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 orderthe right concrete, place it properly, and finish it correctly. Unfortunately, a surprisinglylarge percentage of concrete contractors don’t really know how to do any of those stepscorrectly. So, how do you know that you are getting the job done right? (Especially ifyou are not around to oversee the placement of your concrete?)Well, a little basic concrete knowledge will help you a great deal. When orderingconcrete, there are a few terms you should know that will help insure that you get aquality product.
Strength:Most contractors know that they can specify the STRENGTH of the concrete. Strengthof concrete is measured in PSI, or Pounds per Square Inch (or MPa, MegaPascals, themetric version of PSI). A normal strength of concrete for residential driveways isbetween 3000 and 4000 PSI (20-27 MPa). Commercial driveways and other commercialand industrial pours normally require 5000 – 8000 PSI (34-55 MPa) due to the loads ofheavy tractor trailers and other heavy equipment. This strength is the compressivestrength of the concrete. The higher the compressive strength, the more weight it canwithstand. 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 heavycommercial 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 tothe concrete (rebar) gives the concrete tensional and torsional strength. The amount andsize 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 agood way to go. If you expect a large truck (such as a moving van) or something ofsimilar size and weight, you may want to have the rebar placed in a 16×16 inch grid. Therebar needs to be elevated off the ground so that it sits in the middle of the slab. In mostcases, the top layer of reinforcement in a slab needs to be 2 inches (50cm) beneath the topof the slab. It won’t do any good if it just sits on the ground. For more complicatedprojects, an engineer should be consulted to determine the size and orientation of thereinforcement. In general, the larger the reinforcement, the more tension and torsion theconcrete can handle.The addition of rebar gives a concrete slab structural support, and will be the best thingyou can do to prevent cracking. The other thing to do to prevent cracking is to make surethat the ground underneath the concrete pour is thoroughly compacted. The use of arolling vibrating compactor or a “sheep’s foot” compactor are the best ways to compactthe sub-soil. On most residential driveways, contractors simply scrape the turf off thearea, put up the forms, and start pouring. Guaranteed, you will have cracks if they don’tcompact or put rebar in the concrete.“What about wire mesh?”, you may ask. Well, it is less expensive, and is a greatimprovement over no reinforcement, but if you ever expect a vehicle larger than a car orpassenger 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 ofthe concrete, not at the bottom or top.Fiber reinforcement sounds like a good idea, but really doesn’t significantly helptensional 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 thatdestroy the strength of the concrete. Our advice on fiber and wire mesh is to put yourmoney into rebar. Rebar simply works.
Aggregate:After specifying the strength and reinforcement of the concrete, the next important factoris the aggregate, or rocks, that are used. Unless you want to have a washed or exposedaggregate slab, you will want to make sure that you specify 3/4 inch aggregate in any slabof 4-5 inches in thickness. The thicker the slab, the bigger the aggregate. Mostcontractors leave the aggregate decision up to the concrete plant, which is usually fine aslong 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.
Water:The next, and probably most important, specification is the water to cement ratio. Thisis sometimes specified in terms of “slump,” but should be specified as a ratio. Water tocement ratios of quality concrete range between .36:1 and .55:1 (that’s 0.36 -0.55pounds of water per pound of portland cement). Exceeding these ratios on the high orlow end will produce a product that is severely compromised and will not perform wellover time. The problem this presents to most concrete contractors is that concrete with awater:cement ratio in this range is very difficult to move around – it is quite thick. Thereare ways, however, to increase the workability of the concrete without adding water. Theaddition of a plasticizer to the mix creates a more plastic (workable) mixture withoutadding water.Some contractors only order concrete with a certain “slump.” Slump is simply theamount 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 mostcircumstances, a slump of 4 inches is the MAXIMUM slump that will have the properwater to cement ratio. Once again, the addition of a plasticizer will allow for a properwater:cement ratio and will deliver a mixture with a slump between 7 and 9 inches, whichis very easy to move around and work.A concrete contractor usually gets the concrete delivered to the site at the proper water tocement ratio (because the plant won’t send out a knowingly defective product), and thenthe contractor adds water to achieve a more plastic or workable mix. THIS PRACTICESEVERELY DAMAGES YOUR CONCRETE AND IS UN-FIXABLE. It makes it easyfor 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 tohold water, which, when frozen, expands and pops off the surface creating an unattractiveand sometimes unsafe surface. Excessive water also creates a much weaker (compressivestrength) 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 .55:1water: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 properwater to cement ratio. ALWAYS use a plasticizer on slab placement, and you will standa much better chance at getting a quality result.Plasticizers are a very inexpensive addition to the concrete, usually between $2 and $5per cubic yard. It will add about $20-50 per truckload of concrete. The cost of replacingyour concrete is much higher, and most concrete contractors will not stand behind theirwork 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 producea durable product.
