- Dampen the substrate and formwork and remove any excess water prior to placing the concrete.
- Use of sufficient quantity of synthetic of steel fibres in the concrete to help offset the effect of plastic shrinkage.
- In hot weather, lower the temperature of the fresh concrete by using chilled mixing water or replacing some of this water with crushed ice.
- Install wind brakes.
- Immediately after screeding the concrete, apply aliphatic alcohol over the surface. This water-based finishing compound effectively controls the evaporation of bleed water.
How do you control shrinkage cracks in concrete?
- Restraint to Shrinkage. According to the Portland Cement Association, restraint to shrinkage is the most common cause of concrete cracking.
- Factors Influencing Locations of Crack Development. Temperature differences can contribute to the development of cracks. ...
- Identifying Shrinkage Cracks. ...
How to repair shrinkage cracks?
- SEAL CONCRETE CRACKS, HOW TO - you are currently on this web page.
- FOUNDATION REPAIR METHODS for shrinkage cracks
- POLYURETHANE FOAM INJECTION CRACK REPAIRS for details of the procedure for using polyurethane foam to seal foundation cracks.
- CONTROL JOINT CRACKS in CONCRETE how we prevent shrinkage cracks in poured concrete floors and walls
What are the causes of shrinkage in concrete?
Causes of concrete shrinkage. The are many causes for concrete shrinkage, they can be listed as; High cementitious materials content. Water evaporation. Addition of excessive water in concrete. Contraction of the hydration products. The surface-area-to-volume ratio of concrete. Environmental factors.
How to prevent plastic shrinkage cracks?
Methods of Preventing Plastic Shrinkage Cracking of Concrete
- Erecting Temporary Wind Breaks. Higher wind velocities can greatly affect the rate of evaporation of water from the concrete being placed.
- Providing Sunshades. In case of construction of concrete slabs, it is very effective to provide sunshades over the concrete slabs and thereby controlling the surface temperature of the slab.
- Proper Placement Timing. ...
Should you repair shrinkage cracks in concrete?
If the concrete is properly air entrained it should survive cyclic freezing and thawing. If, however, the cracks are long and caused by drying shrinkage, perhaps they should be repaired or the sidewalk replaced.
How do you deal with shrinkage cracks?
1. Shrinkage Cracks in Masonry. Cracks caused due to shrinkage in masonry walls can be minimized by avoiding the use of rich cement mortar in masonry. It can also be reduced by delaying plasterwork until masonry has dried after proper curing has undergone most of its initial shrinkage.
What will reduce the risk of plastic shrinkage cracks in a slab?
Preventative MeasuresErecting temporary windbreaks and sunshades (if practical),Applying a sprayable evaporation retardant,Covering the flatwork with plastic sheeting between finishing passes,Fog spraying the flatwork,Dampening the base material and forms before placing concrete,More items...•
How deep are plastic shrinkage cracks?
two to four inches deepPlastic shrinkage cracks are typically two to four inches deep and approximately one-eighth inch wide. They may also extend several feet in length adopting a crow's-foot pattern. These cracks form before any bond has developed between the aggregate particles and mortar.
Do shrinkage cracks go away?
They form directly above reinforcing bars in the slab and are due to the concrete continuing to settle below the bar. Vibration can be one cause of these cracks, which get worse for larger bars. Both plastic shrinkage cracks and settlement cracks can be reduced or eliminated by using synthetic microfibers in the mix.
Do shrinkage cracks get worse?
In many, perhaps most, cases, random drying shrinkage cracks function just as well as contraction joints; they just look bad and the edges can become broken down so the crack looks worse than it actually is. But owners usually want some sort of repair, so start by evaluating the cause and repair options.
How can we reduce plastic and drying shrinkage in concrete?
Methods of Preventing Plastic Shrinkage Cracking of ConcreteErecting Temporary Wind Breaks. ... Providing Sunshades. ... Proper Placement Timing. ... Using Fog Sprays. ... Dampening the Subgrade. ... Using Evaporation Retarder. ... Start Curing Immediately. ... Accelerating Setting Time of Concrete.More items...•
How can uncontrolled shrinkage cracking of concrete be avoided?
