A 50-Year Driveway
By Paul Potts
The homeowner should be alert to the following important issues while negotiating with contractors for a new or replacement concrete driveway or sidelwalk: (1.) compaction of the subgrade (2.) quality of the concrete (3.) reinforcement (4.) broom finish (5.) jointing (6.) and the importance of curing concrete.
To clarify, cement is a manufactured product that is used to make concrete; the hardened material is referred to as concrete not cement.
Combined with a bit of enlightened oversight during construction, the explanations in this article and the specification at the end of the article will give the homeowner the tools needed to negotiate with contractors for a high quality concrete driveway at the best price.
Top Soils, Vegetation and Compacting the Subgrade
The dirt under new concrete must be free of top soil and vegetation and compacted soundly to provide a firm support for the new concrete. If top soils or vegetation are widespread, the subgrade will have to be excavated down to compactible soils and the extra space backfilled with sand or stone. The same is true of soils under existing concrete slated for replacement.
Existing soils under a driveway or added sand or stone must be compacted with a heavy vibrating machine about the size of a wheel barrow called a compactor
Ordering High Quality Concrete
The quality and durability of hardened concrete depends on (a.) the water-cement ratio, (b.) size and quality of aggregates, (c.) air entrainment in cold weather climates to improve durability (d) reinforcement and (e.) curing techniques.
Exterior Concrete is a mixture of cement, water, aggregate (and entrained air in cold weather climates) plus some other additives. The ratio of water to cement in concrete (W/C = Ratio), where the weight of water is divided by the weight of cement, must be 0.45 or lower down to 0.40 to make high quality, durable exterior concrete. Proportions are designed by weight, e.g. in a cubic yard of driveway concrete there should be 528 lbs. of cement (5-1/2 sacks), 238 pounds of water plus the weight of aggregate. If you divide 238 by 528 you will get the ratio of 0.45 which is ideal for exterior concrete. Concrete with this proportion will be stiff and pile up 4 – 6 inches high at the end of the discharge chute (see photo above).
Stiff concrete is better concrete, but workers want to liquify it by adding more water making it easier to get in place. The owner must tell the workers up front, before the first truck arrives, that the concrete has been ordered with the right balance of cement and water and under no circumstances should workers add more water. Contractors may choose to use a plasticizer or water reducer to make concrete more workable in which case coming off the truck the concrete will look soupier. Water reducers and plasticizers are sometimes poured in the hopper of the truck at the site, but this is measured in quarts not gallons.
There is a water valve on the outside of the truck just behind the driver’s door; the owner should keep an eye on that valve; if the driver goes for that valve just tell him if he opens the valve, except to wash out his truck, the truck will be sent back to the plant full of concrete.
Don’t forget to ask the concrete truck driver for a copy of the delivery ticket. You will be able to confirm from the ticket that the concrete in the truck is the same as agreed upon.
Concrete contains 1800 to 2200 pounds of large aggregate per cubic yard. The quality of the aggregate is important. In colder climates crushed limestone is preferable, if available, because round natural stone can have soft stone and chert that rises to the surface of concrete due its light weight, absorbs water during deicing, and explodes when frozen causing pop-outs (see photo).
Air Entrainment Admixtures
In my neighborhood many driveways are badly spalled near the street (see photo #3), because deicer salts spread by the city have been pushed into the driveway and have attacked the concrete. Experimentation with concrete over many years has proven that concrete with entrained air is more resistant to freeze-thaw cycles and deicer chemicals.
Air entrainment is an admixture that is added to the concrete at the batch plant.
Contraction Jointing, Welded Wire Reinforcement and Crack Control
Extra water, over and above what is needed to make hardened concrete, is included in the mix design to make it easier to place it in the forms and finish it. This extra water eventually evaporates and the concrete shrinks to make up for the lost volume; this shrinking and pulling toward the center causes long term shrink cracking. The shrinking is directly proportional to the amount of extra water used in the mix that must evaporate before concrete can set.
Contraction joints are hand tooled or saw cut joints in concrete that control cracking. Even concrete with a low water-cement ratio will crack, just less so than concrete with a higher water-cement ratio. Welded wire reinforcement (WWR) is used to mimize the width of jointing cracks and improve the load carrying capacity of concrete. Driveways can be placed without welded wire reinforcement, but the precaution of adding WWR is worth theinvestment. The same goes for 5-inch thickness as opposed to 4-inch.
Even low water cement ratio concrete will crack due to shrinkage. contraction joints provide a natural pathway for shrinkage cracks to follow that are more aesthetically pleasing than random cracks; and welded wire reinforcement holds the joints tightly together.
Some concrete people will try to sell you fiber mesh reinforcement as a substitute for WWR, but fiber mesh in the words of the Portland Cement Association “should not be expected to replace wire mesh in slab on ground”.
WWR is placed on chairs to keep it in the middle of the concrete. If WWR is placed on the ground without chairs, the owner must watch that the workers are using a steel hook to pull the reinforcement up into the slab as the concrete is being placed. WWR should be ordered with 12 by 12-inch box squares so the workers will have a place to step without pushing the reinforcement back on the ground.
