What to know when you are ordering concrete:
How much do you need?
Calculating how much concrete you need can be difficult. We are here to help! Accuracy when you are figuring the dimensions of your pour is very important. If you calculate your slab at 4 inches thick but it is 6 inches in some places it could average 5 inches thick. That is a 20% difference in the amount of concrete needed. It is very important to take the time necessary to get your grade correct and even.
Here is some math to help you calculate it yourself. If you get stuck, give us a call.
For example, a patio slab that is 10 feet x 12 feet and 4 inches thick would be calculated this way:
10 x 12 = 120 square feet
120 x 0.33 feet thick = 39.6 cubic feet.
39.6 cubic feet/ 27 cubic feet per cubic yard = 1.46 yards of concrete
We recommend you order an additional 10% to cover waste and possible inconsistencies in grade. We sell concrete in quarter yard increments with a one yard minimum order. For this example, 1.75 yards would be the appropriate order.
Keep in mind that there will be an additional charge for loads less than five yards. If your placement figures between four and five yards, it may be more cost effective to get five yards and not pay the short load fee. Also, if you do not order enough and need another load the short load fees will apply just like it is a new order.
All concrete mixes contain four basic components. The variation of these ingredients is what makes up the different mixes for a wide variety of uses.
Course Aggregate – 3/8” and larger rock
Fine Aggregate – Sand
Cement – A fine powder made from limestone.
Each mix is first defined by the compression strength it is required to meet 28 days after it is batched. This can range from 2500 psi to 12,000+ psi. The most important factor in making this strength is the water/cement ratio.This ratio needs to be maintained from the time it is batched into the truck all the way until initial cure is complete. (see Slump and Curing below)
Course Aggregate Size (maximum) –
Our mixes can be ordered with 3/8”, ¾”, or 1 ½” course aggregate. Each of these has its own particular use as well as its own challenges. Keep in mind, Duke City uses a combination of these to make our mixes perform above standards while still being affordable. The size of the aggregate mentioned in the mix is the largest size in the mix and cannot exceed 33% of your smallest dimension.
3/8” Course aggregate is easier to pump, better to fill in smaller spaces, but requires more control joints, extra cement to make the same strength and, therefore, more expensive.
¾” Course aggregate is the most commonly used because it is very workable and has great strength.
1 ½” course aggregate is best used in thicker placements (8” thick and over), but is difficult to finish.
The slump of concrete is a way to measure the workability of the concrete. It is most commonly associated with the amount of water in the mix. The test is completed by placing the concrete in a 12” tall hallow cone and then lifting the cone up slowly. The concrete will fall or “slump” down as the cone is pulled up. The number of inches the concrete falls is the slump. If the concrete has a high content of water the slump will increase. Most mixes are designed for a 4” slump. If your project requires more workability additional water reducers can be added to increase slump and maintain strength.
There are a wide variety of concrete admixtures available to help you complete your project in the best and most cost effective way.
Air Entrainment – all concrete exposed to the outside elements needs to have air entrainment. This admixture creates small bubbles inside your concrete to allow moisture in the concrete a place to expand when it freezes. Without this outlet the concrete will fail the first time it freezes. The down side of using air entrainment is the finish. You CANNOT use a steel trowel finish on air entrained concrete because it will peel the surface of the concrete and ruin your project.
Water reducers – All mixes Duke City offers have water reducer in them to help with strength and workability. We are able to use water reducers (aka – Midrange) to increase slump and workability.
Accelerators – In colder weather or if a faster set time is required accelerators can be added to help the concrete set quicker.
Retarders – If you want to slow down the set time or your job site is more than an hour drive time from one of our plants we recommend using retarder.
Fiber – fiber reinforcement is a great way to increase early strength and reduce surface shrinkage cracking by adding small strands of fiber to the mix to help it bond together.
On the Job Site
Job site safety and access are the responsibility of the contractor or owner of the property. Here are a few questions to ask yourself before ordering a load of concrete:
Will the truck fit onto the job site? A mixer truck is 8 feet wide and 12 feet tall and will need additional room on all sides to safely maneuver around the job site.
Do you have a way to get the concrete from the truck to the final location? Every truck can reach about 15 feet directly behind the truck with chutes, but can only flow down from gravity. If the truck cannot access it, do you have access to a concrete pump or wheel barrows and enough help to get the concrete to the placement? Keep in mind you are allotted 10 minutes per yard to unload the truck or you could be charged truck time.
Does the truck have a place to washout when the pour is complete? Our trucks need to remove all wet concrete from the chutes before leaving the job site. This can often be accomplished with a wheel barrow to catch the water and the waste.
Before the concrete arrives
Before the concrete is scheduled to arrive your job site should be ready for the concrete. That means having forms constructed and the ground under the placement compacted and level to the proper grade.Most of the time this will be done by a contractor or by someone who has done the work before.
Duke City Redi-Mix is a concrete supplier and does not finish the concrete for you. You will be responsible for placing and finishing the concrete, or hiring someone that will. Books have been written on this process and takes practice and experience. We do not recommend attempting this without someone who has done it before.
The concrete curing process can take several months, but the first few days are the most important. After the concrete has reached initial set, usually about 10 to 18 hours after placement, the concrete should kept moist as much as possible. This can be done by continually wetting the concrete with a hose or by adding an impermeable material to the surface to keep the moisture from evaporating. The concrete should be kept wet for at least three days, most of the curing will be complete, but the longer you can do it, the better it will be.
Cold Weather Concrete
If your job requires you to pour in the winter it is essential that you keep the concrete from freezing until the strength of the concrete is enough to withstand it, usually 5 to 7 days.
Concrete creates its own heat by the chemical process of hydration and preventing the heat from escaping will be enough to prevent freezing (except under extreme cold conditions). Concrete blankets should be put on to cover the placement at night and removed when the sun is out and the temperature is above freezing. This process needs to continue every night and every day.