Aggregate shape, grading and alkali-aggregate reaction

Effect of aggregate shape on hardened concrete

• Rounded particles pack well, but have weaker bond with paste, so slightly lower flexural strength.
• Angular, rough aggregate has better interlock and bond: higher compressive and flexural strength and a better abrasion resistance, but needs more water and paste.
• Flaky and elongated particles lie flat, trap bleed water beneath them, and cause weak planes, lower strength and lower durability (higher permeability).
• Shape also affects shrinkage and creep indirectly via paste content.

Effect of grading on water requirement

• A well-graded aggregate has fewer voids and a smaller surface area for a given workability, so less water and paste is required.
• Too many fines (fine grading or excess sand) increase the surface area and water demand; too coarse or gap-graded mixes cause harshness and bleeding.
• Hence good grading lowers w/c ratio for the same slump and improves strength.

Alkali-aggregate reaction (AAR)

It is the chemical reaction between alkalis (Na$_2$O, K$_2$O) from cement and reactive silica (opal, chert, strained quartz) or certain carbonates in the aggregate, in the presence of moisture.

• It forms an expansive alkali-silica gel that absorbs water and swells, producing cracks (map cracking), expansion, pop-outs and loss of strength and durability, appearing after several years.
• Control: use non-reactive aggregate, low-alkali cement (below 0.6% Na$_2$O equivalent), pozzolans (fly ash, slag) and keep concrete dry.