Abram's rule and additional factors

Abrams' rule

Abrams (1918) stated that for concrete made with given materials, the strength depends only on the water-cement ratio; a lower w/c ratio gives a higher strength:

$$f_c = \frac{A}{B^{\,w/c}}$$

($A$ and $B$ are constants for the materials and age; e.g. $A = 96.5$ MPa, $B = 7$). It holds for plastic, fully compacted mixes.

Additional factors not covered by the rule

1. Degree of compaction (air voids) Abrams' law assumes complete compaction. Voids left by poor compaction reduce strength severely: 1% of air voids reduces strength by about 5-6%, and 5% voids by up to 30%. The relative strength depends on the density ratio (compacting factor). Stiff mixes with a low w/c need powerful vibration, while a mix with too high a w/c segregates.

2. Age and curing (temperature and moisture) Abrams' law applies at a stated age. Strength increases with age as hydration proceeds (about 60-65% at 7 days, 100% at 28 days, 120-130% at 1 year), but only if the concrete is kept moist (curing) and at a suitable temperature; a lack of curing leaves capillary pores and can reduce strength by 30-50%. A higher temperature gives faster early strength but lower later strength.

Other factors: aggregate properties (size, shape, texture, strength, cleanliness), cement type and content, admixtures, aggregate-cement ratio, and the transition zone.