A Treatise On The Principles And Practice Of Dock Engineering

ACTION OF SEA-WATER UPON CONCRETE. 123 which impermeability is not essential, the excessive time and labour required for such an operation would be wasteful and unremunerative ; and, further, there is absolutely no reason to believe that concrete mixed with a good supply of water is any the less impervious on that account. Available testimony rather demonstrates the contrary, and the following experiment* of Mr. Bamber, F.I.C., is both interesting and instructive. He made three sets of blocks of concrete, in duplicate, with the following proportions:—4 parts of shingle, 2 of sand, and 1 of cement. The first pair were mixed with the full quantity of water that the cement would take up, which proved to be 10 Ibs. for each block. The second were mixed with only 71 Ibs. of water, or three-fourths of the full quantity. The third pair were mixed with 5 Ibs. of water or half the full quantity. After standing for a fortnight, one of each of these pairs was placed on a sea wall, and they were covered and uncovered by each tide. They stood there twelve montlisr and at the end of that time were brought on land and carefully broken through the middle. The results were as follows : —No 1, with the full quantity of water (10 Ibs.) was very hard and perfectly sound and dry quite through to the surface. No. 2, with three quarters of the full quantity of water (7£ Ibs.) was dry in the middle, but, on every side, the water had penetrated about 3 inclies, and had much weakened the block. No. 3, with half the full quantity of water (5 Ibs.) was wet quite through, and was very easily broken up, the water having been able to percolate continually through the block, and having dissolved much of the lime. The fellow pair of each of these was placed in fresh water, and remained the same time, with exactly similar results as to penetration of water and strength of blocks, but in these cases another result could be observed. In the case of No. 1, with the full quantity of water (10 Ibs.), the water in which it stood remained clear. In the case of No. 2 (7j Ibs. of water) the water in which it stood became milky and turbid from the formation of carbonate of lime. In tlie case of No. 3 (5 Ibs. of water) the water became quite white ; and, at the end of twelve months, the whole block was covered with crystals, a quarter to half an inch in thickness. The lime had been gradually dissolved and crystallised on the surface in the form of calcium carbonate. Similar blocks subsequently exposed in the sea wall for nearly three years gave the same results. Action of Sea-Water upon Concrete.—A great deal of discussion has arisen, and many conflicting opinions have been expressed, in reference to the durability of cement concrete in submarine situations. On the one hand, there are those wlio hold, with much practical exemplification, that concrete is in general a thoroughly reliable and durable material for use under such and, indeed, any normal conditions; and, on the other hand, there are those who point to the indubitable evidence of deterioration manifested in several well-known instances. It is a somewhat difficult matter to decide witli any finality whether these failures are due to purely * Bamber on “Portland Cement,” Min. Proc. Inst. G.E., vol. cvii.