Working With SCC Needn’t Be Hit or Miss

Admixture experts and precasters provide important tips to work successfully with self-consolidating concrete (SCC).

By Sue McCraven

Some precasters report that working with self-consolidating concrete (SCC) can be a “hit or miss” affair: sometimes everything goes right and at other times SCC mixes set up too fast.

Or worse, the dreaded root beer float appears, indicating a badly segregated mix. Why do things go wrong with SCC, and what remedies can precasters use to achieve consistently good results? In this article are five SCC case studies from precasters; technical advice from several admixture suppliers; and the best practical SCC tips from an industry expert.

Case study 1: SCC speeds tank production

“It’s a wonderful product to speed production,” claims J. Robert Puddicombe, Winona Concrete & Pipe Products Ltd., Grimsby, Ontario. Puddicombe, who has been using SCC to produce septic tanks since 2004, shares his tips:

1. “Moisture control. If you don’t have moisture control – a moisture sensor or probe in the aggregates and preferably in the mixer – you don’t want to be near SCC. For example, if your sand has a high percentage of retained water that is not compensated, the SCC mix can produce a creamy, bubbly paste at the top like a root beer float, indicating your mix has segregated.”
2. “A good mix design will have proper aggregate gradation, water-to-cement-to-admixture ratios. Have the admix company prepare an initial mix design. Give them your materials – aggregates, cement used, strength needed, percentage air – and then have them prepare your mix for the ideal SCC for your product.”
3. “Expect stiff resistance from seasoned staff who are used to pouring concrete the traditional way. Guys with 20 or 30 years of day-in, day-out ordinary production under their belts may find SCC a difficult transition. You’re essentially throwing a monkey wrench in the works.”
4. “Expect a big positive change in your plant: it will be quiet. There will be no noise except that from the mixer. No vibrator noise. No rushing at 4:30 p.m. to finish up and maybe missing vibration in spots. Guys can talk with one another. No tripping. Floors are clean and staff is not so tired at the end of the day. Guys love it.”
5. “SCC delivers a much denser, stronger product with a smoother finish. Your customer is paying around $20,000 for a septic system. Precast customers deserve a No. 1, Grade-A product that looks as great as it is structurally sound.”

“With SCC,” says Puddicombe, an NPCA member and the technical chair of CPA (Concrete Precasters Association) of Ontario, “we consistently produce a much denser product,”. “We never have missed consolidation, and our septic tanks have fewer bug holes. A smoother, denser surface means that septic tanks are more impervious and resistant to corrosion from biogenic sulfuric acid attack from bacteria byproducts produced inside the tanks.”

Puddicombe uses a VMA (viscosity modifying admixture), a critical agent that thickens up the water in the SCC and stabilizes the mix.

Case Study 2: Don’t give up on SCC if you experience initial problems

“We have used SCC since the Salt Lake City Precast Show in 2003. We love it!” says Bill Bundschuh, president of PRETECH Corp. of Kansas City, Kan. “Vibrators (for placing ordinary portland cement) wear out and require repair, and with SCC you don’t need vibrators.” Bundschuh’s lessons learned:

1. “Don’t rush it; let it flow on its own.”
2. “Don’t use vibrators to move along the SCC mix; that will only serve to work the paste to the top and the aggregates to the bottom for a non-uniform mix.”
3. “Place SCC from one spot in the form and one spot only. SCC is not like ordinary concrete. If you try to place it in two or three spots at once, air pockets will form.”
4. “SCC is about $20 per cubic yard more expensive than ordinary concrete, but it is worth the cost in the savings in labor, vibrator repair and safety. You won’t have guys lugging heavy stinger vibrators all over the forms, and stepping over air hoses. Injuries associated with vibrators just don’t happen with SCC.”
5. “Getting your staff to realize they’re working with a ‘new animal’ is the biggest challenge with SCC. Placement of SCC is different from anything they are used to, and it will take time for them to catch on to new techniques. We tried SCC initially then shelved it, like many precasters. SCC requires persistence, a lot of training and a lot of reminders to the workers to forget vibrators and forget placing the mix in more than one place at once.”
6. “You’ll get fantastic concrete strength with SCC. We routinely get 4,000 psi in 24 hours. We use a lot of cement and a lot of slag in our products (box culverts, sanitary products, stormwater products, utility vaults).”
7. “You can’t use SCC in forms that are cut down on one side, because SCC will seek its own level.”
8. “SCC sets up really fast once the chemical (superplasticizer) dissipates. The SCC coats the concrete particles and allows the mix to move really fast.”

