For concrete pavement areas requiring imminent traffic flow be maintained, accelerated concrete mixes can be provided. These mixes include additional amounts of cement and sometimes other accelerating admixtures like chloride or non-chloride flake.
Moderate Set (MS) Mix has typically around 800lbs cement per cubic yard and also commonly includes other accelerating admixtures. MS mix will often gain sufficient strength for traffic loading in 72 hours of curing. Some claim 24 hours is sufficient with MS mix with light vehicle traffic, but early loading is not recommended if it can be avoided.
Fast Set (FS) Mix typically has around 900lbs cement per cubic yard and also includes accelerating admixtures, often non-chloride or chloride components. FS Mix will often reach sufficient strength for traffic loading in 24 hours given ambient weather/temperatures are conducive, sometimes sooner but at a higher risk of damage. Some claim 4-8 hours is sufficient with FS mix with light vehicle traffic, but early loading is not recommended if it can be avoided.
The concrete strength property most commonly used to identify when a slab is ready for loads or vehicle traffic is flexural strength. The most common test for determining flexural strength is referred to as a ‘beam break’. A beam is typically a 8″ x 8″ x 24″ , is made from concrete on site out of the ready mix truck, and is cast is a mold by the concractor or a testing agency. A common rule of thumb for determining when concrete is ready to load is when the ‘beam breaks’ represent a 600psi or greater modulus of rupture (flexural) strength.
Standard exterior class concrete with no accelerators or added cement will typically have to sit for 7 days under reasonable ambient temperature conditions (above 50 majority of time). Some consider 5 days sufficient time for standard concrete but the decision to load early or without test results should be made by someone who is willing to accept the costs of replacement (in a worst case scenario) and has weighed all the factors influencing the situation. It should also be considered the concrete’s intended use; non-structural elements and non-vehicle traffic elements (sidewalks, etc) or otherwise not expected to receive loading or structural disturbance may be a better candidate for early exposure/opening to puclic.