Tag Archives: reinforcing

Rebar in a cast in place concrete countertop

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I recently got the following question regarding my blog post “Mix design for cast in place concrete countertop in Cayman“:

Q: What is the size of the rebar? Where was it placed in the slab?

Here is what the rebar looked like:

rebar in gazebo

The rebar size and location were dicated by local practices (and the local structural engineer). While I know from practice (and from being an engineer myself) that the rebar used was grossly oversized, politics were the over-riding influence on the reinforcing schedule.

All the rebar was 1/2″ bar, located in the slab so that there was 2″ of cover measured from the top surface of the countertop. The 2″ cover depth is common where corrosion is an issue. Since this is an outdoor countertop on an island, corrosion is always a concern with concrete exposed to the elements.

Note that too little cover depth can lead to problems other than corrosion. See this article about rebar size in concrete countertops.

How to move and transport a concrete countertop safely

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Handling and transportation are risky operations. Too much time and effort goes into making a high quality concrete countertop to risk cracks or damage from improper transport. Keep these important tips and guidelines in mind:

  • Transport on-edge, not flat

Concrete countertops are moved and  transported in a vertical position, much like sheets of glass.

Concrete is strongest in a on-edge, vertical position. Beams are stiffest when they are oriented vertically, and they are the most flexible when they are flat. For example, floor joists and roof rafters are commonly made from 2x material set on the narrow edge, not set flat.

Small, compact slabs can often safely be transported flat (right side up) because they don’t deflect much. However, long slabs have a greater tendency to flex, and a sudden pothole or bump in the road could cause a large shock, overload the concrete and crack it. By orienting the slabs vertically, the deflection of such a deep, stiff beam is negligible. Therefore very low tension forces are developed, so the risk of cracking a slab is reduced tremendously.

 

  • Never turn slabs upside down

Even with very small countertops, slabs should never be carried flat while upside down. Properly designed concrete countertops locate the primary reinforcing near the bottom, which is generally the tension side. If a countertop is turned upside down, it no longer has reinforcing on the bottom.

 

  • Use an A-frame

When transporting slabs, use an A-frame to support and secure the slabs during transport. These frames are often made of galvanized steel rigidly welded to provide a sturdy structure. Sometimes homemade wooden frames are used too, but these must be made very sturdy and rigid to protect the countertops.

 

  • Use padding such as moving blankets to protect and separate slabs from the frame and each other.
  • Use straps or clamps keep the slabs from moving or shifting.

A-frame for transporting concrete countertops

  • Helpful equipment: Lifting straps, carry clamps, rolling carts, dollies, sawhorses

Individual slabs can simply be carried by two or more people, but special lifting straps and carry clamps that grab onto the edges of thinner slabs make handling safer and easier, especially for large and unwieldy slabs. Sometimes rolling carts or dollies are used too. You can place sawhorses along the path from your truck to the installation site to give you places to put down the slabs and rest periodically.

Concrete technology from early 1900s still applies today

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Scientific American magazine has a section called “50, 100 & 150 Years Ago” that features blurbs from the magazine in each of those years. Here is a recent entry for March 1911:

“Concrete for Construction”

“About fifteen years ago serious attempts were made to combine steel and concrete by moulding one into the other in such a way that the resulting product would possess a high resistance not merely to compressive but to bending and tensional stresses. A vast amount of experimental work was done, out of which has sprung our modern reinforced concrete. Not only is concrete found to be available for practically every form of construction which hitherto has been built in brick and stone, but it has now invaded the field which was supposed to be peculiarly reserved for iron and steel.”

If you’ve seen my free video about reinforcing concrete countertops, you know exactly what these compressive, bending (flexural) and tensional stresses are! And you know that your concrete countertop construction methods are based on sound scientific principles. What’s surprising is how recently reinforced concrete was invented, seeing as concrete has been used since Roman times.

Would thinset and fiberglass cloth make a good reinforced concrete countertop?

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You’ve probably seen my reinforcing video on YouTube. Recently I got a question about whether one could use thinset with fiberglass cloth on the bottom as a countertop construction method, and possibly reduce thickness that way. I can see how this idea might have appeal, because it takes a cement-based material and attempts to boost flexural strength by adding a form of reinforcement reminiscent of the glass fibers in glass fiber reinforced concrete (GFRC) to the bottom.

However, the answer is no.

Thinset is not a self-supporting structural material. Yes it gets hard, but its design is to adhere and support tiles to a substrate. That’s not the same function as structural concrete. Concrete needs both a high flexural strength and a substantial compressive strength (plus shrinkage resistance, good aesthetics, hardness, etc) in order to be used for making top-quality concrete countertops. Not even the sidewalk and fence-post grade concrete sold in home centers is good enough.

