Slag Cement Association Executive Director, Drew Burns sat down with the Association’s Technical Director, John Melander to discuss the effects of scaling in concrete and specifically how using slag cement in concrete correctly can help address the issue. John has many years of experience in slag cement and shares his expertise in the following question and answer segment.
What is scaling in concrete flatwork?
Scaling is a surface defect (flaking or peeling of a hardened top surface of concrete) that results from exposure to freezing and thawing conditions. The fundamental mechanism responsible for the flaking or peeling of the concrete is the expansion of water upon freezing. Deicing chemicals (often used in colder environments) cause a significant increase in internal pressure and also increase in the number of freezing and thawing cycles.
Concrete can be more susceptible to scaling due to inadequate mixture proportions, improper finishing of the concrete, lack of concrete curing, and aggressive and early use of deicing chemicals. A proper air void system is also critical to the durability of concrete in a freezing and thawing environment as air voids provide an outlet for the hydraulic pressure that is created when water freezes.
Using appropriate mix designs, proper placement, finishing and curing practices will help mitigate scaling issues.
Does slag cement have different outcomes in concrete when used in colder weather environments?
With all concrete mixtures, temperature and climate play a vital role in the success of your concrete. In colder weather, concrete will set slower which delays bleeding and increases the chances of early finishing. Slag cement will slow the set time of concrete to a greater degree than straight portland cement mixtures. The amount of slag cement used as a replacement for portland cement is usually decreased or eliminated in cold weather to provide a faster setting concrete. If high slag cement replacement levels are used in cold temperatures for other durability reasons, such as mitigation of ASR, care needs to be taken that the concrete has stopped bleeding before final finishing operations are begun.
Does slag cement cause scaling in flatwork concrete applications?
No, it is not slag cement that is the issue, but how it is used. Any concrete mixture can be susceptible to scaling. Proper finishing and curing practices should be followed to ensure a successful project. ACI standards such as 302.1, 305 and 306 on hot and cold weather concreting should be followed. As always, consulting your concrete producer is recommended.
What are some concrete design practices that can be followed to increase concrete’s resistance to scaling?
Working with your local concrete professional is important to the success of your project. They have experience in your specific location and understand how to best use the materials. That said, there are a few rules of thumb that should always be followed:
- A concrete mixture with slag cement should be proportioned at a maximum water to cementitious material ratio (w/cm) of 0.45.
- Concrete mixtures should be properly proportioned (ACI 211) and contain proper graded and clean aggregate (ASTM C33).
- The amount of entrained air should conform to the specified amounts in ACI 318, Table 126.96.36.199 for exposure categories F2 and F3.
- The average spacing factor of the air void system (determined in accordance to ASTM standard C457) should not exceed 0.20 mm (ACI 212.3R)
- Use accelerators and higher cementitious contents in colder weather to decrease set times.
- When allowed, decrease slag cement replacement levels in colder weather to decrease set times.
How does finishing/curing concrete with slag cement effect its scaling potential?
Proper finishing/curing is vital to the success of any concrete system, whether it includes slag cement or not. Some pointers from the SCA’s tech sheet on the subject are helpful here.
- Do not finish while bleed water is still present on the concrete surface (causes an increase in the w/cm at the surface of the concrete).
- Do not sprinkle water on the surface of the concrete as a finishing aid (causes an increase in the w/cm at the surface of the concrete).
- Do not finish air entrained concrete with a steel trowel (decreases or eliminates the air from the surface of the concrete); magnesium or wood floats should be used.
- Be careful when finishing with a vibrating screed (can cause a loss of air and a weaker layer of mortar at the surface of the concrete).
- Be careful when placing concrete on a dry, windy day (can cause incomplete hydration and a weakened surface).
- Do not over finish the concrete (can cause a loss of air at the surface of the concrete).
- Concrete should be cured to maintain temperature and moisture conditions that promote strength gain.
- The curing compound should completely cover the surface of the concrete. Curing should begin as soon final finishing operations are completed.
- Concrete should be allowed to air dry for a period of one month before experiencing a freezing and thawing cycle, and avoid early applications of deicer chemicals.
- Apply a concrete sealer when concrete will be exposed to deicing chemicals before the proper curing period has been completely achieved (concrete sealers, such as silanes, siloxanes, or linseed oil, are not proof against scaling but some studies indicate that their use can provide added resistance to scaling, especially in the first year).
- Recommend finishing: screed, bullfloat, broom finish, minimzie troweling for best results.
What are some resources that would help address issues with using slag cement in cold weather environments?
- NRMCA has an Exterior Flatwork Finishing Certification that addresses properly finishing flatwork with slag cement. There has been great success with various DOTs and other organizations implementing this certification program.
- Concrete International Magazine put together a great article on this subject in August of 2018 from Henry Prenger of Lafarge Holcim. He discusses his experience with various Departments of Transportation and how they worked together to address issues of sidewalk scaling.
- ACI 318 recommendations for freezing and thawing exposure category environments.
- SCA technical information sheet on reducing scaling with slag cement and good concreting practices.
- A webinar by Professor Doug Hooton on slag cement use can be found on the SCA website. This is less about scaling, but a great overview of slag cement use.
As with all concrete mixtures, trial batches should be performed to verify concrete properties. Results may vary due to a variety of circumstances, including temperature, exposure conditions and mixture components, among other things. You should consult your slag cement professional for assistance. Nothing contained herein shall be considered or construed as a warranty or guarantee, either expressed or implied, including any warranty of fitness for a particular purpose.