MATERIALS. 123 



the sea almost immediately after mixing, and while yet soft. It 

 is exceedingly useful for pointing joints, and for other purposes 

 in tide work, also for protecting Portland cement until it is hard 

 enough to take care of itself. 



Mr. Messent, M. Inst. C.E., engineer to the River Tyne Com- 

 missioners, has kindly furnished me with the following par- 

 ticulars respecting the use of Roman cement in the Tyne piers 

 and elsewhere : 



" Throughout the pier work I have been in the habit of using 

 Roman cement mortar in the walls up to about high water, and, 

 except in the very finest weather, the Roman cement concrete 

 hearting is taken up nearly as high, but this depends greatly upon 

 the weather. 



" The strength of the composition of the materials also varies 

 according to the season and exposure. 



" For ordinary work, exposed to a wash immediately after setting, 

 the proportions I have used are 1 cement, 1 sand, and 2 gravel, with 

 as many largish stones or * displacers ' as can be got in and separated ; 

 but towards the end of the season, in any very exposed places, I 

 nearly double the quantity of cement. 



"I have recently constructed a small breakwater at Cullercoats 

 entirely of Roman cement concrete rubble. 



11 1 have also recommended the protection of the outer breakwater 

 at Scarborough by an apron similarly composed, which, although the 

 work was commenced in August last (1890) and finished at the end 

 of November, and has since been exposed to very heavy storms, has 

 not suffered in any way. 



" As to the strength of Roman cement, if it were finely ground I 

 dare say it would not be very much inferior to that of Portland 

 cement ; but I never use it with more than an equal proportion of 

 sand, and frequently, in exposed places, with less." 



In order to protect concrete blocks from erosion, they are 

 often faced with hard stone. This is of great importance where 

 the face is likely to be exposed to the action of rubble or shingle 

 driven and rolled about by the sea (see p. 106). 



Well-built blocks of hard rubble in cement are stronger, and 

 better able to resist wear, than those made of concrete ; but 

 they are more costly, and cannot be made so quickly, 



