3 minute read
How to Determine Concrete Slump
Sakrete
The tools and procedure in testing concrete slump, step by step.
When people hear the term “slump,” most think about posture; think of concrete slump as "concrete posture." In technical terms, slump is a measured indication of the workability, or flow, of a mix. The drier the mix, the lower or “tighter” the slump. Conversely, the wetter or “looser” the mix, the higher the slump. The slump measurement process is defined by ASTM C143 – Standard Test Method for Slump of HydraulicCement Concrete.
Items You'll Need
• Slump Cone Mold—The mold is 12-in. high and known as an Abrams cone. • Scoop—To place the concrete sample into the mouth of the cone. • Steel Tamping Rod—This is a 5/8-in. diameter, smooth-surfaced, 24-in. long rod with hemispherical (rounded) tips. • Baseplate—Or some similar nonabsorbent surface. • Tape Measure—Or a ruler to record the test result. • Sponge or Brush & Water—To dampen the cone and base before the test.
PREPARING THE TEST
• Obtain a sample using ASTM C172 – Standard Practice for Sampling
Freshly Mixed Concrete methods. • Begin slump test within five minutes of collecting and remixing the sample. • Ensure the test base is smooth, nonabsorbent, and on a level surface. • Once the slump test has started, it must be completed within 2 minutes and 30 seconds.
THE PROCEDURE
• Secure the slump cone to the base plate using the cone tabs or by standing on the foot holds. The cone
MUST remain still throughout the test. Pro Tip: While you can do this alone, it will go smoother with help. • Using the scoop, fill 1/3 of the cone with the concrete sample. • Rod the material 25 times with the steel tamping rod. Distribute rodding evenly over the entire cross-section of the sample. You may touch but do not hit or “thump” the base plate.
Pro Tip: A good technique is to angle the rod to match the slope of the cone and rod starting with the perimeter.
Continue in a circular pattern working to the center. • Fill the cone to 2/3 full. • Rod the second layer 25 times, making sure to penetrate the previous layer by about an inch. Continue rodding in a circular fashion, working to the middle. • For the final layer, fill the cone to slightly overflowing. • Rod the final layer 25 times with rod penetrating, but not through, the second layer. Distribute rodding evenly over the entire cross-section of this layer. If the mounded material sinks below the lip of the mold, stop rodding, refill, then continue rodding, picking up where you left off in the 25 stroke count. • Strike off excess concrete from the top of the cone, using the tamping rod as a screed in a rolling or sawing motion. • Clean excess from the base of the cone. Avoid disturbing the cone. • Holding the cone firmly, unlock or step off the cone tabs. Slowly lift the cone vertically in a steady motion, without any rotational movement, straight up to clear the sample. The lifting action should take 5 to 10 seconds.
Pro Tip: Do not to jerk the cone or pull up at an angle. These movements will result in a false reading. • Invert the cone mold (small side down) and set it next to, but not touching, the sample on the base. • Place a straight edge, preferably the tamping rod, across the slump mold base, with one end suspended over the concrete. Measure the distance from the bottom of the straight edge to the displaced center of the slumped concrete. Record your results to the nearest 1/4 of an inch.
Pro Tip: For those unsure of how to judge the displaced center, place a coin onto the center of the concrete after final rodding and strike off. Once the mold is pulled, measure to the spot with the coin.
Unless otherwise specified, readymixed concrete is delivered to the jobsite at a slump of 4 ±1 in. Bagged concrete has its own set of rules as defined by ASTM C387 – Standard Specification for Packaged, Dry, Combined Materials for Concrete and High Strength Mortar. This standard stipulates the slump range of bagged concrete (without waterreducing admixtures) to be between 2 and 3 inches. This is a much tighter or lower slump than you’ll see out of a ready-mix truck chute. However, looks can be deceiving. The smaller coarse aggregate used in bagged concrete mixes makes it much easier to consolidate than most truck-delivered concretes—resist the urge to make your bagged mixes match-ready mix consistency.
Dirk Tharpe is a concrete expert and sales trainer for Sakrete.
