Who's mix design is it?

Concrete is one of the oldest, if not the oldest, manufactured building materials known to man. It predates the Roman Empire by many hundreds of years. Natural deposits of cement compounds, produced by a reaction between limestone and oil shale undergoing spontaneous combustion, have been known to exist as early as 12 Million BC.

Archaeologists have discovered in Syria a form of concrete dating back to 6500 BC. About 600 BC, the Greeks discovered that when a natural pozzolan (a form of fly ash) is mixed with lime and water, it would harden into a very strong mass. Many archaeologists believe the Thracians developed concrete mixes for use in building projects hundreds of years before the foundation of Rome was laid. It is also believed that the Thracians were the first to use concrete to construct aquaducts to move water over long distances. Later the Romans adopted the idea and improved on it.

It is generally agreed that the Romans developed concrete to its highest state of art in the ancient world. Many of their designs would rival today's mixes in strength and durability. While it was the Greeks that discovered the use of pozzolans to produce a crude form of cement, it was the Romans that gave this reactive material its name. The Romans used a volcanic ash they called pozzuolana. It was named for Pozzouli, an area near Naples, where the volcanic ash was found. Pozzolan is the name that is given to fly ash type materials that are used in concrete today.

The Romans, by accident or design, were the first to use an air entraining admixture in concrete. Animal fat and blood were mixed in with the concrete. The concrete, being alkaline, would change the animal fat and blood into soap and during the mixing of the concrete very small air voids would be produced in the cement paste. These very small air voids protected the concrete from the violent forces of freezing and thawing. One of the first admixtures developed for modern concrete were special soaps designed to entrain air into concrete.

About 100 BC a Roman architect, Marcus Vitruvius Pollio, drafted one of the oldest mix design known to exist. He required the cementitious portion of his concrete mix to be made up of two parts pozzolana and one part lime. Not only did the Romans write mix designs, they also required a proof test of the work performed. It was not uncommon to find the architect to be standing under the structure he had designed while it was loaded to design requirements. The architects of that day had a personal interest in what we today call the over-design factor!

With the fall of the Roman Empire, the art of concrete was lost for over a thousand years. In 1756 John Smeation, a British engineer, rediscovered hydraulic cement and built the first concrete structure since the end of the Roman Empire.

The age of modern concrete actually began in 1824 when Joseph Aspdin developed and patented the process for the manufacture of Portland cement. The very foundation of the modern concrete mix design however, did not occur until 1918. That was the year D. A. (Duff) Abrams published what has become known as the water cement ratio law. It is this relationship of the amount of cement to the amount of water in a mix that largely determines the strength of a concrete.

As recently as the 1960's concrete mix designs were very rudimentary. Concrete was specified based on the number sacks of cement in a cubic yard of concrete, and it was not uncommon for a ready mix concrete company to only have three or four concrete mix designs. The most common concrete mix design was a four and one half sack mix. This was specified for most residential concrete projects. The five-sack mix was most often used in the industrial market. Any mix design with six sacks or more was considered a high strength concrete. The major shortcoming of this system was that the water required for a workable mix differed from region to region, therefore the strength of a concrete of the same cement sack factor would vary from region to region. It should be pointed out that the use of chemical admixtures in concrete was in its infancy. The most commonly used chemical admixtures during the 1960's were air-entraining agents and set controlling admixtures. During this time, fly ash was of poor quality and rarely used.

It has been only in the last 25 years that concrete has evolved into the highly technical building material that we have today. The major driving force has been the need for concrete of predictable and desirable performance. This has forced the concrete industry to redefine itself. Not only has the industry had to redefine itself, the men and women designing concrete mixes can no longer think of themselves as only concrete technicians, they must be also able to enter the arena of the chemist and the engineer. The old prescription specifications (specifying concrete by cement content) have been replaced by performance specifications. The mix design of today may look like a recipe for concrete soup, but in reality, it is a chemical equation setting the stage for a series of very complicated chemical reactions.

The concrete technological revolution has given us something new but we still call it concrete. Only a few decades ago we thought of 5000 psi concrete as high strength concrete. Today we have concrete with in-place strengths of 18,000 to 20,000 psi. There was a time when we could not place concrete at temperatures below 40o F. Today, with chemical admixtures, placing concrete at 20o F is commonplace. Not long ago, the strength of concrete was related to its slump. This is no longer true. New curing procedures and surface treatments can reduce surface cracking. In fact, we can now harden the surface of concrete after it has been placed and cured. New surface hardeners can even produce a surface that is tougher than iron.

So what has this technological revolution brought us? It has brought us the world's most versatile building material to date, man's dream, "liquid rock." This new structural building material can be poured like pancake batter and formed into the most interesting shapes. After hardening it develops into a mass that rivals the strength and durability of stone. As far as alternatives, I have yet to see a steel interstate highway or hear of a structural asphalt building.

So whose mix design is it anyway? It is our mix design, and the Thracisans, the Romans, Joseph Aspdin, D.A. Abrams, including the many of us who have worked with it, been up to our necks in it, at times cursed it, and, even been surprised by it. Today, the more current and important question is rather, "who is responsible for the mix design"? The answer: all of us, from the design engineer to the concrete finisher. This answer acknowledges that we are all bound together by the mix design and should have input.

So what is the future of concrete? From all reports that pass this chair, the old guy is getting stronger and better every day. And his advancing age is coming not without a healthy dose of concrete involvement from all members of the building team.

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© 2001 L&M Construction Chemicals, Inc. | ConcreteNews December 2000-January 2001.

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