Concrete Curing & Testing

Concrete gains about 70% of its strength during first week of curing; it takes 28 Days to fully cure, like Sandra Bullock's heart.

TESTING BEFORE CURING

Slump test.

Slump test: measures consistency of concrete; amount of slump desired depends on how concrete will be used, but typically in the range of 2" to 6".

Even a real Kelly ball looks dirty
on an R. Kelly album cover.

Kelly Ball Test: also "ball penetration" test; hemispheric mass of steel with a calibrated stem is dropped onto a slab of freshly laid concrete. Amount of penetration is measured and compared to 1/2 the values of slump test.

K-slump test.

K-Slump test: uses 3/4" tube that contains a floating scale - the distance the scale floats out is read and measures consistency of concrete.

TESTING AFTER CURING

I will crush you.

Cylinder Test: measures compressive strength; tested in laboratory; During

the initial pour, samples of the concrete are poured into 12 inch by 6 inch

cylinders to be tested. After seven days of curing, the first sample is

given a compression test, and again after 28 days (full strength).

Core cylinder extracted from cured concrete.

Core Cylinder Test: used when a portion of the structure is in place and

cured but needs to be tested; a cylinder is drilled out of the concrete and

tested in lab to determine compressive strength.

Impact hammer: non-destructive field test of concrete strength after it has

hardened; rebound of plunger snapped against surface is measured.

TESTING FOR MOISTURE

Moisture is a critical factor in determining the ultimate strength of concrete.

Calcium chloride test (moisture dome test): common test for moisture in

concrete. Contractors use calcium chloride to test the dryness of concrete

before putting down flooring; tester weighs the container again on the same

scale. The difference in weight represents the water vapor emitted.

Hygrometer test (relative humidity test): moisture emission by measuring the

relative humidity RH of atmosphere confined adjacent to the concrete floor;

test standards recommend that moisture sensitive flooring not be installed

unless RH is 75% or less.

Polyethylene test: presence of visible water indicated concrete is

insufficiently dry for application of finishes. Similar to mat test.

Electrical Impedance test: moisture content in slab is read out directly.

TESTING FOR ALKALINITY

pH level of concrete should be tested -  concrete normally has pH of 12.0-13.3 (7 is neutral).

Alkalinity can screw you in 2 ways; high alkalinity on surface of slab

can damage a tile installation by causing the adhesive to re-emulsify, or

revert to its liquid state. Alkalinity is also responsible for Alkali-Silica

Reaction (ASR), in which cement begins to dissolve sand and rock within concrete.

ASR is reduced by using low lime content, aggregates not susceptible to ASR,

proper curing and not finishing with hard trowel surface.

Titration test: lab test of alkalinity in concrete.