Case Study 3: First SCC consideration: temperature

As a professional engineer and manager of quality assurance for The Fort Miller Co. Inc. in New York, Ira Adler lists temperature as the most important factor in producing good SCC.

“Located 30 miles north of Albany, we experience wide temperature swings – both seasonally and day-to-day. Temperatures range from over 90 degrees (F) in the summer to 15 degrees below freezing in winter.” Adler heats concrete mix water to 180 F to produce SCC at a mix temperature of 80 F. Even a drop of 10 degrees can require a change in the admixture dosing rate. The colder the temperature, the less SCC admixture is needed; warmer temperatures require the addition of admixture.

“We batch anywhere from 150 to 200 cubic yards of concrete every day, and we turn our placing beds once a day,” says Adler. “We use SCC in all our products (from median barriers, bridges and burial vaults to manholes, catch basins and pavement slabs), with the exception of forms that are cast on a slope. We like to achieve the maximum in SCC characteristics in our finished products, so we use the right amounts of polycarboxylates.”

For Adler, maximum SCC characteristics mean minimum labor to place concrete, quieter and safer working conditions for staff, and excellent finish quality.
Adler stresses the aspects of working successfully with SCC:

1. “There is no alternative to spending a considerable amount of time working with SCC materials to gain the experience necessary for success in production and finished product.” While many SCC practitioners recommend a mix heavy in sand, Adler manufactures product with exposed aggregates in the finished surface in SCC mixes that use 68 percent stone aggregate (of total aggregate volume).
2. “High daily production volume requires SCC.”
3. “There’s really no substitute for an admixture supply representative who has solid experience in the precast industry, who understands from hands-on work what the precaster requires to produce consistently high-quality product with SCC.”
4. “There is no shortcut for the precaster seeking to become competent at production with SCC than to undertake trial batches.”
5. “There are so many resources out there to get you started on the right track with SCC. PCA’s “Design and Control of Concrete Mixtures” to proportioning concrete is a good place to start. I use a traditional textbook approach to designing SCC mixes using the absolute volume method. We use a uniformly graded mix with three-eighths inch stone typical of New York State DOT requirements and check the FM (Fineness Modulus) of our mix against those recommended by ACI mix proportion guide 211.1. We usually pick a water-to-cementitious ratio (w/cm) of 0.30 to 0.35 and dose with superplasticizer for a 26-inch spread, plus or minus 2 inches. Then we look at the workability of the forms for the job, ensuring that the designed mix is adequate for the production setup.”
6. “SCC means a quieter, safer workplace. This not only is important in terms of liability and insurance, but it is the basis for attracting and retaining quality personnel.”
7. “When it comes to a quality concrete finish, precasters can’t afford not to use SCC.” As an example, Adler recalls that at one time the plant employed six to eight workers per day sack-rubbing or sealing bugholes with grout on the sides of architectural retaining walls after form release. “With SCC, we have reduced staff required to two people working for one-half day to rub our finishes. SCC means significantly lower labor costs and a much improved finish quality for precast products.”

An excellent example of the greater ease of placement possible with SCC is shown in the production of a precast pedestrian box culvert bridge, according to Adler. A typical segment is 50 feet long, 10 feet wide and has 8-inch-thick roof, walls and base, and a core of Styrofoam. “To avoid a ‘glass-bottomed boat’ (where an incorrect mix consistency results in a trapped air bubble, or mixture void, in the form’s bottom), we use SCC,” says Adler. “By placing SCC into the form at the side, the SCC flows down, across and up the form and through the heavy reinforcing, eliminating voids.”

