What solutions can be used to overcome challenges with EcoSmart™ concrete?
With today’s competitive schedule demands in concrete construction, there may be situations when using EcoSmart concrete presents challenges to workers. Some challenges that may be experienced and what you can do to overcome them:
EcoSmart concrete may take slightly longer to set than conventional Portland cement concrete. This is due to the relatively low content of Portland cement, low heat of hydration, and slow pozzolanic reaction. The longer setting time is particularly important when stripping cycle times for concrete forms are of concern, and there are techniques to address this.
- Lower ambient temperatures, higher carbon content of SCM requiring larger doses of air-entraining admixtures, and increased coarseness of SCM particles all contribute to increasing the setting time of EcoSmart concrete.
- Setting time is typically less of a concern in vertical elements (columns, walls) than in horizontal elements (suspended slabs). Portland cement replacement in horizontal elements may be reduced compared to vertical elements.
Long-term strength development is not affected negatively by slower early-age strength development. Actually, EcoSmart concrete becomes stronger and more impermeable over a long period of time. Slow early strength can be addressed by a number of options:
- Keep water and water-to-cementing materials ratio low. This may involve the use of superplasticizers.
- Reschedule so that forms can be left in place longer.
- Reschedule so that finishing activities can be delayed without increasing costs for non-active crew waiting on site. For example, pour concrete in the afternoon, hydrate over night, and finish the next morning.
- Apply proper and timely curing techniques.
- Redesign concrete mixture proportioning to ensure that an optimum SCM content is used in concrete to meet required strength levels at selected ages.
- Adjust specifications to avoid requiring strength more quickly than is actually needed. (Extend the age of specified strength requirement from 28 to 56 days.)
- Modify construction procedures to suit EcoSmart concrete’s increased set times. Use permanent formwork such as insulated concrete forms (ICF) or steel deck and cast-in-place concrete composite flooring system.
- Use high quality SCMs.
- Use a cement with a more rapid strength development.
- Use an accelerator to decrease the setting time.
- Lower the dosage of water reducer to decrease the setting time.
To fully develop its excellent long-term durability properties, EcoSmart concrete, like any type of concrete, needs adequate curing. The importance of applying wet curing or curing compounds for sections or elements made of EcoSmart concrete that are subjected to more severe environmental conditions should not be underestimated.
- The decrease in bleed water may also cause plastic shrinkage at the concrete surface if the surface not kept moist starting immediately after placing and during finishing operations.
- To achieve proper curing on a project, the contractor should be required to submit and follow a plan for protection and curing of EcoSmart concrete, and the quality control on curing should be increased.
Placement of concrete in temperatures less than 5°C usually requires alternate measures to maintain production and quality. This could include reducing SCM content on cold days, increasing cycle times during cold weeks, using accelerating admixtures on cold days, or all of the above.
Steel trowelled finishes of EcoSmart concrete require slightly different techniques than conventional Portland cement concrete. Due to the lower water content, less bleed water rises to the surface than would in a conventional mix. Finishers may not be able to rely on bleed water for finishing and may have to adjust their techniques. There may also be a delay in the time when finishers can start their work.
- When finishers are not able to rely on bleed water, the use of a high-pressure washer to mist the air and keep the sheen on the surface has proven effective.
- Keeping the crew well informed and trained increases the success with EcoSmart concrete and raises the satisfaction of workers on site.
Concrete strength acceptance in North America has historically been based on a 28-day test. To take into consideration the slower strength development of EcoSmart concrete, 56-day compressive strength specification is recommended. In fact, the current CSA A23.1-04 requires that for EcoSmart mixes with fly ash content >40%, or slag content >45% (% mass of total cementing materials), or a combination of fly ash / 40% + slag / 45% > 1.00, the minimum strength requirements shall be specified at 56 days.
EcoSmart concrete, particularly one containing high volumes of fly ash, is more susceptible to carbonation, which may lead to carbonation-induced corrosion of reinforcing steel.
- Proper curing, adequate depth of concrete cover to reinforcing steel, higher concrete strength and lower water-to-cementing materials ratio.
At higher Portland cement replacement levels (e.g., >30%), certain surface finishing techniques, the quality of the concrete, and the curing regime may cause EcoSmart concrete to be more susceptible to surface scaling particularly when exposed to freezing and thawing and deicing salts.