ASTM C 29: Aggregate Strength and Concrete

Study Notes

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.

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 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.

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).

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

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.

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.

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.

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

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