8.3 Beam Molding, Flexural Specimens, and Finishing

Beam Geometry and Aggregate

Beams are molded to measure flexural strength (modulus of rupture), broken later under ASTM C78 (third-point loading) or C293 (center-point loading). The standard beam has a 6 by 6 in (150 by 150 mm) cross section. The minimum cross-sectional dimension must be at least three times the nominal maximum aggregate size, so beams are used for the coarser aggregates common in pavement and structural slabs. The specimen length must be at least 2 in (50 mm) greater than three times the depth as tested, giving enough span plus overhang for the C78 test setup.

Because flexural results are sensitive to the surface condition of the loaded faces, beam molds must be rigid, watertight, and set on a level, firm surface. A beam that twists or rocks during molding can warp and produce an invalid break. Beams are heavy and fragile until well cured, so handling and curing discipline matters even more than for cylinders.

Consolidating and Finishing Beams

Layering for beams depends on width and depth. Beams 6 to 8 in (150 to 200 mm) wide are molded in 2 layers. Beams wider than 8 in or deeper than 8 in are molded so that no layer exceeds 6 in (150 mm) of depth, which usually means 3 or more layers. Rodding follows a surface-area rule rather than a fixed count: rod each layer once for every 2 in² (1,300 mm²) of the top surface area of the specimen. For a standard 6×6 in beam, the top of each layer is about 6 in by the beam length, so the technician computes the stroke count from that area and distributes the strokes uniformly.

As with cylinders, rod the upper layers about 1 in into the layer below, and after rodding tap the mold sides 10 to 15 times per layer to release entrapped air.

When slump is less than 1 in, beams are vibrated. Insert the internal vibrator at intervals of about 6 in (150 mm) along the centerline of the beam and along both sides, keeping each insertion short to avoid segregation; the vibrator frequency must be at least 150 Hz.

After consolidation, strike off the surface and float it to a flat, even finish with minimum manipulation. Identify each beam without marking the as-molded faces.

Beam molding checklist:

• Confirm cross section (standard 6×6 in) suits the aggregate (≥ 3× NMAS).
• Use 2 layers for 6-8 in beams; cap layer depth at 6 in for larger beams.
• Rod once per 2 in² of top surface, then tap sides 10-15 times.
• Vibrate at ≥150 Hz, inserting ~every 6 in, when slump is below 1 in.
• Strike off and float flat; protect the molded faces from damage and marking.

The exam often contrasts the cylinder rule (fixed 25 strokes per layer) with the beam rule (one stroke per 2 in² of surface), so memorize both rather than blending them.

Why Beam Handling Is Less Forgiving

Flexural strength is far more sensitive to specimen condition than compressive strength, which is why beam discipline is so heavily tested. In a compression test the whole cross section resists load, so a small surface flaw is averaged out. In a flexural test the failure starts at the extreme tension fiber on the bottom of the beam, so any defect, dryness, or honeycomb on that face directly drives the result. A beam that is poorly consolidated along the bottom, or whose surface dries before testing, can report a modulus of rupture well below the true value.

The one-stroke-per-2-in² rule produces consistent consolidation across the wide top surface. For the standard 6×6 in beam, the technician computes the area of each layer's top surface and rods accordingly, then taps the sides 10-15 times to drive out wall-trapped air along the full length. Because beams are long and heavy, the molds especially must sit flat and rigid; a beam molded on uneven ground can twist as it sets and fail to seat properly in the C78 loading frame.

After molding, beams stay in their molds during initial curing like cylinders, but their larger surface area loses moisture faster, so moisture protection is even more critical. The final wetting requirement, immersion in lime-saturated water for at least the last 20 hours before testing, exists precisely because a dried beam surface fails early.

The takeaway the exam reinforces is that beams demand the same rules as cylinders plus extra care: correct layering, the 2-in² rodding rule, the side tap, and continuous wetting through to the test, because the flexural test punishes any lapse at the surface.