Prevent extreme changes in temperature. Consider using a shrinkage-reducing admixture. Warm the subgrade before placing concrete on it during cold weather. Consider using synthetic fibers to help control plastic shrinkage cracks.
What is the difference between drying shrinkage and plastic shrinkage?
If the volume reduction occurs before the concrete hardens, it is called plastic shrinkage. The volume reduction that occurs primarily due to moisture loss after the concrete has hardened is known as drying shrinkage.
What are the main causes of plastic shrinkage cracks?
Plastic shrinkage cracks are caused by a rapid loss of water from the surface of concrete before it has set. The critical condition exists when the rate of evapora- tion of surface moisture exceeds the rate at which ris- ing bleed water can replace it.
What do shrinkage cracks look like?
Plastic shrinkage cracks are typically very narrow in width and barely visible. While nearly invisible, it is important to remember that plastic shrinkage cracks don't just exist on the surface, they extend throughout the entire thickness of the slab.
What are the reasons for plastic shrinkage?
Plastic shrinkage is caused by the loss of water by evaporation from the surface of newly laid concrete or by suction of dry concrete underneath. At the surface, plastic shrinkage occurs when the rate of evaporation exceeds the rate of bleeding.
How do you prevent concrete shrinkage?
Minimize drying shrinkage – Keep the total water content of the concrete mixture as low as is practicable for the intended application. This can be achieved by using a high content of hard, rigid aggregates that are free of clay coatings, and by using mid-range or high-range water-reducing admixtures.
Why do shrinkage cracks happen?
Shrinkage cracking: When concrete is mixed, more water than is needed for hydration is mixed with the dry components, such as sand, cement and an aggregate. Most of the water will eventually evaporate, causing shrinkage of the concrete slab.
How the shrinkage cracks in the water tank walls are prevented?
In medium thick structures (mainly walls) usually thermal and shrinkage deformation is prevented by restraint e.g. if a wall is cast against an old set concrete. The cracking may develop due to restraint stresses generated by shrinkage and thermal effects.
What causes shrinkage cracks in plaster?
Drying shrinkage cracks are the result of moisture loss after the plaster has hardened. Plaster will always shrink and crack so it is desirable that is should develop a large number of fine, unnoticeable cracks at close spacing.
How to keep concrete from shrinking?
Start curing the concrete as soon as possible. Spray the surface with liquid membrane curing compound or cover the surface with wet burlap and keep it continuously moist for a minimum of 3 days. Consider using synthetic fibers (ASTM C 1116) to resist plastic shrinkage cracking.
How to prevent plastic from cracking?
Follow These Rules to Prevent Plastic Shrinkage Cracking 1 Dampen the subgrade and forms when conditions for high evaporation rates exist. 2 Prevent excessive surface moisture evaporation by providing fog sprays and erecting windbreaks. 3 Cover concrete with wet burlap or polyethylene sheets between finishing operations. 4 Use cooler concrete in hot weather and avoid excessively high concrete temperatures in cold weather. 5 Cure properly as soon as finishing has been completed.
How far apart are plastic cracks in concrete?
These cracks appear mostly on horizontal surfaces. They are usually parallel to each other on the order of 1 to 3 feet apart, relatively shallow, and generally do not intersect the perimeter of the slab.
What is the critical condition of a concrete slab?
The critical condition exists when the rate of evaporation of surface moisture exceeds the rate at which rising bleed water can replace it. Water receding below the concrete surface forms menisci between the fine particles of cement and aggregate causing a tensile force to develop in the surface layers.
What to use to cover concrete?
If delays occur, cover the concrete with moisture-retaining coverings, such as wet burlap, polyethylene sheeting or building paper, between finishing operations.
Can plastic shrinkage cracks be minimized?
Plastic shrinkage cracks are unsightly but rarely impair the strength or durability of concrete floors and pavements. The development of these cracks can be minimized if appropriate measures are taken prior to and during placing and finishing concrete. (Note: Plastic shrinkage cracks should be distinguished from other early or prehardening cracks ...
Do concrete cracks form when it dries?
