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Do It Yourself

A concrete job done poorly is much more expensive to remove and replace than paying to have the job done by a qualified contractor in the first place. Check with your local building officials. They are there to help you. For exterior concrete, decide where the rain water will go. Proper drainage is essential. Slope the concrete surface at least 1/8” per foot, ¼” is better.

Lay out your project using stakes and a string line. Excavate all vegetation and organic matter from the location. Use a level and string line to guide your form setting. Brace your forms every 3-4 feet and at every joint. Prepare the sub grade so that the minimum concrete thickness is at least 4” for residential flatwork. Foundations must meet local codes. Uniformity of concrete thickness and sub grade is important. Sub grade must be damp, compacted and free of standing water.

Wire mesh is a waste of time and money because it is seldom installed properly. Depending on the application, use steel rebar on plastic chairs, or order fiber in the concrete, or both. A common specification for residential flatwork is 3/8” rebar on 18” centers each way.

Concrete joints control where the concrete cracks. Plan your joints before pouring. Typically, control joints should be spaced 10 to 15 feet apart, making the sections as nearly square as possible. Joints need to be at least ¼ the depth of the concrete. For example, if you have a 4” concrete depth, the joint should be at least 1” deep. Joints can be hand tooled, saw cut, or formed. Formed joints are commonly made from redwood because of its resistance to rotting.

Ready mix producers will help you select the right mix for your job. Depending on your location and application, a typical specification might be: 3000 psi concrete in 28days, air entrained, and a 5” slump. Also, designate the coarse aggregate needed, such as 1” or 1 ½” crushed limestone, pea gravel, or native gravel. Calculate the amount of concrete required by using the online calculator at www.cooperconcrete.com/calculator.htm. Be sure and order enough. It is better to have some extra concrete at the end of the job, rather than wait for the extra load and pay a short load fee. The ready mix producer will dispose of left over concrete still in the truck.

Concrete must be discharged as close to final position as possible. Avoid dragging the concrete long distances. The concrete will segregate, quality will suffer, and it will wear you out. Be prepared to unload the concrete in 30 minutes or less. That means having plenty of people to help, especially if you are using wheelbarrows to move the concrete from the truck to the forms. The concrete will be delivered to the nearest accessible point over stable soil (able to support 65,000 lb.) The truck needs a pathway at least 12 feet wide and 14 feet high (watch for tree limbs and power lines). Existing residential concrete is not usually designed for truck traffic, so avoid bringing trucks over curbs, sidewalks, or driveways. The discharge chutes can reach approximately 12 feet from the back of the truck.

Concrete should be struck off or “screeded” as it is placed. A straightedge (straight 2”X4” or magnesium screed) is used to screed the concrete. Immediately after striking off, and before bleed water appears, the concrete must be bull floated and the edges formed with an edger. After bull floating, no finishing operations should take place until the bleed water has completely evaporated. When the concrete is firm enough that a person’s weight on it will make no more than a ¼” imprint, final finishing operations may begin. Here is where opinions differ. Many times concrete is overfinished. For driveways and sidewalks, broom texturing may be performed after floating and without troweling first. Many contractors prefer to trowel first, then broom. Troweling does smooth and densify the surface, but you run the risk of overfinishing and causing surface defects. For smooth surfaces such as house foundations and garage floors, troweling will give the surface a smooth, hard surface. Begin with the trowel nearly flat, then increase the angle with each subsequent pass. Do not sprinkle water on the surface of the concrete because it will weaken the surface and can cause dusting, spalling, or crazing.

From a quality standpoint, the most important, yet most neglected aspect of placing concrete is curing. The most practical and effective method of curing is spraying a membrane curing compound on the surface immediately after finishing operations are completed. Some people believe using a sprinkler on the concrete helps, but alternately wetting and drying the concrete can cause more problems than it solves. Other methods such as wet burlap, plastic sheeting, or waterproof paper can be used, but may cause cosmetic flaws. Good coverage is essential. Make two passes at right angles to each other. Using a pigmented curing compound will allow you to see that you have covered the entire surface. The only time you would not use a curing compound would be in a situation where you are applying a special finish that is not compatible with the chemicals in the curing compound. These situations are rare, and in those cases you should use the wet burlap, plastic sheeting, or waterproof paper method. Remember, always cure the concrete.

