Fracture mechanics of ASR in concretes with waste glass particles of different sizes

Using waste glass as an aggregate in concrete can cause severe damage becau se of the alkali-silica reaction (ASR) between the alkali in the cement pas te and the silica in the glass. Recent accelerated 2-week tests, conducted according to ASTM C 1260, revealed that the damage to concrete caused by ex pansion of the ASR gel, which is manifested by strength reduction, depends in these tests strongly on the size of the glass particles. As the particle size decreases, the tensile strength first also decreases, which is expect ed because of the surface-to-volume ratio of the particles, and thus their chemical reactivity increases. However, there exists a certain worst (pessi mum) size below which any further decrease of particle size improves the st rength, and the damage becomes virtually nonexistent if the particles are s mall enough. The volume dilatation due to ASR is maximum for the pessimum p article size and decreases with a further decrease of size. These experimen tal findings seem contrary to intuition. This paper proposes a micromechani cal fracture theory that explains the reversal of particle size effect in t he accelerated 2-week test by two opposing mechanisms: (1) The extent of ch emical reaction as a function of surface area, which causes the strength to decrease with a decreasing particle size; and (2) the size effect of the c racks produced by expansion of the ASR gel, which causes the opposite. The pessimum size, which is about 1.5 mm, corresponds to the case where the eff ects of both mechanisms are balanced. For smaller sizes the second mechanis m prevails, and for sizes <0.15 mm no adverse effects are detectable. Extra polation of the accelerated test (ASTM C 1260) to real structures and full lifetimes will require coupling the present model with the modeling of the reaction kinetics and diffusion processes involved.