Article No: 277

2010-07-07 10:54:21
Rebar at the Bar
By: Jeffrey Girard



View down the beam to show the deflection from 500 pounds of sand. There were no visible cracks.

Concrete countertops have stringent performance and aesthetic demands that most structural and decorative concrete doesnt. Aesthetically, concrete countertops compete with other high-end countertop materials such as granite and engineered quartz, where high levels of fit and finish, precision and quality are essential to client satisfaction. Reinforcing plays an important role in achieving this by ensuring the concrete can be processed, transported and used by the client without risk of cracking and breaking.

HOW COUNTERTOPS DIFFER FROM FLOORS
At first glance, concrete countertops seem to be just like floors except they are elevated and installed on top of cabinets. However, concrete countertops have very different aesthetic and structural requirements than floors. This article explains a key structural difference.

Floors, sidewalks, driveways and all other concrete slabs cast on the ground are designed to be fully and uniformly supported by the ground. They are slabs on grade." Any wire mesh embedded in the slab is there only to provide shrinkage control. No other reinforcement is required.

Structurally, concrete countertops are beams, not so much because they are consciously made to be beams, but because of how they are lifted, handled and transported. The very act of moving them turns the piece of concrete into a beam.

Once this is realized, it is possible to design and build the countertops so that they far outperform mere sidewalks. Cantilevered sections and large spans (such as table tops) that can support real weight can be made with confidence. If countertops are beams, what exactly is a beam and how does it work?

A beam is a horizontal structural member that spans some open space and is supported near the ends. The beam can then support some weight placed on top of it somewhere between the end supports. A balance beam is a beam. A floor joist is a beam. Concrete countertops are also beams.

When a beam has weight placed on top, that weight causes the beam to bend. Small weights on stiff beams cause almost no deflection, while large weights on flexible beams cause significant deflection. Countertop slabs will flex and bend from their own weight when they are stored, handled and moved horizontally. This often occurs at various times during processing, transportation and installation.

The deflection in the beam causes two things to happen: The top surface of the beam is compressed and tries to get shorter, and the bottom surface is in tension and tries to get longer. The more a beam deflects the greater the stresses that develop in the beam.

Between the two surfaces something important occurs. Compression is the opposite of tension, so as one progresses down the beam from the top surface to the bottom, the compression stress gradually decreases to zero and then the stresses reverse, go into tension and gradually increase towards the bottom of the beam.

If an unreinforced beam has a symmetrical cross-section (such as a rectangle), the stress switch occurs at the midpoint between the upper and lower faces. This is important because given that there is no tension or compression stress at the midpoint of a countertop, placing reinforcing steel there does absolutely no good.

Reinforcing should never be placed in the middle of a countertop. It simply does not work there. Making a countertop with the reinforcing placed in the middle of the slab is essentially the same as not using any reinforcing at all. The only difference is that an unreinforced countertop will break into two pieces when it cracks, and an improperly reinforced countertop will simply have an ugly crack in it. Neither situation is acceptable to your client.

The maximum tensile stresses occur at the bottom of the countertop. Therefore the most effective location for the placement of reinforcing steel is at the very bottom of the countertop, with a minimal amount of concrete covering the steel.

The measure of a materials bending strength is called the flexural strength. Unreinforced concrete has a low flexural strength that is only about 10 percent of the compressive strength. Combining high compressive strength concrete with high tensile strength steel yields a high flexural strength beam.

Reinforced concrete beams, such as bridges, girders and concrete countertops, rely on the steel reinforcing to resist all of the tension forces. The concretes job is to resist the compression forces and to encapsulate the steel so that the two different materials work together to produce a strong beam. Because the concrete only resists the compression forces, only its compressive strength matters. Take away the steel reinforcing and now the concretes tensile strength determines whether the beam will break or not. Concretes tensile strength is always many times smaller than its compressive strength.

If a countertop is made of concrete (with no reinforcement), any significant weight placed on top of it will cause it to fail at the bottom of the countertop because the tension stresses in the bottom of the countertop will exceed the tensile strength of the concrete. Without reinforcement, concrete cracks easily. Because unreinforced concrete is brittle, it cannot tolerate large deflections (bending) and could fail suddenly and without warning when lifted.

WHY THIS IMPORTANT TO CLIENTS
Using reinforcing in concrete countertops provides several key benefits that translate into happier clients and higher profitability:
Countertop slabs can be made longer. This means fewer seams for the client.
Cantilevers can be made longer and used with more confidence.
Larger, more complex pieces can be made with greater confidence that they will not crack.
Reinforced concrete can be moved, handled and transported much sooner and with a greater degree of confidence than an identical piece of unreinforced concrete can. Time is money. Projects that take less time to finish mean more jobs and happier clients.

Jeffrey Girard, P.E., president of the Concrete Countertop Institute, is a pioneer of engineered concrete countertops. Girards mission is to raise the standard for concrete countertops through intensive training courses, membership programs, events, material testing, and guidance to manufacturers and designers. For additional information, see ConcreteCountertopInstitute.com.