What is concrete made of?
Concrete is the mixture that results from blending hydraulic portland cement (or blended cement), sand, stone, water and admixtures.
What controls the strength of concrete?
The primary components that control the strength of concrete are the water and the cement contents. These two components are commonly referred to as the water/cement ratio. While properly graded sand and stone, and their relative amounts can affect the overall strength of the mix, the water/cement ratio has the greatest effect on the strength of the mix.
Why does concrete crack?
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 every 2 ½ to 3 times in feet the depth of your structure in inches (4″ slab should have joints every 10 to 12 feet). Control joints formed or sawed into the concrete will minimize random cracking. This will enable the cracks to occur in the control joints, which should be approximately ¼ the concrete thickness. Uneven shifting of the substructure or subgrade can also cause cracking. This is a structural failure, as opposed to improper curing or jointing as mentioned above.
How long can concrete remain in the delivery truck before it’s no good?
The recommended time from when concrete is batched until it is considered “no good” is 2 hours, when the temperature is between 60 F and 80 F. If the temperature is lower, it may take a little longer for the concrete to begin accelerated hardening. When the temperature is higher, it is wise to monitor the concrete temperature, and if it rises above 90 F, discard the balance of the load.
How much water can I add to the concrete?
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. Every gallon of water added to a cubic yard of concrete, above the designed water/cement ratio, will decrease the compressive strength of that concrete by approximately 200 psi. So by adding an extra 5 gallons of water to one cubic yard of concrete, you will decrease the compressive strength by about 1000 psi!
Why can’t I pour the concrete rather than place it?
Normal concrete is designed with a certain workability, which means it can be conveyed to the pour site using smooth rounded chutes and still maintain its composition and not segregate. Adding more water to make it pourable or looser usually causes segregation, not to mention the adverse effects of the additional water on the strength of the concrete, slower setting times and increased cracking. To make concrete more flowable or “pourable”, without adversely affecting the concrete, one would add a chemical admixture such as a midrange water reducer. Here again there is a limit to the amount that can be added before the concrete begins to segregate.
What are the different kinds of concrete?
Concrete mixtures are designed for specific strengths and with certain ingredients. Most residential concrete is designed for 3000 psi with a ¾” maximum stone and a 5” slump. Commercial and industrial concrete is typically 4000 psi or higher with a ¾” or 1 ½” maximum stone and a 3” slump. Mixes can also be adjusted depending on how they are going to be placed, the time of year they are being placed or special applications. Check with our Quality Control Department if you have questions about your application.
What tests do you make on concrete?
Tests on fresh concrete include slump (a measure of workability), air content, temperature, and unit weight. Tests on hardened concrete specimens include compressive strength, and in some cases flexural strength.
What is better, a HIGH SLUMP or a low slump?
This depends on the application and the design of the concrete mix. For normal concrete a moderate slump of 3″ to 5″ is desirable. High slump is desirable when the concrete must be placed around a high concentration of reinforcing steel (high slump should never be attained through the addition of water). Low slump is desirable when concrete is placed in large open forms, or when the form is placed on a slope. The concrete mix must be designed for these special applications. Always check with our Quality Control Department when you question your application.
If the slump is too low, what can be done to make the concrete workable?
It depends on what you mean by too low. Concrete for commercial or industrial applications is usually designed with a maximum slump of 3 inches. Residential concrete is designed with a maximum slump of 5 inches. If the concrete delivered to the project is so stiff that it cannot be discharged from the truck – contact the concrete plant. To add additional water to concrete, above the maximum designed value, will cause problems in the hardened concrete.
If the slump is TOO HIGH, what can be done to make the concrete usable?
Not much – send the truck back!
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 32 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. Check with our Quality Control Department for suggestions.
Where do I buy my concrete and supplies?