Thoughts on Cast Stone and Concrete
By Craig E. Barnes, P.E., SECB
The changes in the composition of concrete are somewhat like the change in rock structure. Rocks have been with us since the beginning of time and the ingredients in concrete have been with us since the beginning of time.
The basic ingredients for concrete have changed very little through the years. Concrete is the simple combination of cement, aggregate, and water.
The most complicated part of the mix is the cement. The cement we use today is a manufactured product using some of the raw materials that the ancients used in making their version of concrete. The first cement manufacturing plant in the U.S. was built in 1875 in Coplay, Pennsylvania.
When I was at Northeastern University enjoying my undergraduate studies in civil engineering, the head of the Civil Engineering Department, Professor Ernie Spencer, enjoyed two things he liked to impress upon his students. First, the pouring of concrete is a misnomer he insisted. Concrete is a refined product that needs to be placed. Ernie was clearly fighting a losing battle because you very seldom hear instructors, engineers, or contractors referring to the placing of concrete and with the advent of super plasterscizors and self-consolidating concrete, the term pouring is probably more appropriate. Second, it used to drive Professor Spencer crazy to hear students, contractors, and the like refer to concrete as cement. If you made that mistake you heard Ernie say, “Cement is an argillaceous conglomerate ground and pulverized and then heated to incipient fusion.” If you managed to indulge in that faux pas when Ernie had time on his hands, you would then be admonished that it was our duty as young engineers to straighten the world out on the misused terms.
Today you can make concrete only slightly easier than the ancients.
You can go to the local hardware store, buy a bag of cement, bring that bag of cement home to our backyard, and using the most basic of the equipment, make concrete. All we need do is gather some rocks from the backyard, add water to the cement and rocks, and we’ve started the process of making concrete. The ancients would do the same thing, only their version of cement was not as refined as what we use today.
As soon as water is introduced to cement a chemical process called hydration starts. Hydration is an exothermic process, meaning that it gives off heat, where cement is transformed from a dry particle through a gel to eventually become a hardened material. The aggregate does not participate chemically, and in other words you might say the aggregate goes along for the ride. These are the basic steps for making concrete.
If one thinks simply beyond the making of concrete to the concrete elements we see around us every day an entirely different picture emerges.
Through the years there have been tremendous changes to the basic constituent parts of concrete. Changes in the type of cement, how finely ground a cement clinker and the color of the natural cement are some of the more basic modifications.
Utilizing chemical to reduce the amount of water, using chemicals to increase or reduce the rate of hydration, introducing natural material with cementitious properties, using waste product such as slag and fly ash, varying aggregate size, shape, and color are all useful for tinkering with basic concrete.
Concrete in its many constituent forms needs to be delivered in some fashion to the end user, and confined in some way to be truly useful.
The backyard batch that you just made may simply be turned out of the mortar box and then smoothed over with a piece of wood or the trowel to result in a concrete slab for your patio. In that case the mortar box was turned into the delivery system and the wood or the trowel was the rudimentary confinement vehicle shaping it to the intended use.
Everybody has seen these large lumbering ready-mix trucks on our highways, often referred incorrectly to as being cement mixers. These are perhaps the most ubiquitous delivery system that is in use today.
The ready-mixers that we see on the roadways usually are dispatched from a ready-mix plant that loads the trucks with constituent parts of concrete, cement, sand, coarse aggregate, and water from large silos.
A new generation of compound delivery vehicles consisting of the ready-mix truck discharging to a concrete pump allows concrete to be delivered directly to the spot where workmen are placing the concrete. A concrete mix for this type of operation is tuned to be pumpable.
One of the earliest uses of formed concrete was utilized in a one-story warehouse building in Port Chester, New York in 1875. Concrete form work has gone through many changes since wood boards were used to construct the first concrete multi-story building in the United States, completed in 1904 in Cincinnati.
Concrete form work now comes in plywood, overlay plywood, fiberglass, metal, and plastic.
Precast T’s make wide use of metal forms. Look above your head in over half of the parking garages in the New England area and you will recognize the concrete in a T shape indicating a prestressed member which utilizes concrete cast in a metal form. For reasons of better function, your upward gaze will probably encounter two stems, what we call double T’s.
An interesting process used very frequently today is the process of extruding concrete. Apartment buildings and hotels make wide use of precast hollow core planks. These planks are made by extruding concrete around stressed steel wires.
Summary: The making of concrete is a multifaceted issue. Basic constituents of which concrete is made cement, aggregate, and water have been around for thousands of years. Mix them and then allow the mixture to cure and concrete is the result. That is a thirty second explanation of how concrete is made. Just how concrete gets from where the constituent parts are combined through the mixing process and finally to the delivery point may be considered part of the manufacturing process. As you have learned, there are many avenues for that delivery system. Lastly, how the concrete that at one time is much like a fluid is transported, shaped, and confined through many techniques is part of the fabrication process.