Concrete
Concrete Cylinder Strength and Water-Cement Ratio Calculator
Concrete strength is measured by crushing standard cylinders under ASTM C39. Enter the maximum load and cylinder diameter to get strength in psi and MPa, and enter mix water and cement weights for the water-cement ratio.
Formula
Compressive strength f = P ÷ A, with A = πd²/4 (ASTM C39). 1 psi = 0.006894757 MPa. Water-cement ratio w/c = weight of water ÷ weight of cementitious material. ACI 211.1 Table 6.3.4(a) relates w/c to 28-day strength for non-air-entrained concrete: about 0.41 for 6,000 psi, 0.48 for 5,000 psi, 0.57 for 4,000 psi, 0.68 for 3,000 psi and 0.82 for 2,000 psi.
Worked example
A 4 in by 8 in cylinder breaks at 50,000 lb: area = 12.57 in², strength = 3,979 psi (27.4 MPa). A mix with 280 lb of water and 564 lb (six sacks) of cement has w/c = 0.50, which by interpolating ACI 211.1 corresponds to roughly 4,800 psi for non-air-entrained concrete.
Assumptions and limits
- Single cylinder result; acceptance under ACI 318 Section 26.12 uses the average of two cylinders (or three 4x8) per test and running averages.
- Length-to-diameter ratio of 2.0; shorter cores need ASTM C39 correction factors.
- w/c strength relationships are approximate and vary with materials; trial batches establish real strength.
Frequently asked questions
What are the ACI 318 acceptance criteria?
Strength is satisfactory if every average of three consecutive tests is at least f'c and no single test falls below f'c by more than 500 psi (for f'c up to 5,000 psi) or 0.10f'c above 5,000 psi.
Why does lower water-cement ratio mean stronger concrete?
Excess water leaves capillary pores when it evaporates, weakening the paste. Abrams' law describes strength falling as w/c rises.
How long until concrete reaches full strength?
Design strength f'c is specified at 28 days. Typical concrete reaches roughly 65% to 75% of it at 7 days and continues gaining slowly after 28 days.