The Forth Bridge/Cement
Cement.
The cement used exclusively was Portland cement manufactured on the Medway. This also came by water, but required to be stored for a specified number of days before it could be used. The contractors purchased an old hulk—called while in its prime the Hougomont—in which ten to twelve hundred tons could be readily stored. It was moored off Queensferry, and the ships bringing cement from the south were moored alongside and discharged. From this ship the cement was brought ashore and stored in such quantities as might be required at any time.
Subsequently the Hougomont was moored close to the west side of the South Queensferry jetty, and thus a more direct and speedy communication established. While the caissons were being sunk at Queensferry a large number of foreign workmen were lodged on board this hulk, and when, shortly after new year, 1886, an epidemic of small-pox broke out at Queensferry, the ship was towed round into Port Edgar, and moored in an isolated position, and converted into a small-pox hospital. As such it proved of signal service in speedily stamping out the disease.
Cement Tests.—The cement is described in the specification to the contract as having to be of the best quality and ground so fine that the residue on a sieve containing 50 divisions on the inch, equal to 2500 meshes per square inch, should not exceed 5 per cent, by weight. It had to be kept in a dry store, and was not to be used until a certain number of samples out of every cargo had been tested. Neat cement was not to be set within less than one hour. The weight had to be between 112 lb. and 116 lb. per bushel.
For tests, the cement to be mixed with three times its weight of sand which has been passed through a sieve of 400 meshes but retained upon one of 900 meshes to the square inch (20 divisions and 30 divisions to the lineal inch). About 10 per cent, of water had to be added, and briquettes made, which were to be put into water after twenty-four hours and remain in water twenty-five days, when they had to bear a stress of not less than 170 lb. without breaking. For briquettes of neat cement the breaking stress after four days had to be not less than 300 lb., and after seven days to be not less than 400 lb. per square inch.
A few tons of quick-setting or Roman cement were used in making good the joints of the circular iron cofferdams or caissons for the Inchgarvie north piers. This cement is not very strong, but if the joints are well packed with bags of clay puddle and loaded with stones they answer the purpose of keeping the water out for a while. If all preparations are made beforehand, Roman cement can be used in a plastic state, and is thus very useful to divers working under water; but all the work requires to be done very quickly.
Mortar.—The mortar used in the masonry was in the proportion of one of cement and one of sand in the foundations, where rubble-work was used, and one of cement to two of sand in the piers. The pointing in the joints of the granite blocks was done in pure cement.
Of the other materials used, except the steel, it may be as well to state here that timber in baulk was brought from Grangemouth, ten miles up river, where it was rafted and towed to the works wherever required.
Planks, battens, and boards were brought from the same place in lighters.
Hardwood, such as oak, beech, and ash, used for packings and other temporary purposes, was got in the neighbourhood and dealt with at the saw-mills.
Coal and coke for the three main piers were brought in barges—mostly from Charleston, close by; but for the shops and yards they were brought by rail.
Creosote oil for the Lucigen lamps, and rivet and other furnaces, came in specially constructed tanks by rail.