Hand tooled contraction joints are better looking than saw cut joints, but contractors prefer to saw cut joints because it’s easier. Saw cutting joints should be done in the first 6 – 18-hours to be effective. Hand tooled joints will often fill with dirt and cultivate weeds unless caulked every 5 -10 years.
Spacing contraction joints is an aesthetic choice but it has practical effects. The smaller the squares, the better the crack control; but more jointing work costs extra so there is an economical middle ground to be reached. I required 6-foot by 6-foot hand-tooled joints in my driveway (see photo #4). In recent years a small corner crack developed but that is the only crack I have seen in 23 years.
Troweled concrete can be really slippery in wet or icy weather, for that reason a broom finish is safer than a smooth surface. The worker gently pulls a 36-inch wide broom across the surface after it is partially set to give it a safer walking surface.
Hardened concrete is made by the reaction between water and cement particles called hydration. Much of the hydration takes place in the first few hours and continues for years as long as there is free water and particles of cement that haven’t found a mate. After concrete starts to harden, the hydration process slows down, and if the free moisture evaporates too quickly the hydration is halted altogether leaving the concrete under strength. The longer the process is encouraged to continue the better the hardened concrete.
Curing is the process of keeping the extra water from evaporating prematurely. Curing methods can be divided into four classes: (1.) Membrane-Forming Curing and Sealing Compounds sprayed on the surface (2.) rewetting techniques including continuous flooding of the concrete for 7 to 10 days (3) absorptive covers kept continuously wet for the same period of time and (4.) Moisture-retaining covers of polyethylene film. Application of the curing treatment should not be delayed more than a few hours after the concrete has been placed.
Membrane forming curing compounds are the most common curing method used for residential and most commercial exterior concrete projects. It is sprayed on the surface of concrete in two passes starting as soon as bleed water and finishing operations have ended. It is the easiest to maintain over the required period of curing.
Continuous water flooding is one of the best curing methods but requires a continuous operation of keeping the surface thoroughly flooded for 7 – 10 days. Covering the surface with a commercially available burlap kept continuously wetted, produces great results but is expensive and while it is a little less intensive than continuous flooding still requires a lot of attention.
Moisture-Retaining polyethylene film works very well but is probably the least used method owing to the problem of keeping the plastic sheet in place for 7 – 10 days without having it blown off by the wind.
Concrete must not be placed on frozen ground. Provide clear slope for water to get off the driveway to so that it does not trap water that can turn to ice. Slope driveways or sidewalks away from structures so that it does not channel water into your garage or house.
In colder climates some thought must be given to the elevation of the new concrete at the point where the driveway meets the garage floor. The ground under the driveway will freeze in the winter and heave to some depth depending on how cold it gets; the garage floor will be protected perhaps even heated and will remain in place. Water in the soil under the driveway will freeze and expand pushing the driveway up as much as an inch causing a hump at the intersection with the gargae floor. There is no formula that would work in every climate, so this will be something that the homeowner will need to discuss with the contractor.
Getting the Best Price
Negotiating with contractors to get the best price while all the time checking references and forming your personal opinion of their character is a tedious activity; but getting the right contractor is all important. Avoid using the terms “bid” or “bidding” when negotiating with contractors, this implies that like a government agency you will take the lowest price. You want to stay in control of who you give the contract and the lowest price is not the always the best choice. Give each contractor the same list of requirements (see short specification below).
Keep in mind if you are replacing a driveway the contractor will not know what the subsoil looks like and they will not want to give a price for something they don’t know. If your current concrete driveway has sunken in places, it is likely the subgrade will need to reworked; but this is something you are probably going to have to negotiate after the old driveway has been removed.
For additional information visit the Portland Cment Association website on driveway placement https://www.cement.org/cement-concrete-applications/paving/buildings-structures/concrete-homes/products/driveways
Concrete Driveway Specification
The contractor must remove existing asphalt or concrete.
The contractor must remove vegetation and topsoil, replace voids with acceptable subgrade material and compact the resulting soil or gravel.
Strength: 4500 lbs.
Water-Cement Ratio: 0.45 (adding more water at the site is not allowed).
Cement Content: Minimum 5-1/2 Sack (94 lbs. per sack) Maximum 6 Sack.
Aggregate: ASTM C 33: Maximum size 1-1/4-inch (Gravel); Maximum size 1-inch (Crushed Limestone).
Slump: 3-inch plus or minus 1-inch.
Thickness: 4-inch or 5-inch (select one: 5 – inch is recommended for heavier loads)
Entrained Air 6-percent plus or minus 1-percent (for cold weather climates).
Mid-Range water reducer is allowed for convenience, but the owner must be advised of the contractor’s intentions with regard to methods to ease placement.
Welded Wire Reinforcement: WWR 12 x 12 inch, W1.4 by W1.4. WWR must be mounted on chairs or concrete bricks to keep it in the center of the pour; if placed on the ground, workers must be actively pulling WWR into the center of the concrete during placement with tools made for the purpose.
Curing Concrete: Membrane Forming Curing Compound.
Contraction Joints: Saw cut or hand tooled contraction joints with areas not to exceed 10 by 10 foot. Cut extra joints where uncontollable cracking is likely. Discuss the choices with the owner.
Disclaimer: The author disclaims any and all responsibility for the applications of the principles discussed in this publication or for the accuracy of the sources other than information developed by the author.