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The navigation lock of Lower Monumental Dam, Washington required 1.2 million dollars 1n repairs as the concrete 1n the walls showed severe scaling because of lack of air entrainment and the presence of smectite (montmorillonite) in the Mabels Pit fine aggregate. Smectite because of its adsorptive-absorptive nature robs the mixture of water thus causing slump loss. It also increases drying-shrinkage volume changes.
Methylene blue staining of grain thin sections was used for quantifying the smectite 1n the Mabels Pit sand. The smectite turns blue with the staining, whereas muscovite nor biotite and most of the rock forming minerals do not stain. Utilizing this procedure indicated the No. 50 – No. 100 sieve size at Mabels Pit contains 23.8 percent smectite.
The methylene blue adsorption (MBA) is a titration process whereby the grains of a sieve fraction of a sand are ground to pass a No. 200 sieve and 2 grams of the powder is tested with methylene blue dye and a spot tested on filter paper until a light blue ring forms around the central dark blue spot. MBA values of greater than 2.5 m1/2 grams are associated with unsound concrete sands. The sieve size with 23.8 percent of smectite has an MBA value of 3.5.
smectite, methylene blue staining and adsorption
petrographer, U.S. Army Corps of Engineers, Portland, Oregon