The Concrete Handbook

Concrete's performance is defined by its properties in two distinct states: fresh (plastic) and hardened.
These properties influence how it behaves during construction and throughout its service life.

Fresh Concrete Properties

Fresh concrete refers to the state immediately after mixing, when it is workable and can be shaped. Its properties are critical for proper handling, placement, and achieving desired performance.

Workability is the ease with which concrete can be mixed, placed, compacted, and finished without segregation.

  • Important Factors:
    • Water-to-Cement Ratio (w/c) - higher ratios increase workability but reduce strength while lower ratios improve strength but can hinder flowability without admixtures.
    • Aggregates - rounded, well-graded aggregates improve flow and cohesiveness.
    • Admixtures - superplasticizers enhance flow without increasing water; air-entraining agents improve stability and durability.
    • Temperature - high temperatures reduce workability by accelerating hydration; low temperatures slow hydration but may require accelerators.
  • Testing Methods:
    • ASTM C143 Slump; low-slump concrete is ideal for pavements to minimize shrinkage.
    • ASTM C1611 Slump Flow / Spread; high-slump or self-consolidating concrete (SCC) is useful in congested reinforcements or complex formwork.
    • ASTM C1621 J-Ring; for passability.

Air Content is critical for durability, especially in freeze-thaw environments.

  • Important Factors:
    • Entrained Air is introduced with admixtures intentionally for durability.
    • Entrapped Air is naturally occurring during mixing and placement, usually not beneficial for concrete.
  • Testing Methods:
    • ASTM C231 Pressure Method for normal weight concrete.
    • ASTM C173 Volumetric Method for lightweight concrete.

Temperature affects hydration and setting time. Proper temperature control ensures consistent strength development and durability.

  • Important Factors:
    • Ambient conditions will affect the performance of concrete and admixtures; high temperatures accelerate hydration, low temperatures slow hydration.
    • Aggregates temperature directly influences concrete temperature.
    • Water temperature directly influences concrete temperature.
  • Testing Methods:
    • ASTM C1064 using a temperature probe or thermometer.

Unit weight confirms the density of fresh concrete, which ensures proper proportions and consistency across batches. Yield calculates the actual volume produced versus the designed volume.

  • Important Factors:
    • Aggregate Density - heavier aggregates increase unit weight, while lightweight aggregates decrease it.
    • Air Content - higher entrained air reduces unit weight, impacting yield.
    • Water-to-Cement Ratio (w/c) - excess water reduces density and affects batch volume consistency
  • Testing Methods
    • ASTM C138 for Unit Weight.
    • ASTM C29 for Dry Rodded Unit Weight of aggregates.

Setting Time is the transition from fluid to semi-solid state is essential for proper placement and finishing.

  • Initial Set Time is when hardening begins (~500 psi).
  • Final Set Time is when concrete hardens enough to bear minimal loads (~3,000–4,000 psi).

  • Important Factors:
    • Cement type such as Type III for faster setting.
    • Temperature: such as higher temperatures accelerate setting; lower tempteratures delay it.
    • Admixtures such as retarders delay setting; accelerators speed it up.
  • Testing Methods:
    • ASTM C403 (Penetration Resistance Test for Mortar and Concrete).

Stability is the resistance of fresh concrete to segregation and bleeding, ensuring uniformity and preventing weak spots.

  • Important Factors:
    • Aggregates properly graded help minimize segregation.
    • Water-to-Cement Ratio (w/c) to limit excessive water content, reducing bleeding.
    • Admixtures such as air-entraining admixtures improve stability and reduce bleeding.
  • Testing Methods:
    • ASTM C136 for aggregate gradations.
    • ASTM C1610 - Column Segregation Test for SCC.
    • VSI - Visual Stability Index.

Hardened Concrete Properties

Hardened concrete properties dictate its long-term strength, durability, and resistance to environmental challenges.

Strength is the ability of hardened concrete to withstand applied loads without failure.

  • Important Factors:
    • Compressive Strength is the primary measure of concrete's ability to withstand loads.
    • Tensile Strength measures concrete's ability to resist forces that pull it apart, such as tension.
  • Testing Methods:
    • ASTM C39 for compressive strength.
    • ASTM C78 for flexural strength.

Durability is the concrete's ability to resist environmental damage and maintain its intended performance over time.

  • Important Factors:
    • Freeze-Thaw Cycles - Air-entrained concrete improves resistance by preventing cracking due to water expansion.
    • Chemical Resistance - Sulfate-resistant cement (Type V) or SCMs improve resistance to aggressive environments.
    • Abrasion Resistance - Surface hardeners or denser mixes prevent wear in high-traffic areas.
    • Curing ensures hydration and strength development, reducing shrinkage and increasing resistance to environmental challenges.
  • Testing Methods:
    • ASTM C66 used to evaluate freeze-thaw resistance.
    • ASTM C805 for the use of the rebound hammer.