Understanding Beam Molds in Construction Testing

Concrete Testing Products Concrete Testing Testing

July 14, 2025

Stacked metal Beam Molds arranged in rows on a factory floor, showcasing their durable construction and uniform design.

Beam molds are essential tools in construction testing, used to measure the flexural strength of concrete, a critical property for structures exposed to moving loads. From airport runways to highway pavements, knowing how to make and use beam molds properly helps ensure your concrete performs under stress.

Why Flexural Strength Matters in Construction

When concrete is subjected to moving loads, such as airplanes landing or cars crossing bridges, it needs to withstand bending or flexing forces. This is where beam molds come into play. Unlike cylinders that measure compressive strength, test beams made in beam molds help determine flexural strength, the material's ability to resist bending. In projects involving pavement, runways, or bridge decks, flexural strength data provides more accurate performance indicators than compressive strength.

For a hands-on demonstration, watch Beam Molds and Flexural Strength Explained. Using beam molds to produce reliable concrete test beams ensures your material meets design specifications, regulatory requirements, and safety standards. Inadequate flexural strength could lead to premature surface cracking or even structural failure under repeated dynamic loading.

Choosing the Right Beam Mold Size

Selecting the correct beam mold size is more than a matter of convenience; it's about accuracy and compliance. Deslauriers offers two standard sizes:

4 x 4 x 14 inches: Suitable for concrete mixes with aggregate sizes up to 1 inch. Shop the Econ-O-Beam steel mold now to get started.
6 x 6 x 21 inches: Required for mixes containing aggregate sizes over 1 inch. Shop the Econ-O-Beam plastic mold for a lightweight, budget-friendly alternative.

According to ASTM standards, the beam mold size used for field testing should match the size used during the original concrete mix design. This helps ensure that test results reflect the mix's actual performance. If you're unsure of your aggregate size, consult your mix design documentation or testing lab.

Beyond aggregate considerations, smaller molds (4x4) are easier to handle, require less concrete, and simplify the curing and transport process. Larger molds (6x6) are bulkier but necessary for higher aggregate mixes and potentially stronger concrete beams.

Step-by-Step: Making a Concrete Test Beam

Making a concrete test beam with a beam mold involves several key steps to ensure a high-quality specimen for accurate flexural testing:

1. Prepare the Mold

Make sure your mold is clean and lightly oiled for easy removal. Place it on a level, stable surface.

2. Fill the Mold Halfway

Pour concrete halfway into the mold. For a 6x6x21 mold, you'll need to rod the mix 63 times (1 rod per 2 square inches of surface area). For a 4x4x14 mold, rod 28 times.

3. Rod and Tap

Rodding consolidates the concrete, but it creates air voids that must be eliminated. Use a rubber mallet to tap the sides of the mold, releasing trapped air and ensuring a denser specimen.

Use a 5/8-inch tamping rod for the larger mold
Use a 3/8-inch rod for the smaller mold

4. Add the Second Layer

Fill the mold to the top with more concrete. Rod again using the same method, making sure the rod slightly penetrates the first layer to bond them.

5. Strike Off and Finish

Use a strike-off bar or straight edge to level the top surface. A sawing motion works well to achieve a flat finish. Do not worry about making it perfect; the beam will be tested on its side, not on the struck surface.

6. Moisture Retention

Unlike cylinders that have lids, beam molds require wet burlap and plastic sheets after the concrete sets to retain moisture. Proper curing is essential for obtaining accurate flexural strength data.

7. Curing Conditions

According to ASTM C31/C31M, beams must remain on-site in their molds for the first 24 hours after casting. This period ensures proper hydration and prevents moisture loss.

ASTM Standards to Follow

When working with beam molds and making concrete test beams, the following ASTM standards are critical:

ASTM C31/C31M: Standard Practice for Making and Curing Concrete Test Specimens in the Field
ASTM C78/C78M: Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading)
ASTM C192/C192M: Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory

These standards provide detailed procedures for mixing, placing, curing, and testing beams to ensure consistent and accurate results.