Ordering Concrete

How to order/specify concrete

Here’s how most people order concrete:  Call the concrete contractor and tell him they want 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 you insure that you will get a quality product.  

Strength:

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 4000psi (20 – 27 MPa).  Commercial driveways 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 13,000psi or 90MPa), but you will only see this kind of strength in heavy commercial applications.

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 driveway 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.

 Aggregate:

 After specifying the strength 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, and they are usually ok to do so as long as the 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.

Water:

 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 .36 -.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 adding water.

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 him 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), 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 provide help you insure that your concrete pour will produce a durable product.

Supplemental Cementacious Materals (SCM’s):

A lot has been learned in the past 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 would want to speak with the engineer at the concrete plant to determine the exact amount you would want to use for your project.

In addition, 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 to portland cement.

Micro-silica has been found to be a significantly useful product in increasing concrete durability.  Micro-silica is used as an addition 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:

  • First, 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 4000psi mix is loses 25% of its strength, and becomes a 3000psi mix.  
  • Another issue is that 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 would 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 only.

Finishing:

 The most common mistakes concrete contractors make are to improperly consolidate (remove air pockets) from the concrete and to finish the concrete before it is ready.  Like most people, 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.  He’ll have to work fast.  If his crew is big enough, that won’t be a problem.

 Time:

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 the time the concrete was 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 this.  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®:

CreteDefender is a product that substantially increases the density of concrete by filling the pore and capillary structure of the concrete, thereby preventing water, salts, and other chemicals from entering the concrete and creating damage.  CreteDefender will not correct mistakes in the concrete mixture, such as excessive water, failure to consolidate, or early finishing of the concrete.  However, in properly mixed and place concrete, CreteDefender will significantly increase the durability of the concrete and prevent damage due to salts, freeze/thaw cycles, and even some chemical attacks.  Best of all, it works just as well with air-entrained concrete as it does with non air-entrained concrete.  

Summary:

That’s it.  Now just to summarize:  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 is elevated 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 are pouring
  5. Order the mix with a Plasticizer or a Super Plasticizer, and make sure that the water:cement ratio is not over .55:1.
  6. Make sure the slump is not greater than 4 inches without the Plasticizer, 9 inches with the Plasticizer
  7. Use SCM’s to create a more durable mix, but consult with a concrete engineer to design a mix that will meet your needs.
  8. Don’t take any concrete that has been on the truck for more than an hour without a 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 to substantially increase the durability of and prevent damage to your concrete.

Follow these steps and you will have the best opportunity for a durable, high quality concrete slab.