Q: What is System-K™?
A: System-K is a combination of Komponent® mineral additive, small synthetic fibers known as K-Fibers, and portland cement. The K-Fiber replaces temperature and shrinkage steel (steel in place solely for crack control) in concrete. K-Fiber provides the necessary restraint in System-K Shrinkage-Compensating Concrete. Price spikes in steel reinforcement have made System-K a very economical choice to create shrinkage-compensating concrete. The most common applications for System-K are concrete toppings and slabs on grade, but it can be used in any concrete application where minimizing shrinkage cracking is desired.
Q: What are the specifications that are now in effect that can be used for guidelines?
A: CTS Cement provides a master guideline specification for System-K Shrinkage-Compensating Concrete on our website. (See Specifications above.) CTS Cement Engineers and our Technical Support team can assist with project recommendations for specific applications, like concrete containment structures, slabs-on-grade, parking structures, geotechnical, etc. The following publications may also be helpful:
- ASTM C806 (Test Method for Restrained Expansion of Expansive Cement Mortar).
- ASTM C845 (Standard Specification for Expansive Hydraulic Cement).
- ASTM C878 (Test Method for Restrained Expansion of Shrinkage-Compensating Concrete).
- ACI 223 (Standard Practice for Use of Shrinkage-Compensating Concrete).
- ACI SP-2 (ACI Manual for Concrete Practices) Page 374-38 1.
- Ohio Department of Transportation (Expansive Hydraulic Cement).
- Ohio Turnpike Commission – 5P5 1 1A (Class S Concrete, Using Shrinkage Compensating Cement).
- U.S. Corps of Engineers – Standard Practice for Concrete (Emi 110-2-2000).
Q: What makes shrinkage-compensating concrete different from ordinary concrete?
A: When regular portland cement concrete hardens and begins to dry out, a negative volume change occurs in the total mass. This process is called drying shrinkage. Shrinkage of a concrete sets up internal tensile stresses, and if these stresses exceed the tensile strength of the concrete, cracking, called drying shrinkage cracks, occur. The shrinkage-compensating concrete expands slightly during the early stages following placement and, when restrained by reinforcement, puts mass into compression. The mass returns to its original size when the effects of drying shrinkage occur, relieving the compressive stresses related to regular portland cement. The result is a concrete that can minimize drying shrinkage cracks and ensure dimensional stability of the concrete, as well as many other design and construction advantages over conventional portland cement.
Q: Where can I use shrinkage-compensating concrete?
A: Shrinkage-compensating concrete can be used in any application where regular portland cement concrete is used, including, but not limited to reinforced and post tensioned structural slabs and slabs on grade, walls, topping, grouts, below-grade/underground and pre-cast elements.
Q: What can’t shrinkage-compensating concrete do?
A: Shrinkage-compensating concrete cannot eliminate all types of cracking. There are many types of cracking such as settlement cracks, structural cracks, and cracks caused by a variety of influencing factors, like creep of concrete over long periods, plastic shrinkage cracks while concrete is in the semi-plastic state, and cracks related to environmental conditions or poor installation practices. Shrinkage-compensating concrete cannot overcome all cracks, but it can overcome the most common – drying shrinkage cracking, which contributes significantly to costly deterioration and early failure.
Structural Design Questions
Q: What is the recommended joint spacing?
A: Typically joint spacing of 130’ x 130’ to 150’ x 150’ is used. Areas exposed to thermal movement may be limited to 100’ x 100’ feet. For longer, narrower placements, maintain the three-to-one (3:1) length to width ratio recommended in ACI223. Consult a CTS Cement representative for additional information and assistance with joint layouts.
Q: What about System-K™ and structural steel?
A: The K-Fiber in System-K cannot replace structural steel reinforcement. However, the structural steel will not hinder the effectiveness of System-K.
Q: Where is System-K™ available?
A: Nationwide. It can be shipped anywhere in North America that it is desired in bags, by bulk, or by rail.
Q: Can we guarantee a crack-free project?
A: Due to the wide range of influencing factors in concrete construction, no project can be guaranteed crack-free. However, Type K shrinkage-compensating concrete was engineered to overcome the effects of shrinkage and minimize and potentially eliminate shrinkage cracks. If shrinkage cracking does occur, the cracks will be minute and tight, and markedly inconsequential to the long-term performance the concrete.
Q: How does shrinkage-compensating cement gain strength compared to Type I?
A: Shrinkage-compensating concrete has a slower strength gain from one to seven days than normal Type I cement. Type K is ground finer to compensate for this. The seven to 28-day strength gain after ettringite crystals have grown is comparable to regular PCC. The water cement ratio is the greatest factor effecting shrinkage-compensating cement strength, as it is with portland cement.
