A Fiber-reinforced Concrete Testing Primer for Ready Mixers (Part 1)
March 18, 2015
In this and the next issue of Ready Mixer News Memo we will provide a general primer for ready mixers as well as engineers, contractors and commercial laboratory personnel. Part 1 in this issue includes:
- Sampling of Fiber Reinforced Concrete (FRC)/Fiber Reinforced Shotcrete (FRS)
- Fabrication of FRC/FRS test specimens.
Part 2 in the next issue will cover:
- Testing FRC/FRS specimens
- References to sources with more detailed information.
For ready mixers to ensure that the product delivered to each and every one of their projects is sampled properly and specimens are fabricated and tested properly, they must first fully understand the importance of these three critical areas. And, of course, the FRC’s performance tests must meet with local DOT or other codes/specifications or with ASTM or other national or international consensus procedures.
In most cases the ready mixer employs a Quality Control Manager. The Quality Control (QC) Manager is charged with making certain that all concrete leaving the plant meets the appropriate specification(s). To protect the interests of the ready mixer, it is the responsibility of the QC Manager to monitor the sampling, fabricating of specimens and testing of the concrete on each project. Without this policing of project inspectors and commercial laboratory technicians the potential exists for the concrete test data generated for a given project to be rejected, ultimately resulting in non-payment of the concrete.
It is, therefore, mandatory that the ready mixer protect his interests by providing personnel who can monitor field and commercial laboratory work affecting the concrete produced. The American Concrete Institute (ACI) offers schools that enhance the concrete knowledge of the ready mixer’s personnel when it comes to sampling, fabrication of specimens and testing of specimens. These programs can support the ready mixer’s efforts and deserve consideration.
Part 1 – Proper FRC Sampling and Fabrication
Correctly Sampling and Testing the FRC/FRS Is Absolutely Critical.
If the FRC sample-gathering process, test specimen fabrication and testing are not conducted per standard consensus practice the test results may not accurately reflect the parent FRC. If the finished product is not approved, this could result in significant penalties for the contractor, ready mixer and vendor.
First, Let’s Define Sample:
A sample is a representative portion of the parent concrete or shotcrete generally recovered at the point of discharge from the mixing vessel at the jobsite. The ASTM International Practice for Sampling Freshly Mixed Concrete is C172.
IMPORTANT NOTES for Sampling and Fabricating FRC/FRS Test Specimens
In determining the properties of hardened Fiber Reinforced Concrete (FRC) as well as Fiber Reinforced Shotcrete (FRS), attention to accuracy is paramount. It’s imperative to review both these and the following FRS-specific notes prior to sampling and fabricating FRC/FRS test specimens.
- One of the concerns with the three-dimensional/randomly distributed fibers in the concrete is that modifications in the orientation of the fibers may occur when the FRC/FRS is placed and consolidated in the mold. The FRC industry now considers it very critical how the FRC is placed in both the cylinder and beam molds. New thinking has brought a major modification to the standard practice in filling both cylinder and beam molds. The concern is specific to fiber orientation; by using standard cylinder and beam fabrication for plain concrete the fiber orientation in the concrete can be reoriented. Thus the fabrication of FRC test specimens requires a single introduction of the FRC to the mold with no multiple layers or the use of a rod to compact the FRC.
Modifying the fibers’ orientation can result in test results that are not representative of the parent FRC sampled. Thus the cylinders and/or beams fabricated could produce results that do not meet the required specifications
ASTM, ACI and FRCA have eliminated one of the major culprits by discontinuing the use of the tamping rod to internally consolidate the FRC. ACI Committee 544 states that beams and cylinders shall be consolidated with external energy, like a rubber mallet or vibrating table. Here’s the wording:
For concrete cylinders: Place fiber reinforced concrete in the mold and use the standard rubber mallet or another comparable instrument to strike the exterior of the mold to consolidate the concrete. A vibrating table may also be used to consolidate the FRC. The reasoning behind this amendment focused on consolidating the concrete is to ensure that the “natural” fiber orientation is not modified in the cross-section, which could potentially alter the test results.