Case Study 4: Brewing up a root beer float

“SCC can really get away from you,” says Chris Matson of Dura-Kast Products LLC, Springfield, Mo. “For us it’s really been a hit-or-miss experience over the last year or so working with SCC.” Matson describes the root beer float effect, when SCC foams up at the top. “You can end up with a poor product if things go wrong, and there are so many variables in an SCC mix that have to be just right. But when SCC is right, it could be used in every product; there’s less labor, less waste, a cleaner shop, a better-looking product.”

Because some precasters have had a hit-or-miss experience with SCC, producers like Matson are looking for that magic formula that works every time, like the perfect cake recipe.

Case Study 5: Supplier must get his hands dirty
Armen Alajian of Arto Brick California Pavers in Gardena, Calif., says the important thing for him is to reduce rejects, reduce labor and have consistent, repeatable results with SCC.

“The one thing for precasters to know is that the relationship they have with their admixture supplier will determine their success or failure with SCC,” stresses Alajian. It is the admixture representative who will provide staff training with SCC and help perfect the mix design to work in a specific product. The timing of SCC introduction to the mix is crucial to success.

“The admix guys you work with must be willing to get their hands dirty,” he says. For Arto Brick, the smooth, almost defect-free surface produced with SCC is the goal for many of their architectural products.

Admixture supply experts uniformly agree that SCC should work right every time if the precaster understands the many factors that affect SCC and, secondly, if the precaster uses the correct dosage in a good mix design. The following admixture suppliers provide SCC advice from a material technology perspective:

BASF: SCC quality control: a must

“As with any consistent mix design, quality control is important in SCC production,” explains Kenneth Kruse, director of industry development for BASF Construction Chemicals. Publications such as ACI 237 R 07 outline the proper process in formulating a concrete mix design that will be suitable for your application.

“SCC use was initially adopted by the precast/prestressed producer because of the need for increasing production capacity without the need for additional capital,” explains Kruse. “Approximately 35 percent of structural precast/prestressed is manufactured using SCC producing a variety of structural components including bridge girders, double tees as well as components for parking garages. There is good reason, however, to use SCC to manufacture utility precast elements such as box culverts, utility structures, burial vaults and septic tanks.” Kruse advises the following:

1. “Procedures and controls are key. Quality control/assurance programs are critical for producing SCC precast products with consistently good results.”
2. The precaster must have a good understanding of local material properties. Cement, fly ash, and coarse and fine aggregates – each play an important role in quality SCC production. The producer must control aggregate moisture content.
3. “SCC benefits your labor force because SCC does not require vibrating forms or working with hand vibrators. Facilities that use SCC are much quieter. SCC eliminates not only the noise but minimizes the need for air pressure hoses, which are tripping hazards. Plant safety is increased.”

Sika: SCC moisture control is #1

The biggest issue precasters have with SCC, according to Bruce Strickland, national marketing manager for Sika, is moisture control. Fine aggregates must have good moisture control to work in SCC mixes. “Too much water in the mix is usually the biggest problem for precasters who have inconsistent results with SCC,” explains Strickland. Too much water is what causes the root beer float (too much polycarboxylate or superplasticizer causes bubbling). Here are Strickland’s pointers:

1. “Always involve your admixture supplier and ask what you should expect of SCC. What performance can be achieved? Do hands-on SCC trial mixes with your admixture supplier and production staff.”
2. “Use proper aggregate gradation.”
3. “Determine the slump flow (30- or 20-inch diameter) needed to produce a product. Less slump generally translates into higher-strength concrete.”
4. “When there are gaps in coarse or fine gradation, water demand goes up; those gaps are filled with cement paste.” What can the precaster do if he is working with gap-graded aggregates? A fallback step is to use a Viscosity Modifying Admixture (VMA) to bind up the excess mix water. “I don’t routinely use VMAs—only as a last resort.”

