Precasters must consider order when establishing proper mix designs
Precasters spend plenty of time understanding the materials that go into a concrete mix design. But what about the order in which those materials are added to the mix? Sequencing, or the order precasters use when introducing components into a concrete mix, is critical. Proper sequencing can help admixtures work, save wear and tear on mixers, compensate for mixer age or performance, help avoid problems such as balling or inhomogeneous mixing, and allow producers to attain the desired final properties for their products.
Tailor to the mixer
The most important factor affecting sequencing is the type of mixer used. According to Paul Ramsburg, technical sales specialist at Sika Corp., precasters use a variety of mixers but most are central, stationary, batch-type mixers located at the plant. At computer-controlled central mix plants, initial blending occurs on conveyor belts and mixing occurs on one batch while the next one is simultaneously created.
The number and type of mixing cycles in a sequence specify how long it takes to introduce different materials into the mix. Mixing times will vary depending on the mixer, batch size, admixture recommendations, and the plant’s basic philosophy about wet and dry cycles during batching.
The order materials are introduced into the mixer and the energy of mixing determine concrete’s microstructure and performance, as well as mixing methods and mixer conditions. Mixing order affects efficiency, and every mixer and mixing method has advantages and disadvantages.
Batching affects sequence
Batching, or measuring raw ingredients before mixing by weight, volume or both, is another important factor.
“Sometimes, weight and proportions change based on sequencing choices made during batching,” Ramsburg said. “So mixes should be designed to be batched. Since sequencing affects optimizing the mix design, they should go hand in hand.”
Frank Bowen, director of quality assurance at Piedmont Precast in Atlanta, noted that batch size matters. For larger, 2-yard batches in their pan mixers, Piedmont creates a dry mix first and then adds the wet ingredients. But when making small test batches less than 2 cubic feet – where a 2-yard mixer doesn’t effectively produce conclusive results – the company changes the mixing order. Using small shear mixers, Piedmont starts with about 80% of the wet ingredients and then adds the dry ingredients.
Most common sequences and rules of thumb
The most common sequence is an initial dry mix cycle followed by a final wet mix cycle. First, coarse and fine aggregates are checked for moisture content, weighed and introduced into the mixer. If an air-entraining admixture is necessary, it is introduced with the aggregates. Then, cementitious ingredients are weighed and added to the mix. In the most common mix designs, the initial dry mix cycle blends a homogenous mixture in about 30 seconds.
Ramsburg said dry cycle time depends directly on mixer efficiency. Brand new, aggressive pan mixers can batch quickly, but it can take more than 1 1/2 minutes in older mixers with reduced efficiency. Producers must determine the timing for themselves.
Timing also depends on aggregate type. Some are more easily blended during the first mix, allowing short dry cycle times. Complicated mixes may need longer initial cycles. All are specific to the producer.
After the initial mixing cycle, other admixtures and the remaining water are added and mixed a second time. The wet cycle’s final mix time depends on the efficiency of the mixer and what the precaster wants from the mix.
“Sometimes, rash decisions are made during the final cycle to speed up production or quicken the batch,” Ramsburg said. “The concrete may look the same when it comes down the chute after trimming thirty seconds of the final mix, but the slump, strength and initial set will be impacted.”
Bowen added that the mixer manufacturer will typically provide sequence information, but other parties may assist as well.
“Admix companies and other suppliers with knowledge about local ingredients will provide input,” he said. “If using an old or handed-down mixer, analyze it to see its capabilities and if it is appropriate for the goals.”
At Piedmont, wet materials are introduced very slowly into the most agitated part of the mix.
“In our planetary counter-current mixers with counterrotating arms, that spot is at the dead center of the mixer,” Bowen said. “We add water slowly at the most violent part in the middle to distribute it evenly and avoid clumping. We add plasticizer and churn the final mix cycle for around one minute.
“Twin-shaft mixers and counter-rotating pan mixers from other manufacturers may use different sequences, especially if they have proprietary differences like more mixing paddles or different rotation speeds that impact the sequence or times.”
