Concrete and Reinforcing
Mix design and placement
Concrete is specified by 28-day compressive strength, water-cement ratio, air content, and slump. Air entrainment (typically 5 to 7 percent) provides freeze-thaw durability; lowering the w/c ratio raises strength and reduces permeability. Place, consolidate with internal vibrators, screed, float, and cure. Moist curing generally runs about 7 days, and cold or hot weather requires protection, accelerators or retarders, and adjusted curing.
Reinforcement
Cover requirements follow ACI 318 and the exposure condition: about 3 inches cast against earth, roughly 1-1/2 to 2 inches on exterior surfaces, and 3/4 inch for interior slabs. Lap splice length depends on bar size, grade, and splice class, with Class B running about 1.3 times the development length. Chairs, bolsters, and bar supports keep steel in position, and a 4,000 psi mix reaches design strength at 28 days, not 7.
Testing and joints
Slump (ASTM C143), air content, and temperature are checked at placement. Cylinders are typically sampled once per day, per 150 CY, or per 5,000 square feet of slab. Control joints create a weakened plane so shrinkage cracks occur where intended.
Traps
Assuming the finisher's trowel closes the surface enough to skip curing; misunderstanding curing duration; placing control joints too far apart; and forgetting that formwork removal is tied to strength gain, not to the schedule.
Rule of thumb: control-joint spacing is 24 to 36 times slab thickness in inches, so a 6-inch slab gets joints roughly 12 to 18 feet apart.