Supplemental Cementacious Materals (SCM’s):A lot has been learned in recent years about the addition of certain materials into theconcrete mixture to either replace a portion of the portland cement in the mix or add tothe mix to provide more durable characteristics. These products include Fly Ash, groundblast furnace slag, and micro-silica (also called silica-fume). The use of these productscan produce some very positive results in the concrete, as well as increase the durabilityof the concrete.Fly Ash is the most common additive in a concrete mix. Fly Ash is a waste product fromthe burning of coal, usually from a power plant. Most of the Fly Ash produced at apower plant ends up in a landfill. But, Fly Ash works a lot like portland cement, and ajudicious use of it can create concrete that is more durable than concrete created withportland cement alone. Typically, you can replace up to 25% of the portland cement in abatch of concrete with fly ash, but you should consult the engineer at the concrete plant todetermine 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 whenit comes to concrete. The more dense, the more resistant to freeze/thaw damage and saltdamage your concrete can be. However, the down side to slag is that it makes the surfacemuch more difficult to finish, often leaving a network of unsightly spider web-like cracksand small bumps in the finish. Unless your finishing contractor has experience workingwith concrete that has slag in it, it may be best to stay away from this addition, as it maycreate more issues than it is worth. If you are pouring foundations or wall forms, Slag isa good option, using about 15% replacement of portland cement.Micro-silica is useful in increasing concrete durability. Micro-silica is added to theconcrete mix, and does not replace any quantity of portland cement. For mostapplications, the addition of about 8% by weight of micro-silica to the mix will yield ahighly dense, strong, and durable mixture.Air-Entrainment is a proven modification to concrete that really works to protectconcrete from freeze/thaw cycle damage. Air-Entrainment additives do not introduce airinto 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 apercentage. The effective range of air entrainment is 4% – 6%. Numerous studies havedemonstrated 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 theconcrete and the distance between those bubbles. Unfortunately, there is no wayto determine if a particular slab has the proper size and distribution of airentrainment 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 airsignificantly, so we advise speaking with a concrete engineer to determine theamount 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 pocketsonly.
Finishing:The most common mistakes concrete contractors make are:1. To improperly consolidate (remove air pockets) from the concrete and2. To finish the concrete before it is ready.Concrete contractors understand that time is money. If they can get done quickly, theycan do more projects and make more money. Skipping the consolidation step, whichhappens all too frequently, even on large commercial projects, can leave large voidsinside the slab, reducing the strength significantly and inviting early failure.Premature finishing seals the surface of the concrete and prevents air (which is a normalby-product of the setting process) from escaping the concrete. These relatively largebubbles get trapped just beneath the surface, and create a place for water and salts tocollect. When the water freezes, the pressure from the expansion of water changing intoice will pop off the surface of the concrete.It is critical that the contractor wait to finish-float the concrete until the off-gassingprocess is complete. Leveling it with a board or “screeding” device is OK as the concretecomes off the truck, but floating it out needs to wait. It is hard to know when this stateoccurs, but usually it is just after the water gets sucked back into the concrete. Thecontractor must work fast. If the 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 theconcrete plant, and the total time from when the concrete is put on the truck and mixeduntil it comes off the chute is ONE HOUR or greater, the concrete will not be goodbecause it has already begun to set, and you should not accept it. Don’t be shy aboutthis.However, if you are close to or over an hour away, you can have the plant add aRETARDER to the mix, which will keep it in good working condition. The length of theretarder can be specified (e.g. 1 hour, 2 hours, 3 hours). If you don’t know where theplant is, ask your contractor and then call the plant to see what the normal delivery time isfrom that location. Don’t forget to include the amount of time the truck will sit at the jobsite waiting for the contractor to be ready for it.
- CreteDefender P2 is a product that substantially increases 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 P2 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 P2 will significantly increase the durability of the concrete andprevent damage due to salts, freeze/thaw cycles, and even some chemical attacks. Best ofall, it works just as well with air-entrained concrete as it does 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 P2 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.