Sequencing and admixtures
Admixtures are the most precise part of the mix design. If not sequenced properly, they could have no effect, or worse, their period of effectiveness may occur too early or too late, which could be detrimental to the concrete. Proper sequencing, storage and use information – as well as known incompatibilities – are listed on technical data sheets that precasters should request from manufacturers. Because they have the largest and most immediate impact if mixed incorrectly, there are some rules of thumb to keep in mind.
Reactive air-entraining admixtures should be introduced into the mix separately without touching other admixtures. They are discharged onto the aggregates or entered into the mix water early, and their doses may change if they are used in cold weather or with hot water. Ramsburg suggested deciding if adjustments are necessary using the second batch of the day instead of making conclusions immediately after the first batch.
Water-reducing admixtures are added after the cement is wetted. The earlier they are added, the longer the mix will hold slump and the easier it will be to distribute throughout the mix, but more chemicals are necessary. If added later, the mix won’t hold slump as long, but lower doses can be used, making them more cost-effective.
In the southern United States, retarding admixtures are used throughout the year. The earlier retarders are added to a batch, the more they impact and extend the initial set time. Adding retarders later affects the slump life more and set time less.
Liquid pigments are typically introduced at the beginning of the final cycle with other admixtures. Powders or granular pigments are introduced at different times.
Compensating for weather with sequence changes
Changing climates may affect the amount of air in the concrete, so doses of air-entraining agents may need to change to achieve targets. Typically, they are put on the sand, but some put them in the mixer. According to Ramsburg, sequencing air-entraining agents differently may help move the air around to achieve more consistent concrete.
Some plants use retarders and accelerators in hot weather. Ramsburg warned not to add both into the mixer at the same time. Whether the accelerators go into the end of the batch or up front may change throughout the season.
When it’s time to change the sequence
If there’s a problem in the mix, such as clumping or balling, there’s likely an issue with sequencing.
Cement and water can clump together and form solid balls that float to the surface and don’t mix thoroughly with the aggregate. They prevent the cement from interacting with the rest of the mix and cause inconsistent mixing, lower strength and aesthetic issues. The clumps can form if the cement is added too early or if free moisture in any exposed aggregates isn’t compensated for on a wet day. If compensating for free moisture doesn’t solve clumping problems, a sequence change could help. Adding superplasticizers before the mix is well blended isolates and highlights the balls. Adding it too late can also create balls.
When the mix is inhomogeneous or if there’s foaming, bubbles or segregation, it’s a good time to investigate a sequence change. Aesthetics and engineering properties will vary if the sequence is changed, so changes should be made systematically and scientifically.
Ramsburg explained that sequences are typically well established, so any changes should be made conservatively, one at a time.
“Document the mix before the change, and test and document after the change,” he said. “Test multiple points from four or five samples in three or four batches to verify results and analyze the effect on strength.”
Taking the necessary steps
Ramsburg stressed that precasters should consider sequencing as part of the mix design and add the concept to their existing design processes. Many producers include sequences on their mix design printouts. He added that companies should allocate resources, including people and processes, to understanding sequencing.
“If a company has a great system now, dedicate some QC time to document what the sequence is,” he said. “Several years from now, when a new QC manager takes over or if any questions arise, a process document exists.”
If you’re just getting started with sequencing, mixer manufacturers – who have literature with recommendations – are a good place to begin. Cement and admixture companies are also great resources. Even though sequencing is viewed as equipment-specific, it impacts everyone’s role and the concrete’s performance.
Debbie Sniderman is an engineer and CEO of VI Ventures LLC, an engineering consulting company
Resource:
C. Ferraris, “Concrete Mixing Methods and Concrete Mixers: State of the Art”, Journal of Research of the National Institute of Standards and Technology, Vol. 106, No. 2, 391-399, March-April 2001. fire.nist.gov/bfrlpubs/build01/PDF/b01012.pdf