Published: Jan 1990
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Concrete piers supporting towers at the corners of an electrical substation are expanded and severely cracked. Eight intermediate piers show no signs of expansion, cracking, or surface deterioration. All piers were constructed (1969) of the same concrete mixture.
The corner piers are connected to both ground and the above steel structure. The intermediate piers are grounded only and not connected to the above steel structure.
The coarse aggregate (CA) is a siliceous gravel composed essentially of quartz, quartzites, and cherts. The fine aggregate (FA) is a natural sand of essentially the same composition.
Cores taken from the corner piers exhibit: a) internally cracked CA particles with cracks extending into the mortar and connecting with cracks from other particles; b) reaction rims around CA particles with adjacent pockets of dried Alkali-Silica Reaction (ASR) gel that has breached through the reaction rim; c) peripheral separations around CA particles, with peripheral cracks in the adjacent mortar, and CA particles loose in their sockets; and, d) dried ASR gel in voids, cracks, and CA sockets, commonly in association with ettringite crystals.
The determined alkali contents of the corner pier, and intermediate pier concretes were the same; .06% NA2O and .05% K2O in each, as weight percents of the concrete.
Samples from the intermediate piers show these features of ASR to a minor degree: a) rim development on some chert and quartzite CA particles; b) small gel deposits within, and near, chert and quartzite CA particles; and, c) some chert and quartzite CA particles with internal cracks that do not extend out of the particles.
Since both concretes have ASR, one to an inconsequential degree and the other to a serious degree, the difference in the condition of the two concretes is attributable to a difference in the rate at which the ASR proceeded in one versus the other.
On-site measurements of electrical potential, by Cu/CuSO4 reference cell, between the steel structure and both the pier concretes and the surrounding soil show current flow from the concrete to the soil, with strong implications that, because of the corner piers being connected to the superstructure, the potential differences and current passing through the corner concrete was larger than through the intermediate concrete.
It is proposed that the reason for the difference in the degree of the ASR in the concretes is that the ASR has been accelerated in the corner piers by greater diffusion of alkali ions under the influence of a greater electrical potential gradient than is present in the intermediate piers.
Alkali Silica Reaction, alkali-aggregate reaction, electrical effects, concrete
consulting geologist/petrographerAssociates, concrete and concrete materials, Baltimore, MD