Fiberglass cloth for resin-based fiberglass structures is typically made from E-glass. GFRC glass fiber is made from alkali-resistant (AR) glass. E-glass (and any other glass that’s not AR glass) will, over time, be weakened from the cement in concrete.

As for thickness, the best way to reduce that is to use GFRC. Traditional precast concrete countertops are often 3/4″ to 1″ thick. GFRC that uses AR glass as reinforcement is typically 1/2″ to 1″ thick depending upon what is actually being made. It’s this reduction in thickness, and the resulting reduction in weight, that make GFRC a very popular form of concrete in the concrete countertop industry.

Conventional precast concrete that uses structural steel reinforcing is often 1.5″ thick or greater. This is because reinforced concrete is a composite that relies on the concrete for the compressive strength and the steel for the tensile strength. As the total concrete thickness decreases, the compression and tensile faces move closer together. This greatly increases the stresses in those faces, so the amount of steel (and the strength of the concrete) needed to resist those forces increases proportionally. At some point it becomes physically impractical to create very thin precast countertops with enough steel in them to resist the loads imposed from moving and using the countertops. That thickness is around 1.5″.

It is possible to make thinner pieces of concrete that use less or no steel. But those pieces of concrete must be smaller, often shorter, under 4 feet in length, and the actual strength of the concrete is quite low. Think of a wall panel or backsplash versus a free-spanning countertop. The wall panel can be made thin because it only has to support itself, and in use it’s stuck to a wall. The countertop could be sat or stood upon (a realistic possibility some time after installation), and breakage is very likely.

Do I need to put plywood on cabinets under concrete countertops?

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“Is it necessary to have plywood tops on cabinets?”

The answer to this question really depends on whether the concrete countertops are being cast in place or if they are precast.

If they are cast in place, then the answer is yes. The plywood for cast in place concrete countertops is a permanent portion of the formwork. The plywood simply serves only as a form bottom, to keep concrete out of the cabinets, and it ceases being necessary once the concrete cures.

Note that you do need to protect the plywood with a moisture-proof barrier, or it will swell. You could also use a sheet good other than plywood that is already moisture-proof.

Precast concrete countertops do not require installing plywood on top of open cabinets. However, plywood does make installing large, awkward or heavy pieces easier, since the slab can be slid along the smooth top without catching on a cabinet wall.

Whether precast or cast in place, properly designed and fabricated concrete countertops are structural beams fully capable of supporting well over 300 lbs once installed (even 1.5” thick countertops). They don’t need the plywood at all. Plywood of any practial thickness (including ¾” material) is far more flexible than the concrete placed above it, and thus provides little or no structural support. Plywood should never be relied upon as the structural support for an improperly reinforced countertop.

Fibers as Secondary Reinforcement in concrete countertops

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Fibers are used in concrete for a variety of reasons, and not all fibers do the same thing or have the same effect on concrete. The size, shape, material and amount of fibers used has a significant effect on the concrete, and using too little or the wrong fiber type can result in disappointment or failure.

Fibers are generally added to concrete as shrinkage control (also known as secondary reinforcement; structural reinforcement is primary reinforcement). As the concrete sets and transforms from a workable paste into a hard solid, plastic shrinkage can occur. This is especially true in concrete slabs exposed to heat or wind. The matrix of fibers helps to stabilize the wet concrete and distribute the shrinkage stresses so that large cracks are minimized or eliminated.

Fibers are often advertised as capable of replacing welded wire mesh. This is true, but only when the welded wire mesh is used only as plastic shrinkage control. The confusion stems from the fact that welded wire mesh can also be used as structural (primary) reinforcement, while synthetic fibers cannot.

Most commonly used fibers are synthetic, either polypropylene or nylon, but some are natural, like cellulose fibers. None of these fiber materials is stiff or strong enough to provide any significant tensile reinforcement to uncracked concrete. And they simply stretch too much to do any good once the concrete cracks. After the concrete hardens, these fibers don’t contribute anything (nor can they) to resisting external structural tensile stresses.

There is a class of fibers that provides some resistance to external structural tensile stresses, but these are more esoteric and not generally necessary if you use proper steel primary reinforcing.

This information applies to precast concrete countertops, not to GFRC concrete countertops (glass fiber reinforced concrete). In the case of GFRC, fibers do provide the primary reinforcing because there are so many of them, they are aligned two-dimensionally by rolling, and there is enough polymer in the concrete to provide a great deal more flexibility than normal concrete. Be aware though, GFRC is a system. You cannot take one element of GFRC and add it to regular precast concrete countertops and expect it to convey the properties of GFRC to the countertops.

Structural AR glass fibers in GFRC:Fibers in GFRC

Rebar size in Concrete Countertops: It can be too big.

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The size of reinforcing steel in a countertop is an important consideration. Concrete countertops that are made with rebar that is too large in scale for the slab thickness are susceptible to telegraph cracking.

