How to Order/Specify Concrete
Here’s how most people order concrete: Call the concrete contractor and tell him theywant a new driveway, porch, sidewalk, foundation, or slab.
Here is the problem: You put a great amount of faith in your concrete contractor to order
the right concrete, place it properly, and finish it correctly. Unfortunately, a surprisingly
large percentage of concrete contractors don’t really know how to do any of those steps
correctly. So, how do you know that you are getting the job done right? (Especially if
you are not around to oversee the placement of your concrete?)
Well, a little basic concrete knowledge will help you a great deal. When ordering
concrete, there are a few terms you should know that will help insure that you get a
Most contractors know that they can specify the STRENGTH of the concrete. Strength
of concrete is measured in PSI, or Pounds per Square Inch (or MPa, MegaPascals, the
metric version of PSI). A normal strength of concrete for residential driveways is
between 3000 and 4000 PSI (20-27 MPa). Commercial driveways and other commercial
and industrial pours normally require 5000 – 8000 PSI (34-55 MPa) due to the loads of
heavy tractor trailers and other heavy equipment. This strength is the compressive
strength of the concrete. The higher the compressive strength, the more weight it can
withstand. Concrete can be engineered to achieve extremely high compressive strength
(over 13,000 PSI, or 90 MPa), but you will only see this kind of strength in heavy
Concrete is quite strong in compression, but very weak in tensional or torsional strength,
meaning it will break or crack when bent (tension) or twisted (torsion). Adding steel to
the concrete (rebar) gives the concrete tensional and torsional strength. The amount and
size of the rebar you need depends upon how much weight and movement you expect.
For most driveways, 1/2 inch (#4) rebar placed and tied together on a 2×2 foot grid is a
good way to go. If you expect a large truck (such as a moving van) or something of
similar size and weight, you may want to have the rebar placed in a 16×16 inch grid. The
rebar needs to be elevated off the ground so that it sits in the middle of the slab. In most
cases, the top layer of reinforcement in a slab needs to be 2 inches (50cm) beneath the top
of the slab. It won’t do any good if it just sits on the ground. For more complicated
projects, an engineer should be consulted to determine the size and orientation of the
reinforcement. In general, the larger the reinforcement, the more tension and torsion the
concrete can handle.
The addition of rebar gives a concrete slab structural support, and will be the best thing
you can do to prevent cracking. The other thing to do to prevent cracking is to make sure
that the ground underneath the concrete pour is thoroughly compacted. The use of a
rolling vibrating compactor or a “sheep’s foot” compactor are the best ways to compact
the sub-soil. On most residential driveways, contractors simply scrape the turf off the
area, put up the forms, and start pouring. Guaranteed, you will have cracks if they don’t
compact or put rebar in the concrete.
“What about wire mesh?”, you may ask. Well, it is less expensive, and is a great
improvement over no reinforcement, but if you ever expect a vehicle larger than a car or
passenger van, or didn’t have the sub-soil compacted, or you have any seismic activity,
you will likely have cracks. Once again, the reinforcement needs to be in the middle of
the concrete, not at the bottom or top.
Fiber reinforcement sounds like a good idea, but really doesn’t significantly help
tensional or torsional strength. The big issue with fiber is proper dispersion in concrete.
More often than not, fiber will come off the chute in clumps, which creates voids that
destroy the strength of the concrete. Our advice on fiber and wire mesh is to put your
money into rebar. Rebar simply works.
After specifying the strength and reinforcement of the concrete, the next important factor
is the aggregate, or rocks, that are used. Unless you want to have a washed or exposed
aggregate slab, you will want to make sure that you specify 3/4 inch aggregate in any slab
of 4-5 inches in thickness. The thicker the slab, the bigger the aggregate. Most
contractors leave the aggregate decision up to the concrete plant, which is usually fine as
long as they communicate to the plant what the anticipated thickness of the slab is to be.
But, it is always good to ask what size rock the plant uses on projects like yours.
