Get the Best Results with Concrete Admixtures

• A new concrete floor has dark, moist spots, even after it has cured. Floor coverings are ruined by this moisture.
• A driveway poured at 7:30 in the morning under ideal conditions doesn’t cure until 4 o’clock the next morning.
• A stamp project turns out with little tears across the texture because the surface crusted over too fast.

The culprit in each of these nightmare scenarios is an admixture improperly used. Of course, admixtures are intended to enhance the installation and/or performance of a finished project and in the vast majority of cases they do. However, when things go wrong, the remedy can be difficult and costly.

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In the first instance, dark discoloration was caused by calcium chloride, used to accelerate the cure. Rick Smith of Richard Smith Custom Concrete, Canoga Park, Calif., says, “The number one nemesis is calcium chloride. It will draw moisture even after the concrete is cured. It can ruin a wooden floor laid over it.”

Gabriel Ojeda explains, “The main problem with calcium chloride is the chloride ion. The chloride ion plus water causes corrosion. Alternative accelerators do not contain this ion, so they do not corrode steel or affect colors.” Ojeda is president of the admixture manufacturer Fritz-Pak Corporation of Dallas, Texas. He goes on to explain that accelerators are used to save time, such as when pouring concrete in cooler temperatures. During a short winter day with high temperatures in the 40s, the concrete can take all day to set unless an accelerator is used. While calcium chloride can be used in applications like a foundation or a warehouse where the floor will be dry, for decorative applications Ojeda recommends using a non-chloride alternative even though the material cost can be four to five times as much. Bob Harris of the Decorative Concrete Institute, Temple, Georgia, concurs. He says, “As a general rule, we use non calcium-based accelerating admixtures.”

Too much accelerator, even the right one, can cause trouble too, according to Chris Sullivan, national sales manager for QC Construction Products, Madera, Calif. “When there is too much accelerator in a mix and it cures too fast, the slab gets away from the contractor before he can stamp it. Most problems with admixtures are due to overdosing or underdosing.”

Admixtures for retarding the cure sometimes have unexpected outcomes too. Smith recalls walking the beach near his customer’s home all night, wondering if the concrete he poured early that morning would ever set or if he would have to rip the whole project out and begin again. The problem was the interaction of two admixtures. The concrete contained a water-reducing agent and a retarder. “The water-reducing agent reduced the water by 30 percent, but the amount of retarder used was based on a full water load,” Smith explains. “So the slab didn’t cure until 4:00 the next morning.”

According to Bob Harris, a good percentage of concrete mix designs already include a mid-range water-reducing admixture, which gives you a water reduction of somewhere between 5 percent and 10 percent. He explains that most mid-range water reducers have a slight effect on the setting times, making the underlying concrete set more slowly while the surface crusts over. “When you stamp a mix like this,” he says, “you inevitably end up with little tears across the surface of your texture. My personal experiences when pouring color hardened concrete with two or more admixtures in the mix has proven to be a challenging ordeal.”

Set retarders must be added to the mix in the right proportions. Ojeda says that not adding enough will make the concrete start hardening before the decorative finish can be applied. Adding too much has two possible outcomes, depending on the chemistry. “With an overdose, it is either a matter of time or a matter of life or death. Either it takes too long to set, or it never sets. The water evaporates, the concrete dries out and it just crumbles — you end up with sand and gravel. There is no remedy for this catastrophic failure.”

Two other common admixtures should be mentioned here. Air-entraining admixtures increase durability during freeze-thaw cycles. However, too much air weakens the concrete and it loses strength. Chris Sullivan adds that too much air can lighten the color as well.

Superplasticizers increase workability without additional water, but too much plasticizer can result in material segregation and excessive bleed water. It can also affect final color.

What’s a contractor to do?
In windy, sunny conditions when the surface is curing too quickly, Bob Harris suggests using a spray-applied evaporation retarder to lock in moisture. “I will sometimes cover the slab with 6 mil plastic for 10 or so minutes between dry shake applications to help draw moisture to the surface without adding more water,” he adds.

One way to slow down the cure without an admixture is to ask the ready-mix supplier to use chilled water in the mix. Rick Smith remembers trying this on a challenging project. He says, “We didn’t have one crack at 106 degrees and high winds.”

