Concrete
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Surface Scaling of Residential Concrete Slabs

Scaling is a very common surface defect often associated with, but not limited to, concrete slabs exposed to weathering and deicing salts. It is characterized by peeling or pitting of the concrete surface and can be caused by several factors.

How to Avoid Surface Scaling

Strength, Air Entrainment, and Resistance to Weathering and Deicing Salts
Concrete needs a minimum amount of strength and entrained air to withstand the damaging effects of freeze-thaw cycles and deicing salts. The repetitive cycles of freezing and thawing will cause the concrete surface to scale if it does not possess adequate strength or the required volume of entrained air. When water penetrates a concrete slab and then freezes, it expands and exerts terrific tensile pressures inside the pores of the slab. The presence of air entraining, a network of small bubbles of a certain size and spacing, allows the expanding water to follow the path of least resistance, enter a bubble, and harden. This action relieves the pressures normally exerted by freezing water.

 

Deicing chemicals used for snow and ice removal exacerbate the buildup of internal stresses in concrete and thereby contribute to the tendency to scale. Deicing salts are almost always applied to streets and highways in northern climates, and even though homeowners might not put them directly on their driveways, automobiles will accumulate snow and ice laden with salt on their undercarriages, which eventually ends up on driveways or garage floors.

 

Deicing salts cause a significant increase in osmotic and hydraulic pressures over and above those pressures produced when water freezes in concrete. Additional pressure develops when dissolved salts recrystallize in the concrete pores as the water evaporates away. Entrained air in the concrete helps relieve these pressures too, but again, the mix must also possess enough strength to withstand the damage.

 

So, how much air entrainment and how much strength are needed? According to the Portland Cement Association and American Concrete Institute (ACI) standard 318, unreinforced concrete exposed to freezing and thawing and deicing chemicals must have a maximum water-cement ratio of 0.45 and a minimum specified compressive strength (f 'c) of 4,500 psi. Both trade groups specify an air content of 6.0 percent by volume for 1-inch maximum size aggregate. Of course, it is recommended that adequate drainage be designed into the surface and that the concrete be cured properly, as well.

 

To bring this into perspective, in the northern United States many residential contractors ask for a six-sack mix with air when placing driveways. Most mixes containing six sacks of cement, air entraining, and 1-inch maximum size aggregate will require about 295 pounds of water (35.4 gallons) per cubic yard to achieve a 3- to 4-inch slump. This translates to a water-cement ratio of 0.52, already above the 0.45 requirement. Unfortunately, many residential finishers and contractors will not place concrete at a 3- to 4-inch slump, usually preferring to work with 6- to 7-inch slumps, adding even more water to the mix. It is not surprising, then, that scaling driveways and sidewalks are a major problem associated with residential construction.

 

Premature Finishing and Scaling
Finishers can also cause surface scaling. The correct sequence for placing and finishing a concrete slab is to consolidate, strike off, and immediately bullfloat to smooth out the surface further. After this, all bleed water must be allowed to rise to the surface and evaporate. Only after the bleed water has risen and evaporated can further finishing take place.

 

For most outdoor pavements, only a broomed finish is needed, which can be applied after bullfloating whenever the concrete has set enough to retain the pattern. But, if the owner insists on a smoother surface, floating and troweling can be applied. The difficult part comes in deciding when the concrete is ready for floating.

 

Floating brings fine particles of cement and sand to the surface and packs them tightly so as to make the layer impermeable. If the finisher floats the surface before all the bleed water has risen (premature finishing), the water will accumulate directly beneath the floated layer, creating a weakened plane between it and the rest of the slab. Whenever the floated layer is weakly bonded, it is prone to scaling, usually during the first or second winter.

 

If the outdoor pavement is to be floated as part of the finishing procedure, there is a rule of thumb for determining when to start floating: The concrete is ready for floating when all the bleed water is gone and the footstep of a man makes an indentation of only ⅛ to ¼ of an inch. However, during hot, windy days, the water can evaporate so quickly from the surface that it looks dry when in fact water is still working its way up. Premature finishing often happens under these conditions. It is the finisher's responsibility to protect the slab from rapid evaporation by covering it with polyethylene sheets, putting up windbreaks, or spraying it with a monomolecular film. Adding large quantities of water to the concrete surface to reduce the effects of drying is not recommended because water can severely weaken the concrete surface.

 

Summary
To avoid surface scaling, make certain the concrete has sufficient strength and contains air entrainment. Don't let the finishers add large amounts of water to the concrete after it has arrived at the jobsite. In addition, the finisher has to know when to begin work on the slab if floating or troweling is to be part of the finishing procedure. Don't let the finisher prematurely finish the surface or add large amounts of water to the surface during finishing operations.

 

 

For more information about preventing surface scaling in concrete, please contact your Lafarge sales representative or technical sales engineer.

 

 

Lafarge North America is committed to providing the best possible service to our customers. For more information about concrete or concrete materials, please contact your Lafarge sales representative or technical sales engineer. 

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