When concrete shrinks due to drying out, high tensile stress levels develop around proportionally oversized reinforcing. These stresses cause cracks that run along the length of the steel. Smaller diameter reinforcing has less of an affect on the concrete, so the same amount of concrete shrinkage develops much less tensile forces, thus the likelihood of cracks caused by the steel itself drops dramatically. Because all concrete shrinks to some degree, and shrinkage occurs over a long period of time, the occurrence of telegraph cracking may not appear until long after a countertop is installed.

In addition, oversized reinforcing occupies so much space inside a thin countertop slab that there is very little cover between the steel and the slab surface. Generally, reinforcing is fabricated in a grid arrangement, with strands running along the length of the slab and overlapped strands running across the width of the slab. When stacked, larger rebar can take up half of the total slab thickness, while smaller rebar takes up much less space.

 Oversized rebar can cause telegraph cracking

The sheer size of rebar that is too big places a significant amount of the steel closer to the visible surface of the countertop instead of down near the bottom of the slab. For example, a grid made from 3/8” diameter rebar held only 1/4” away from the bottom (the bare minimum cover for such a size) would place the top of the rebar at the midpoint of the slab, leaving only 1/2” of concrete cover between the steel and the visible surface. Contrast this with a grid made from 3/16” diameter wire held ¼” away from the bottom of the slab, and now there is 7/8” of concrete between the surface and the steel, a 75% increase in cover.

In addition to causing large stress concentrations in the concrete (and therefore increasing the likelihood of telegraph cracking), using oversized reinforcing actually decreases the load capacity of the concrete.

In the drawing above, the same concrete is reinforced with equal amounts of steel reinforcing (based on cross sectional area). The top drawing shows one piece of 3/8” (#3) diameter reinforcing steel while the bottom drawing shows four pieces of 3/16” structural reinforcing wire. The cross sectional areas are the same, so the tensile capacity of the steel is the same.

But because the four pieces of wire can be located lower in the slab, the load capacity of the wire-reinforced concrete is now 13% (lower reinforcing layer) to 78% (upper reinforcing layer) greater than the slab with the single #3 rebar, even though there is the same amount of steel in the concrete. In actuality, the structural wire has a higher strength than the rebar, so the difference in capacity is even greater.

Are concrete countertops still at risk of cracking if they have high compressive strength?

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“My concrete has a high compressive strength. That means I don’t need reinforcing to protect against, right?”

Not necessarily, and not in all circumstances. All concrete, regardless of the mix design or the magnitude of the compressive strength, is much weaker in tension than in compression. And cracks are caused by tensile failures of the concrete.

The tensile strength of concrete is often only about 10% that of the compressive strength, but this is a rough average. Tensile strength is highly variable, difficult to predict, and is dependent not only on the ingredients and their proportions, but on the casting technique, the curing history, the amount, size and distribution of voids and defects (microcracks), etc. Even the amount of pigment or other admixtures can significantly reduce the tensile strength of the concrete.

Unreinforced countertops rely completely on the tensile strength of the concrete itself to hold everything together. Since it is already known that the tensile strength is variable and difficult to predict, relying solely on it is tantamount to gambling with a finished piece.

Often experimenters will initially try out small pieces with success, because small pieces don’t develop large tensile stresses when handled. However, success leads to bolder ventures, and longer slabs result. Eventually the sheer weight and size of the slab generate tensile stresses that overcome the concrete. Because there is no reinforcement to resist the tensile load after the concrete cracks, the slab suddenly fails and snaps in half with little or no warning.

Reinforcing steel adds ductility, the ability for a material to absorb deflection and overloading without falling apart. Unreinforced concrete fails in a brittle fashion. That is, it fails suddenly and without warning; one moment it’s fine, the next it’s broken. Don’t take these kinds of risks with your concrete countertops.

Compression/Tension: Concrete countertops are beams

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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 floor joist is a beam. Concrete countertops are also beams.

When a beam has weight placed on top of it, that weight causes the beam to deflect (bend). Small weights on stiff beams cause almost no deflection, while large weights on flexible beams cause significant deflection. 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.

Between the two 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 (like 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. The point at which this switch occurs is called the neutral axis, and can be thought of as an imaginary line that runs parallel to the length of the beam.

compression-tension in beam

If a countertop is made out 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. A crack will form at the bottom and progress upward literally at the speed of sound.

Some argue that because concrete countertops usually actually span only the width of a cabinet box (usually a maximum of 36″), they are rather short beams, and therefore the stresses involved are not that high. This is true, but what about when an 8 foot long precast slab is picked up in the shop and loaded onto a truck for transportation? The largest stresses and biggest risk of cracking occur in the shop. Once the slabs are installed, only settling of the cabinets or building would impart much stress.

This video goes much more in depth on the subject of compression and tension forces in concrete countertops.