How to Increase Flexural Strength in Concrete


Concrete’s flexural strength is the ability of a slab, wall or beam of concrete to resist failure under pressure to bend. The compressive strength of concrete, or its ability to withstand compression, is generally much greater than its flexural strength. This is why concrete is often reinforced with welded wire, wire mesh or steel rebar. Substituting slag cement, or the blast-furnace slag byproduct of steel manufacturing, for a portion of the portland cement in concrete greatly increases both compressive and flexural strength. This is because the combination works together to create extra calcium silicate hydrate, the “glue” that holds concrete together.

Things You'll Need

  • Slag cement
  • Portland cement
  • Sand
  • Gravel
  • Safety glasses
  • Dust mask
  • Waterproof gloves
  • Mixing bucket or tub
  • Trowel
  • Wheelbarrow
  • Hoe
  • Stiff scrub brush
  • Measure the dry ingredients by volume -- except for the slag cement substitution, which should be done by mass or weight. General ingredient proportions for concrete are 60 to 75 percent aggregate, or sand and gravel or crushed rock; 10 to 15 percent portland cement; 10 to 20 percent water; and air.

  • Prepare for making this substitution by measuring the amount of portland cement you would use if that was the only cement used.

  • Remove the percentage of portland cement you want to replace with slag cement. For example, if you want to mix a concrete with 25 percent slag cement, remove 25 percent of the portland cement you just measured. Weigh that 25 percent of portland cement and note the weight.

  • Weigh the same amount of slag cement. Because the specific gravity of slag cement is lower than portland cement, the identical weight of slag cement will produce a larger volume than the portland cement it replaces.

  • Mix the dry ingredients -- sand, gravel, portland cement and slag cement. Gradually add water to your slag concrete mix, starting with a small amount of water and mixing thoroughly before adding more. You don’t want soupy concrete. It should be stiff yet moist and workable.

  • Wash out your containers and scrub and clean your tools immediately after pouring your strengthened concrete to make sure concrete doesn’t set and ruin your equipment.

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