Common
Concrete Misconceptions
- Water will flow uphill.
- Concrete curing is not important.
- Reinforced concrete will not
crack.
- I saw concrete poured once, so I dont know need to know anything else.
- One person can place concrete.
- The concrete truck driver will tell me what to do when he gets here. (He
or she will do their best to help, but you need to be prepared with the proper
knowledge and tools.)
- Concrete will flow to wherever it needs to go.
- A concrete truck can go anywhere a pickup truck can go.
- A garden wheelbarrow is good enough. (You need a heavy duty contractors
wheelbarrow.)
- My husband will spread the concrete when he gets home.
- Forms dont need to be braced very much.
I hope you enjoyed reading the misconceptions listed
above. Keep in mind that 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. Remember "Proper planning prevents poor performance."
Before You Start
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, Ό is better.
Layout, Forming and Site Preparation
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.
Reinforcement
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.
Jointing
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 Ό 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.
Ordering Concrete
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.
Placing
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.
Finishing
Concrete should be struck off or screeded as it is placed. A straightedge
(straight 2X4 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 persons weight on it will make no more
than a Ό 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.
Curing
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.
Definitions
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.
Fly Ash 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.
Back to top
|
