Guidelines for Strong Concrete
Summary: Are you getting ready to install a new concrete driveway, concrete sidewalk, concrete patio and/or steps that may be exposed to freezing temperatures and deicing salts? If the job gets done right, your concrete will last longer than you!
Seriously, there are two associations that promote concrete and its correct use. They are progressive and their recommendations are backed up with tens of thousands of hours of scientific investigation and testing. The two groups are the Portland Cement Association (PCA) (www.cement.org) and the American Concrete Institute (ACI)(www.concrete.org). Here are some guidelines they suggest you follow, if you want trouble-free concrete in cold climates:
Low Water to Cement Ratio
The cement in concrete is the glue that holds everything together. The more water you add when you mix, the weaker the concrete will be for a given amount of cement. Think of cement as sugar in iced tea. If you add just a little sugar the sweetness level is weak. Add lots of sugar and you get some sweet stuff!
A Four Inch Slump - The MAXIMUM Allowed
Slump refers to the stiffness of the mix. The lower the number the stiffer concrete is. You can actually mix and pour concrete with a 1 or 2 inch slump! Have you seen highway median work where the crash barrier is done with a forming machine? The concrete is so stiff that it can stand four feet tall moments after it is poured - yet it attains a high strength once cured and dried. You don't need lots of water to make concrete strong. The addition of water to a 5 inch or higher slump just makes it easier to pour.
Six Bag or 4,000 PSI Mix
You must have enough cement in each cubic yard to make sure it is strong. For concrete exposed to freezing temperatures you want a minimum of six bags (564 lbs.) of cement per cubic yard or a design strength of 4,000 pounds per square inch.
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Air Bubbles
Special chemicals can be added to concrete as it is mixed. These chemicals create micro-air bubbles within concrete. The air spaces become shock absorbers as water freezes within concrete. Make sure your concrete is ordered with air - often called air entrained concrete.
Proper Finishing
Just as you can't add extra water to mixed concrete, you can't add water during the finishing process. This added water dilutes the cement paste in that localized area.
Slope
Concrete slabs need to have good drainage. This means that they need a slope of 1/8 inch per lineal foot minimum to shed water. Pooled water that freezes on the surface is bad....bad.......!
Adequate Curing
As concrete dries, it needs water to complete the chemical transition called hydration. If you let too much water escape from the slab before the chemical reaction is complete, it simply doesn't become as strong as it should be. You can cure concrete by covering it with plastic, wet burlap that stays wet or by spraying it with a liquid curing compound. Don't forget this important step!
No Silicone!
If you feel the need to seal your concrete, do not use silicone sealers. These are film formers. You need a breathable sealant or water repellent. Top performers are those that contain silanes and siloxanes. These are chemicals that are similar to silicone.
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Comments:
ray 13 Dec 2007, 11:06
give some more info on haw to make concrete stronger
AsktheBuilder 15 Dec 2007, 10:33
Ray,
Sure. Add more Portland cement, steel and follow ALL of the tips in ALL of my concrete columns.
judy Ludwig 18 Dec 2007, 12:06
Do you have to seal your new concrete drive way? Is it necessary!
Do you always have to use metal with concrete or is there such a thing as fiber mesh added to the concrete that does as well. Thanks, Judy
AsktheBuilder 18 Dec 2007, 16:42
Judy,
All of these questions and more are answered in my past Concrete columns. Please take the time to read all of them. You will discover a wealth of information.
Ross Jordan 11 Jan 2008, 00:00
I am considering on making a headstone for my self (I not planning on
needing it soon) and I wonder if concrete could be used, will it last very
long? I want to make one that looks like a tree stump maybe 5 feet tall,
18 inches thick with realistic cut off limbs and bark. Have any
suggestions?
AsktheBuilder 11 Jan 2008, 10:17
Ross,
I have had some strange posts, but this one takes the cake! Concrete will work, but it will not last as long as solid granite. If you do the concrete, make the mix at least 8,000 psi and try like the dickens to get sand that has as much pure silica sand in it as possible. You can buy fine silica sand in bags, but you need a coarse silica sand. The other stones in the mix should be rounded pieces of granite gravel. You can get all of this, it just may be some work.
Tom 31 Jan 2008, 03:38
Hello. Is a 4" thick concrete-slab (driveway) beefy enough to use rebar as
reinforcement? I've been getting bids from different contractors on
replacing my driveway (residential) and I've had 3 of them tell me that
they use wire mesh rather than rebar for this purpose because residential
drives are not deep enough to use rebar. Does wire mesh do as good of a job
as rebar? That is, will the driveway-slab be as strong using wire mesh as
it would had they used rebar instead? I've made it clear to the contractors
that regardless of which metal they use I want them to use fiber-mesh mixed
concrete in addition to the metal (for added strength). Thanks, Tom
AsktheBuilder 31 Jan 2008, 08:39
Tom,
Are you pouring the concrete on bedrock? Is the backhoe broken? Why can't 5 or 6 inches of concrete be poured? I have experimented for over 30 years using 21-pound mesh vs. 1/2-inch steel rods. I have gotten great service from both. My own driveway apron that is 21-years old is mesh and is in perfect shape except for some shrinkage cracks. The apron has never pulled apart or the cracks offset. But my advice to all is to use the steel bars. I feel they are better. They are most certainly easier to get into the center of the slab more consistently. If a contractor really takes their time, they can get mesh in the center. You just need to watch them do it or hire one you trust will do it.
Dan 03 Mar 2008, 16:32
Hi Tim,
Great article. I am in the middle of trying to put together a retrofit on a house. The sill plate was not bolted to the block wall foundation at all. My structural engineer calls for 8000PSI concrete for the fix but I can't find it in dry mix form anywhere. Can you tell me exactly how to make it or where I might find some in a dry mix? Thanks!
AsktheBuilder 08 Mar 2008, 13:06
Dan,
I can't but your closest Concrete Ready-Mix plant can..... View all comments |