The next, and probably most important, specification is the water to cement ratio. This
is sometimes specified in terms of “slump,” but should be specified as a ratio. Water to
cement ratios of quality concrete range between .36:1 and .55:1 (that’s 0.36 -0.55
pounds of water per pound of portland cement). Exceeding these ratios on the high or
low end will produce a product that is severely compromised and will not perform well
over time. The problem this presents to most concrete contractors is that concrete with a
water:cement ratio in this range is very difficult to move around – it is quite thick. There
are ways, however, to increase the workability of the concrete without adding water. The
addition of a plasticizer to the mix creates a more plastic (workable) mixture without
Some contractors only order concrete with a certain “slump.” Slump is simply the
amount the concrete will sag when concrete is put into a device called a slump cone,
turned upside down, properly consolidated, and the cone removed. Under most
circumstances, a slump of 4 inches is the MAXIMUM slump that will have the proper
water to cement ratio. Once again, the addition of a plasticizer will allow for a proper
water:cement ratio and will deliver a mixture with a slump between 7 and 9 inches, which
is very easy to move around and work.
A concrete contractor usually gets the concrete delivered to the site at the proper water to
cement ratio (because the plant won’t send out a knowingly defective product), and then
the contractor adds water to achieve a more plastic or workable mix. THIS PRACTICE
SEVERELY DAMAGES YOUR CONCRETE AND IS UN-FIXABLE. It makes it easy
for the contractor to work, but is the most common problem with concrete placement.
Excessive water in the mix creates a pore structure in the concrete that is large enough to
hold water, which, when frozen, expands and pops off the surface creating an unattractive
and sometimes unsafe surface. Excessive water also creates a much weaker (compressive
strength) product, and invites rapid deterioration when exposed to de-icing salts.
Nothing can be done to correct the addition of water to a concrete mix past the .55:1
water:cement ratio after the concrete is off the truck. The use of a plasticizer or super-
plasticizer will allow the contractor to have a workable mix without exceeding a proper
water to cement ratio. ALWAYS use a plasticizer on slab placement, and you will stand
a much better chance at getting a quality result.
Plasticizers are a very inexpensive addition to the concrete, usually between $2 and $5
per cubic yard. It will add about $20-50 per truckload of concrete. The cost of replacing
your concrete is much higher, and most concrete contractors will not stand behind their
work if there is spalling or a surface failure (They will claim it is “salt damage”).
Specifying a plasticizer in your mix will help insure that your concrete pour will produce
a durable product.
Supplemental Cementacious Materals (SCM’s):
A lot has been learned in recent years about the addition of certain materials into the
concrete mixture to either replace a portion of the portland cement in the mix or add to
the mix to provide more durable characteristics. These products include Fly Ash, ground
blast furnace slag, and micro-silica (also called silica-fume). The use of these products
can produce some very positive results in the concrete, as well as increase the durability
of the concrete.
Fly Ash is the most common additive in a concrete mix. Fly Ash is a waste product from
the burning of coal, usually from a power plant. Most of the Fly Ash produced at a
power plant ends up in a landfill. But, Fly Ash works a lot like portland cement, and a
judicious use of it can create concrete that is more durable than concrete created with
portland cement alone. Typically, you can replace up to 25% of the portland cement in a
batch of concrete with fly ash, but you should consult the engineer at the concrete plant to
determine the exact amount you would want to use for your project.
Ground blast furnace slag is also used to create a denser mixture. Density is good when
it comes to concrete. The more dense, the more resistant to freeze/thaw damage and salt
damage your concrete can be. However, the down side to slag is that it makes the surface
much more difficult to finish, often leaving a network of unsightly spider web-like cracks
and small bumps in the finish. Unless your finishing contractor has experience working
with concrete that has slag in it, it may be best to stay away from this addition, as it may
create more issues than it is worth. If you are pouring foundations or wall forms, Slag is
a good option, using about 15% replacement of portland cement.
Micro-silica is useful in increasing concrete durability. Micro-silica is added to the
concrete mix, and does not replace any quantity of portland cement. For most
applications, the addition of about 8% by weight of micro-silica to the mix will yield a
highly dense, strong, and durable mixture.
Air-Entrainment is a proven modification to concrete that really works to protect
concrete from freeze/thaw cycle damage. Air-Entrainment additives do not introduce air
into the concrete. They simply take the air and break it up into very small bubbles,
stabilizing the air in the mixture of the concrete. Usually, air entrainment is specified as a
percentage. The effective range of air entrainment is 4% – 6%. Numerous studies have
demonstrated that there is no benefit to having additional air entrainment beyond 5%.