For concrete beams: Recommended practice proposes that a single dumping of the FRC into the mold is needed to ensure that the fiber orientation is not disrupted.
DO NOT modify fiber orientation in test cylinders or beams.
- Paramount to this whole process is ensuring that the fibers have been thoroughly distributed within the concrete or shotcrete matrix during mixing
- It is important that the samples be representative of the fresh fiber reinforced concrete/shotcrete. A key point: the sampling and fabrication of specimens for laboratory prepared mixes as well as concrete plant or field prepared mixes is considered part of this discussion. Both ASTM C192 (laboratory) and ASTM C31 (field) practices for making and curing test specimens relate to this discussion. The use of the modified fabrication and compaction process for FRC cylinder and beam specimens applies to both.
- When macrosynthetic fibers, steel fibers or blends, are specified, to optimize the mix and to verify that all of the required engineering properties are achieved, it is mandatory that all of the ingredients approved for use on a given project be included in the trial mix.
- It is important to achieve consistency in the following processes:
- Sampling the parent material
- Preparation of the test specimens
- Handling and aging of the test specimens
- Testing of the test specimens.
If any change or modification in the above processes, procedures and/or practices occurs, the resultant deviation in test results can be costly! This includes changing the technician who extracts the samples and/or fabricates the specimens or conducts the tests.
Additional Concrete-reinforced Shotcrete-specific Notes:
- When testing FRS samples for compression strength, flexural strength and post-first crack, testing must be recovered from shotcrete test panels containing the parent material. The orientation of the ingredients, including the fibers, is different in a shotcrete-applied concrete than in a cast-in-place concrete.
Therefore, to secure a true representative sample of the FRS, panels must be shot. Specifications can call for either or both vertical or overhead shot test panels. Test specimens are then sawed from the test panels.
- When shooting test panels with FRS, the actual equipment and a certifiednozzle man (certification is mandatory) must be part of the process.
When shooting test panels with Fiber Reinforced Shotcrete (FRS), the actual equipment and a certified nozzle man must be part of the process.
Variables in the Sampling, Fabrication of Specimens and Testing that Can Affect Results
- Changing the point at which the samples are extracted from the parent material can produce deviations in results produced; for example, taking some samples at the ready mix plant and then sampling at the project site.
- Changing the field or laboratory technician who takes the sample and/or fabricates the test specimens and/or tests the specimens can produce variations in the results.
- Changes in any of the ingredients in the mix, like the cement or admixtures, typically should require additional trial mixes. The cement compression strength can vary from silo to silo within a single vendor source. Variations can be much more pronounced if the cement’s vendor source is changed. This is also true for the coarse and fine aggregate.
- Sampling procedures are detailed and intended to be followed exactly step by step. Having the correct tools and molds is imperative.
- Fabrication of test specimens should follow the uniform practice established in the appropriate ASTM International standard, including the size and construction of the molds.
- Handling and storage of test specimens must be uniform to reduce variations in results. Field practice requires that test specimens be properly stored to ensure no loss of moisture or significant temperature swings. Test specimens must be removed and placed in the laboratory for storage within 24 to 48 hours after they have been cast.
- Follow the exact procedures for removing hardened concrete test specimens from storage and preparing specimens for testing. CAUTION! DO NOT ALLOW the test specimens to dry out.
- Review the ASTM Standard Specification for Moist Cabinets, Moist Rooms and Water Storage Tanks Used in the Testing of Hydraulic Cements and Concretes … ASTM C511.
The above notes and cautions reinforce the critical need for absolute consistency throughout the testing program. It is very easy to introduce erroneous data when details are ignored.
We in the Fiber Reinforced Concrete industry believe that all parties are responsible in the process of securing accurate test results. Therefore all parties should be educated as to the proper procedures related to the process.
Need help to ensure that your FRC sampling and specimen fabrication are being performed properly?