  • Admixture – An ingredient in concrete other than water, aggregates, or cement that enhances certain properties of the concrete. Some examples are air entrainment, accelerators, retarders, and water reducers.
  • Aggregate – Rock, stone and sand.
  • Air Entrainment – Microscopic air bubbles introduced in to the concrete to improve freeze/thaw durability.
  • Beam – In a foundation, a structural member that supports a slab. May be included in the slab or separately formed.
  • Bleeding – The movement of water to the surface of the concrete caused by the settling of the solid materials.
  • Bull float – A concrete finishing tool with a large, flat, rectangular piece of wood, aluminum, or magnesium connected to a handle.
  • Calcium Chloride – A common accelerating admixture.
  • Cement – The powder which reacts chemically with water to bond aggregate together to form concrete.
  • Chair – A device used to hold reinforcement in the proper position while placing and working concrete.
  • Chute – A trough which carries the concrete from the truck to the forms.
  • Cold Joint – A joint in hardened concrete where the fresh concrete has not bonded correctly with the previously placed concrete because too much time has elapsed between placements.
  • Concrete - A composite material made of cement, water, aggregates, and admixtures. The most versatile building material in the world.
  • Control Joint – A joint designed to accommodate movements in concrete caused by temperature changes, volume changes, and drying shrinkage. The joint forms a weakened plane designed to control where the concrete cracks, rather than having cracks in random locations.
  • Cure – To retain moisture in concrete during the early stages in order to allow the cement to chemically react with water and reach the highest strength.
  • Dusting – Powder on the surface of hardened concrete coming from the concrete itself. A surface defect.
  • Edging – The operation of tooling the edges of fresh concrete to provide a rounded corner.
  • Fiber – Secondary reinforcement that can be made from polypropylene, nylon, steel or other materials. It is added to the concrete truck and mixed integrally with the concrete. Fibers provide three dimensional secondary reinforcement.
  • Finishing – Operations such as floating and troweling that produce a surface with the desired characteristics.
  • Flatwork – A general term referring to driveways, sidewalks, patios, and other slabs on ground.
  • Floating – The finishing operation involving a concrete tool called a float. Floating opens up the surface of the concrete so that bleed water can come to the surface.
  • Flyash – A mineral admixture used in concrete to react with cement and modify or enhance the properties of concrete.
  • Form – The mold, usually made from lumber, to support and contain concrete until it has gained sufficient strength to support itself.
  • Grout – A mixture of cement, water, and sand.
  • Honeycomb – Voids left in concrete when the forms are pulled.
  • Monolithic slab – A block of cast in place concrete with no joints.
  • Plastic Concrete – Concrete that has not hardened enough to resist penetration.
  • Plastic Shrinkage – Cracks that form in concrete before it is finished. Usually caused by rapid evaporation due to low humidity, high winds, high temperature, or a combination of all three.
  • Pozzolan – A fine material that is not a cement but which reacts with cement to form a cementitious binder. Flyash is a pozzolanic admixture.
  • PSI – Abbreviation for pounds per square inch.
  • Ready mixed concrete – Concrete batched at a concrete plant and delivered in a mixer truck ready to pour.
  • Rebar – Steel reinforcing bars imbedded in concrete to reinforce it.
  • Scaling – Flaking or peeling of the surface of concrete, sometimes caused by using deicers on fresh concrete. Similar to spalling, but it peels off in thinner layers.
  • Screed – An established grade which is used as a guide for striking off the surface of fresh concrete at a desired level.
  • Screeding – The operation of striking off concrete at the desired level.
  • Segregation – Undesirable separation of the materials in plastic concrete during the process of handling and placing it.
  • Set Time – A measurement of the time it takes concrete to harden enough to resist penetration.
  • Slump – An indication of the consistency of plastic concrete. It is the distance that freshly mixed concrete subsides when a conical mold (slump cone) is lifted from the test specimen. Increasing the amount of water in concrete will increase the slump, but an increased slump is not always an indication of higher water content.
  • Spalling – The breaking away of part of the concrete from the surface due to the concrete being weaker near the surface. Often caused by a lack of curing or poor finishing practices.
  • Striking Off – The process of shaping the surface of freshly placed concrete by using a straight edge to level it to the elevation of the desired grade.
  • Sub Grade – The prepared surface on which a concrete slab is cast. Proper sub grade preparation is essential for quality concrete work.
  • Superplasticizer – A high range water reducing admixture that increases the strength and workability of concrete.
  • Troweling – Smoothing and compacting the surface of concrete by strokes of a trowel.
  • Water Reducer - An admixture that lowers the amount of water needed to achieve a desired workability. The use of a water reducer can produce higher strength and denser concrete.
  • Wire Mesh – Also known as welded wire fabric. A mesh made of wires crossing at right angles and welded together for use in concrete. Wire mesh is supplied in sheets and roles.
  • Workability – The ease of response of concrete in mixing, placing, compacting and finishing.