Implementing SCC: Part 1

By Sue McCraven

Editor’s Note: This condensed, two-part overview of self-consolidating concrete (SCC) is based on a recent NPCA White Paper. Part 1 explains SCC characteristics, aggregates and equipment, plus a cost/benefit analysis of SCC compared with conventional concrete. Part 2, to be published in the March-April issue of Precast Inc., will guide the precaster in SCC test methods and troubleshooting. Parts 1 and 2 are designed to highlight the critical advice of industry experts to assist precast concrete producers who are using SCC for the first time.

If you’ve been following the three-part series in Precast Inc. about diversifying and growing your company, you may be planning to modernize your plant for the greater production efficiencies of self-consolidating concrete (SCC).

If so, this article provides the answers you need to the following questions:

1. What makes SCC a “different animal” compared with conventional concrete?
2. Can I produce SCC with my existing mixer and forms?
3. If I do need to purchase more equipment to use SCC, which already costs more than conventional concrete, how can it possibly save me money?
4. What do industry experts say about SCC?

What is SCC, and what can it offer me?

SCC was first developed by Japanese precasters more than 30 years ago as a means to produce concrete with less labor due to a serious shortage of qualified workers. Therefore, SCC’s primary benefit is the significantly reduced need for labor. Unlike conventional concrete, SCC does not require vibration. SCC flows like lava to fill forms on its own, even for structures with a lot of reinforcing steel and intricate forming. During pouring, SCC’s large aggregates remain uniformly distributed (no settling).

The three material attributes of SCC are:

1. Flowability: Fills form spaces with no external assistance
2. Passing ability: Flows through rebar and around obstructions
3. Stability: Large aggregates stay evenly mixed throughout production

The production advantages of SCC are:

• Less labor: Vibration and the noise it creates are eliminated, along with the cost of vibration equipment
• Increased safety: no walking on forms and reinforcement steel with hand-held vibrators)
• Decreased form maintenance
• Improved product finish and less rework
• Reduced electrical bills

The way to find out if SCC is viable in your plant – and the only way – is to give it a try. Run some trial batches of SCC with your crew and products and see what happens. There’s a pretty steep learning curve for SCC, so be prepared to give your staff enough time to experiment and get a good feel for using the material. Do trials with flat slabs and structures with complex reinforcement.

While the crew sizes up this new experience, do a cost/benefit analysis of SCC versus conventional concrete. Be forewarned, SCC mixes can be temperamental – even delivery methods (ready-mix truck or bucket) must be addressed in mix design.

SCC experts recommend a plant-wide meeting to present your SCC initiative and the particular vagaries of SCC production. Host lunch-and-learn presentations for your crew with your admixture supplier or industry experts on what to expect with SCC.

Learn from the pros

Consider the following advice from industry experts in the areas of aggregates, cement, admixtures, equipment, QC and mix trials:

Aggregate sources and grading: critical to SCC performance

• Coarse aggregates retained on a 200 sieve/pan may be advantageous
• Light aggregates can be challenging; closely monitor air content
• Grading of fine aggregate may deviate from the norm
• Coarse aggregate volumes may need to be altered due to SCC’s increased paste requirements
• A higher percentage of fines retained on a 200 sieve/pan may be advantageous
• When using >50% fines (by aggregate volume), hardened shrinkage testing may be required
• Software programs are available for optimum aggregate grading
• Choosing the cheapest source of aggregates is a common mistake
• Trial batching is the only way to find optimum aggregate grading
• Follow ASTM except for fines gradation(i)
• Ensure that there is no crossover contamination between aggregate storage bins
• When in doubt, consult with an SCC software/mix-control expert

Cement & supplementary cementitious materials (SCMs)

• Follow ASTM standards, DOT specs (some states require AASHTO M35) for all SCMs
• Paste volume and cementitious particle size may affect SCC
• Re-qualify mix if cement source or cement characteristics change
• Blended cements and variable SCM particle sizes and shapes aid SCC mix stability
• Choose low-carbon fly ash to avoid bleeding and air-content fluctuation
• Grade 120 slag is preferred by producers due to its high reactivity
• Silica fume can contribute significant paste and mix stability
• When in doubt, call in an a professional concrete mix specialist

Admixtures

• Due to the viscosity of the paste, SCC is particularly vulnerable to small changes in the amount and character of SCM
• High Range Water Reducers (HRWR) are essential to SCC’s flowability and consolidation; polycarboxylates are highly recommended
• Potent HRWR are part of mix water (w/c) and require accurate metering; overdosing may cause surface bubbles (“champagne effect”) and aggregate segregation
• Air-entraining admixtures improve workability, flowability and reduce bleeding
• Calcium chloride accelerators are not recommended as they can degrade reinforcing and promote drying shrinkage
• Corrosion inhibitors can delay accelerate set time and cause rapid loss of spread
• HRWR plus corrosion inhibitors can cause water reduction in the mix
• Viscosity Modifying Admixtures (VMA) bind water into the concrete matrix; VMA overdosing can cause incomplete self-compaction, sticky finishing and bug holes
• Intricate formwork and heavy reinforcing requires a less viscous mix to ensure complete coverage and consolidation
• When in doubt, call your admixture supplier for technical advice

