Unsoundness of Aggregate due to Volume Change

Water entering porous aggregate expands when it freezes, generating hydraulic pressure inside the pores. This pressure eventually disintegrates the aggregate, causing permanent volume changes that render it unsound. The size and continuity of pores control water movement and determine frost damage severity.

Salt crystallization within aggregate pores during wetting and drying cycles creates internal stresses that disintegrate the aggregate. This process results in permanent volume changes, particle size reduction, and weight loss, indicating the aggregate's unsoundness. These mechanisms are similar to deleterious substances in aggregate that compromise durability.

Soundness is evaluated by subjecting graded and weighed aggregates to repeated cycles of immersion in sodium or magnesium sulfate solution followed by oven drying. Salts crystallize within pores, simulating natural deterioration and wetting and drying cycles. Weight loss and particle size reduction indicate unsoundness and reduced durability.

Pore shape and continuity control the rate and amount of water absorbed and released by aggregates, making them more critical than total pore volume. These characteristics determine how aggregates respond to environmental changes like freezing, thawing, and moisture cycles. Understanding porosity and absorption of aggregate is essential for predicting performance.

Porous flints, cherts, and aggregates containing clay minerals are especially susceptible to unsoundness from volume changes. These materials undergo significant physical alterations during freezing and thawing, thermal changes, and wetting and drying cycles, leading to permanent disintegration and reduced concrete performance.

Concrete with unsound aggregates typically exhibits pop-outs, scaling, and surface cracking. These defects result from the permanent volume changes and disintegration of aggregates subjected to freezing, thawing, and moisture cycles. Such deterioration compromises the structural integrity and appearance of concrete structures.

Frost susceptibility is analyzed by subjecting loose aggregates and aggregates embedded in concrete to repeated freezing and thawing cycles that simulate field conditions. These laboratory tests reveal whether aggregates will deteriorate when exposed to natural environmental stresses, helping engineers select durable materials for cold climates.