Some of the blocks
were allowed to harden for twelve months in sand before being
placed, and these gave better results than the others. Two
brands of German Portland cement were used in these tests, one,
from which the best results were obtained, containing 65.9 per
cent. of lime, and the other 62.0 per cent. of lime, together
with a high percentage of alumina. In this case, also, the
addition of finely-ground silica, or trass, improved the
resisting power of blocks made with poor mortars, but did not
have any appreciable effect on the stronger mixtures.
Professor M. Moller, of Brunswick, Germany, reported to the
International Association for Testing Materials, at the
Copenhagen Congress previously referred to, the result of his
tests on a small hollow, trapezium shape, reinforced concrete
structure, which was erected in the North Sea, the interior
being filled with sandy mud, which would be easily removable by
flowing water. The sides were 7 cm. thick, formed of cement
concrete 1:2 1/2:2, moulded elsewhere, and placed in the
structure forty days after they were made, while the top and
bottom were 5 cm. thick, and consisted of concrete 1:3:3,
moulded _in situ_ and covered by the tide within twenty-four
hours of being laid. The concrete moulded _in situ_ hardened a
little at first, and then became soft when damp, and friable
when dry, and white efflorescence appeared on the surface.
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