SCC equipment and placement

• Mixers used for conventional concrete can usually handle SCC
• Forms may need alteration because for lower viscosity
• mixes — formwork gaps may leak paste (blowouts), resulting in honeycombing and air pockets
• Hydrostatic (outward) pressure may be higher than that of conventional concrete and additional bracing for wooden and semi-rigid forms may be required
• Pouring too rapidly may cause entrapped air and bug holes; pouring too slowly (especially for more viscous mixes) may lead to insufficient head pressure, decreasing SCC’s ability to fully compact in the form
• Proper timing of pour is important to ensure that HRWR effects are not diminished
• Concrete conveyors and chutes must have tight dispensing gates, both out of concern for safety as well as loss of paste
• If your concrete delivery bucket traverses rough ground, post-transport testing is required to ensure proper elastic properties (not required of ready-mix truck transport)
• Typical vibration and finishing equipment is not needed for most SCC mixes
• Over-vibration causes: course aggregates to sink (segregate), loss of entrained air, and paste and bleed water to rise to the surface
• Finishing may be delayed by small entrapped air bubbles; likewise, screeding may be delayed (or eliminated entirely)
• Just like conventional concrete, SCC’s surface will crust if exposed to outside wind and heat, leading to finishing problems; use an evaporation retardant where needed
• Note that any delays in placement, as with conventional concrete, can have a profound effect on SCC
• When in doubt, call your equipment suppliers for expert advice and assistance

QC: increased testing, calibration and monitoring

• Carefully review and record technical data in mill test reports; record fineness (Blaine) when qualifying mixes
• Establish SCC mix design parameters for slump flow and air content
• Set minimum frequency for calibration of scales, meters, admixture dispensers and probes
• Increase minimum testing frequency for aggregates, gradation, and moisture content
• Increase minimum testing frequency of fresh and hardened concrete
• Designate the person who will decide if and when inadequate SCC mixes should be discarded; ideally, the responsible person has experience with SCC, or – at a minimum – a good understanding of SCC characteristics, how numerous factors affect SCC quality and troubleshooting knowledge
• It is critical that the designated control person be present during all stages of SCC production, and this person must have total control over batching operations (good communication and mutual SCC understanding with mix-control and aggregate-storage personnel)
• Establish post-casting inspections for paste leakage, dimensional integrity, static-mix segregation, air content, foaming and surface bleeding
• It is important to note: SCC is generally more sensitive to water variations. For this reason alone, automated moisture controls are strongly recommended.

For specific information on moisture control systems for SCC production, see the sidebar “New Tools for SCC Consistency – A Case Study.”

The SCC linchpin: mix trials

• Mix trials should be performed with multiple sources of raw materials
• The cheapest materials can be the most costly in the long run
• Always use actual plant mixer and equipment in mix trials
• Trials may reveal any reaction between SCC head pressure and form release agents
• Early trial batches in the lab may minimize material waste and labor

Perform a cost/benefit analysis of conventional concrete versus SCC

Precasters know that the No. 1 and most costly resource in the production of precast concrete is labor. The process efficiency of workers and staff is just as critical for small- to medium-sized producers as it is for the largest precast concrete conglomerates.

Perform a time study at your plant to determine the amount of time spent placing, consolidating, floating and patching a conventional-mix product. Record the time (minutes per cubic yard). Then, using a matrix similar to Table 1, calculate your labor cost per cubic yard. The labor rate is an average of the hourly rate (including overtime) plus the cost of health benefits, profit sharing and pension plans. Any step in the production process that does not add value to the product must be identified and eliminated.

The second direct manufacturing cost is materials. Tables 2 and 4 illustrate a theoretical cost/benefit evaluation of material costs of conventional concrete versus SCC.

Increased costs of using each type of concrete are highlighted in red, while cost savings are highlighted in green.

The total cost (labor + materials) of producing conventional precast concrete in the theoretical examples shown in Tables 1 and 2 is $63.46 per cubic yard.

Once you know the costs associated with the production of conventional concrete, use the same procedure to determine the cost for producing SCC. A trial SCC production run is necessary to obtain actual time-study data. Time estimates from another precast plant will not accurately reflect the efficiencies – or lack thereof – of your crew and equipment.

The total cost (labor + materials) of SCC in the theoretical examples shown in Tables 3 and 4 is $58.29/cy, resulting in a theoretical savings ($63.46/cu yd – $58.29/cu yd) of $5.17/cu yd.

