What is concrete?
Concrete is a material made from cement, aggregates (sand and stone), water, and admixtures (chemicals that enhance or modify the properties of concrete).
What is cement?
Cement, or Portland cement, is a dry powder made from limestone and other materials. It is burned in a kiln and then ground finely. Cement reacts chemically with water to cause concrete to harden. There are several different types of Portland cement, designated as Type I, Type II, Type III, etc.
What is flyash?
Flyash is a byproduct of coal-burning power plants. When used properly in concrete, it improves the quality of the mix. Concrete with flyash will typically have a higher ultimate strength, although early strength will be lower than with straight cement.
What is ground granulated blast-furnace slag (GGBF)?
GGBF is the nonmetallic float on a cook of iron, water quenched, and ground to a cementitious product. Concrete with GGBF improves finishability, achieves higher strengths, and slightly delayed set times which can be an asset in warm weather placement.
What is the difference between a “sack” mix and a “performance” mix?
A sack mix uses Portland cement for strength. The amount of cement used in the mix is specified by the purchaser. 5.0 sack, 5.5 sack or 6.0 sack will specify the weight of cement used in each yard of concrete. A sack of cement is defined as 94 lbs. so a 5.5 sack mix will contain 5.5 x 94 or 517 lbs. of cement per cubic yard. More cement will generally make stronger concrete. More cement will always make a faster setting concrete. If the option of a sack mix is selected, the producer is only responsible for supplying the prescribed weight of cement in the mix.
A performance mix is specified by the compressive strength of the mix design. The producer is supplying a combination of materials which, when placed within reasonable limitations for slump and when tested according to the ASTM test standards, will yield a compressive strength above the specified rating. A performance mix will often utilize fly ash, granulated ground blast furnace slag, and/or chemicals to assist in the development of strength. Because cement is replaced by these more economical materials, a performance mix is less expensive than a comparable “sack” mix.
What is air-entrainment and why would I want it in my concrete?
Air-entrainment is a network of bubbles dispersed through the mortar portion of the concrete mix. The bubbles range in size from microscopic up to the size of a pencil dot. In the winter, when the water trapped in a wet or saturated mass of concrete freezes, it expands. The network of bubbles provided by a proper air-entrainment system allows room for the expansion without rupturing the concrete. Exterior concrete, especially flatwork, must be air-entrained if it is to survive more than a single winter.
In addition to freeze-thaw durability, air entrainment imparts several positive characteristics to a concrete mix design. It makes a mix more homogeneous. Segregation of the different size and types of materials in a mix is a significant problem. Air entrainment will help to prevent this segregation and keep the total concrete more workable. Further, the air bubbles are spheres. They work as ball bearings in the mix making the mix move more easily and smoother.
A non air-entrained (regular) concrete should be selected if a finishing machine is to be used on the surface of the concrete. Air will make the mix stickier. It tends to bond and pull up as the blades of the machine pass over it. Additionally, the aggressive kneading action of the machine tends to move the bubbles in the paste around. They coalesce (join) within the mix and form large bubbles that will create a bulge or “blister” on the finished surface. Sometimes the blisters can be popped, but there is generally a debonded layer within the mass of concrete. If it is close enough to the surface, the top can scale off. More commonly, you are left with a surface that is blemished with humps where the air blisters have formed and hardened into the finished surface.
Why did my concrete crack?
All concrete cracks. It is the nature of the beast. Through a knowledge of how and why it cracks we can create situations where the cracking is either disguised or minimized.
A crack develops whenever the stress on the mass of concrete is greater than the strength it has developed at that particular time. Plastic concrete has no strength. Under a perfect cure condition it will take 28 days to develop “full” strength. During the first 24 hours, while strengths are at a minimum, concrete is likely to develop microcracks which will control its breakage for life. This is because, during the first 24 hours, concrete is also losing water to evaporation faster than it ever will again. When water is lost, the mass shrinks in an attempt to re-fill the voids. The stresses brought on by this shrinkage can easily be greater than the strength of the fresh concrete.
Minimize moisture loss! Minimizing moisture loss will slow the early shrinkage of the mass and reduce the stress on the system. This means that you should wet the subgrade immediately before the placement, place concrete in sheltered areas (windscreens at a minimum, enclosed building are the best), mist over the slab if necessary to maintain a light sheen (do not finish this water into the surface of the slab, let it evaporate away) and finally cure the finished slab to stop moisture loss after you walk away from the project.
Joining. Joints are placed in concrete to control where it breaks. A joint can be tooled in during finishing, sawn with a light saw immediately after finishing, or sawn with a standard concrete saw sometime before 24 hours. Much later than that and sawing is a waste as the microcracks will probably control the breakage anyway. Concrete should be cut into 36 times the slab thickness. For a 4 inch thick slab, 4 x 24 = 96 or 8 feet. 4 x 36 = 144 or 12 feet. Thus, a 4 inch slab needs to be joined every 8 to 12 feet. Further, the joints should extend _ of the depth of the slab. Generally, if you are using a joint system and your slab is still cracking, the system is not working. Get the slab joined sooner. Make the joints deeper so that they will control the breaks.
Isolation. Anytime a slab will contact a fixed object, the fresh concrete should be isolated from the existing object. Concrete alongside a house. Going to butt into an existing walk. Placing a turnaround on the edge of an existing driveway. All of these locations will bond the new concrete and restrain its movement. Restrained movement creates a stress which will result in a crack.
