Concrete testing companies refer to the ASTM 1856 standard when testing UHPC concrete. However, in practice, we have seen situations where even slight deviations from the standard can introduce inaccuracies into the results so that the actual strength of the concrete is misrepresented.

The following checklist will ensure that your results are accurate.

#1: Field Specimen Preparation, Reporting, and Transport

• Before transporting, cure and protect the specimens as described in ASTM C31 Section 10, which states that specimens shall not be transported until at least 8 hours after the final set.

• During transport, protect specimens with suitable cushioning material to prevent damage and jarring. If transported during cold weather, the specimens should also be protected from freezing with suitable insulation material.

• Prevent moisture loss during transport.

• Transportation to the laboratory should not exceed 4 hours.

• Before and during the testing, make sure you record the following:
  
  • Sample identification number.
  • Location of the pour.
  • If field testing, report air temperature and relative humidity.
  • Curing conditions for the specimen.
  • If thermal curing is specified, report the specimen age when the thermal treatment is started and the time/temperature profile of the specimen, including heating and cooling periods.
  • Results of the tests conducted.
  • Any deviations during specimen preparations or testing.

#2: Proper Preparation of Cylinders for Laboratory Testing

• Grind the cylinder ends properly (see #4 below).

• The diameter used for calculating the cross-sectional area of the cylindrical test specimen should be determined to the nearest 0.1 mm (0.04 in). Average the two diameters at right angles, about mid-high on the specimen. This will give you greater accuracy than the Test Method recommended in ASTM C39/C39M.

• Remove any surface moisture and measure the mass of the specimen.

• Measure the length of the specimen to the nearest .05 inch at three locations spaced evenly around the circumference, compute an average, and report to the nearest .05 inch.

• Set the machine load rate to 145 psi +/- 7 psi.

• Adjust the peak percentage or break percentage according to the manufacturer’s recommendation due to cylinders possibly having a faulty break.

• Certify that the laboratory setting is set to 90 percent.

#3: Measuring the Specimens

• Specimens should not be tested if any individual cylinder’s diameter differs from the diameter of other cylinders by more than 2 percent.

• If a specimen’s length-to-diameter ratio is 1.75 or less, correct compression results by multiplying by the appropriate correction factor mentioned in ASTM C39.

#4: Grinding the Ends

• Use a grinding machine. Do not cut the ends with a saw.

• Grind both ends of the cylinders so that the ends don’t depart more than 0.5 degrees from perpendicularity from the axis.

• Make sure the ends are ground plane to within 0.050 mm (0.002in) (Figure 2).

• ASTM recommends that only specimens measuring 75 mm (3 in) in diameter and 150 mm (6 in) long should be used for compression strength testing. However, it also mentions that “if molds in SI units are required and not available, the inch-pound mold should be permitted.”

#5: Centering on the Break Machine

• Place the specimen on the lower bearing block and align the axis of the specimen with the center of the thrust of the upper bearing block (Figure 2).  

#6: Setting the Break Schedule

• The break schedule may vary from state to state or job to job.

• We have gotten the best results by scheduling breaks at 4 days, 7 days, 14 days, and 28 days. 

• We hold three samples in reserve in case they are needed at points along this timeline.

#7: Optimizing the Safety Factor

• The “safety factor” in a compression machine refers to the ratio between the maximum compression force the machine can exert and the calculated force needed to potentially cause failure. The safety factor thus acts as a buffer that ensures the machine operates safely well below its potential breaking point.

• Typically a safety factor of 2 to 3 is considered standard in many compression applications, which means that the machine can withstand at least double the expected operating force before reaching failure.

• For best results, confer with the calibration technician or refer to the manufacturer’s recommendations.

An Example Case

In one situation with a reputable testing agency,  a series of UHPC test samples broke correctly on days 4 and 7, then rested for 10 days, were re-tested, and had started gaining significant strength by day 14. But on the 21-day test, the PSI strength was lower than it was on day 14. The 28-day strength was even weaker.

After several conference calls, we realized that the ends of the cylinders were cut, not planed, as recommended in the second item on this checklist, invalidating the results.

This is a good example of what can happen if either end isn’t ground so it is 90 degrees from the axis. The cylinders can crack sooner than they would in regular use, and the resulting break pattern will not reflect reality.

When it comes to compression testing of UHPC, details matter.

———-

Many thanks to RNDr. Miles Zeman PH.D, who provided source information for this article.