FAQs & Resources
Concrete in Practice, written by the National Ready Mixed Concrete Association, is a series of information sheets on important technical topics, written in a non-technical "What, Why and How?" format. The following Concrete in Practice (CIP) sheets are available for you to view.
What is Concrete?
Concrete is basically a mixture of two components: aggregates and paste. The paste, comprised of Portland cement and water, binds the aggregates into a rock like mass as the paste hardens because of the chemical reaction of the cement and water. Supplementary cementitious materials and chemical admixtures may also be included in the paste.
How much water can I add to the concrete?
Concrete should contain enough water to produce a mix that has a relativity stiff consistency, which works readily and does not segregate. Normally, concrete delivered to a job has a slump of 3-4 inches.
Since the water/cement ratio controls the strength of the concrete, water cannot be added above the maximum established water/cement ratio for that particular concrete mixture without sacrificing strength and durability. If you add only one gallon of water to a yard of properly designed 3000 PSI concrete mix you will:
Increase the slump approximately one inch
Cut the compressive strength by as much as 200 PSI
Waste the effect of ¼ bag of cement
Increase the shrinkage potential approximately 10%
Increase the possibility of seepage through the concrete by up to 50%
Decrease the freeze-thaw resistance by 20%
Decrease the resistance to attach by de-icing salt
Lower the quality of the concrete in many other ways
Why is temperature important?
Concrete gains strength through a complex chemical reaction that involves hydration of the cement paste. This reaction is temperature sensitive. If it is too cold, below 35 degrees F, the cement will not hydrate fast enough, the water in the mix may freeze, and the concrete will not set and develop the designed strength. If it is too hot, above 90 degrees F, the cement will hydrate rapidly, and the concrete may set before it is properly placed and finished. The recommended temperature range for proper concrete placement is between 50 F and 85 F. Placement of concrete outside of these ranges requires special preparation.
How long must my concrete driveway cure before I can drive on it?
Seven days after the placement of concrete.
How much does concrete weigh?
Normal concrete weighs about 4000 pounds per cubic yard.
Do I need to seal my new concrete driveway?
You should apply a high-quality sealer to all exterior concrete slabs exposed to freeze-thaw conditions. The sealer helps protect the concrete from moisture absorption, exposure to chemicals, and grease and oil stains. On decorative colored concrete, a sealer will also help enhance the color.
Generally, there are two categories of sealers for exterior concrete: film-formers and penetrants. Penetrating sealers tend to offer the best protection from moisture absorption. Regardless of the sealer you use, be sure it's applied according to the manufacturer's instructions.
My concrete is cracking after only a short period. Is there something wrong with it and can it be repaired?
All concrete cracks. It has to crack because it contracts during the drying, curing, hardening process, and the bond between the cement paste and the aggregates is not strong enough to withstand that stress. The best way to prevent unsightly cracking is to put joints in your concrete at regular intervals. A good formula is to measure the depth of your structure and multiply the number by three. Use this number to determine the approximate number of feet between joints. For example, a 4 inch slab of concrete should have joints every 10 to 12 feet. Uneven shifting of the substructure or sub grade can also cause cracking. This is a structural failure, as opposed to improper curing or jointing as mentioned above. Before repairing any concrete cracking, determine the source of the cracking and remedy that first. Epoxy grout is an excellent crack repair agent.
My concrete has oil/grease stains. How can I get rid of them?
Commercial products are available in paint/hardware home centers.
Sprinkle with tri-sodium phosphate. Allow to stand for 30 minutes, and then scrub with a stiff brush and hot water. Rinse with clean water.
Scrub stain with concentrated detergent using a stiff brush. Rinse well with water. Dry and repeat if necessary.
Sprinkle dishwasher detergent on wet concrete. Let it stand a few minutes, then pour boiling water on the area. Scrub and rinse.
Dissolve a cup of tri-sodium phosphate in 1 gallon of hot water. Pour over stained concrete surface and allow soaking for 25 minutes. Scrub with stiff brush or broom. Rinse with clean water. Repeat if necessary.
Can I use deicer chemicals on my concrete?
The use of deicing chemicals during the first year of service is not recommended, especially if concrete is installed late in the year. Sand is an acceptable alternative anytime. Deicing chemicals used for snow and ice removal can cause and aggravate surface scaling. Therefore, judicious use of these products with regard to amount and frequency of application is strongly advised. Remember deicers can also reach concrete surfaces other than by direct application -- for example, drippings from the under-carriage of vehicles.
During and after the concrete's second winter, deicing chemicals containing sodium chloride (common salt) or calcium chloride may be used judiciously.
NEVER use deicers containing ammonium sulfate, ammonium nitrate, potassium sulfate, and magnesium chloride, as they will chemically attack and rapidly disintegrate concrete.
Protecting your concrete from deicers and the harsh winter environment can be accomplished by using a sealer.
How far do the concrete chutes reach?
The chutes on our mixers reach up to 16 feet straight out the back, and 13 feet on an angle.
How big is your ready mix truck?
Our mixers are 8 feet wide wheel to wheel, 10 feet wide mirror to mirror, and 12.2 feet high.
How much time do I have to unload the truck?
Concrete is a perishable product, and most specifications require it be discharged on the job site within 90 minutes or 300 revolutions of the truck barrel, after the addition of water to the concrete mix at the batch plant. Allowable unloading time at the job site is 10 minutes per yard. Well-staffed job sites typically unload a truck in 30 minutes. Truck time charges will apply in excess of allowable time.