During the summer months, the action of the water from day to day was not perceptible ; the surface still remained tolerably hard ; occasionally portions of the fine fit•ing spa_ rated from the rest Of the block, owing, it was said, times to want of care in the original construetion, sometimes to injuries caused by boats or vessels striking du. wall ; in these cases, however, a new facing of cement was applied, :mil before the winter, the general appearance of the wall was to a certain extent satistitctory.
During a hard frost, however, evidences of failure began to show themselves ; • and as soon as the thaw allowed a thorough inspection of the face of the wall to be made, it was found that hardly a single block had escaped damage ; in many instances, the whole face had peeled off to the depth of half' an inch ; at one spot, where a drain discharged itself into the river from a height of about six or eight feet, the back action of the water atter its fall, had worn away the lower courses to the depth of some inches. These were the evidences of the action of frost and water combined, upon the best constructed wall at 'Woolwich. At Chatham, they were of the same character, but the damage done to the wall was much greater.
The portion of river-wall at Woolwich, which was built with rough concrete, was severely injured by the common action of the water before frost, and the same result was observed in the walls of a school near Blackheath, which were built of concrete some years ago : at the ground-line, where the drip of the water hadacted, the concrete was soft, and yielded easily to any force applied, while the walls above were very fairly hard, and seemed to have stood very well. The results or the observations made at that time, on the use of concrete in con structions of a kind similar to those above mentioned, are summed up by Captain Denison in the opinion " that in climates like ours, in situations exposed to the alternate action of water and air, concrete cannot he advantageously used as a building material. the apparent economy, caused by the cheapness of the material employed, being more than compen sated for by the frequency of repairs." In the report (dated 18.16) and of the " Committee on the Harbour of Refitge to be constructed in Dover Bay," a great deal of valuable intOrmation is afforded on the use of concrete. Amongst the various plans submitted to the com
mittee, Captain Denison, Colonel Jones, and Mr. Vignoles proposed to eonstrnet breakwaters of blocks of concrete. The first of these gentlemen recommended that the blocks should be manuthetured at Dungeness, and thence floated to Dover by means of camels. The French adopted a similar plan in their works at Algiers. where large blocks of betas, or hydraulic concrete, were floated out to the required spot, and then allowed to drop into their places from slings. These blocks were rectangular in form, and measured 324 cubic feet. At the works at St. Joilette, at Marseilles, also, immense blocks of concrete, 13 yards cubic measure in size, have been sunk ft T the foundation. The form suggested by Captain Denison was that of an hexagonal prism, and it was considered each block would weigh from 20 to 30 tons. The concrete would be made in the following manner :—the gravel of sea-beach to be mixed with the best hydraulic lime in the proportion of ten or twelve parts of gravel to one of lime ; and with the view of causing it to set more speedily tinder water, a proportion of puzzolana should be added, vat•ying in quantity according to its quality ; half the quan tity of puzzolana to that of lime would make very hard, sound concrete, which would set rapidly ; but if desirable to make it set very quick, the quantity of puzzolana might be increased till it equalled that of the lime. The concrete used by Mr. Ranger at Woolwich was nearly the same, except that he used no puzzolana.
In the course of Captain Denison's evidence he refers to the works at Chatham and Woolwich, to which we have already alluded, and states that he had again examined the wall at Woolwich, and found the interior as hard as could be wished. Those parts, however, of the concrete ftcing, which were exposed to the mechanical action of the water, were injured by it ; and therefore, though recommending concrete below low-water mark, he was bound to admit that it was not adapted to those situations where it must be exposed to such action.
The speak gravity of concrete, as compared with that of other materials, is as follows : Concrete weighs about 140 lbs. to the cubic foot.