The testing methods for UHPC concrete are different than testing for traditional concrete. It’s important for testing facilities and contractors to understand UHPC fabrication and testing regulatory standards before they embark on a project.
Contractors know that tests will confirm that their mix was done correctly and that it is compliant with design specifications. Test results can make or break a project, so it’s vital to have a well-planned testing program to complement diligent construction.
Proper sample collection, laboratory certifications, and analysis are essential to this process. Because tests are performed after the concrete has already been poured, it raises the stakes for bad test results. Inadequate results may indicate the need for significant rework including demolition, removal, and replacement of newly installed concrete.
Testing Typical Concrete vs. UHPC
Most construction groups and laboratories are familiar with concrete strength testing. Standard concrete tests are usually performed using ASTM C39 and related preparation procedures. At a basic level, the routine is to collect samples during the field pour, submit them to a familiar lab, and run the usual cylinder tests at key intervals (days 7 and 28 are common). Labs run these tests regularly, but not all ASTM methods used for conventional concrete apply directly to UHPC.
ASTM C1856 is the industry standard for UHPC testing, requiring specific certification, sample preservation, and analysis. This test method is applicable to mixes with a compressive strength of at least 120 MPa (17,000 psi).
The sample preparation and testing procedures are similar to those for common concrete but with some key differences. Typical concrete tests would fail to account for the steel fibers and lower water content in the UHPC matrix. Curing and sample preservation might be compromised, in addition to the timing and methods of testing. UHPC projects should include education for field crews and also laboratory confirmation that the proper tests are being performed under the right circumstances.
To ensure proper testing, facilities should obtain the proper UHPC testing accreditation by undergoing the days-long process of proofing to the correct standards. ASTM C1856 is a unique accreditation, not just a simple addition to common concrete testing. A manufacturer’s representative will also help with the process to confirm that the testing agency understands the material and procedures.
“Fresh” Property Measurements
Prior to submitting material to a lab, some mix characteristics should be documented on the spot. Measuring fresh UHPC properties helps catch any fundamental oversights in the mixing procedure prior to a full schedule of lab work.
Field-flow testing is performed by filling a slump cone mold with a standard volume, lifting the mold, and allowing the mixture to slump for two-plus minutes. The mix will flow outward and its diameter signifies the test result. Notably, the mold should not be tamped and the table should not be raised and lowered as might be done with traditional concrete.
Other “fresh” properties such as setting time might be also requested by an engineer, though additional tests may not be required. Contractors should always document mix ratios, procedures, and observations to ensure the field conditions are well understood.
The next step in the process is to make and test hardened UHPC specimens. Contractors submit discrete samples for testing to demonstrate compliance with design specifications. The fate of the material used on site will depend on these results.
Test specimens are collected in cylindrical molds and rectangular prisms. Molds should be filled in one continuous layer. Tamping rods and internal vibrators should not be used in fabricating and consolidating specimens from UHPC. This is a key difference. Tamping and vibrating normally performed with common concrete could compromise the fiber distribution in this case.
The filled molds need to cure in appropriate temperature and moisture conditions. Typical concrete has enough water to allow for some time to get samples covered and protected from drying out.
However, UHPC contains less water, so it must be covered within one minute of finishing the sample surface and/or preserved in a lime water bath. Specimens must be stationary (i.e., not transported) for a full day and thermally treated after demolding. The temperature and duration of curing are dependent on the type of fibers within the UHPC mixture.
Lab Testing Program
ASTM C1856 includes modifications to a typical list of concrete laboratory tests. The specific program applied for a given project or mix design may depend on the engineer’s or manufacturer’s requirements. An overview of the tests is provided below.
- Compressive Strength – Surfaces of the cylinders must be ground so that the ends achieve a virtually exact plane without the use of capping compounds or unbonded neoprene pads. Axial loading is applied to 3” x 6” cylinders from either end until various points of failure occur. Diameter measurements and the load rate/testing time are adjusted from that of standard concrete testing using ASTM C39. An initial peak strength will be determined when the cement matrix gives way, but the tensile strength of the UHPC fibers will prolong the test and performance of the sample until a final failure occurs.
- Flexural Strength – Prism beam specimens are used for flexural strength tests, in which third-point loading is applied until first-peak, peak, and residual strengths are determined. This property is enhanced by UHPC’s fiber reinforcement. The length and alignment of fibers have significant effects on the material and corresponding test results. Larger or smaller prism samples should be used depending on fiber size.
- Static Modulus of Elasticity and Poisson’s Ratio – Cylindrical specimens are analyzed by recording their deformation and change in diameter under applied stress. Compressometers and extensometers are used in conjunction with highly accurate gauges and a modified loading rate from typical concrete testing.
- Creep in Compression – Samples are loaded into a frame that applies a load (not more than 40 percent of a given compressive strength). Strain measurement devices are used to take readings immediately before and after loading and at increasing intervals afterward to observe the creep characteristics of the sample under continuous compressive loading.
- Length Change – Volumetric expansion or contraction can occur from causes other than applied force or temperature change, which can introduce permeability or cracking. Length change testing assesses this risk by measuring strain in concrete due to shrinkage or expansion. A comparator device provides length measurements, complemented by an atmometer used to regulate moisture conditions.
- Resistance to Abrasion – The wear resistance of UHPC wear resistance is measured by mounting the sample to an abrasion device (such as a drill press) with a rotating cutter device that applies a constant grinding action. The average loss in sample mass and depth of wear on its surface can be measured to gauge the material’s ability to withstand abrasive forces over a project’s lifecycle.
- Resistance to Freezing and Thawing – Rectangular specimens undergo 300 freeze-thaw cycles in a controlled chamber. The procedure is done in fully submerged conditions or by freezing in air and thawing in water. Cement in UHPC mixes can potentially become more hydrated by the moisture conditions of the test, resulting in a change to its modulus of elasticity.
- Penetration of Chloride Ions – Common concrete is evaluated by measuring the amount of electrical current passed through a sample specimen. The total charge passed in coulombs is related to the resistance of the specimen to chloride ion penetration. However, if the UHPC mixture includes metallic fibers, this test method is not applicable because the metal fibers can short circuit and result in invalid results. An alternative permeability test should be sought instead.
Reporting and Quality Control
Reports will be issued at separate intervals, depending on the types of tests and requested confirmations from the engineer. UHPC often receives a 24-hour compressive strength result to provide an initial assessment of the quality of the pour. Additional results come in at 7 days, 16 days, 28 days, etc. Some test specimens can also remain with the lab long term, such as creep and freeze-thaw testing specimens, but these may be more applicable to a research batch than a particular project.
There are various potential outcomes at the stage of reporting. If the samples were not handled correctly during collection or upon receipt by the lab, inconsistent results might come back. If an accurate and complete chain of custody wasn’t used, disclaimers may negate the validity of the tests. In some cases, a pour may have been plagued by a mixing error in the field or unintentional vibration, causing unsatisfactory results. All of these possibilities can be avoided by consulting the manufacturer and laboratory ahead of time.
Cor-Tuf has experience implementing UHPC in a variety of project types and locations. Fortunately, we have a track record of aligning quality control with real-world project success. Sometimes we may suggest a pre-pour or mock-up to ensure the batch is being prepared and tested correctly. This saves everyone time and money.
Our products meet UHPC testing requirements by following straightforward procedures. Whether you need an all-in solution or prefer to combine UHPC components with your local mix supplies, we can support any contractor or laboratory in locking down the testing component of the process. Your sand, your cement, our Cor-Tuf UHPC.