ASTM C 29: Aggregate Strength and Concrete

ASTM C 29

• Relates aggregate strength to concrete, considering factors like compaction, shape, size distribution, and porosity.
• Concrete strength is influenced more by the quality of the cement paste and its bond with aggregates rather than aggregate strength alone.
• Weak aggregates can lead to lower concrete strength while strong aggregates generally contribute to stronger concrete.

Physical Properties of Aggregates

• Moisture Content: Impacts water requirements and can lead to issues like local scaling and surface cracking.
• Absorption: Ratio change in weight indicating the aggregate's ability to retain water, affecting the concrete mix ratio.
• Specific Gravity: Critical for concrete design; typical values range from 2.4 to 2.8 for various aggregates.
• Soundness: Measures aggregates' resistance to volume changes due to physical conditions like freezing and thawing.

Grading and Sieve Analysis

• Grading affects concrete workability more than strength; maximum aggregate volume is 55% in mortar and 85% in concrete.
• Sieve analysis helps determine aggregate graduation by dividing samples into size fractions.
• Fineness modulus quantifies the grading of aggregates using standard sieve sizes.

Water & Admixtures

• Quality of water is vital; acceptable options include potable water without impurities, while seawater is under exploration for mixing.
• The typical water requirement for concrete ranges from 140 to 200 liters per cubic meter, modified by aggregate size.
• Admixtures: Enhance concrete properties; include set retarders (slow hydration), water-reducers (improve workability), and superplasticizers (reduce water without losing workability).

Mineral and Chemical Admixtures

• Pozzolans: React with calcium hydroxide to improve strength and durability, reducing bleeding, temperature rise, and enhancing resistance against chemical attacks.
• Accelerators: Speed up the hardening process, allowing for earlier form removal and reduced curing times.

Curing and Moisture Control

• Curing is essential for hydration; techniques include controlling temperature and moisture movement.
• Retempering restores workability if moisture loss occurs during mixing or transport.

Mechanical Properties of Concrete

• Compressive Strength: Indicated through standard tests on cylindrical or cubic samples, typically showing elastic behavior until a significant stress level is reached.
• Tensile Strength: Generally achieves 10-15% of compressive strength after 14 days, assessed via a splitting tensile test.
• Shear and Impact Strength: Shear strength is a combination of shear and flexural forces, while impact strength can surpass compressive strength.

Deformation Properties

• Creep: Long-term deformation under sustained loads affecting structural integrity.
• Shrinkage: Primarily drying shrinkage causes loss of adsorbed water and is a detrimental property of concrete.

Summary of Key Formulas

• Modulus of Elasticity: Influenced by aggregate properties, represented as (E_c = 4700 f'_c) for normal weight concrete.
• Calculating specific gravity (SSD) and (OD) essential for understanding aggregate performance.

Poisson’s Ratio

• Reflects the relationship between transverse strain and axial strain, generally ranging from 0.15 to 0.20 in concrete.

Transporting and Handling

• Central-mixed and transit-mixed methods enhance quality control, especially in congested areas, but require careful maintenance of workability during transport.