Compressive Strength Test (ASTM C 39)
The compressive strength of hardened concrete isdetermined from compression tests on standard cylindrical specimens. As you know, compressive strength tests are used during concrete mix design to evaluate the performance of the materials and to establish mixture proportions that will give the required strength. The tests are used also to control the quality of the concrete in the field.
"Compressive strength" is defined as the average of the strengths of all cylinders of the same age made from a sample taken from a single batch of concrete. At least two cylinders, or preferably three, are required to constitute a test. So, if tests are to be made at 7 and 28 days, you will need four or six specimens. The standard specimen is 6 inches in diameter by 12 inches long and is capped with a suitable material to provide a smooth-bearing surface on each end of the specimen. You learned the procedures for preparing and capping compressive strength specimens in the EA3 TRAMAN. If necessary, you should review those procedures. The following paragraphs discuss only the procedures usedto perform compression tests on the prepared specimens.
The equipment you will use to perform the compression test is a compression-testing machine, having a capacity of 250,000 pounds. An example of that machine, shown with a test cylinder in place, is illustrated in figure 13-22.
The procedures for conducting the compression testare as follows:
1. Prepare the testing machine by cleaning thebearing plates and, if needed, cleaning and lubricating
Figure 13-22.—Compression-testing machine.
the spherical seat. Check the operation of the machine. Keep the previously prepared test specimen moist by covering it with wet burlap during the period between removing the specimen from the curing environment and testing. That applies to each specimen you will test.
2. Determine the diameter of the test specimen tothe nearest 0.01 inch by averaging two diameters measured at right angles to each other at midheight of the cylinder. Measure the length, including caps, to the nearest 0.1 inch. Record the dimensions on a prepared data sheet.
3. Place the specimen on the lower bearing block,bring the upper block almost to contact, and align the axis of the specimen with the center of thrust of the spherical head. Carefully and slowly bring the spherical head into contact with the specimen, rotating the movable portion gently by hand so that uniform seating is obtained. Apply the test load continuously and
Figure 13-23.—Normal fracture of concrete test cylinder in compression.
without shock at a rate of 20 to 50 pounds per square inch (psi). Observe and record the maximum load during the test. Observe the type of fracture and record any unusual features. The normal cone type of fracture is illustrated in figure 13-23.
4. Calculate the compressive strength of theconcrete using the following formula:
f´c = compressive strength (in psi)
P= maximum load (in pounds)
A= cross-sectional area of specimen (in inches)
Figure 13-24.—Flexural-strength testing assembly for 6-inchby 6-inch by 21-inch concrete beam.
For each tested specimen, your test report shouldinclude the identification, diameter, length, maximum load, compressive strength, irregular fracture or defect in caps or specimen, and age of specimen at test. Report the average compressive strength of all cylinders from the same concrete sample.