What is the formula to calculate the square yardage of concrete?
Calculate the volume (length x width x height) then divide by 27 to get cubic yards. Remember to always round up!
What is the formula for mixing concrete?
The ‘rule of thumb’ mix design for concrete is 1 part cement, 2 parts sand, and 3 parts gravel or crushed stone. If you want to strengthen the mix, add additional cement without changing the proportion of sand and stone.
What is the best way to control cracking in my concrete slab?
First, it is important to understand that ALL concrete cracks. The best solution to controlling cracks is to install joints in the concrete. Joints are basically straight cracks that let the concrete crack while maintaining a good appearance. The joint groove should have a minimum depth of ¼ the thickness of the slab, but not less than 1 inch, so if the slab is 4 inches thick, the joint would need to be 1 inch deep.
What is the rule of thumb for placing joints on an exterior slab?
For 4 inch concrete slabs on grade, the suggested join placement would be 10’x10’. The guideline is 2 to 2 ½ times the slab thickness, in feet. For a 4 inch slab, the suggest spacing would be 8-10 feet. For the joints to work properly, the depth of the joint will need to be at least 1 inch.
What type of sealer should be used on an exposed aggregate driveway?
Use a clear, high-solids acrylic sealer (greater than 20% solids) on an exposed aggregate driveway. An individual should be able to find such a sealer at a building supply store or at a contractor supply house. This material cannot be sprayed; it is best applied with a long napped roller.
What is the best way to cure my newly poured concrete patio/driveway?
A great method for a DIY-er is to apply a cure-n-seal product. As soon as an individual can walk on new concrete without marking the surface, the product can be applied. Look for an acrylic cure-n-seal product with at least 15-18% solids. There are several brands available. The product can be applied with either a sprayer or a roller, but it is not recommended spraying them through a cheap unit that has a plastic tank and want (acrylics tend to gum up these kinds of sprayers).
Water curing is a great way to cure the slab, but it is hard to do well. It is very important that the entire surface of the slab be kept wet continuously, and this is hard to accomplish without the use of burlap. 3 days of wet curing should be sufficient, but more is better. If you water cure, you can still apply cure-n-seal after the surface of the slab is dry.
There is a gray film on my exposed aggregate driveway after the retarded was washed off, what should I do?
The slab may have needed to be washed more thoroughly during the construction phase. This is necessary to make sure the gray film does not remain on the surface. Acid washing the surface of your slab should remedy the problem. Keep in mind that acid is a dangerous material and proper precautions need to be taken with its use. When the acid is rinsed off (the slab needs to be thoroughly rinsed to insure that no acid remains!) you need to be aware of where the spent acid is flowing. The acid could harm or kill your vegetation, and you don’t want it pooling where children and/or pets could be exposed. Also, please prevent the acid runoff from flowing into a public storm sewer.
Will misting and covering a new patio help it cure?
Curing concrete is an essential step for concrete to gain its full potential strength and it is also essential for long-term durability. Water curing is the best possible cure for concrete, provided the concrete is kept wet at all times. This means that the surface should never be allowed to dry out. Here’s why:
- If you cure the concrete by using a sprinkler or hose, but only wet the slab intermittently, the stress of the alternate cooling and then heating as the water evaporates can actually increase the stress on young concrete to the point of causing thermal cracking. This is more of a concern when the ambient temperature is high.
Covering concrete with plastic is also an acceptable method of curing in many situations, but it is not recommended in extremely hot weather, as the plastic tends to increase the temperature of the concrete. Plastic sheeting can also cause discoloration of the concrete in some instances.
If you chose to water cure, do so for 3-5 days, making sure the surface is wet at all times. After that, it is recommended that you seal the concrete with an acrylic sealer to extend the curing period and to help protect the slab from water intrusion, dirt, etc. Look for an acrylic cure-n-seal product with at least 15-18% solids. Higher solids content will bring an increase in the gloss or shine that the sealer brings to the slab, and if the slab does not have a textured finish, the use of an acrylic sealer will make the slab more slick, especially when wet.
I had a driveway poured and now I have scaling and a white powder on the surface. What can I do?
This is difficult to diagnose without more information, but the driveway may have experienced carbonation of the surface. This normally occurs when the fresh concrete is placed in cold weather, then covered and heated to prevent freezing or to promote settling. If the heat source burns fuel and is not vented, the exhaust fumes from the heater will react with the fresh concrete in a process called carbonation, resulting in a dusty surface. The rest of the slab will not be affected as carbonation only occurs at the surface. Sometimes the dusty surface can be remedied by applying a fluorosilicate floor hardener. If this does not work, sand blasting the surface down to sound concrete will remove the dusty layer.
It is also possible the concrete was placed too wet or finished too early. Both of these practices can cause dusty concrete surfaces. If too much water was added to the concrete (this is usually done to make placement of the fresh concrete easier) both the strength and durability of the concrete would have been dramatically reduced. Much of this excess water will come to the surface of the concrete (this is called bleed water) and will dilute the concrete even further at the surface. This results in a weakened surface layer that is prone to dusting.
Exterior concrete needs only a minimum of finishing. It should be placed, screeded, floated, and broomed, then cured properly. If the contractor performed more operations than this, the risk of producing a dusty surface (along with other surface problems) is greatly increased. If the mix was also placed too wet, the problem is compounded. Any finishing operation performed while bleed water is present on the surface of the concrete will weaken the surface of the concrete, and often result in a dusty surface.
What should I know when asking a contractor about the concrete mix and finishing practices he will use for my new driveway?
The contractor should be using a 4000psi concrete mix, with 5% air entrainment, as a minimum. You may not need this strength of concrete for a residential driveway, but stronger concrete is generally more durable concrete. ***Consult you local ready mix producer for more specific recommendations*** Fibers can also be included in the mix at the rate of 1.5 pounds per cubic yard of concrete.
If the contractor wants to add a lot of water to make placement of the fresh concrete easier, insist on using a plasticizer instead. This is a chemical admixture added to the concrete to increase flowability (normally referred to in the industry as ‘slump’) without the addition of water. There will be a charge from the ready mix producer to use this material, but it will allow the contractor to have concrete that is easier to place and you will maintain the quality of your concrete. With today’s concrete technology, there is no excuse to add excess water to concrete.
Finishing operations should be kept to a minimum, and the final finish should be a light broom. If you don’t want a broom finish, consider either a burlap drag or just a plain float finish. If you want a very smooth concrete surface that can only be produced by troweling with a steel finishing trowel, you will have to accept the possibility of dusting, crazing, or lamination in your surface. Plus, this smooth surface will be very slick when wet.
The final step is curing the concrete. For most situations, the use of a cure-n-seal product will produce good results. This material should have a solids content of about 15-18% and can be applied with either a sprayer or roller. If you are spraying, the material should be applied to the concrete surface right after the brooming.
My car left rust spots on my newly constructed, not yet sealed driveway. How do I clean it?
From less complicated to most, here we go. It is suggested that before tackling the entire project with any of the suggestions listed below, test a small area to make sure they work and that they don’t have any unintended consequences.
First try soap powder, water, and a stiff broom – this method may work for light stains. It’s easy, you probably have the materials, and it’s cheap. Try it first, but don’t be too disappointed if it doesn’t work. Apply the soap and water, scrub and leave in place for 30 minutes. Add more soap, scrub again and rinse.
The next step would be to try an off-the-shelf rust remover that you might find at a big box hardware store or janitorial supply store. Directions should be included on the product and it should at least claim to remove rust stains from concrete and/or stucco or masonry. If your stains are light, this may do the trick.