143 



TIk- problem is lari;fly one of liahiiuing the cost of cement and materials at;ainst the 

 labor cost of mani|nilation, the richer mixes reciuiring less tamping and compacting 

 than the poorer mixes. But no increase in cement will adequately offset excessively 

 wet consistencies and careless manipulation, because of the separation of ingredients 

 and ft)rmation of laitance which occur under such circumstances. Experience has 

 proved that concretes of the proportions designated in these specifications and of such 

 consistency that an average tamper can work two yards per hour into a uniform plastic 

 mass are satisfactory for sea water use. 



Methods of delivery which deposit concrete more rapidly than it can be tamped, 

 or which are liable to recjuire wet consistencies, are not suitable for use in marine 

 concrete construction. For this reason small containers, the dumping of which can 

 be controlled at the point of deposit, are preferable to chutes. 



With regard to tamping, it is advisable to require an excess rather than a de- 

 ficiency, in order to insure uniformity. Enclosed shapes having restricted openings 

 should receive more than the specified amount of working. Mechanical tamping by 

 means of pneumatic hammers, or by utilizing the impact of pile hammers, is sometimes 

 advisable under special conditions. When such a mechanical means is used, however, 

 the forms should be watertight; otherwise, owing to intense vibration, there is a 

 tendency to draw the mortar away from the coarse aggregate. 



Joints with set concrete are frequently sources of disintegration. This results, 

 not only from failure to clean laitance and dirt from the old surface, but also because 

 concrete deposited against rigid surfaces requires additional tamping and spading 

 to make it plastic and requires more attention than when deposited against unset 

 concrete, which vibrates and puddles with the new deposit. 



Proper curing is probably of more importance for air exposed than for sea water 

 exposed structures. Concrete immersed in sea water cures and sets under ideal con- 

 ditions. While it is preferable to leave forms in place in order to protect the surface 

 from impacts, no detrimental effects resulting from early exposure to sea water have 

 been observed at San Francisco, either in construction work or in laboratory tests. 

 For facilitating special construction, exposure in 48 hours may be permitted. Pre- 

 cast concrete is often reduced in strength and checked with surface cracks by exposure 

 to the sun and wind, and should be protected against too rapid drying. 



"Simple" concrete structures, whether subjected to protected "harbor" exposure 

 or to "ocean" exposure, may be relied upon to resist sea water permanently if the 

 concrete is intelligently mixed and deposited in accordance with the provisions of 

 these specifications. The principal abuses to guard against are flooding the mix with 

 excess water and failure to tamp and compact the mass thoroughly in the forms. 



Another condition which makes for defecti\e concrete in this class of structure 

 is the flooding of the forms with sea water during concreting. When freshly mixed 

 concrete comes in contact with sea water, magnesium salts are precipitated in con- 

 siderable quantities, increasing the deposit of laitance. This, combined with separation 

 and the laitance ordinarily caused by excess water, produces concrete of questionable 

 quality, unsuited for sea water exposure. Systems of construction should not be used 

 which are liable to subject the concrete to seepage of sea water during placing. Flood- 

 ing of concrete protected by forms immediately after placing, as in the case of con- 

 crete deposited during low tide, is not objectionable provided the spreading and 

 compacting is completed before the flooding occurs. 



As previously stated, tremie concrete shoukl not V)e depended upon to resist 

 sea water unless protected by an impervious outer layer of concrete or other material. 

 While concrete is sometimes tremied through small pipes into the restricted openings 



