Concrete Basics

basicsBelow are our recommendations which should make your concrete experience productive, cost-effective and enjoyable.

Planning & Preparation

Specifications

Placing & Finishing

Curing

Other Tips

 

Planning & Preparation
Before you begin your project, it is important to properly plan and prepare.  Proper planning will save time in end, and helps to assure your satisfaction with the end product.

Thickness
For common household projects - driveways, walkways and patios - 4 inches thick should do the trick.  You may require a thicker pour should the area see large amounts of traffic.

Base
In the general Portland area we recommend using a ¾ minus crushed gravel compacted base..

Drainage
Industry standard calls for a slope of ¼ inch per foot away from structures.  Some projects may be able to use ⅛ inch slope.

Excavation
While taking care to only dig as deep as necessary (keeping an eye out for power lines), ensure you remove tree roots, leaves, wood and any other organic matter in the desired area.

Compaction
To ensure your slab doesn't settle or vary in thickness, make sure to compact the soil/gravel base in the desired area.

Forming
Take care to stake your forms securely to the ground, while paying close attention to the slope (for proper drainage).  2x4s are perfectly adequate for use as forms, though make sure you have the desired thickness (usually 4") available at the edges.

Isolation
Install premolded joint material anywhere concrete will come into contact with existing slabs, steps or buildings.  The joint most reach the bottom of the slab, and ensures the new slab will be separated from existing structures.

Now that you have your planning down, we suggest you review our Concrete Calculator to determine your exact needs.

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Specifications
Specifying the correct mix for your job can be the difference between long-term satisfaction or disaster.  Concrete specifications consist of Strength, Slump (basically the thickness) and Chemical Additives.

Strength
Concrete Exposed to freezing and thawing temperatures should have a compressive strength of 3500 psi and have at least 5%  entrained air (+/- 1.5%).

Slump
For our environment we recommend a slump of 4 inches (+/- 1"). Note: a slump greater than 4" may prolong the time you have to wait before finishing, particularly in cool weather. It will also increase the tendency for shrinkage cracking.

Chemical Additives
In addition to air entraining admixtures, chemical additives may be used to accelerate or retard setting, reduce water or plasticize the concrete.

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Placing & Finishing

Placing
Concrete delivery should be closely coordinated with placing and finishing operations.  Concrete should not be poured faster than it can be worked - spreading, consolidating, bullfloating.  Instead, concrete should be poured continuously as near as possible to its final position.  You should never dump concrete into separate piles and work together.

Consolidation
Placing concrete in forms and then consolidating is popular for some types of construction.  The consolidation process uses vibration and gravity to compact fresh concrete within the forms and around embedded items and reinforcement.  This process eliminates unwanted pockets of trapped air, honeycomb and stone while maintaining significant amounts of entrained air.  The vibration temporarily suspends the natural friction between the aggregate particles, causing the concrete to behave as a liquid.  Internal friction resumes as the vibration is stopped.

Finishing
Visible concrete - slabs used for driveways, patios, etc. - will often need finishing.  You can finish your concrete in many ways, including color tinting and texturing.  Patterned-stamped finishes are very popular on projects of all sizes.  Some surfaces may require just strikeoff and screeding to proper contour and elevation.  Others may be better off with a broomed, troweled or floated finish.  Strikeoff and screeding is the process of cutting off excess concrete to bring the top surface of the slab to proper grade.  A straight edge is moved across the concrete with a sawing motion and progresses forward a short distance with each movement.

Bullfloating eliminates high and low spots and embeds large aggregate particles immediately after strikeoff. This looks like a long-handled straight edge pulled across the concrete. Jointing is required to eliminate unsightly random cracks. Contraction joints are made with a hand groover or by inserting strips of plastic, wood, metal, or preformed joint material into the unhardened concrete. Sawcut joints can be made after the concrete is sufficiently hard or strong enough to prevent raveling. After the concrete has been jointed, it should be floated with a wood or metal hand float or with a finishing machine using float blades. This embeds aggregate particles just beneath the surface; removes slight imperfections, humps, and voids; and compacts the mortar at the surface in preparation for additional finishing operations. Where a smooth, hard, dense surface is desired, floating should be followed by steel troweling. Troweling should not be done on a surface that has not been floated; troweling after only bullfloating is not an adequate finish procedure. A slip-resistant surface can be produced by brooming before the concrete has thoroughly hardened, but it should be sufficiently hard to retain the scoring impression.

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Curing & Drying

Curing
Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only if water is available and if the concrete's temperature stays within a suitable range. During the curing period-from five to seven days after placement for conventional concrete - the concrete surface needs to be kept moist to permit the hydration process. The most common method of curing (and one of the simplist) is to utilize a liquid membrane which is sprayed or rolled on the surface of a slab after finishing to prevent premature drying of the surface. Water may be used when you are able to insure constant wetting for at least 3 days.

Curing in extreme weather
Temperature extremes make it difficult to properly cure concrete. On hot days, too much water is lost by evaporation from newly placed concrete. If the temperature drops too close to freezing, hydration slows to nearly a standstill. Under these conditions, concrete ceases to gain strength and other desirable properties. In general, the temperature of new concrete should not be allowed to fall below 50°F during the curing period.  Membrane curing does not keep the concrete warm enough in freezing temperature, nor does calcium chloride accelerator prevent concrete from freezing. Only adequate insulation or heating will maintain proper curing temperatures during freezing weather.

Curing shortcuts to avoid
Avoid any curing method that lets the surface dry in a short time. Quick drying stops the hardening process, thus making a weak surface that is likely to scale.

Drying
After curing, newly placed outdoor concrete needs time to dry in warm air. For best results, plan to place your concrete early enough in the season so that it has one month of temperatures above 40°F for curing and still another month for drying out before hard freezes are expected.

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Other Tips

Sealing
Water-repellent coatings or sealers can help prevent damage from freeze/thaw cycles and salting. They deter water from getting into the surface pores. Newly cured concrete should have a period of air-drying before being sealed.

First Winter
Do not use salt or other deicers during the first winter. Use sand instead to improve traction. Even light applications of salt, or salt carried on cars may cause severe scaling of newly placed conc- rete. Fertilizers are not an acceptable deicer at any time.

Saturation
Proper drainage should be maintained to avoid saturation of concrete.

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