“The key to successful SCC use is to start with the right mix design. Use an appropriate sand-to-aggregate ratio and an adequate water-to-cement ratio. I start with 10 cubic feet of cement paste by the absolute volume, which contains three elements: water, cement and air.

To proportion the aggregates, begin with a 50:50 sand-to-aggregate ratio based on absolute volume. From this point, I use a computer program to fine-tune the SCC mix by plugging in the screen sizes for the fine and coarse aggregate for a combined FM. Using a graph from a Sika computer program, I then adjust the sand-to-aggregate ratio to obtain the best gradation with the selected aggregates.

“You must adequately ‘wet out’ the SCC mix.” Proper mixing time is necessary for the polycarboxylate admixtures to work effectively; if the polycarboxylate admixture is added at the end of the process, it may not adequately disperse, or reach what is called a “wet out” in the mix. The superplasticizer must have time to sufficiently disperse into the concrete mixture before it is dumped into the forms.

“If you add superplasticizer in the production trim water, realize that you can short yourself on admixture. Superplasticizer in the production trim water to adjust slump can cause problems in SCC mixes. For example, say that there has been a heavy rain the night before; there won’t be need for much trim water, and the first SCC batch can potentially be shorted on admixture. Then the second batch of SCC receives the regular dispensed amount of admixture in addition to the admixture left over in the trim water line from the first batch. You now have an overdose of admixture in the second batch.” It is therefore important for the precaster to ensure that all of the required admixture ends up in the mix and is not left in the water line.

“An ounce of image is worth a pound of performance. Precast product with a beautiful, smooth, bughole-free finish speaks volumes for the pride and quality control practice of the producer. While it is true that underground utility structures, septic tanks, pipe and manholes are not any less structurally sound because of bugholes, the company name stamped on a smooth, high-quality precast finish is advertising with immediate results.”

General Resource Technology: Listen

Denny Krizan of General Resource Technology is on the road, working daily with precasters ro perfect SCC mixes that produce consistent results. “The most important thing for precasters to do is to listen,” says Krizan. “Precasters are sometimes set in their ways of working with mixes and need to learn that SCC is a different animal.” SCC is a more expensive mix but needs to be used correctly to obtain consistent results. Typically an SCC mix will contain more than 700 pounds of cementitious material per cubic yard. Krizan recommends a half-inch top size (maximum size) aggregate for successful SCC mixes.

Why you can’t afford not to use SCC

“I wouldn’t use anything else.” “SCC is wonderful.” “The guys love working in a quiet, clean shop where they can talk with one another in a normal tone of voice.” “It creates a safer job site.” “It’s just so much faster.” “We never have missed spots.” “We get a consistent product every time.” “SCC gives us a Grade-A product every time.” These quotes are from the precasters, not the admixture suppliers.

It is clear from the admixture manufacturers and precasters that inconsistent results with SCC can be remedied using the advice and tips provided here. For first-time users, these lessons learned from their peers can supply the needed confidence to begin using SCC to produce high-quality precast products in a much improved work environment. SCC has many advantages, from faster production to a denser product with a smoother finish – important competitive assets for building a stronger, smarter industry.

Sue McCraven, NPCA senior technical consultant, is a civil engineer, technical writer and editor, and environmental scientist who has contributed numerous articles and studies to prominent scientific journals.

BASF’s Joe Daczko: Don’t expect a cookie-cutter recipe for SCC

According to Joe Daczko, product manager at BASF, precasters need to determine what they need. SCC needs to be appropriate to the application: “What performance is needed, and by performance, I mean the finished product. There are no simple recipes. There are a wide range of mix designs with various cementitious and aggregate contents. There is no simple ‘cookie-cutter’ recipe for SCC – no single recipe that works in every application.”

Daczko explains that mix materials vary from region to region. “Materials are different: If I go to Texas, Florida or Boston, precasters use different ingredients, different cements and different aggregates. This is why we can’t expect a high-performing SCC concrete in each of these different areas to use the same cookie-cutter recipe. The development of the correct mixture, however, can be achieved for each circumstance.” During that development process, the concept of mixture robustness must be evaluated, he says. “Robustness refers to the ability of the concrete mixture to withstand slight fluctuations in material properties, such as in gradation and moisture. Certain proprietary software developed by BASF allows for this development of specific and robust SCC mixtures.”