When conditions are variable and the set time can’t be accurately controlled or predicted, Smith recommends having extra crew on hand for finishing. “It may increase labor costs,” he says, “but it’s like paying for an insurance policy, even if they just stand there and look at it. One time we had to stamp 15 minutes after we poured, the concrete was curing so fast.”

Microtoppings and overlays offer an alternative to tearing out a slab. When problems with a set retarder result in poor stamping quality, a contractor gets a second chance with a stampable overlay. An overlay gives the customer the added benefit of an extremely strong surface.

A pound of prevention…
Of course, avoiding problems in the first place is much better than fixing them later. “Successful decorative concrete installations are a total team effort, starting with the ready mix producer,” Harris says. “As a contractor, it is your responsibility to understand mix designs that include admixtures and make sure that you order the appropriate mix. We have six or seven mix designs we use on a regular basis because we are able to partner with our ready-mix producer. The bottom line with mixes is, keep it simple! Don’t try to re-invent the wheel.”

Smith agrees. “Get to know the chemist at your ready mix plant,” he says. “Try to stick with proven mixes.” Ojeda takes a serious view of the contractor’s responsibility. “The contractor should tell the ready mix supplier exactly what he wants the concrete to do.” Being specific about requirements and understanding what is going into the mix are key to avoiding admixture problems.

While there can be some hair-raising moments when admixtures are used improperly, in the vast majority of cases they make decorative concrete installations easier and more successful. “Admixtures are a means to get from point A to point B,” Smith says. “They are not a fix-all, but they are another tool in the arsenal.”

Chris Sullivan goes further. “There aren’t that many issues created by admixtures. Admixtures are your best friends in decorative concrete — they give you more time or speed things up so you can control the pace. They increase strength. The benefits of admixtures far outweigh any problems.”

What is a Flowable Fill ready mix?

• Flowable fill is a self-compacting low strength material with a flowable consistency that is used as an economical fill or backfill material as an alternative to compacted granular fill. Flowable fill is not concrete.

What is a concrete retarder?

• Concrete retarders are additives that slow down the chemical process of hydration so that the concrete remains workable for a longer period of time (typically used in hot weather).

What is a ready mix accelerator?

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• Accelerators make concrete set faster, also known as increasing the rate of hydration. At the same time, they promote strength development so it happens earlier in the set time of a slab Typically used in cold weather or when strength tests need to be met in a short period of time.

What are the types of concrete accelerators?

• Calcium Chloride (1% & 2%) & Non-Chloride Accelerator (NCA) (1% & 2%) are the most commonly used types in ready mix concrete.

How to determine which accelerator to use?

• If steel or rebar is being used NonChloride Accelerator should be used. Calcium chloride causes corrosion that leads to premature failure. NCA has become the standard and is always safe to use.

How many 80 pound bags of concrete is 1 yard?

• 45 bags

How many 60 pound bags of concrete is 1 yard?

• 60 bags

How much does 1 yard of concrete weigh?

• Wet concrete can weigh as much as 4,000 lbs +. Dry will be about 3, lbs. Mix design and psi can drastically change the weight.

How many wheelbarrows does it take to unload 1 yard of concrete?

• Now, this is a load question because it depends on how much the pusher can push and the strength of the wheelbarrow. Generally speaking, we have noticed it takes 19-24 wheelbarrow trips per yard of ready mix.

How long are the Quick Mix Concrete truck’s chutes?

• 12 feet.

What is the height & width clearance for Quick Mix trucks?

• Height: 9 ft 6in Width: 8 ft 0 in. Note this spec could vary from truck to truck and is with the funnel in the travel position.

Will sugar affect the ready mix concrete?

• Absolutely! It all depends on the amount used. Too much will cause the concrete not to set up or cure. Below the too much point it will act as a retarder! Also, depending on the amount used it can increase the final strength! What are these amounts? Well, we don’t know the exact and nor do we recommend using it.

What is 1 cubic yard of ready mix concrete and how to calculate?

