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Q. Why does concrete crack?

A. Concrete, by nature, shrinks as it hardens. When concrete is placed on supporting soil or around steel reinforcement, the concrete mass is prevented from shrinking. This restraint creates internal forces exceeding the strength of concrete. Cracks form to relieve these forces.

Q. Does the presence of cracks indicate a structural problem?

A. In most instances, the answer is no. Very narrow "hairline" cracks are aesthetic in nature and do not indicate any structural problem. Cracks that have movement where one side of the crack moves relative to the opposite side should be investigated by a professional engineer.

Q. Can I replace wire mesh with fibers?

A. The answer depends on the intended purpose of installing wire mesh. If control joint spacing exceeds 30 times the concrete thickness, then wire mesh should be used to hold random cracks together. If control joint spacing does not exceed 30 times the concrete thickness, wire mesh can be omitted and fiber can be used to reduce surface cracking resulting from rapid evaporation (plastic shrinkage cracking) and improve fatigue strength.

Q. Can jobsite addition of water increase cracks?

A. Yes. Concrete is designed and delivered by the Ready Mix Concrete Producer according to industry standards that define mixes to meet the intended performance. Adding water at the jobsite can decrease the strength and increase cracking in concrete. If a higher slump is required consider using water reducing admixture or superplacticizer.


Construction Issues - General

Q. My finishers are always telling me that the concrete is delivered too stiff. Friends tell me that adding water to make the concrete easier to place damages the concrete. How much water can I add to an eight yard load of concrete without causing problems?

A. No more than 1-1/2 gallons of water should be added per cubic yard of concrete. Concrete should be ordered based on strength and workability. Increased workability can be achieved by using admixtures.

Q. What can be done to make concrete easier to place?

A. Chemical admixtures have been developed in recent years that make concrete user friendly.

These chemicals can produce concrete that will almost flow into place or that will increase or decrease the set time to meet your specifications. Contact a Knight's Redi-Mix representative at (843) 821-7600.

Q. How much concrete do I need?

A. Concrete is sold by the cubic yard. Calculate this volume by multiplying the area (ft) by the thickness (ft) then divide by 27. If math is not your favorite subject, contact a Knight's Redi-Mix representative at (843) 821-7600.

Q. Where can I get a list of reputable concrete contractors?

A. Contact a Knight's Redi-Mix representative at (843) 821-7600.

Q. I am building a patio in my fenced-in back yard. How can I get the concrete back there?

A. Consider these two options. Concrete pumps are used to place concrete in hard to reach places. There are also mechanical transport devices such as Georgia buggies that are used to deliver the concrete from the truck parked on the street to the desired location.

Q. Are there any special requirements that I should know about the concrete truck making a delivery to my project?

A. Fully loaded concrete trucks are heavy and may cause damage to driveways designed for lighter vehicles. An area must be provided for the driver to wash out the truck's chute.

Q. The tile in our house will not stick to our concrete floor due to moisture. What causes this problem?

A. Many of the adhesives used today are more water-sensitive due to restrictions on the use of volatile organic emissions (VOX) products. Concrete slab moisture can originate from the following sources: 1. When the slab is below the ground water table or when it comes in contact with wet soil from rain, irrigation systems, or broken plumbing. 2. Fine-grained soils can draw water from considerable distances allowing the water to saturate the sub grade through capillary action and move through the slab. 3. Water vapor from damp soil can condense on the slab surface especially when the air on the concrete surface is cooler and at a lower humidity than the damp soil below the slab. 4. Placing floor covering materials before a slab has had time to dry to an acceptable level or after the slab had recently gotten wet from rain, etc.

Q. What are joints?

A. Joints are pre-planned cracks to accommodate the expansion and shrinkage of concrete from changes in moisture and temperature.
Although irregular cracks are unsightly and difficult to maintain they generally do not affect the integrity of the concrete. Cracks in concrete can be Controlled and minimized by properly designed joints. There are three types of common joints:

  1. Contraction joints
  2. Isolation or expansion joints
  3. Construction joints


Decorative Concrete - General

Q. What types of colored finishes are available?

A. There are four methods for coloring concrete. 1. You can add color pigment to the entire slab. This adds some expense but color will be throughout slab if a crack should occur. Using white Portland cement produces brighter, cleaner colors and the color should be mixed with the dry cement before adding to the concrete mix. 2. The color pigment can be added to a top layer of concrete only. The base slab should have a rough texture to provide a better bond then a colored topping can be added as soon as the base slab is firm enough to hold the finisher's weight. This method saves some money but a good finisher is a must with this method. 3. Dry-Shake colored materials can be added to the concrete after the concrete has been bull floated and excess moisture has evaporated. The material is shaken evenly by hand over the surface and then floated into the surface. The dry-shake material must be evenly applied or no uniformity in color and/or peeling could result. Most dry-shake compounds contain a curing compound that will help the concrete to cure properly. 4. Existing slabs can be colored using stains and paints. Stains typically give a no uniform color but that can also be part of the appeal. Stained concrete pavements are very popular right now. Paints can wear away with heavy traffic and must be renewed periodically.