4.4 ASTM C143 Scope, Equipment, and Setup

Slump Test Scope

ASTM C143/C143M measures the slump of plastic hydraulic-cement concrete. Slump is a field indication of consistency and workability — not a direct measure of water content, strength, or quality by itself. The result is meaningful only when the concrete is within the method's scope and the test is run on a representative C172 sample with correct equipment.

The slump test is not suitable for every concrete. It applies to plastic, cohesive mixtures. The method states it is not applicable when aggregate larger than 1.5 in. (37.5 mm) is present; such samples must be wet-sieved over the 1.5 in. sieve before testing. Very stiff, nonplastic concrete will not slump usefully, and very flowable or self-consolidating concrete needs a different test such as slump flow (ASTM C1611). These scope points are classic written-exam traps because candidates assume any fresh concrete can be tested by C143.

It is worth being precise about what slump does and does not tell you. Two batches at the same slump can have different water-cement ratios, different admixture dosages, and different strengths. Slump is a consistency indicator that flags batch-to-batch variation and verifies the mix matches what was ordered; it is not a quality grade. A sudden slump increase on a job, for example, may signal added water at the truck and prompt the technician to flag the load even though the slump number alone looks acceptable.

Typical ordered slumps fall in a familiar range. Many structural mixes are specified around 3 to 5 in., pump mixes and admixture-modified mixes run higher, and stiff pavement or low-slump mixes run lower. A Grade I technician is not asked to judge whether a slump is "good," only to measure it accurately and compare it to the specified target and tolerance — often a target plus or minus 1 in. or a maximum allowable value set by the project.

The Mold, Rod, and Base

The standard slump mold is a frustum of a cone. Its inside dimensions are 8 in. (200 mm) diameter at the base, 4 in. (100 mm) diameter at the top, and 12 in. (300 mm) high, each with a tolerance of about plus or minus 1/8 in. The mold is made of metal not thinner than 1.5 mm, with foot pieces and handles, and the interior is smooth and free of dents.

The tamping rod is a straight, round steel rod 5/8 in. (16 mm) in diameter with a length of about 24 in. (600 mm) and at least one end rounded to a hemispherical tip of the same 5/8 in. diameter. The hemispherical tip is required so the rod consolidates rather than cuts the concrete.

Setup and Sample Control

Setup is part of the performance exam. Dampen the mold and base so concrete does not stick or lose water to dry surfaces. Place the mold with the larger 8 in. opening down on a flat, rigid, level, moist, nonabsorbent surface. Hold it firmly in place by standing on the two foot pieces throughout filling and rodding. If the cone shifts, leaks, or lifts during filling, the specimen geometry is disturbed before measurement even begins.

The same 5/8 in. by 24 in. hemispherical-tip rod is used in ASTM C138 density and ASTM C231 air-content testing, so a Grade I technician carries one rod across several methods and must keep its rounded end undamaged. A flat-ended or worn rod cuts and shoves the concrete instead of compacting it, changing the consolidation energy and biasing the slump. Inspect the rod and the inside of the mold for dents before each test set.

The base is more than a convenience. A flat, rigid, level surface keeps the cone from tilting, which would throw the slump measurement off; a moist, nonabsorbent surface keeps the bottom of the specimen from losing water to dry plywood or soil, which would stiffen the base of the cone and reduce the apparent slump. A common field setup is a clean, damp steel base plate or a smooth, nonabsorbent board. Sand, bare soil, or warped plywood are not acceptable bases because they absorb water, flex, or sit unevenly.

The sample must be remixed and uniform before filling starts. Do not begin filling until the C172 sample has been remixed. Once filling begins, C143 has its own uninterrupted timing requirement, so the rod, scoop, and measuring device should be within reach before the first scoop.

Setup checklist:

• Confirm the concrete is within C143 scope (no aggregate over 1.5 in. unless wet-sieved).

• Verify the 8-4-12 in. mold and the 5/8 in. by ~24 in. hemispherical-tip rod.

• Use a clean, damp mold on a flat, rigid, moist, nonabsorbent base.

• Stand on the foot pieces before adding concrete.

• Stage the rod, scoop, and ruler so the test runs without interruption.