5.2 Sampling, Filling, and Consolidation Choice

Getting a Representative Sample

The density result is only as good as the sample, so C138 begins with a sample obtained and handled per ASTM C172, "Standard Practice for Sampling Freshly Mixed Concrete." The composite sample is remixed with a shovel just before testing to redistribute aggregate and paste, and the test should start within 5 minutes of obtaining the final sample. Concrete is time-sensitive — slump loss and stiffening change the density as the mix ages, so promptness is part of correct procedure.

The technician selects the measure size, confirms it is clean, damp, and calibrated, and weighs it empty if that hasn't already been recorded. From here the critical decision is how to consolidate: rodding or internal vibration.

Choosing Consolidation by Slump

C138 ties consolidation method to the workability of the concrete, measured by slump (ASTM C143):

This mirrors the rule in ASTM C231 and C173 for air testing, so the exam tests it across multiple methods. The principle: stiff concrete needs energy input to consolidate, while fluid concrete consolidates under rodding and would lose entrained air or segregate if vibrated.

Rodded Consolidation — Three Equal Layers

When rodding, fill the measure in three approximately equal layers (by volume). The mistake to avoid is filling in unequal lifts — the standard says equal layers so each receives consistent consolidation energy.

Rodding strokes per layer:

• 25 strokes for measures of 0.5 ft³ (14 L) or smaller — the size Grade I technicians use most.
• 50 strokes for measures larger than 0.5 ft³.

Distribute the strokes uniformly across the cross-section. Rod the bottom layer throughout its depth without forcing the rod hard against the bottom of the measure. For the second and third layers, penetrate the rod about 1 in. (25 mm) into the layer below so the lifts knit together.

After rodding each layer, tap the outside of the measure 10–15 times with the mallet. The taps close the holes left by the rod and release large air bubbles trapped during filling. Over-tapping can densify the sample artificially, so stay within the 10–15 range.

Vibrated Consolidation — Two Equal Layers

), use an internal vibrator rated at ≥9,000 vibrations per minute and fill in two approximately equal layers rather than three. Insert the vibrator vertically at a small number of representative points per layer, allowing it to penetrate roughly 1 in. into the underlying layer, and vibrate only until the surface becomes relatively smooth and large bubbles stop rising — typically when mortar appears around the vibrator head. Over-vibration drives out entrained air and segregates the mix, raising density and falsely lowering the gravimetric air result.

Whichever method is used, the goal is full consolidation without segregation, so the recorded density truly represents the concrete as placed.

Rodding Technique Details the Exam Tests

Grade I candidates are graded on the quality of their rodding, not just the count. A few specifics:

• Hold the rod vertically and use the rounded (hemispherical) end to consolidate, which limits damage to the entrained-air system that a flat or sharp end would cause.
• For the first (bottom) layer, rod throughout its depth but do not forcibly strike the bottom of the measure — that can dent it and change Vm.
• For the second and third layers, penetrate about 1 in. into the underlying layer so the lifts knit rather than form a cold joint between lifts.
• Spread the strokes evenly over the surface; clustering all 25 in the center under-consolidates the perimeter.

The mallet taps (10–15 per layer) are not optional polish — they release the larger entrapped (not entrained) air pockets that rodding leaves behind. Entrapped air is an unintended void you want to remove; entrained air is the intentional microscopic bubble system you must preserve, which is why both rodding and tapping are deliberately gentle and bounded.

Common Sampling and Filling Mistakes

Memorize the two-line summary for the exam: rod three equal layers (25 strokes for ≤0.5 ft³, 50 for larger), tap 10–15 times per layer; vibrate two equal layers, just until consolidated. The number of layers is the single most-confused fact on this method, so anchor it firmly before test day.

Consolidation choice quick recap

• Rod when slump is greater than 3 in.; vibrate when slump is less than 1 in.; either method is allowed from 1 to 3 in.
• Rod each layer 25 strokes with the 5/8-in. rounded-tip rod, penetrating about 1 in. into the layer below.
• Tap the sides 10–15 times with the mallet after rodding each layer to close rod holes and release trapped air.