Common Concrete Problems

Causes & Preventions

Crazing - A large number of interconnected small cracks just in the surface of the concrete. The are nonstructural, and most visible on a nearly dry surface.

Causes:

Rapid drying of the concrete surface due to high temperatures, low humidity, drying winds, or any combination of these.
Overuse of placing tools, such as the jitterbug vibrator, darby, or bull float, will bring excess mortar to the surface.
Premature finishing with a float & trowel will bring excess moisture and paste to the surface. Rapid loss of this moisture may cause crazing.
Use of a dry cement or cement and sand mix as a mop coat to absorb excess water.

Prevention:

Use evaporation retardants or a fog spray immediately after the screeding to maintain the water to cement rates at the surface.
Use a fog spray or cover with wet burlap or canvas as soon as possible after strike off and darbying. Maintain spray or keep covers in place until surface is nearly ready for finishing.
Begin curing immediately. Where possible, continue mist curing to lower temperature by evaporation.
Use placing tools only to bring the surface to its proper plane, in preparation for finishing. Do not overwork surface paste.
Do not begin finishing operations until all free water has been evaporated or removed and the slab can support the finisher with only slight indentation.
Use low-slump, air entrained concrete and finish at the proper time.

Discoloration - Changes in color from pour to pour, spots, mottled light or dark patches, can be caused by a number of factors.

Causes:

Variations in any placing or finishing operations.
Delayed finishing or early finishing can affect color.
Finishing in bleed water will lighten and weaken the surface.
Delayed finishing/hard troweling almost always darkens the surface.
Job-site-added calcium chloride flakes are very difficult to dissolve in cold concrete and can result in dark spots.
Calcium chloride retards the ferrite phase of the cement. The ferrite phase tends to remain dark in the presence of calcium chloride. Cold weather increases this effect.
Plastic sheeting will cause high, and dark spots in areas of contact and non contact (greenhouse effect).

Preventions:

Avoiding the use of field-added flaked or pellet calcium chloride.
Maintain constant subgrade, placing, finishing, and curing conditions.
Finishing time is controlled by the concrete set time not bleed water. Surface water is controlled by wind speed, humidity, mix design, subgrade, admixtures, and temperature.
Products are available to treat minor discoloration and stain removal.

Cracks - Breaks that occur in areas other than those placed intentionally.

Causes:

Shrinkage after hardening (drying shrinkage).
Poor subgrade preparation and drainage.
Uneven settlement.
Hydrostatic pressure or structural loads (premature back filling).
Thermal cracking (in thick sections).

Preventions:

Place or cut control joints at recommended distances.
Put expansion joints where concrete meets other sections or slabs, over grade beams, and other recommended locations.
If existing material cannot be properly compacted and drained, add 8 to 15 centimeters of sound granular fill.
Prepare subgrade properly and build construction joints where required.
Provide for adequate drainage.
Use adequate control and expansion joints.
Lower the water to cement rates.
Use the largest aggregate size allowed.

Plastic Shrinkage Cracks - Small cracks in the surface of concrete. These can combine drying shrinkage cracks.

Causes:

High rate of evaporation of water from the concrete surface.
Affected by:
- Ambient temperature
- Relative humidity
- Wind velocity
- Temperature of concrete

Preventions:

Dampen subgrade and forms
Erect windbreaks to reduce wind velocity
Minimize placing and finishing time
Begin curing as soon as possible after finishing, using membrane curing compound, curing paper, wet burlap, sand, or other acceptable methods.
Use evaporation retardants or a fog spray immediately after the screeding to maintain the water-to-cement ratio at the surface.

Questions & Answers

1. What is the process of cement being made:

It starts with the raw materials of limestone, shale, clay, iron ore and sand. The raw materials are then heated in a rotary kiln to approximately 2700 degrees Fahrenheit to form a partially molten mass. This is called a clinker.
The clinker is cooled and ground into a fine powder called Portland cement.
The right amounts of raw materials are fed into the kiln. A chemical process called hydration occurs when water is added to cement; the paste is used as an adhering binder, which glues together the stone and sand. This paste then hardens to a stone-like mass called concrete.

2. “Set Time” - What does this mean?

The concrete gets to a point at which it can no longer be worked, or finished.
The strength necessary for concrete to support a finisher without his boots sinking in more than 1/4” is 20-40 psi.
The point in which a wall contractor can place the next lift of concrete without causing a form failure, while still being able to penetrate the previous lift with a concrete vibrator.

3. “Cure” Concrete - What does this mean?

The maintenance of moisture and temperature within the concrete for a period of time after it has been poured is curing.
In winter it may take two weeks, but in summer, it might only take four days.
Curing of concrete should be as long as it takes for the concrete to develop 75% of its design strength.

4. What are the reasons concrete cracks?

Super heavy loads that are greater than the compressive strength of the concrete.
Rapid thermal changes
Shrinkage cracking is the most common cause. This is due to volume changes within the concrete.
A long time after or even before the concrete has hardened, shrinkage cracks can occur.

Common Concrete Problems

Causes & Preventions

Questions & Answers

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