• Measure the area in feet. Multiply length by width, and then by thickness. Divide the resulting number by 27 to find cubic yards.
• Additional information:
  • 1 cubic yard is 27 cubic feet.
  • One yard of concrete, also known as a cubic yard, covers: 81 square feet at 4 inches thick. 65 square feet at 5 inches thick. 54 square feet at 6 inches thick
  • Concrete Calculator

What is the difference between Interior and Exterior mix?

• Indoor concrete is generally meant for foot traffic and should provide a smooth look. Indoor concrete is typically polished down to a nice sheen and finished to allow for good grip and a continued shine. Interior concrete is typically not exposed to natural changes of the environment and often does not require any air entrainment as it has less of a tendency to crack than exterior concrete.
• Finishing exterior concrete too early (before the excess water has time to rise to the top) can trap the bleed water, setting the stage for other surface problems.
• Outdoor concrete is often rougher since it is often meant for creating grip for car traffic and pedestrians, reducing the chances of any slipping or falling.

What is ready mix slump and how is it measured?

• Slump is the measure of concrete consistency and fluidity. It shows the flow and overall workability of freshly mixed concrete. Simply put, the higher the slump, the more fluid the mix. Four-inch (4”) slump is very common with normal weight concrete and is good for pumping. Slumps that are above mix design can cause reduced strength, durability, and permeability of the concrete. Admixtures can be used instead of water to achieve higher slumps and can maintain the quality of your concrete.
• Slump is tested with a slump cone (aka Abrams cone). Simply put the ready mix concrete is packed into the cone and then the cone is slowly lifted away from the ready mix. The concrete slump is measured by measuring the distance from the top of the slumped concrete to the top of the slump cone.

What is a ready mix concrete line pump?

• A pump that moves concrete to an inaccessible location. They can be mounted to a truck bed or tailgate, trailer, or a whole truck. They can save time, money, and energy. They do require a special mix to ensure the ready mix will flow.

What is the difference between line pump mix and the standard mix?

• The ideal ready mix concrete for pumping typically contains air-entraining agents, a little extra sand (compared to standard mix), fly ash, and a smaller aggregate blend.

When should a line pump be used?

• Line pumps are generally used to reach projects that are in “hard to reach” places (ie: basements, backyards, tight spaces in between houses or buildings and areas on a steep incline). Line pumps are a convenient way to get your concrete exactly where you need it.

What are ready mix concrete additives and when should they be used?

• There are hundreds of different additives. Here are the most common:
  • Fiber: increases the concrete’s durability. It reduces crack growth and increases impact strength. Fiber-reinforced concrete improves resistance against freezing and thawing.
  • Retarder: Slows down set time. Typically used in warm weather.
  • Accelerators: Speeds up set time in cold weather or when strength is need quicker.
  • Water Reducer: Significantly decrease the amount of water needed in a mix design.
  • Flowable fill is defined by the American Concrete Institute (ACI) as a self-compacting cementitious material that is in a flowable state at placement and has a compressive strength of 8.3 MPa (1,200 lb/in2) or less at 28 days. Generally used in utilities or in construction where the material will be removed.

What does PSI stand for?

• Pounds per square inch (psi) measures the compressive strength of concrete. A higher psi means a given concrete mixture is stronger, so it is usually more expensive. But these stronger concrete are also more durable, meaning they last longer. Most common are between 3,000 & 4,500 psi

How do you determine the PSI you need for your project?

• An engineer would need to provide a definite answer, but here are general concrete strength (PSI) for common structures:
  • Concrete footings and slabs on grade typically require a concrete of 3,500 to 4,000 psi.
  • Suspended slabs, beams, and girders (as often found in bridges) require 3,500 to 5,000 psi
  • Traditional concrete walls and columns tend to range from 3,000 to 5,000 psi.
  • The residential workhorse of concrete, 3,000 PSI can be used for driveways, patios and sidewalks if you aren’t expecting to drive or park heavy equipment on it.
  • If heavy vehicles such as RVs are being placed on your driveway or parking area PSI should be used.

What do I need to know when ordering concrete?

• Basics: Name, Number, Location, & Payment
• Amount needed in cubic yards. It is a good idea to order 4-10% more than need for waste and over excavation.
• The PSI
• Where you plan on unloading the truck. It is highly recommended that the truck stays on a hard surface.