Precaster needs to be engaged

There must be a technical transfer of SCC wisdom to the precaster’s employees, says Daczko. “The precaster needs to be engaged in the process. If we want to make concrete better, if we want to improve performance, the precaster must be engaged in the process. Working successfully with SCC means that we meet a certain standard, that we train to do it well. Just like a person trains for a marathon, a precaster must train to use SCC. There is no simple cure or single recipe in applying new technology like SCC, there’s no cheating to get to the finish line. At the same time, success with SCC is possible to do as can be seen by the great number of producers successfully applying this technology.”

Approximately 35 percent of precast/prestressed production uses SCC, and it is the leaders in the precast industry who have embraced the technology. Why? “The primary motivator for the precast/prestressed producer was initially the labor savings per piece manufactured. The number of non-structural precasters who use SCC is not known. The precaster must have his eyes wide open in dealing with SCC and must also have appropriate expectations of the product. SCC is the poster child for performance concrete,” he says.

“If I am casting a manhole cover that is 12 inches thick, for example, or a jail cell with walls that are 3 to 4 inches thick and 12 to 14 feet high, I need to ask what performance I want for this product using SCC. What do I want, and is it appropriate? Another example would be to take 5,000 psi strength concrete; would the quality control for producing 5,000 to 6,000 psi strength concrete be the same as the QC to produce 30,000 psi concrete?

Do you need 28-inch or 30-inch slump SCC concrete for your product? What slump flow do you require, and why? What mix stability do you need? What fluidity do you need?”

Other SCC benefits

One of the reasons precasters use SCC has nothing to do with saving labor costs, safer work conditions, a quieter work place or ease of placement – it has to do with pride, says Daczko.

“When finished surfaces on precast products look like glass, the product speaks a new message to customers: ‘Here is quality control, here is finishing expertise, here is a precaster who takes great pride in his or her product.’ Even for engineers, who know a bug-holed surface does not necessarily reflect on inherent concrete strength, a beautifully cast precast product implies structural quality and process control.”

Your finished product will reveal whatever is on the inside of your forms. Because SCC has such high fluidity, every mark, every imperfection on the inside surface of the form will be revealed in the finished product. Different strategies are used depending on whether you are using a 20-inch-diameter or 30-inch-diameter spread SCC mix. “Working with SCC is not like working with a traditional 5,000 psi strength portland cement mix. It’s a different animal. I use the analogy of the difference between a hammer (as the typical portland cement mix) and the nail gun (the SCC mix): A mistake with a hammer might result in a sore thumb, but a mistake with a nail gun might be much more painful. You have to understand the tool that you are using. SCC is a tool that must be used appropriately to deliver consistent success for the precaster.”

Placement timing and production techniques

Placement techniques are critical to successful SCC products. “For example, say a precaster wants to cast jail cells; one needs to look at the time lag between successive batches of mix that will go into the form. If the time lag is too great, the SCC may not flow with consistent hydraulic head into the forms, and a dark line on the finished surface may result. In this way, it is easy to understand that SCC requires not only knowledge of how the admixture technology works, but also operational technology to make sure all the facets of production are in sync to produce consistently good results.”

Aggregate moisture can be a killer

“Variations in the moisture content of aggregates can create significant variability in SCC production. For example, say it rains all night and the aggregate stockpile in the precast yard is wet. If aggregates bring the amount of water in the SCC mix above what is optimum, the spread will exceed the design parameters and the aggregate can settle in the forms.”

Sidebar: Mel Marshall’s Practical SCC Tips

By Mel Marshall

Self-Consolidating Concrete (SCC) was developed in Japan in 1987 and although it took time to reach North America, this amazing product has rapidly gained acceptance. In 1983, only about 8 percent of all precast products used SCC, but by 2005 that number had increased to 33 percent. Currently, approximately 50 percent of all precast products are manufactured from SCC.

