Accelerators are one of the most popular kinds of chemical admixtures. Like water reducers, retarders and plasticizers, when added to a concrete batch either immediately before or during mixing.
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To kick the set time of a batch of concrete into high gear, hit the accelerator.
Like water reducers, retarders and plasticizers, accelerators are one of the most popular kinds of chemical admixtures, added to a concrete batch either immediately before or during mixing. 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.
If a contractor is using an accelerator, the odds are good that the weather is wintry. Accelerators counteract the influence of cold weather, which slows down the curing and setting process.
But accelerators aren&#;t just for cold weather. A contractor can use one anytime a curing process needs a kick. The admixture may allow a concrete worker to remove forms earlier, get onto a concrete surface earlier for finishing, and sometimes even put loads on it earlier, such as when diverting foot traffic to do patching.
&#;Decorative guys will use accelerators in summer. It helps control the process when stamping,&#; says Terry Collins, concrete construction engineer with the Portland Cement Association.
If an accelerator is added to concrete on one half of a pour, a contractor can start stamping on that side, work his way to the other side, and enjoy a relatively consistent level of workability throughout. Decorative contractors have been using retarding admixtures for several years to achieve this effect, says Gabriel Ojeda, president of concrete admixture manufacturer Fritz-Pak Corp., and accelerators can pull off more or less the same trick.
Because accelerators cut set time, they can reduce labor costs, Ojeda says. They can also hasten the time an indoor remodel job, such as a cast-in-place countertop, takes to set, Ojeda says. &#;You don&#;t want people to have to wait seven, 10, 15 days without being able to use their kitchen,&#; he says.
And while they do cost a little extra, accelerators may still be a better bargain than, say, natural gas. &#;If you don&#;t accelerate, and you have a short set time, the other option is heat,&#; Ojeda says. &#;Heat is now more expensive because of energy consumption. It may be cheaper to use accelerators than to heat a building.&#;
Calcium chloride pros and cons
A number of chemicals qualify as accelerators, but the most common is calcium chloride. It&#;s cheap, plentiful, and readily available from huge chemical companies.
However, while calcium chloride may be the cheap favorite for concrete in general, it is not necessarily the best option for decorative concrete.
Calcium chloride slightly increases workability and reduces the water required to achieve a given slump in a mix, according to a report from the Federal Highway Administration. It reduces initial and final setting times, and it improves compressive and flexural strengths of concrete at early ages.
Guidelines published by the Portland Cement Association list colored concrete among the jobs in which calcium chloride &#;should be used with caution.&#; The guidelines also state that slabs intended to receive dry-shake metallic finishes should not take calcium chloride or admixtures that contain soluble chlorides, and neither should most slabs poured in hot weather.
Decorative concrete contractors are going to be discouraged from using calcium chloride, says Collins of the PCA. It can inhibit the ability of acid stain to react with cement in the concrete. And it increases the potential for efflorescence. These aren&#;t significant problems on generic concrete slabs, but on decorative jobs, they can be distressing.
Most people see a little bit of white powder on the sidewalk, they sweep it off and forget about it,&#; Collins says. &#;But they see a little bit of white powder on decorative concrete and they tend to believe the world is ending, there&#;s something wrong with it.&#;
Excessive amounts of calcium chloride may cause rapid stiffening and shrinkage while drying, creating cracks in the cured surface. Calcium chloride may promote corrosion in steel reinforcements and increase the potential for scaling.
Perhaps most troubling for decorative contractors, calcium chloride may darken their slab. The chemical is hydroscopic. Just as table salt gets hard absorbing water from the air, calcium chloride literally liquefies. &#;If you put a pile of it on a table and come back in the morning, it will be all water,&#; Ojeda says.
That&#;s essentially how calcium chloride can make concrete darker, Ojeda says. Say a colored slab with calcium chloride is half in the shade, half in the sun. Rain or moisture will linger longer on the shady portion, get sucked into the slab by the calcium chloride, and make that part darker. The change won&#;t be significant, he acknowledges, but &#;it&#;s still enough to make a difference in color between the shaded area and sunny area.&#;
Calcium chloride&#;s potential to oxidize might also change the tint of colors and pigments that are based on iron oxide, he says.
