119 



these failures caused the material as a whole to fall into disrepute, as has Ijeen the 

 case with many other good materials. The use of concrete has persisted however and 

 sufficient time has elapsed to disclose the reasons for failures and the means by which 

 most of them may he a\-oidcd and successful results obtained. 



COMBIX.Vno.XS OF CONCRETE .\ND TIMBER 



Combinations of these two materials have been used in a wide range of t>pes, 

 from those in which timber supporting units are protected by concrete shells to those 

 in which concrete supporting units are superimposed on timber pile foundations. 

 The former, from the standpoint of concrete itself, is a secondary use, the latter 

 falling with the use of concrete as a primary construction material. When used as a 

 protection for timber the concrete may be applied either during initial construction 

 or as a repair protection to existing timber. 



The substructure uses of concrete, of both the above classes, may farther be 

 di\-ided into those cast-in-place and those pre-cast. 



The cast-in-place types all involve the common procedure of placing a form and 

 depositing the concrete in place in the structure. The types differ in the details of 

 form material and construction and in the method of depositing the concrete. Good 

 results are possible if the forms are tight so that salt water and mud can be excluded 

 from the concrete while it is being placed. This usually involves driving or sinking 

 the form into the substrata, pumping out the mud and water, and sealing the bottom, 

 unless the substrata are sufficiently solid that the form will remain empty without it. 

 As the depth of water increases it becomes increasingly difficult to obtain sound 

 concrete and, in pile jackets, even thickness and good reinforcement in so thin a shell. 



The pre-cast types invoke units completely constructed on shore and subse- 

 cjuently placed in the structure. The great advantage of this method is that the unit 

 can be carefully constructed and a good quality of concrete obtained, a condition 

 which is difficult to secure when the work is done in place in water. Pre-cast types 

 include the timber pile protections in which a pre-cast reinforced concrete sleeve or 

 jacket is slipped over the pile after it has been driven, and those in which the concrete 

 or cement cover is fabricated on the pile before driving; pre-cast "socket" columns 

 in which a concrete column is cast with a long socket at its lower end such that it can 

 be slipped over a single timber pile cut off near the mud line: large pre-cast reinforced 

 concrete caissons, either bearing directh' on hardpan or enclosing a pile cluster, which 

 are ultimately filled solid with concrete; and pre-cast reinforced concrete piles. 



Concrete Protections for Wooden Piles 

 Inasmuch as this use of concrete forms is in reality a phase of the external pro- 

 tections for wooden piling, which have already been discussed for other materials 

 than concrete, its discussion will here be undertaken before that of the use of concrete 

 as a primary construction material, with the understanding that the considerations 

 in respect to fabrication of the concrete and its pouring, setting, protection from 

 deterioration, etc., to be presented in the discussion of the latter apply in general 

 with equal force to the pile protection form of concrete construction. 



protections c.\st in peace 

 There are several different methods, which are quite distinct from each other. 



The Stationary P'orm Type 

 Placing full-length, stationary forms around piling and filling the forms with 

 concrete has been tried many times, usually as a protection for piling in place which 



