Verify your concrete meets spec

ASTM C39 is the standard method for testing the compressive strength of cylindrical concrete specimens. Concrete cylinders (6×12 inches or 4×8 inches) are cast from fresh concrete, cured under controlled conditions, and crushed at 7 and 28 days. The 28-day result is the acceptance criterion for most specifications. A 7-day break typically reaches 60–70% of 28-day strength for Type I cement. Use the compressive strength calculator to project the 28-day result from your 7-day break early in the project.

Target slump depends on the application. For slabs and footings, 3–4 inches (75–100 mm) is typical. For walls and columns with dense reinforcement, 4–5 inches (100–125 mm). Pumped concrete generally requires 4–6 inches. A slump above the specified maximum usually means excess water has been added, which directly reduces 28-day strength — every additional 10 lbs of water per cubic yard reduces strength by roughly 200 PSI. Use the slump test calculator to interpret your reading against ASTM C143 workability classes.

Drying shrinkage occurs as water evaporates from the hardened cement paste, causing the concrete to contract. Typical shrinkage strains range from 400 to 800 microstrain (0.04–0.08%). The main causes are high water-cement ratio, high cement content, large aggregate size variations, and rapid drying in hot or windy conditions. Plastic shrinkage cracks form within hours of placement before the surface sets, typically caused by evaporation exceeding bleed water rate. Control joints should be spaced at 24–36 times the slab thickness to manage cracking. Use the shrinkage calculator to estimate strain and recommended joint spacing for your mix conditions.

Carbonation is the reaction between atmospheric CO2 and calcium hydroxide in the cement paste, which lowers the concrete’s pH from ~13 to below 9. When the carbonation front reaches the rebar depth, the passive protective film on the steel breaks down and corrosion begins. Carbonation depth follows a diffusion law: depth is proportional to the square root of time. Lower w/c ratio and adequate concrete cover are the primary defences. The carbonation depth calculator estimates how long before the front reaches your rebar based on cover depth, w/c ratio, and exposure class.

The most reliable site check is to weigh each batch constituent — cement, sand, gravel, and water — and compare the proportions to your target mix design. For volumetric batching, use calibrated measuring boxes. The slump test gives an indirect indication: a consistently higher slump than expected suggests excess water, which implies a lower effective cement ratio. For formal verification, cast cylinders and test at 7 and 28 days against the specified strength. The mix ratio checker lets you enter your actual batch quantities and immediately compare them against M5–M50 standard proportions.