridge it.
Edge undermining. Rain running off the slab edge erodes sand from under the perimeter, and the edges are exactly where wheel loads land. Either turn the slab edge down 8 to 12 inches or set pavers and grade to carry water away.
Rain between prep and pour. A base compacted on Thursday and poured on Monday after a weekend storm is a different base. It gets rechecked and recompacted, or you are pouring on something you have not tested.
Costs and where they land
Base work runs $1.50 to $4 per square foot depending on depth and whether limerock is imported. It is typically 15 to 25 percent of a driveway job and it is where a low bid gets its margin. A 3,000-square-foot driveway can be bid $6,000 apart on the same concrete spec purely on what happens below the forms.
The question worth asking any bidder is what they will do if the proof roll shows soft spots, and whether that is included or an extra. A bid that has no answer has not planned to proof roll. When I am helping an owner compare numbers I want a Jacksonville concrete contractor to write the base depth, the material and the compaction method into the proposal, because those three lines are what separate a ten-year drive from a thirty-year one.
Frequently asked questions
Do I need a vapor barrier under an exterior slab? No. Vapor barriers belong under interior slabs where floor coverings and moisture-sensitive finishes are involved. Under a driveway a barrier traps bleed water and can make curling worse.
How is compaction actually verified? On residential work, rarely by testing — it is proof rolling and experience. On anything larger, a nuclear density gauge or sand cone test against a Proctor value is the standard, and a few tests run $300 to $800. On a large or unusual site it is cheap insurance.
Can I pour over an old driveway instead of removing it? An unbonded overlay of 4 inches or more over a stable old slab can work, but if the old slab is moving, the overlay will crack over every one of its joints within a couple of years. Overlays fix surfaces, not bases.