Mix Design Questions
Q: Why the higher water/cement ratios?
A: Due to the hydration mechanism used to hydrate the Komonent® used in shrinkage-compensating concrete, and to ensure appropriate expansion, it requires 10% to 15% more mixing water than standard cement to accommodate the formation of the ettringite. Water cement ratios are typically 0.45 to 0.50 for interior concrete and 0.50 to 0.55 for exterior. We like to see the concrete batched at a seven-inch slump at the concrete plant because of the high demand for water, which usually occurs initially when mixing water into shrinkage-compensating cement. This high demand is caused by the (1) hydration of the cement, (2) the ettringite growth which starts immediately upon introduction of water, and (3) the wetting of materials. We can look for a one to three inch slump loss within the first 30 minutes. Shrinkage-compensating concrete should be placed at a five plus/minus one inch slump.
Q: Does air entrainment react differently with shrinkage-compensating cement?
A: With very few exceptions, air entrainment admixtures that comply with ASTM C260 may be used for the same purpose with shrinkage-compensating concrete as with other types of portland cement concrete. Generally, the same amount of a given air-entraining admixture will produce a comparable percentage of entrained air, all other conditions being equal. (ACI 223-98 4.4.1). Please call your CTS Cement representative for guidance in selecting an air entraining admixture for your shrinkage-compensating concrete mix design.
Q: Why is there no bleed water?
A: The formation of ettringite uses the extra mix water thus eliminating the bleed water.
Q: How is System-K™ incorporated into the concrete mix?
A: System-K components are added at the batch plant via a silo through the central mixing system, or added at the job site using a slurry machine. Add the K-Fiber™ after combining the water, cement, aggregate, and Komponent. The slurry machine can be fed via bags or portable silo. These methods help ensure complete incorporation of the Komponent into the mix and prevent balling and pop-outs.
Q: I hear that we have two yards less concrete in the truck.
A: With normal procedures and good batching practices the same size loads can be hauled. Some Departments of Transportation require two yards less on projects within their state. Check local regulations.
Storage and Handling Questions
Q: Where are CTS shrinkage-compensating cement products available?
A: Everywhere. Contact your CTS Cement representative for details.
Q: Will shrinkage-compensating cement disrupt my operation?
A: Producers who dedicate a silo to the Komponent mineral additive can incorporate it via normal operations. For those who will be using a portable silo or slurry machine, pre-planning is needed to ensure minimal disruption. There are a number of ways to incorporate Komponent into the operation. Planning ahead always makes the process much smoother. Contact your CTS Cement representative for assistance.
Q: What do I do with the leftover shrinkage-compensating cement?
A: Shrinkage-compensating cement can be used in footers or other non-spec work. Type I can be pumped right on top of it and blended if there is not much left over. Calcium chloride can be added to Type K to prevent ettringite formation, reverting the remainder of the mix to standard portland cement
Q: Type-K sticks to the fins of my drums on my concrete trucks.
A: Fin build-up is primarily due to insufficient mix water or lack of enough mixing revolutions. Use about 1,000 pounds of stone and enough water to let the mix act like a scouring pad in the truck. The stone can be reused.
Q: Do we need special tools or knowledge?
A: Standard finishing tools and equipment are used. Any distinctive characteristics can be covered during the pre-construction and pre-placement meetings. It is also suggested that a representative from CTS be contacted prior the initial pour for consultation.
Q: Can we add water at the jobsite?
A: With Type K mix water is added at the batch plant. For increased workability on the job site, water reducers can be added. Exceptions must be approved by the test lab or engineer. If slump adjustments are necessary, a superplasticizer may be used on site.
Q: Will the shrinkage-compensating concrete blow up the forms with this expansion?
A: No, the shrinkage-compensating concrete does not expand that quickly or that much. These are very small controlled expansions (0.04% to 0.1% at 7 days). As the cement expands, the reinforcement resists the expansion of the concrete, limiting it to the designed expansion requirements.
Q: What about curing? Can we just use a spray-on curing compound?
A: Proper curing is very important to ensure a successful shrinkage-compensating concrete project. Curing compounds DO NOT provide sufficient curing for the concrete. The surface of the concrete must remain continuously wet for a full seven days to ensure maximum expansion of the ettringite crystals. There are many ways to accomplish this. The simplest is to dam the sides of the concrete and flood the surface. Alternatively, the surface can be wet with a hose and then rolled plastic sheets placed on top. Periodically the slab will have to be re-wet. Other options are available. Contact your CTS Cement representative for more information.