IS-14

Compressive and Flexural Strength

Why Are Improved Strengths Important?

Concrete made with slag cement provides higher compressive and flexural strengths com- pared with straight portland cement concrete. Improved strengths make it easier to achieve specified safety factors of the concrete mixture and can provide engineers with a tool to optimize concrete element designs. It provides enhanced material properties allowing producers to optimize concrete mix designs. Owners may realize decreased life cycle costs.

How Does Slag Cement Improve Strength?

Slag cement increases the compressive and flexural strength of conventional concrete (Figures 1 and 2) and is often a vital component in producing high strength concrete. 28-day strengths generally increase as the percentage of slag cement increases, up to about50 percent slag cement as a percent of cementitious material. When portland cement reacts with water, it forms calcium silicate hydrate (CSH) and calcium hydroxide (Ca(OH)2). CSH is the glue that provides strength and holds concrete together, Ca(OH)2 is a byproduct of portland cement hydration that does not contribute to strength. When slag cement is used as part of the cementitious material in a concrete mix, it reacts with water and Ca(OH)2 to form more CSH. The additional CSH densifies the concrete matrix, enhancing strength.

How Does Slag Cement Affect Strength Development?

When slag cement is used in concrete mixes, early strength development may be slower while ultimate strengths will be higher than straight portland cement mixtures. If more rapid strength development is required, the concrete mixture can be modified with conventional technology, for example, the use of accelerating admixtures, or use of heated materials or curing conditions. Concrete made with slag cement will have higher strength growth over the lifetime of the concrete element compared with straight portland cement concrete mixtures. Several factors influence the strength development of the mixture. They include:

  • Chemical composition of the slag.
  • Proportions of the slag component.
  • Temperature of the curing environment.
  • Chemical composition of the cement component.
  • Temperature of the concrete.
  • Fineness of the slag component.
  • Availability of soluble alkali.

Slag cement has a particularly significant effect on the flexural strength of concrete. Flexural strength (or modulus of rupture) is one of the principal factors in concrete pavement design. Increased flexural strength is evident in Figure 2, where 50 percent slag cement achieved a 20 percent strength improvement, even though total cementitious content was decreased by 52 lb/cu yd. Improved flexural strengths are attributed to the increased denseness of the paste and improved paste-aggregate bond.

What Are The Benefits Of Improved Strength

Feature Benefit Compressive Strength Benefit Flexural Strength
Improved Safety Factor Greater reliability Greater reliability
Optimized element design Thinner members
Lighter members
Fewer members
Less dead load
More usable floor space
Thinner section
Optimized mix designs Lower cementitious factors
Less shrinkage
Lower heat
Lower cementitious factors
Less shrinkage
Less curling
Life cycle cost Increased service life Increased service life
Lower maintenance costs
Greater ability to handle unexpected increases in traffic volumes


Figure 1: Effect of Slag Cement on 7 and 28-day Compressive Strength


Figure 2: Effect of Slag Cement on 7 and 28-day Flexural Strength

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