Contractors can take steps to minimize cracking caused by calcium chloride&#;s shrinkage, says Tim Reason, president of Chargar Corp., a manufacturer of concrete and masonry products. &#;You may try to keep the surface wet and damp during the curing process. That might help a little bit.&#;
Reason also downplays the concern that calcium chloride may promote corrosion of reinforcing steel, noting that, even if it occurred, the corrosion would take a long time to become a real problem. &#;The corrosion issue is something they&#;ve been saying for a hundred years, but I don&#;t know if they&#;ve definitely proved it,&#; he says. &#;It may. But how long is it going to take? It&#;s not like it&#;s going to deteriorate in a year or so.&#;
Alternatives
In any case, contractors who want to accelerate their concrete aren&#;t forced to use calcium chloride. There are alternatives. &#;Although calcium chloride is an effective and economical accelerator, its corrosion-related problem limited its use and forced engineers to look for other options, mainly nonchloride accelerating admixtures,&#; states the FHA report.
Sodium thiocyanate, triethanolamine, and calcium formate, nitrate and nitrite are among the &#;nonchlorides&#; that have been successfully used to accelerate concrete set times.
There are four of the most common:
Accelerators are typically added at the manufacturer level, and with many manufacturers, the exact chemical makeup will be closely held, Ojeda notes. &#;We use calcium formate as a base, but we add other materials to boost performance of the formulation. That&#;s proprietary.&#;
Again, none of these alternative admixtures costs as little as calcium chloride does. &#;Calcium chloride is the least expensive of all the materials,&#; Ojeda says. &#;That&#;s the main reason people would not use a nonchloride accelerator.&#;
Of course, decorative concrete contractors also have other techniques for accelerating concrete besides adding an accelerator. Using cement that promotes high early strength will work. So will adding a water reducer, curing at higher temperatures, and lowering the water-cement ratio by putting more cement into the concrete.
&#;It&#;s very common for decorative concrete guys to just increase the cement content by 100 pounds and just call it a day,&#; Collins says. &#;That will have the same effect.&#;
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Cement hydration is temperature and time dependent. The higher the temperature, the faster the reaction happens. As time passes, strength continues to develop, even beyond initial set, as the hydration reactions continue. Accelerators act as a catalyst for hydration reactions.
Accelerator Basics
The main reason to use accelerators is to achieve early strength gain and early setting. As soon as water comes in contact with cementitious materials, hydration reactions begin. Likewise, most accelerators also begin performing shortly upon their addition to a concrete mix. During the dormant period when the fresh concrete is plastic and able to be handled, prior to initial set, the action of set accelerators can last anywhere from 10 minutes in a hot mix to 2 hours for a cold mix. During the setting period, after initial set but prior to final set, accelerators can provide rapid strengthening for up to 10-12 hours, especially if heat is applied. Lastly, during the hardening period, strength gain is typically very slow and can take many hours to complete. In some cases, however, flash set, or rapid stiffening accompanied by excessive heat generation, can occur due to an insufficient amount of gypsum in the cement. False set, which involves rapid setting and very little heat generation, could also occur.
Typically, accelerators are custom blends of chemicals that do exactly what the precaster wants with the specific cement used. Some have accelerating and water-reducing properties (ASTM C494 Type E chemical admixtures), while some are only accelerators (ASTM C494 Type C chemical admixtures). Some even help improve concrete&#;s workability while others can help increase corrosion resistance. One way that manufacturers classify accelerators is based on the materials from which they are produced. Ketan Sompura, director of concrete technology and admixture and fiber products manager at Sika, said there are four types of accelerators; soluble inorganic salts, soluble organic compounds, quick-setting admixtures and miscellaneous solid materials. Admixture companies may combine several raw materials from all of these categories.
There are several soluble inorganic salts used for accelerators. The most effective is calcium chloride. Chlorides are the most economical accelerators and perform well. Chlorides, however, promote corrosion of steel, so they aren&#;t used in steel-reinforced concrete. Other inorganic salts contain nitrates, nitrites and thiocyanates instead of chlorides. These accelerators are slightly more expensive and can be used with steel reinforcement. Soluble organic accelerators use triethanolamines and calcium. Quick-setting accelerators aren&#;t typically used in precast or ready-mix applications because they cause such a rapid loss of plasticity and workability. Other accelerators may use compounds such as silicates and carbonates.
&#;Usually, accelerators are a blend of organic and inorganic materials,&#; said Sompura. &#;Each raw material has a different reaction time. One may not do anything for 30-60 minutes and another will.
&#;Manufacturers combine them in different ratios and tailor the start and stop times before they react and provide results so they are suitable for precast applications.&#;
How accelerators work in concrete
Set accelerators work by acting as a catalyst for hydration reactions. The products of hydration reactions continuously accumulate, and the reactions continue until either all of the cement or all of the water in the mix has been exhausted. However, the products of hydration reactions can also surround unhydrated cement particles and create a barrier for the unreacted cement to come in contact with water. Chloride- and salt-based accelerators weaken the barrier created by these products and allow certain compounds in cement to hydrate, speeding up the hydration process. Accelerators containing triethanolamines act on a different compound in the cement and increase the rate at which the reactions occur with that compound. Triethanolamines can also encourage ettringite formation.