What is SCC?

An official SCC definition from Khayat, Hu and Monty, is “A highly flowable, yet stable concrete than can spread readily into place and fill the formwork without undergoing any consolidation and without significant separation.” SCC is the most exciting and beneficial development in concrete in decades, but seems to be a product that folks either love or hate.

Many precasters have abandoned SCC early on because of unpleasant experiences. SCC is different from conventional concrete and requires that particular attention be paid to the development of mix designs. It is imperative that precasters work closely with their admixture supplier, who will ensure a mix design that is suitable for their specific applications.

When first introduced in North America, SCC was recommended only for larger products such as jail cells, but has since proved to be very effective for manholes, septic tanks and a variety of other products.

Useful tips for achieving success with SCC mixes are:

1. Moisture control: Since SCC mixes are extremely sensitive to even slight changes in moisture content, one needs to utilize moisture probes in the aggregate bins (particularly sand bins) and moisture sensors in the mixer. Batches that are too dry will set up very quickly and ones that are too wet will segregate and foam up (root beer float). Don’t even try to batch SCC unless you have mix moisture content control!
2. Aggregate gradation: SCC mixes are also very sensitive to changes in aggregate gradation, so it is imperative to do a regular sieve analysis to ensure gradation consistency. Remember that a change in gradation (aggregates sizes) changes the total surface area of the aggregates that needs to be surrounded with the paste (CSH gel that “glues” particles together). If the total aggregate surface area is increased, there will not be enough glue, and if the total surface area is reduced, there will be too much glue.
3. Temperature control: Excessive aggregate temperatures can create problems. The hot summer sun will remove absorbed moisture for the aggregates, and the resulting aggregate moisture content will fall below SSD (Saturated Surface Dry). When placed in the mixer, these dry aggregates will absorb some of the mix water and dry out the batch, causing it to set too quickly. A simple solution is for the precaster is to use a lawn sprinkler system to wet down and cool the aggregates.
4. Fly ash: The addition of up to 20 percent fly ash improves the stability of SCC mixes.
5. Mixing time: Keep in mind that SCC batches take longer to mix because of the additional mix time required to ensure proper disbursement of the admixture throughout the batch.
6. Form design: Forms utilized for SCC need to be designed for full hydraulic pressure as an SCC pressure head is similar to that of water: the pressure on the forms is much higher than with that of conventional concrete. SCC batches will distort poorly designed forms.
7. Pour location: When fabricating a septic tank or manhole base in the inverted position (with the base at the top), avoid pouring directly onto the top (doing so can cause the release agent to wash off and result in the set concrete sticking to the form). For these products, the recommended practice is to pour into the wall element and allow the SCC flow up the sides and over the top.
8. Lines between pours: It is not uncommon to observe a black line in the sides where two pours meet – especially if there has been a delay between pours. This dark line can be eliminated by simply tapping the side of the form with a rubber mallet – no metal hammers, please.
9. Let it flow: When pouring SCC mixes, pour in only one location, and let the concrete flow throughout the form under its own force. Pouring in different locations will result in excessive bug holes.
10. Bug holes: Pouring SCC too quickly is a common cause of bug holes.
11. Voids: Accelerating curing before initial setting of the concrete can increase the number of large voids.
12. Admixture supplier: Probably the most important consideration for achieving successful SCC mixes is the need to work closely with the admixture supplier. Some admixture companies supply the polycarboxylate admixture separately from the Viscosity Modifying Agent (VMA) admixture, while others formulate one admixture that already includes a blend of both the polycarboxylate and VMA. When changing admixture suppliers, precasters will be starting from scratch with the new supplier, because suppliers don’t all use the same admixture recipes.

SCC is the future, so why not climb on board?

Those who have not enjoyed success with SCC mixes really owe it their organization to take a second look. There is absolutely no doubt that within five to 10 years, virtually all precast products will be manufactured with SCC mixes.

Mel Marshall, P.Eng., a longtime educator and principal of Mel C. Marshall Consultants, has more than 40 years of experience in the precast industry.