If the surface dries very rapidly, the concrete may still be plastic, and cracks do not develop at that time; but plastic cracks will surely form as soon as the concrete stiffens a little more. ...
What to do when a crack is too tight?
When a crack hasn’t been abused and is tight ( thinner than a dime), leave that crack alone and polish the floor. Alternatively, you could fill the thin void with a grout coat. Photo courtesy of Jennifer Faller
Why do concrete slabs get wider?
As the slab ages, these cracks grow with continued stressful influences. They grow wider as the concrete experiences traffic, resulting in chipped, brittle edges. And PSCs get longer during curing, settling of the slab, and natural expansion and contraction due to environmental conditions.
Why does concrete crack?
Because the interior concrete restrains the shrinkage of the surface concrete, tensile stresses form on the surface. When these stresses exceed the tensile capacity of the concrete, surface cracking occurs. Cracks start on the surface and grow downward creating V-shaped cracks. Typically, plastic shrinkage cracking occurs when bleeding ceases ...
What is shrinkage cracking?
Typically, plastic shrinkage cracking occurs when bleeding ceases and the concrete is neither a liquid nor a solid. At this stage of hardening, the concrete has essentially zero tensile capacity.
What causes cracks in concrete?
As the name implies, this form of cracking occurs when the concrete is still plastic and caused by surface shrinkage of the concrete. Occurring primarily in concrete flatwork, these cracks can also occur on other exposed, horizontal surfaces including beams, foundations, and tops of walls. Plastic shrinkage cracking is commonly associated ...
How wide is a crack in concrete?
Cracks widths at the surface can be fairly wide (as much as 1/8-inch) but rapidly diminish with depth. Plastic shrinkage cracks appear in the first few hours after concrete placement and typically before the finishing operations are complete. As the name implies, this form of cracking occurs when the concrete is still plastic ...
How to lower the temperature of concrete?
Dampening the base material and forms before placing concrete, Lowering the concrete temperature by using chilled water or chipped ice, Including microfibers in the concrete mixture to increase the tensile capacity of the plastic concrete, Protecting the concrete from evaporation during construction delays, and.
Can fog be used on concrete?
Fogging can also help replace the bleed water that prematurely evaporated from the surface. However, be careful not to work accumulated surface water from fogging into the top surface of the concrete. Fog nozzles should create a blanket of fog — not a water spray that can wash away the surface of the concrete.
Can plastic cracks be repaired?
Plastic shrinkage cracks are usually superficial and seldom repaired. However, they may create aesthetic concerns, especially for architectural concrete. Depending on the cracking severity, crack width and depth, and the concrete’s exposure conditions, plastic shrinkage cracks may create durability concerns.
Why does plastic shrink in concrete?
Cracking caused by plastic shrinkage in concrete occurs most commonly on the exposed surfaces of freshly placed floors and slabs or other elements with large surface areas when they are subjected to a very rapid loss of moisture caused by low humidity and wind or high temperature or both.
Why is plastic cracking?
Since cracking because of plastic shrinkage is due to a differential volume change in the plastic concrete, successful control measures require a reduction in the relative volume change between the surface and other portions of the concrete.
When does plastic shrink?
Plastic shrinkage usually occurs prior to final finishing, before curing starts. When moisture evaporates from the surface of freshly placed concrete faster than it is placed by curing water , the surface concrete shrinks.
How to prevent plastic cracking?
These include using sunshades and windbreaks, dampening the subgrade and formwork, and initiating the curing process promptly.
What causes plastic to shrink?
Plastic shrinkage cracks occur when the rate of evaporation is high during construction. Conditions that contribute to rapid evaporation are high temperature, low humidity, wind, a slow rate of bleeding or some combination of these.
What is a short crack in concrete called?
If so, they are perhaps what are usually known as plastic shrinkage cracks. Plastic shrinkage cracks occur when the rate of evaporation is high during construction.
Can you seal plastic shrinkage cracks?
But the usual result is to accentuate their presence. If the cracks are in fact plastic shrinkage cracks, the chances are that they won't get any worse and that the best thing you can do for their appearance is to leave them as they are.
