Materials (Breadth)
Concepts
Concrete: mix design by absolute volume, w/cm ratio, slump, air content, admixtures, curing, and 28-day compressive strength. Steel: grades, weldability, cover, lap splices, and inspection. Asphalt: mix design, compaction temperature, and density. Aggregates: gradation, fineness modulus, absorption, and deleterious materials. Testing: cylinders, slump, air, and field density.
Traps
Strength is governed by w/cm ratio, not cement content alone; adding water on site raises slump and lowers strength. Air entrainment improves freeze-thaw resistance but reduces strength. Nominal versus actual dimensions and unit weights. Curing duration and temperature affect strength gain. For reinforcing steel, development length and cover are separate checks.
Worked example
A mix has 600 lb of cementitious material and 300 lb of water per cubic yard. w/cm = 300/600 = 0.50. If 30 lb of water is added at the job site, w/cm = 330/600 = 0.55 and strength drops. As a rule of thumb, each additional gallon of water per cubic yard can reduce compressive strength by roughly 200–300 psi; verify against the mix design and specification.
Rule of thumb
Normal-weight concrete weighs about 150 lb/ft³. Cylinders are tested at 7 and 28 days; 7-day strength is typically about 65–75% of 28-day strength.