Less than 4% air entrainment is of no value.
There are, however, a few things to be aware of regarding air entrainment:
* For every 1% of air entrainment you put in your concrete, the concrete loses 5%
of its compressive strength. So at 5% air entrainment, a 4000 PSI mix loses 25%
of its strength, and becomes a 3000 PSI mix.
* Effective air entrainment depends completely on the size of the air bubbles in the
concrete and the distance between those bubbles. Unfortunately, there is no way
to determine if a particular slab has the proper size and distribution of air
entrainment until the slab is hardened and a sample is cut out to inspect.
* The use of Fly Ash in an air-entrained mix reduces the percentage of air
significantly, so we advise speaking with a concrete engineer to determine the
amount of additional air you need to include in a mix that also includes Fly Ash.
* The use of vibrating consolidation can also remove air from the mixture.
Vibrating consolidation should be done minimally to remove large air pockets
The most common mistakes concrete contractors make are:
1. To improperly consolidate (remove air pockets) from the concrete and
2. To finish the concrete before it is ready.
Concrete contractors understand that time is money. If they can get done quickly, they
can do more projects and make more money. Skipping the consolidation step, which
happens all too frequently, even on large commercial projects, can leave large voids
inside the slab, reducing the strength significantly and inviting early failure.
Premature finishing seals the surface of the concrete and prevents air (which is a normal
by-product of the setting process) from escaping the concrete. These relatively large
bubbles get trapped just beneath the surface, and create a place for water and salts to
collect. When the water freezes, the pressure from the expansion of water changing into
ice will pop off the surface of the concrete.
It is critical that the contractor wait to finish-float the concrete until the off-gassing
process is complete. Leveling it with a board or “screeding” device is OK as the concrete
comes off the truck, but floating it out needs to wait. It is hard to know when this state
occurs, but usually it is just after the water gets sucked back into the concrete. The
contractor must work fast. If the crew is big enough, that won’t be a problem.
Time is the other thing that can really hurt a concrete job. If you are far away from the
concrete plant, and the total time from when the concrete is put on the truck and mixed
until it comes off the chute is ONE HOUR or greater, the concrete will not be good
because it has already begun to set, and you should not accept it. Don’t be shy about
However, if you are close to or over an hour away, you can have the plant add a
RETARDER to the mix, which will keep it in good working condition. The length of the
retarder can be specified (e.g. 1 hour, 2 hours, 3 hours). If you don’t know where the
plant is, ask your contractor and then call the plant to see what the normal delivery time is
from that location. Don’t forget to include the amount of time the truck will sit at the job
site waiting for the contractor to be ready for it.
- CreteDefender products like P2 and CP substantially increase the density of concrete byfilling the concrete’s pore and capillary structure, thereby preventing water, salts, andother chemicals from entering the concrete and creating damage. CreteDefender products willnot correct mistakes in the concrete mixture, such as excessive water, failure toconsolidate, or early finishing of the concrete. However, in properly mixed and placedconcrete, CreteDefender products will significantly increase the durability of the concrete andprevent damage due to salts, freeze/thaw cycles, and even some chemical attacks. Best ofall, most work just as well with air-entrained concrete as they do with non air-entrainedconcrete.
Summary:When placing a new concrete slab:1. Compact the sub-soil.2. Use rebar to provide structural strength to the concrete, and make sure it iselevated to the center of the slab.3. Order concrete with a compressive strength that meets your needs.4. Make sure the aggregate is appropriate to the thickness of the slab you arepouring.5. Order the mix with a Plasticizer or a Super Plasticizer, and make sure that thewater:cement ratio is not over .55:1.6. Make sure the slump is not greater than 4 inches without the Plasticizer, 9 incheswith the Plasticizer.7. Use SCM’s to create a more durable mix, but consult with a concrete engineer todesign a mix that will meet your needs.8. Don’t take any concrete that has been on the truck for more than an hour withouta retarder in it.9. Make sure the contractor consolidates your concrete properly.10. Wait until the concrete is ready before floating and finishing the concrete.11. Use CreteDefender products to substantially increase the durability of, and preventdamage to, your concrete.Follow these steps and you will have given yourself the best opportunity for a durable,high quality concrete slab.