Goldsboro concrete, from the ground up
Wayne County was carved out of Dobbs County and established on November 2, 1779, and took its name from Anthony Wayne, the Revolutionary War general. Goldsboro became the county seat later: acts passed in 1845 and again in 1847 moved the courthouse there from Waynesborough, on the condition that the people voted for it. A courthouse sits on a foundation, and so does every driveway, patio, walk and garage floor in town. This page is about getting that foundation right on a residential slab, using weather figures measured right in Goldsboro.
The station numbers
NOAA keeps engineering weather data for Seymour Johnson AFB, in Goldsboro, at an elevation of 108 feet. For Seymour Johnson AFB, NOAA lists:
- about 36 freeze-thaw cycles in an average year;
- a 100-year rainfall rate of 3.8 inches per hour;
- a 50-year basic wind speed of 100 mph, as a 3-second gust at 33 feet;
- a 99% winter design temperature of 24°F;
- a 1% summer design temperature of 92°F, with a mean coincident wet-bulb temperature of 76°F;
- about 1,240 hours a year with the wet-bulb temperature above 73°F, and about 2,687 hours above 67°F.
Your yard's shade, slope and soil will shape what your own slab sees, but these give a contractor something concrete to plan against.
What 36 freeze-thaw cycles ask of a mix
Each cycle at Seymour Johnson AFB is a chance for water in the top of a slab to freeze, swell and thaw. Repeat that about 36 times a year and a weak surface starts to flake and pop. Questions for each bidder:
- Will outdoor concrete be air-entrained, and at what strength?
- How will the crew finish it? Troweling bleed water back into the surface weakens the top layer.
- When should the first coat of sealer go on, and how often should it be renewed?
- Which deicers should stay off the new slab, and for how long?
3.8 inches an hour, and where it goes
NOAA's 100-year rainfall rate for Seymour Johnson AFB is 3.8 inches per hour. Rain at that pace runs off a hard surface almost as fast as it lands. Ask:
- what slope each section gets, in fractions of an inch per foot, and in which direction;
- where the runoff lands, and whether that spot can take it without washing out;
- whether a downspout should be extended, or piped under or around the slab;
- how the slab edge will meet the soil, so water drains off rather than undermining it.
Pouring in summer
The 1% summer design temperature at Seymour Johnson AFB is 92°F, with a mean coincident wet-bulb temperature of 76°F, and NOAA counts about 1,240 hours a year above a 73°F wet-bulb there. Fresh concrete in heat stiffens faster, gives finishers less time, and can crack if the surface dries ahead of the slab below. Ask what time of day the truck is booked, how many finishers will be on site, whether the base gets dampened first, and how the slab will be cured: wet burlap, plastic sheeting, a curing compound, or a mix, and for how many days.
Sealers have their own limits on surface moisture and air conditions. Ask which product the contractor uses and what the label requires before it goes on.
Pouring in winter
The 99% winter design temperature at Seymour Johnson AFB is 24°F. Concrete that freezes before it gains strength can be ruined at the surface. Ask what forecast makes the contractor postpone a pour, whether they use blankets or heated water, and how long the slab stays covered.
Wind and anchored loads
For Seymour Johnson AFB, NOAA lists a 50-year basic wind speed of 100 mph. The slab won't move, but whatever is bolted to it pulls on its anchors: a carport, a shed, a pergola or an equipment pad. If something will be fastened down, ask how thick the concrete will be at those points, which anchors go in, and whether the maker of the structure sets its own slab requirements.
Under and inside the slab
Ask what gets dug out, what base goes in, how deep, and whether it is compacted in lifts or all at once. Ask how they handle roots and soft pockets found mid-dig, and whether that is priced ahead or billed as an extra. Ask for slab thickness by area, since a truck driveway needs more than a garden walk. Ask whether the plan uses rebar, wire mesh or fibers, and how steel is held off the dirt. Finally, ask for a drawn joint layout with depths. Control joints choose where cracking happens; they do not stop it.
Before any digging, ask who calls in the utility locate and how meter boxes and cleanouts will stay reachable.
Paperwork: the $40,000 line and permits
The North Carolina Licensing Board for General Contractors says that by law a general contractor must hold a state license when the total cost of a project is $40,000 or more, and the board's website has a public license search. Ask every bidder: what state license, if any, covers this job, and what is the number? Then look it up on the board's search.
Ask which City of Goldsboro permits the job needs, who pulls them, and whether an inspection is part of it. Ask for proof of liability coverage and workers' compensation, and for addresses of slabs the company poured a few winters ago that you can go look at.
Reading the quotes
Line up each written quote against the others:
- square footage, thickness and finish per area;
- breakout and haul-off of old concrete;
- excavation, base and compaction;
- reinforcement and the joint plan;
- slope, runoff and downspout work;
- curing method and sealer;
- permits, locates and cleanup;
- the warranty and what it excludes.
If one total is well below the others, find the line it skipped before calling it a deal.