GUIDELINES FOR APPROVING AND TESTING
SCC MIX DESIGNS FOR FIELD PLACED CONCRETE
A. Develop and provide self consolidating concrete (SCC) for cast in place structural concrete. SCC is defined as a concrete mix that provides the following:
· Filling ability to flow and fill completely spaces within formwork, under its own weight.
· Passing ability to flow through tight spaces between reinforcement without segregation or blocking.
· Ability to resist segregation by remaining homogenous during transport and placement.
Typically aggregates are well graded with a maximum top size of ¾” or less. Aggregate angularity and shape can affect the slump flow. Typical sand to aggregate ratio is 0.40 to 0.50. Paste volume can range from 28 to 40% depending on slump flow required.
Water to cementitious ratio is typically in the 0.25 to 0.44 range.
If the producer has no previous experience with SCC, it is recommended that a technical representative of the admixture company be present during initial trial batches.
Meet the requirements of Division 41 for the appropriate materials and the following:
· Use a high range water reducer (HRWR) from Material IM 403 Appendix D.
· When a viscosity modifying admixture (VMA) is used, manufacturer shall provide documentation indicating compatibility with HRWR.
· Use maximum nominal aggregate size no larger than one third the minimum clear spacing between reinforcing steel
· Maximum w/c ratio of 0.45
· Minimum cementitious content shall be 624 pounds per cubic yard
· When required to maintain plasticity during a placement, use a retarding admixture or hydration stabilizer.
Mix designs will be approved by the District Materials Engineer (DME). New mix designs for SCC shall be verified through trial batches. Other mix designs will be qualified by previous performance. Field validation shall be required for all new mixes.
· Work with the admixture supplier representative to develop the mix design
· Slump flow in accordance with Materials IM 389. The target slump flow value is 23.0 inches. The allowable tolerance range of the slump flow is plus or minus 3 inches. The contractor may submit a target slump flow, if placement requires different flow characteristics.
· Target Visual Stability Index (VSI) in accordance with Materials IM 389. The VSI Rating shall not exceed 1.0.
· Passing ability by J-Ring in accordance with ASTM C 1621. Calculate the difference between slump flow and J-Ring flow. The maximum allowable difference is 2 inches.
· Static segregation using hardened cylinders in accordance with Materials IM 390
Producer shall submit material sources, proportions, individual gradations of each aggregate, combined aggregate gradation, slump flow, visual stability index, air content, and compressive strength for the proposed mix design.
Trial Batch Validation
1. Allow the District Materials Engineer ample opportunity to witness the trial batching. Provide the District Materials Engineer notice and mix proportions 7 calendar days prior to this event.
2. Mix the trial batch with a minimum of 3 cubic yards at least 30 calendar days prior to planned placement. Establish the batching sequence of the materials during the trial batch.
3. Transport the concrete a distance comparable to the distance from the ready mix plant to the placement site.
4. Test concrete samples that are representative of the entire batch for air content, slump flow, visual stability index, J-Ring, density (unit weight), static segregation and temperature. Cast specimens from each sample for compressive strength tests. Modify the consolidation method of all materials test procedures, including IM 315, Materials IM 316, Materials IM 318, and Materials IM 340 by placing the concrete in the molds in one layer without vibration or tamping.
5. Cast a minimum of eight 4 inch by 8 inch cylinders for testing. Trial batch concrete will be tested for strength and static segregation. All samples will be cast, cured, and handled according to Materials Materials IM 315.
6. Strength samples will be stripped of their molds and wet cured until their break age. Strength samples will be tested according to AASHTO T 22. Three cylinders will be tested for strength at each age of 7 and 28 days. The District Materials Engineer may witness the strength testing. The samples for static segregation may be sent to the Central Materials Laboratory for sawing.
7. Submit a trial batch report to the District Materials Engineer no later than 7 calendar days after trial batching. Approval will be based on successful trial batch mixing and properties. The District Materials Engineer may waive the trial batch testing provided satisfactory mix properties have been achieved through testing of previous trial batches or production placements.
Quality Control Plan
Submit for approval a written Quality Control Plan describing the procedures to be used to control the production and placement of SCC. Submit the Quality Control Plan at least 30 calendar days before the first intended structural concrete placement. Do not place structural concrete before receiving written approval from the engineer of the Quality Control Plan and having all equipment and materials necessary to facilitate the plan on site and ready for use.
Include the following in the Quality Control Plan:
· Develop mix design that meets the design criteria for strength, flowability, passing ability, and consistency.
· Define concrete batching sequence, mixing time , and minimum revolutions to prevent cement balls and mix foaming. Include procedures for ensuring wash water is removed before batching.
· Define concrete placement pattern and methods. Include maximum horizontal flow distance from point of discharge.
· Describe additional quality control procedures at the plant to ensure consistent delivery of concrete.
· Define field procedures to accept or reject concrete during production.
· Describe procedures used when continuous placements are interrupted.
· Other information as needed.
Provide stability analysis of proposed formwork for full static pressure and proposed methods used to prevent leakage.
Deliver concrete without any interruption of flow such that a continuous placement is achieved. Deposit concrete continuously or in horizontal layers of such thickness that no new concrete will be placed on concrete that has hardened enough to cause seams or planes of weakness. Do not exceed 30 minutes between placement of successive batches unless engineer has reviewed placement conditions. If a section cannot be placed continuously, provide construction joints as specified.
If deemed necessary by the Engineer, construct a mock-up of the section to verify placement procedures.
Do not re-temper SCC.
Do not vibrate SCC without permission of the Engineer. If Engineer approves vibration, maximum insertion time is 2 seconds or less. If emergency delay occurs, concrete may be rodded with a piece of lumber or conduit if the material has lost its fluidity prior to placement of additional concrete. The DME may approve other methods of consolidation, if necessary.
Validate drop distance to demonstrate that separation does not occur.
Notify the Engineer 48 hours prior to placement of production concrete. Use only approved SCC mixes for production concrete. Ensure mix has the same materials, proportions, and properties established in the trial batch.
Perform air content testing in accordance with Materials Materials IM 318, except place SCC in one layer, without consolidation or tapping. Cast cylinders in accordance with Materials IM 315, except place SCC in one layer, without consolidation or tapping.
The Engineer will perform air content testing at sampling at testing rate described in IM 204. The contractor will perform quality control testing of slump flow in accordance with IM 389 at rate of 1/30 cubic yards. Slump flow range shall be ±2 inches of the mix design target value. The visual stability index shall not exceed 1. If the slump flow exceeds the range up to a maximum of 28 inches, the concrete may be placed provided the visual stability index does not exceed 1. The producer will make adjustments to move the slump flow back into range.
The District Materials Engineer will obtain verification strength samples on a minimum of two random placements. Strength samples will be tested at the District Materials Laboratory according to AASHTO T 22. A set of five cylinders will be cast, cured, and handled according to Materials IM 315. Three cylinders will be tested for strength at 28 days. The remaining two cylinders will be checked for static segregation of hardened cylinders in accordance with Materials IM 390.
Since SCC mixes are highly sensitive to moisture, the Producer should perform aggregate moistures at a minimum of once per day prior to mixing. The DME may adjust moisture testing depending on weather conditions and aggregate storage.