Remember that the costs and savings realized in this example are based on hypothetical data and actual savings are specific to each plant. A savings of more than $5/cu yd is a substantial sum for a plant producing 50, 75 or 100 cu yds of concrete per day, and savings can be realized even for a smaller operation with an output of 10 cu yd per day or less.

Adjustments in workforce staffing hours probably will need to be addressed in order for any savings to be realized. A complete overhaul of post-cast handling may be needed. If patching is no longer a standard practice with SCC, staging areas and secondary handling may be eliminated. These changes can have huge implications on the bottom line, but only if staff adjustments are recognized and acted upon. Sales staff, particularly, must be cognizant of lower SCC production costs for success in bidding and winning new work.

Additional SCC savings

Reduced labor and material costs are not the only financial savings offered by SCC. Other savings include reduced cost for:

• Electric power for consolidation
• Purchase and maintenance of vibratory equipment
• Concrete waste due to overfilling of forms (SCC is self-leveling)
• Housekeeping (plant cleanliness) due to spilled concrete
• Form repair needed for damage during demolding
• Health issues and injuries (including lost time and replacement training) related to climbing on forms and reinforcing to haul and operate vibrators, and tripping over electrical cords and air hoses
• Health issues relating to hearing loss from high-decibel vibratory noise

As you can see, there are a great many advantages of using SCC, and a lot of precautions that go along with them. Is SCC right for you? There is only one way to find out, and that’s to try it. Arm yourself with information, and expect some failures along the way, but you may find, as other precasters have, that little failures can lead to big successes.

Sue McCraven, NPCA technical consultant and Precast Inc. technical editor, is a civil and environmental engineer.

Watch for Part 2 of Implementing SCC in the March-April issue of Precast Inc. to learn about SCC testing protocol and troubleshooting.

Sidebar – New Tools for SCC Consistency – A Case Study

By Wayne Faulkner and Rob Piosik

Self-consolidating concrete (SCC) provides the concrete producer with improved product quality at reduced costs. Like most production enhancements, however, improvements do not occur without some investment. Many concrete producers have already discovered that producing SCC is not a walk in the park. Factors related to mix design proportions, aggregate gradations and moisture control require tight product specifications if consistent SCC quality is to be achieved from batch to batch.

SCC success depends on controlled aggregate moisture content. New moisture probe technology allows the precaster to control total water in the mix.

The first requirement for consistent SCC is a consistent yield of the raw materials. Your goal should be to achieve consistent aggregate material yields by measuring the moisture as the material flows across the probe during weigh up and then correcting the required weight target in real time. This comprehensive process delivers the exact material amount as specified in the mix design.

In addition, your process can be set to measure all of the sources of moisture content in the mixer before intake of cement, including a final measurement of blended aggregate, silica, admixtures and color. Total measurement of moisture content of all the “dry” aggregate moisture includes any rinse water left from the last batch or other foreign sources of water.

Foreign water sources could be rainwater on a material transfer belt during mixer charging or unmelted ice in the material. Often small amounts of ice will melt during the aggressive dry-mixing time. Armed with accurate data on total water, producers are able – for the first time – to produce consistent SCC spreads.

This same technology accommodates high-absorption aggregates. Prior to SCC innovations, the producer’s only option was to constantly sprinkle the aggregate with water, hoping that doing so would produce an aggregate at Saturated Surface Dry (SSD) conditions. However, there was no way to ensure SSD conditions as material moved through the plant, was stored overnight in bins, or exposed to hot or windy conditions. The solution is to introduce a small amount of water during the dry-aggregate mixing cycle to ensure the aggregate reaches at least the SSD state.

Another important improvement in the new SCC toolset further reduces errors in moisture measurement. Using sensor physics, the precaster can specify a mix calibration based on dry-aggregate volume (cubic yards) in the mixer. Additional tools for SCC include the ability to graph and store the mix moisture information with the batch weights. The graph produced during batching has proven to be an invaluable tool for QC analysis of the concrete quality. These tools allow producers to monitor moisture, temperature, air content, wattage draw and other relevant sensors, and plot this data in real time with the final batch report for storage.

These technological improvements in SCC production control systems give precasters the advantage of being able to request SCC concrete at the touch of a button.

Wayne Faulkner is the products plant specialist for Command Alkon and has more than 30 years of experience in engineering and building concrete batch plants. He may be reached at [email protected].

Rob Piosik is the global CP delivery manager for Command Alkon and began his career in concrete plant automation 24 years ago as a support engineer. He may be reached at [email protected].

Endnotes

(i) Fine aggregate for use in SCC should conform to ASTM C33 with the exception of gradation requirements. Grading may deviate to achieve a more ideal grading curve when blending with other aggregates to develop a robust SCC design. Variations in the fineness modulus (FM) should not deviate from the qualification design by more than +/- 0.20