"Near the middle of the last century I entered the concrete industry as a technician. There were a number of concrete facts and rules of thumb that I was required to know. This knowledge has served me well over the years."

Abram's Law:
(Water:Cement Ratio)
For a given amount of cement, as the water content is increased the strength of concrete or mortar will decrease.

Fact:

Slump is only an index of the plasticity of concrete and does not determine its strength. The strength is determined by the water:cement ratio and additives added to the concrete mixture, such as admixtures and steel fibers.

Rule of Thumb:
Adding 1 to 1.5 gallons of water per cubic yard of concrete will increase the slump about one inch.


Air Entrainment

Fact:
Billions of air voids are entrained into concrete to protect the hardened concrete from damage caused by freezing and thawing.

Rule of Thumb:

It should also be pointed out that at a midrange cement factor of approximately 500 pounds of cement, the strength will decrease approximately 5 % for every 1% increase in air content. As the cement factor increases, the strength loss increases as the percent of air content increases. Also, when concrete is air entrained to 5% to 6% by volume of concrete the water content can be reduced by 3% to 5% as compared to a non-air entrained concrete of equal slump.


Hydration and Strength

Fact:

Not all of the cement in a concrete mix hydrates.

Some estimations place the rate of hydration at about 60% to 70%. The reason is that there is not enough water in the concrete. About 50 years ago the Portland Cement Association determined that when the water cement ratio is below .45 there is insufficient water to hydrate all of the cement. It would seem that we have a conflict with the water cement law but we do not.

Cement in plastic concrete exists as individual particles known as cement grains. The distance between the cement grains before hydration plays a large role in determining the strength of the concrete. As the distance between the cement grains is reduced, the strength is increased. When the water content is reduced the distance between the cement gains is reduced and the strength will increase, but fewer concrete grains will hydrate. A reduction in the distance between the cement grains allows the hydrated cement to gain a stronger bond.

Fact:

Under normal conditions, the strongest ingredient in concrete is the aggregate and the factor controlling the strength of the concrete is the strength of the hardened cement paste.

However, when an ingredient that has a lower strength than the cement paste is introduced into a concrete mixture the strength of the concrete will be reduced. The amount of strength reduction is a function of the strength of the weaker ingredient and the amount of that ingredient.


Temperature Fact:

The optimum temperature for curing concrete is 68°F.

When concrete is cured at temperatures higher that 68°F the early strength will be higher but the later strength will be less. When concrete is cured at temperatures lower that 68°F the early strength will be lower but the later strength will be greater.

Fact:

When concrete temperature approaches 50°F, hydration slows to about one half of that of concrete having a temperature of 68°F and the setting time is about twice long. As the concrete temperature approaches 40°F, the rate of hydration is very slow. At 30°F, all hydration stops. The freezing point for most concrete is about 29°F to 30°F.


Concrete Samples

Fact:

You cannot make bad concrete test good but you can make good concrete test bad.

One of the biggest problems in the concrete industry is the proper casting of concrete samples and transporting them to the laboratory. All concrete samples cast in the field should remain at the point of fabrication for 18 to 24 hours and cured at a temperature of 68°F. The samples should then be transported to the laboratory using the same care as if they were eggs.

Note: More information on testing concrete can be found in ASTM Volume 04.02 Concrete and Aggregate. The Portland Cement Association has publication "Design and Control of Concrete Mixtures". It is one of the best, if not the best, publications of its kind.


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© 2007 L&M Construction Chemicals, Inc. | ConcreteNews January 2007.

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