The KCMMB approved concrete mix designs are not the same as previous mix designs in the area. Different concrete means different characteristics and different concrete characteristics mean different treatment. Let’s pursue what makes KCMMB concrete different, and what is required to place the mix successfully.
KCMMB Concrete Requires:
Why? To mitigate the potential for Alkali Silica Reaction (ASR).
Water Demand. Water demand varies with the blend of slag or “F” fly ash with different cements. Usually water reducers are introduced to hold the integrity of the water/cement ratios and help produce a more workable concrete.
Workability. Workability is generally improved, but slag or “F” fly ash can make the mix stickier at times.
Bleed Water. Bleed water is usually reduced, sometimes tricking finishers when they are not used to the reduction.
Set Time. The set time is much slower than before. Add cool conditions and the concrete set is considerably longer. The finisher must be aware of the conditions and not rush the finishing process. Finishing too soon increases the potential for scaling in the future. The sawing window will change too, the time to initial sawing will be delayed.
Why? Toproduce a more durable concrete and alleviate the potential for freeze-thaw distress.
Sawing. Sawing is delayed as a higher concrete strength is needed to cut “harder” rock without raveling.
Why? To mitigate potential for freeze-thaw distress. The air pockets in the hardened concrete allow for the expansion pressures from freezing (and thawing) water to move in and out of the void system.
Why? To maintain satisfactory moisture and temperature in concrete immediately following placing and finishing so the desired properties may be developed. The surface area is susceptible to evaporation before surface water has hydrated with the cementitious materials. Curing has a strong influence on the properties of hardened concrete.
The KCMMB material mix specification came about as a result of the need for more durable concrete.
Area cities expressed concerns about the condition of concrete curb & gutter, sidewalks, and pavements placed in recent history that were showing signs of premature deterioration.
Concrete industry representatives joined together with local city public works departments, forming a team to improve the quality of concrete.
The conclusion of the team based on observations in the field as well as impartial scientific investigations identified several reasons for premature deterioration. These included: the large number of freeze/ thaw cycles in Kansas City area, the porous nature of locally available aggregates and the challenging placing conditions in the area due to extreme weather conditions.
- Be aware of weather conditions, invest in a portable weather station. Proper curing is a function of the specific concrete mix and the environmental conditions.
- Hot ambient temperatures will quicken the initial set and therefore increase the need for the crew to react faster for proper finishing and curing.
- Cold ambient temperatures will slow the initial set and therefore slow down the finishing process, slow the jointing and increase the demand for proper curing.
- The use of an evaporation retarder can help insure the moisture retention (for proper hydration) after the screeding step and before the final finishing step. Do not confuse the evaporation retarder step with the curing step, they are different.
- The possibility of plastic shrinkage cracks and scaling are increased. Increasing the need for proper curing.
- Steel trowels should not be used on concrete exposed to freeze/thaw conditions and deicers.
- Overfinishing may reduce the amount of entrained air in the surface of slabs - thus making the concrete surface vulnerable to scaling.
- Curing KCMMB concrete is critical as is the timing of getting the cure immediately applied. Curing can take place in a multitude of methods, from white pigmented curing agents, to fogging, wet cure with burlap, concrete blankets, etc…
- Note that problems with the concrete may not show up immediately, sometimes mortar flaking and scaling show up months or years down the road. But also note, that proper curing can potentially eliminate them.
- Note weather conditions in your project diary
- Keep close tabs on the weather conditions, discuss conditions for that day with the contractor
- Keep a current print out of the KCMMB specification on hand
- Make sure the batch ticket mix design identification number matches the official approved mix design list from the KCMMB website
- Wash out a handful of concrete to verify KCMMB approved aggregate, as a visual check
- Check to see that air tests meet specification
- Check on the setting time in relation to finishing
- Talk to the contractor about the curing regiment and see that the appropriate materials and equipment are on the job site
- Make sure the concrete is properly cured
- Check for proper timing for saw cutting/jointing