5.2 Sampling Fresh Concrete & Slump (ASTM C143 / AASHTO T119)

Sampling Fresh Concrete: Getting a Representative Portion

Every test described in this chapter - slump, air content, unit weight, and the cylinders used for strength - is only as good as the sample it was run on. ASTM C172 / AASHTO T141 governs how a technician pulls a sample of fresh concrete, and the rule that matters most for an inspector is where in the load the sample comes from: the middle portion of the batch, never the first or last material discharged from the truck's chute. The first concrete out of the chute tends to be short on paste and coarse-aggregate rich (segregated); the last concrete out tends to be wetter. A sample from either extreme misrepresents the batch and can produce a false pass or a false fail.

A proper sample is a composite of at least two portions collected at regularly spaced intervals during discharge (never a single grab), combined and remixed with a shovel to ensure uniformity before testing. The minimum composite sample size is 1 cubic foot, and the standard limits elapsed time on both ends: sampling from first to final portion must be completed within about 15 minutes, and fresh-property testing (slump, air, temperature) must begin within a short window after the composite sample is obtained - commonly cited as starting within 5 minutes of obtaining the composite sample for slump, air, and temperature, with strength-cylinder molding (Section 5.4) started within 15 minutes of the same sample. An inspector who lets a sample sit in the sun while chasing down a paperwork question is compromising the test before it starts, because concrete continues losing slump and changing temperature the moment it leaves the truck.

Slump Test - ASTM C143 / AASHTO T119

The slump test measures the consistency and workability of fresh concrete - nothing more. A common exam trap is treating a slump result as a proxy for strength; it is not. Two batches with identical slump can have very different compressive strengths if their w/c ratios differ, and a batch can be well within its strength requirements while still failing on slump because it was placed too stiff or too wet for the intended method (hand placement versus slipform paving, for example).

Equipment and procedure:

• The slump cone is a frustum of sheet metal, 8 in. diameter at the base, 4 in. diameter at the top, 12 in. tall.
• The cone is filled in three layers of approximately equal volume, each rodded 25 times with a 5/8-in.-diameter, 24-in.-long rounded-tip tamping rod.
• After the top is struck off flush, the cone is lifted straight up in one smooth motion (5 +/- 2 seconds), with no lateral or torsional movement.
• The slump is the difference between the height of the mold (12 in.) and the height of the displaced original center of the top surface of the concrete, measured to the nearest 1/4 in.

If the concrete shears or slides to one side rather than settling evenly (a shear slump), the test is invalid and must be repeated on a fresh portion of the same sample - only a true, even settlement is reportable.

Reading Results Against the Spec

Different placement methods call for different target slumps, and the inspector's job is to compare the field result against the mix-design or spec range, not a generic number:

A slump reading outside the specified range is grounds to reject the load before it is placed - accepting an out-of-tolerance load and hoping the strength cylinders pass later is not acceptable inspection practice, because a too-wet load also raises w/c beyond the mix design's approved maximum, undermining durability even if it happens to break in spec at 28 days.

Worked example: A slipform paving mix specifies a slump of 0.75 in. +/- 0.5 in. (i.e., 0.25-1.25 in.). A field slump test reads 1.75 in. - a full inch above the upper limit. Because slipform paving relies on the concrete holding its edge shape immediately behind the paver, this load should be rejected even though a 1.75-in. slump would be unremarkable for hand-placed structural work; the acceptable range is a function of the placement method, not a universal constant.

Field Judgment on Marginal Results

A slump reading right at the tolerance edge is not automatically a pass or a fail by eye - it is a documented measurement compared against the written spec range for that placement, on that day, for that mix. Inspectors who "eyeball" acceptance instead of running the cone lose the ability to defend a rejection decision later, and they lose the paper trail that protects the agency if a pavement section underperforms and the contractor disputes the cause.