&#;The majority of accelerators react with tricalcium silicate, the major component of cement,&#; Sompura said. &#;Usually accelerators don&#;t work with dicalcium silicate, and only some of the accelerators work with tricalcium aluminate to make them be very quick setting.&#;
What&#;s important about accelerators
Although all accelerators start out as powders, most are supplied in liquid form because it is easier to dispense and evenly integrate into the concrete. Admixture suppliers provide bagged, dry forms to smaller precasters who don&#;t use high volumes, don&#;t have liquid dispensers and metering systems, are in remote locations or don&#;t receive weekly deliveries. Some accelerators, however, must be shipped in liquid form because they would be too dangerous as powders.
Liquids are easy to dispense and a homogenous mix is attained easier with liquids than with powdered additives. Sompura said there are two ways to integrate liquid accelerators into fresh concrete. One uses an automatic dispensing system which directly feeds accelerators into concrete mixtures. This metered batch process that takes liquid from a bulk tank to the mixer is usually used with large volumes of chemical admixtures. For lower volumes, a dispensing tube and a glass dispensing bottle provide visual confirmation for the person in charge of batching.
Admixtures are dosed by weight of the cement in the batch. The dosage of chloride accelerators typically follows the industry standard of 1-to-2% of the cement weight. Non-chloride accelerators have more variable dosages. Each accelerating admixture has a manufacturer-recommended dosing rate on its product data sheet. Sompura said these recommended dosages are guidelines and starting points for precasters to determine how much of the product to add. Most precasters create trial mixes and test a few doses to see which gives their product the required strength while also exhibiting a desirable level of workability and maintaining plasticity long enough to place the concrete and finish the exposed surfaces.
Accelerators are used more frequently in low ambient temperatures because the hydration reactions occur at a slower rate in colder temperatures. Accelerators also have a greater impact in colder temperatures. The warmer the ambient temperature, the less effective the accelerator. Using 2% accelerator by weight of cement at 45 degrees Fahrenheit may provide 5 hours of acceleration, but at 70 degrees Fahrenheit, the same amount of the same admixture may only provide 2-3 hours of acceleration. The accelerators-decreased effectiveness at higher temperatures is a nonissue since the hydration reactions occur at a faster rate at warmer temperatures &#; making accelerators unnecessary except for special circumstances.
Accelerators can also pose challenges for precasters, such as long-term strength gain and shrinkage. These are easily dealt with as long as the precaster is aware of the challenge and the solution.
&#;Although accelerators provide higher early strength, many people don&#;t realize that they lower the ultimate strength,&#; said Jesse Osborne, precast segment and admixture manager at Euclid Chemical. &#;Most precasters accept this and over-design a little bit, adding 5-to-10% more cement to reach the specified strength.&#;
Osborne said many admixture producers also include components that help offset shrinkage, which is a concern for precasters. Expansive agents and surface tension-reducing components can be included in accelerators. ASTM C494 dictates the maximum allowable amount of shrinkage and water reduction caused by the use of accelerating admixtures.
Sompura said accelerators affect the set time, but don&#;t necessarily affect the strength. Precasters should also be aware of how much time will elapse between mixing and placing the concrete, how the concrete will be transported to the forms, ambient temperature and the mix water temperature. Some precasters use hot water to increase the rate at which the hydration reactions occur. In some cases, if approximately 30 minutes will elapse between mixing and casting, a set accelerator could make the concrete too stiff to place. Instead, an accelerator that doesn&#;t take effect for the first half hour after mixing should be used.
Combinations of different accelerators mixed in manufacturer-specified ratios could also be used, depending on job specifications and how all the raw materials and chemicals interact. It is important to test different products in test batches to determine which combinations work best with the materials being used.
How accelerators advance precasters&#; work in the plant
According to Mark Celebuski, partner at Trinic, a manufacturer supplying powdered accelerating admixtures, precasters often use accelerators to offset the delaying effects of other additives. Some common additives like water-reducing admixtures or supplementary cementitious materials can decelerate the set of concrete. Using accelerators helps hydration occur at an elevated rate and increases the rate of set and strength gain. Precasters can also use accelerators to turn forms faster, which translates into greater efficiency and a reduction in total days and labor costs for production.
Debbie Sniderman is an engineer and CEO of VI Ventures LLC, an engineering consulting company.
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