Concrete and Steel: Complementary Opposites

Rock and Metal

Like the opposing ends of a teeter-totter, concrete and steel – two main ingredients of a Monolithic Dome – complement and contradict each other, all at the same time.

In a Monolithic Dome, concrete and steel complement each other by working together to give the dome its strength, durability and longevity.

But a look at the characteristics of each suggests contradiction. For example, concrete is a rock; steel is a metal. Concrete has enormous compressive strength. It can be compressed or squeezed! But it has very little tensile strength.

On the other hand, steel has enormous tensile strength: the ability to resist a force tending to tear it apart. So steel can resist tearing or tension.

That squeezable concrete

Concrete is usually made by combining portland cement with an aggregate, then adding water.

In 1824 John Aspdin, a Scotsman, invented portland cement. He burned finely ground chalk with finely divided clay in a lime kiln until the carbon dioxide was driven off, then ground the sintered product. But today in the U.S. various types of portland cement, for various types of construction, are produced.

Aggregate refers to broken or crushed stone or gravel, and, as with portland cement, in today’s construction industry there are various kinds of aggregate.

Americans are used to living with concrete. We use it in our sidewalks, driveways, roads, bridges, tunnels, swimming pools, floors, etc. The addition of steel to the concrete produces what we call reinforced concrete.

Concrete is tough stuff.

It has great compressive strength – the ability to be squeezed without breaking. Unfortunately, concrete does not have much tensile strength.

But tremendous loads can be put on concrete without crushing it. This compressive strength of concrete can be measured in pounds per square inch or PSI. In other words, one square inch of a concrete surface can hold XX number of pounds before the weight causes crushing.

Most concretes can handle 2,000 to 3,000 PSI; some can withstand up to 12,000 PSI before crushing. Even the weakest concretes – the kind used to fill holes – can handle several hundred pounds of weight per square inch.

Concrete is a universal material, literally found all over our planet.

In dome construction, Monolithic specifies concrete with a 4,000 PSI for the dome shell. It’s called shotcrete and it generally has aggregate with a diameter of three-eights inch or smaller, so it can be sprayed.

For the dome’s footings and floor, Monolithic specifies concrete with a 3,000 PSI; it contains aggregate with a three-quarter inch diameter or smaller.

When mixing concrete, the amounts of added cement and the amounts of added water can make the concrete weaker or stronger. More water and/or less cement generally makes weaker concrete.

In dome building, we have to be really careful not to use too much water. A normal yard of concrete takes about 40 gallons of water to process. The first 10 gallons of that 40 begins the set-up process. The remaining 30 make the concrete pourable.

Making concrete

The process of making concrete begins by adding water to the cement powder. That immediately starts the linking-up. I like to think of it as linking fingers. I imagine molecules with fingers that grab onto each other and hang on tight. As the concrete in a mixer drum turns, the fingers that have taken hold of each other break off. As more fingers grab on, that process continues. Eventually, all the fingers grab on and break off. There is nothing left to set up and you have a mass ready to begin solidifying. For that reason, state highway departments keep a metered count of a concrete truck’s drum turns. They don’t want to end-up with a truck of solid concrete without fingers to properly set it up!

Concrete hardens over time. The chemical process that hardens concrete begins very quickly, then gradually slows. For example, a normal pour of concrete, such as one for a driveway, takes about a week to reach 50 percent of its hardness or ultimate strength. In 30 days it reaches 80 to 90 percent. Each year, for 25 years, the concrete’s increasing strength can be measured, but we do not know when the concrete actually reaches its ultimate strength and stops hardening. Some experts believe that it never does – that the strength-increasing process just continues.

That force-resisting steel

In Monolithic Dome construction, steel or reinforcing bars of steel, commonly called rebar, supplies the tensile strength that concrete lacks.

Monolithic uses rebar around which concrete is poured. We do this because we know that as concrete cures, it shrinks, and when concrete shrinks it cracks.

One of the most often asked questions is: What about cracks in the concrete? Does the concrete crack? Are the cracks going to create leaks?

The answer is that there will be cracks in concrete. There is an old saying, “If you don’t want concrete to crack, you leave it in the sack.” Concrete is one of the strongest materials in the world when it is put in compression.

The reality is that normal concrete cracks quite easily. Concrete is not a good tension material. That is why we use reinforcing in the concrete. Reinforcing helps to control the cracking, and in some cases totally eliminates it. By using the reinforcing and planning for cracking, there is very little problem associated with cracks in concrete. We routinely build concrete water tanks. We control the cracking; therefore we don’t have leaking water tanks. It is the same with the Monolithic Dome — we control the cracking by use of reinforcing; therefore we don’t have a leaking concrete dome.

Consider: A Monolithic Dome’s concrete has rebar placed in it every few inches in both directions. This means that any crack that occurs cannot propagate more than a few inches before crossing a rebar. Crossing the rebar won’t prevent the crack from continuing, but it keeps the concrete from shifting position and keeps it permanently in alignment.

Concrete is one of the strongest materials there is for compression. Steel rebar is one of the strongest materials for tension. The two, working together, create the magic for thin-shelled concrete structures.

I like to explain to some people that working with concrete is a lot like working with bricks. Bricks stacked up on top of each other without mortar will carry a tremendous load in compression. Obviously if we try to live on unmortared brick they very quickly come apart. However, if we put a rebar down through the stack of brick, fasten the rebar on each end, and tighten it up, we create a composite that acts as if it were a single piece. It’s not, but it still acts as if it were a single piece.

Concrete is much like that. When we pour a driveway, if we use reinforcing steel in it the driveway can crack in many, many pieces and still function perfectly. Hence, that is why we put in the reinforcing material.

Rebar keeps its shape and holds its strength. Moreover, rebar expands and contracts at about the same speed as concrete. That makes it a great reinforcement for concrete.

Generally, the tensile strength of rebar is measured in pounds per square inch. In other words, if you grabbed a piece of rebar with a machine and began pulling one end while the other end was fastened in a vice, the rebar would tend to stretch. The point at which the rebar actually began stretching is the yield point and it’s measured in thousands of PSI, often abbreviated KSI.

One KSI equals 1,000 PSI. Most general purpose rebar, called Grade 40, has a tensile strength of 40,000 pounds per square inch or 40 KSI. In general we use Grade 60 or 60,000 psi rebar in the construction of Monolithic Domes.

Working together

Obviously, while concrete and steel have very different characteristics, in dome construction they work very well together. Like that teeter-totter I alluded to at this article’s beginning, what one lacks, the other provides. And the result of this joint effort is the Monolithic Dome.

October 16, 2009