Proper Concrete Practices Can Avoid Surface Scaling
In the last few years, complaints of spalling (scaling) of exterior concrete surfaces have been on the rise. There are several non-contractor related reasons scaling issues are on the increase. In many regions, the quality of existing aggregate sources is decreasing, despite producer efforts.
Due to climate changes, many areas are experiencing more freeze-thaw cycles than in the past. And perhaps, most importantly, municipalities and owners are using more aggressive salts to remove ice and snow, which creates more exposure to scaling damage. And in many areas, the temperature of placed concrete is often much higher than before.
What Causes Spalling?
Spalling is the local flaking or peeling away of small particles of cement paste from the near-surface portion of the finished slab. In most instances, the defect does not affect the slab’s service life if untreated.
Finished concrete is often porous and can absorb rain water. Spalling occurs on exterior slabs when water that is trapped in the concrete goes through multiple freezes and thaws. When water freezes it expands by about 9%, which creates tremendous pressure inside the slab.
If entrained air bubbles are not present near the surface of the concrete to act as an internal pressure-relief system, the hydraulic pressure can easily exceed the tensile strength of the surrounding cement paste. Eventually, microcracks start to form and the surface begins to separate from the body of the slab. Successive freeze-thaw cycles eventually cause the top surface to flake and scale off.
The American Concrete Institute (ACI) has created a rating system to classify the severity of spalls. Light scaling does not expose the coarse aggregate particles. Medium scaling involves the loss of mortar to depths from 3/16 to 3/8 inch with exposure of the coarse aggregate particles. Severe scaling includes the loss of mortar surrounding the coarse aggregate particles from 3/8 to 3/4 inch in depth. If the depths of mortar loss exceed 3/4 inch and coarse aggregate particles have been lost, the scaling is rated as very severe.
Six Ways To Prevent Spalling
To combat these challenges and prevent callbacks and customer complaints, concrete contractors should periodically review their practices. Let’s look at six areas that can improve slab-on-ground quality and improve contractor profitability.
1. Check your placing and finishing tools
After a busy construction season, tools and equipment may have become worn. Check your floats for uneven edges. Make sure that surfaces of jitterbugs and surface rollers are undamaged and that they roll evenly. Check the blades of your riding trowels for wear and broken edges. Even check your hand tools for wear, looking for sharp edges, worn grooves, and broken handles.
2. Review your concrete mix design order
Durable concrete starts with well-designed mixes. Work with your ready-mixed concrete supplier to have a group of mixes that will cover all seasons and temperature variations.
There are some basics to follow for scale-resistant concrete. Use a maximum w/cm ratio of 0.45 and a minimum 28-day concrete strength of 4500 psi. Placing concrete with a low w/cm ratio, adequate strength, and proper curing will reduce the permeability along the top surface. These denser concrete limits the amount of water and deicing chemicals that can penetrate the concrete also increases the scale resistance of the concrete.
For exterior slabs, sidewalks, and patios, use air-entrained concrete that meets the ACI requirements. The total air content requirements increase as the maximum size of the coarse aggregate decreases. It is important to check the air on the project before placing. Air content can vary due to changing production plant conditions, especially early in the morning.
Also develop an approach with respect to retarders and accelerators with your crew and producer. Finishing crews want to get on the concrete as soon as possible. As the concrete and ambient temperatures change during placement, be flexible on changing dosage rates.
3. Review your placing and finishing procedures
The best designed concrete can suffer surface defects when not properly placed and finished. One of the greatest contributors to scaling can be finishing concrete too early. While the increased productivity gained by using laser screeds and riding trowels has accelerated production, let the concrete set properly.
Early finishing creates problems that can create scaling. This action prematurely seals the concrete surface trapping rising bleedwater. The early action can also trap air directly beneath the surface. Both conditions create a weak layer of concrete that is prone to break upward when exposed to freezing.
Another contributor to scaling can be overworking of over-wet concrete. Excessively working the surface can damage or destroy the entrained air bubbles placed into air-entrained concrete. Overworking can also create a near-surface zone of weakened concrete.
Placing concrete on a cold subbase can contributes to the scaling problem. The cold temperature causes the concrete to set more slowly and thus bleeds longer.
Prohibit steel troweling of air-entrained concrete. Instead, call for a float, broom, or burlap-drag finish.
Don’t perform any finishing operations when water is on the concrete surface.
4. Proper Curing
To retain moisture and gain strength, all concrete must receive proper curing. For flatwork it is particularly true that the top surface be well cured. Membrane curing compounds applied by spray or paint rollers are recommended. Cure the concrete with a curing compound or watertight covering, then allow it to dry uncovered for at least 30 days before deicing salts are used.
5. Design landscaping to keep concrete dry
Another cause for scaling occurs during final grading, or in new landscape projects. There must be adequate slopes to properly drain surface water away from the slab. Saturated concrete is more susceptible to damage from freezing and thawing than drier concrete.
6. Follow up with the owner about good preventative maintenance
Many owners are unaware of their involvement in maintaining concrete. The first step is to restrict any deicer use on the concrete surface for as long as possible. In most circumstances six months is an adequate period.
When the concrete achieves full strength, normally in 56 days, a simple reminder to have their exterior and garage floors sealed a few months after installation can help maintain the surface for years.
Check here for info on repairing spalled joints and flat areas: