By Sue McCraven
Editor’s Note: Part 1 of this two-part series covered self-consolidating concrete (SCC) characteristics, aggregates, equipment and instructions for conducting a plant-specific cost/benefit analysis (Precast Inc. Jan/Feb 2014). Part 2 details test methods and troubleshooting based on a recent NPCA White Paper from SCC industry experts. Armed with this knowledge, you can confidently transition to higher-quality SCC products, cut labor costs and successfully compete for greater market share.
After reading “Implementing SCC – Part 1” in the last issue of Precast Inc., you understand the main advantages of self-consolidating concrete (SCC) and its sensitive nature, and you are prepared for typical SCC production challenges. You’ve calculated SCC’s labor and energy savings and reviewed the capacity of your current equipment to handle the transition to SCC. Industry experts explained SCC’s moisture-content sensitivity and its dependence on the source, type and grading of aggregates and cement. Now, if you are ready to take your operation beyond the familiarity of conventional precast concrete products, two remaining topics are presented here in Part 2: SCC test methods and troubleshooting.
Test methods to qualify an SCC mix
SCC Part 1 identified the three material characteristic specific to SCC as flowability, passing ability and mix stability. SCC tests measure these characteristics. Some SCC tests are approved – or are in the process of approval – by ASTM, and yet others are methods used in the precast concrete industry but not yet officially recognized. Here are the six most common SCC tests used in the United States:
- Slump flow is the form-filling ability of the mix. Measurement results of the concrete spread from an Abram’s cone onto a level, nonabsorptive surface may range from 20 in. to about 30 in. The larger the circular spread, the greater the flowability. Target slump flow for a given SCC mix formulation can vary by ± 2 in.
- VSI stands for visible stability index and is a subjective visual determination of the general relative mix stability based on the appearance of the slump flow. An SCC-experienced plant operator validates VSI. A VSI < 2 is generally acceptable(i). With experience, the assigned plant operator will gain sufficient skill in evaluating mix stability.
- J-RING is a slump flow test that uses a circular steel ring with downward-facing radial tines or spokes; the ring is set around the bottom of an Abram’s cone. Once the cone is lifted, the sample’s dispersion between the tines is meant to simulate reinforcement and demonstrate how larger aggregates may restrict mix flow during placement. A difference of ≤ 2 in. between two J-RING tests is acceptable. The J-RING and slump flow tests are usually performed at the same time.
- L-BOX is an open-topped, L-shaped (vertical and horizontal sections) apparatus with a movable gate, all of predetermined dimensions. SCC is poured into the vertical section, the gate is opened, and the fresh concrete’s flow rate and slope are measured. Product-specific reinforcing can be added beyond the gate to assess any blockage resistance that may be encountered when filling forms. As you may expect, VSI and L-Box appraisals are subjective and results vary between plant operators.
- Static segregation (ASTM C1712) is a quick and easy assessment of the likelihood that segregation of an SCC concrete will occur. (See the sidebar “ASTM Standards Relating to This Article” for the titles of each standard.)
- Column segregation is a static test that measures aggregate segregation of SCC mixtures at rest. A sample from the mass of concrete placed in form work (not concrete from the bottom of a bucket or wheelbarrow) is placed in a vertical tube constructed of three lengths. After a set time, the top portion is sieved to isolate, weigh and record the coarse aggregate. The middle tube portion is discarded. The remaining aggregate in the third section is then weighed. The relationship between data from the top tube to that from the bottom tube is an indication of the static stability of the mix, provided that the difference in aggregates from the two sections ≤12%.
ASTM test methods altered to accommodate SCC
In addition to the six tests discussed, existing ASTM test methods for conventional concrete must be modified, or altered, to accommodate the material properties of SCC. (NOTE: These alterations all require one lift, no rodding and only slight tapping of the sample container or cylinder mold.) These SCC-specific changes are an ongoing discussion at the national level. For the present, these are the five ASTM test methods with this specific alteration for SCC mixes:
- ASTM C31
- ASTM C138
- ASTM C173
- ASTM C192
- ASTM C231
SCC test methods
The following fresh concrete testing standards for SCC production are typically considered as mandatory testing requirements:
- Slump flow (ASTM C1611)
- VSI (ASTM C1611)
- Temperature (ASTM C1064)
- Air content (ASTM C231 and C173 – amended)
- Unit wt./yield (ASTM C138 – amended)
- Compression specimens (ASTM C31 – amended)
- Compressive strength (ASTM C39)
Hardened testing, depending on project-specific applications, may include the following ASTM tests:
- Freeze-thaw resistance (ASTM C666)
- Chloride permeability (ASTM C1202)
- Shrinkage (ASTM C157)
- Hardened air analysis (ASTM C457).
Staff training determines SCC success
If you have followed the experts’ advice in SCC Part 1, you know that a successful transition to SCC requires more than exacting moisture probes and mix-control systems. Staff must be thoroughly trained in the correct methods to mix, test, place and finish SCC products.
In addition to an SCC-trained and experienced batch operator, who must be in total control of SCC operations, all plant staff should soon be able to recognize and address problems quickly in fresh and hardened SCC. SCC is more moisture-sensitive than conventional concrete, and its material idiosyncrasies require more attention to detail and testing throughout production.
After initial placement, all staff – particularly the production crew – should understand the five attributes of sound SCC:
- Coarse aggregate at or near the top surface
- Stable paste with no air percolation, or “champagne effect”
- No bleed water
- Absence of foaming at form edges
- Complete compaction in forms with heavy, intricate reinforcing and/or blockouts (tapping or light vibration is a possible consideration)
Testing frequencies and final product inspections will likely increase, at least until everyone is SCC-knowledgeable and mixtures are consistent and stable over time. In early stages of production, weekly meetings will address staff questions and provide a means to gauge workforce progress. To capture the savings of reduced labor for SCC, foremen must anticipate and address man-hour changes.
Because production staff has a more intimate relationship with fresh concrete, they must be able to spot poor concrete right out of the mixer or during placement, and quickly alert the plant operator for evaluation and system adjustments. Workers also need to know when certain SCC mix designs may lead to slump flows over or under the prescribed limits. To avoid defective products, vibration may be necessary in situations where SCC has stiffened up or has lost flow prior to or during placement.
Test batches, water fluctuations and mock-ups
Test batches. It is important to run a series of test batches prior to full implementation of SCC. Test batches should be as large as your standard batch to ensure that the tests can be duplicated. Start trial production runs with SCC in conjunction with plant training and presentations from admixture suppliers.
Before beginning trial runs with SCC, make sure that your aggregate suppliers are consistent with their sieve analyses. Sieve analyses must fall within ASTM C33 guidelines.Aggregates and bin maintenance are critical to SCC as discussed in Part 1. Never mix stockpiles from different aggregate producers. Also check the mechanical ability of your mixer and set up a maintenance schedule for adjusting blades and scrapers before running trials.
SCC test batching helps train staff and reveals optimal plant-specific procedures, problems, required adjustments and other information, including:
- Optimal sequencing of admixtures
- Computer timing adjustments
- Mixer cleanliness and required maintenance or alterations
- Optimal charging point to the mixer
- Best mix timing for HRWR admixtures
- Timing sequences for ideal mix stability and consistency
- Presence of concrete balling
- Low compressive strength
- Poor hardening performance
- Seasonal changes in raw materials and aggregate moisture content
- Opportunity to call in an admixture specialist to help retool the mix design or recommend SCC monitoring equipment and software fixes
- Experience needed for successful SCC production
- Necessary workforce reduction due to less post-cast handling and finishing
- Operational savings in labor, production efficiencies and energy costs
- Plant-specific data for a realistic cost/benefit analysis of conventional concrete versus SCC
Moisture variation testing. After attaining consistency in fresh concrete test results, monitor – on a daily basis – all standard raw materials, equipment alterations, performance and any potential source of moisture fluctuation that may be detrimental to the stability of the SCC mix design. Because local aggregates vary and mixers and equipment conditions are specific to each plant, the only way to monitor moisture fluctuation is to run a trial SCC batch and test for water sensitivity.
Begin with an SCC mix batched to the designed water-to-cement ratio and determine its slump flow and VSI. Then add water in progressing amounts and continue to perform slump flow and VSI tests until the mix becomes unstable and the VSI result exceeds the allowable limits. In this way, you can determine how any day-to-day fluctuations in raw materials and equipment performance affect SCC moisture content in typical production conditions. If the allowable SCC test parameters are exceeded, you know that the mix design, aggregates or plant equipment (or all three) requires modification or adjustments.
NOTE: Industry experts stress that SCC mixes are sensitive to water variation from any source and therefore recommend the use of automated moisture control.
Mock-ups. The idea behind a mock-up is to simulate the final manufactured precast concrete product. Mock-ups can be a scaled-down or full-size version of the final product, but they must mirror the specified formwork, height and finish requirements, including form finish. Mock-ups provide important and specific information, including:
- Hydrostatic pressure of the SCC mix
- Additional formwork bracing and whalers needed
- Potential for blowouts
- Ability to meet specified dimensional tolerances
- Best placement point for desired product finish
- Potential for trapped air and honeycombing
- Opportunity to test form oils for potential SCC reactions (bug holes, staining and ease of demolding) and EPA and local VOC compliance
The precast SCC plant operator must address any problems that occur in production and identify the root of any trouble in the production and placement process. There are five SCC problem categories:
1. Material changes – Whenever any suspicion of a problem with SCC arises, first check aggregate stockpiles and bins for compliance with ASTM C33. Cement supplies should also be checked, including the type purchased. Any change in cement supplier, fineness or chemical properties can affect the quality and consistency of SCC. Constant communication between the plant operator and the individual(s) responsible for aggregate stockpiles/bins is imperative.
2. Batching operations – SCC is susceptible to any inconsistency in batching and mixing operations. Changes in discharge settings or sequence can alter the entrained air content, reduce slump flow, or contribute to segregation potential of the concrete. Here are some requirements and precautions:
- Calibrate and monitor aggregate scales, cement scales, moisture probes, admixture dispensers and water dispensers to ensure accuracy of mix proportions
- Keep an eye on discharge settings and sequence, as they must remain consistently correct
- Adhere to prescribed mixing times
- Mix HRWRs for the appropriate length of time to ensure proper dispersion throughout the SCC matrix
- Avoid short batching, as it will result in reduction of slump flow and inconsistent flows within the same batch and restrict full self-consolidation
- Watch for lack of thorough consolidation, as it will decrease SCC strength and increase porosity of the finished product
3. Moisture control – Most problems associated with SCC are caused by moisture fluctuations during the course of daily production. Here are some tips on how to avoid them:
- Use automated adjustments for moisture control; probes are recommended
- Use regular testing for slump flow and VSI to alert the plant operator to moisture variations in the absence of automated controls
- Monitor excess moisture, as it decreases SCC mix stability
- Watch for severe mix instability indicators: “champagne effect,” foaming, bleeding and segregation
4. QA procedures – Increased testing, calibration and monitoring were covered in SCC Part 1.
5. Pouring operations – Take heed of these precautions:
- Intricate reinforcing requires a less viscous SCC mix
- Placement timing is critical to ensure HRWR effects are not diminished
- Pouring too rapidly can lead to bug holes by entrapping air on the finished surface of the forms
- Pouring too slowly may decrease head pressure and SCC’s ability to fully compact
- As with conventional concrete, SCC should not be poured from excessive heights, as this leads to mix segregation
- Mix qualification testing should include average placement time of a given form to evaluate performance
- Even though SCC is known for not requiring vibration, forms with heavy reinforcing or blockouts may require light vibration or tapping for full compaction
See the “Potential Problems and Remedies” table as a quick reference chart for using SCC.
5 main points to remember
With SCC Part 1 and 2 articles, you are now informed, armed and prepared to take on SCC to sharpen your competitive edge. If you have decided to transition your company to SCC production, remember these five points:
- Your peers in the industry advise you to expect a steep SCC learning curve.
- Trial runs and thorough staff training in all aspects of SCC are critical.
- Obtain SCC technical advice from you admixture supplier.
- SCC delivers significant production savings.
- Yes, SCC is the smart way to go, but don’t forget that it is a “finicky” material requiring exacting material and production controls.
Sue McCraven, NPCA technical consultant and Precast Inc. technical editor, is a civil and environmental engineer.
(i) VSI Level 0 is a homogeneous mass with no evidence of bleeding; Level 1 concrete shows slight bleeding or surface sheen; Level 2 concrete has a mortar halo plus water sheen; and Level 3 concrete reveals course aggregate in the center plus a mortar halo.
(ii) Thixotropy is the property exhibited by certain gels of becoming fluid when stirred or shaken and returning to the semisolid state upon standing.