Discussion on Portland Cement and Concrete.

Abstract

Vice-President, remarked that in a discussion five years ago on Concrete as applied to the Construction of Harbo~rs,'~ he drew attention to failures of concrete made with Portland cement.1 Mr. Bamber was led, he believed, by those failures, and by conferences with himself, to undertake the investigations he had recorded. Mr. Bamber, in his Paper, dealt chiefly with three questions: (1) the specific gravity of Portland cement; ( 2 ) the ingredients of Portland cement; and( 3 ) the quantity of water that should be used in Portland cement mixtures. The primitive method of taking the weight per bushel might give results, from the same heap, varying as much as 2 per cent. Mr. Bamber stated that a properly clinkered and ground cement, when new, will have a specific gravity of 3 1 to 3 . 1 5 ; " and, with this knowledge, it was open to any one to adopt this more scientific test of weight, which was also advocated by Mr. Carey. As regarded the ingredients, he had always advocated analysis; but this had been hitherto precluded by uncertainty about the proper constituent parts, and especially as to the percentage of lime. Mr. Bamber, whilst stating that 50 per cent. of lime seemed theoretically correct, said that, with the present form of kilns, a better cement was obtained, having 61 to 62 per cent. of lime, which was unsatisfactory, as there was danger in an excess of lime. Engineers had demanded excessive tests at the outset ; and the manufacturers had consequently to over-lime the cement, whilst protesting against such high tests too soon after manufiacture. One of the consequences of an excess of lime was that it might not be all taken up by the silica and alumina; and the free lime in a concrete mixture would rise to the surface and form a slurry " ; and if this was not effectually removed, which was difficult to ensure, the next layer would not unite with the lower one; and there was also the waste of so much of the cement. Again, after a while free lime would absorb carbonic acid from sea-water if the work was in the sea, or from the atmosphere if on land, forming a carbonate of lime which might be injurious to the work. The effect was more specially seen in pavings, which were often cracked all over from this cause, he believed. But the most mischievous l Niuutes of Proceedings InBt. C.E., vol. Ixxxvii. p. 162. _ _ _________Downloaded by [] on [11/09/16].