A Treatise On The Principles And Practice Of Dock Engineering

66 DOCK ENGINEERING. gives the following équation,* from experience gained in drawing a number of piles against the frictional resistance of clay. WH P P 500 (5) the weight of the ram (W) being taken in tons. P is the extreme resistance of the pile, also in tons. H and D, as before, are the height of fall and the depression under the last blow respectively, both in feet. Concrète Mixers.—Concrete can be very efficiently mixed by hand, but the process is slow and only suitable in dealing with small quantities. When the requirements are large, as in block and mass work, it will be much more economical and expeditious to employ mechanical agency. So many varieties of concrete mixers, each with its own particular merits, are on the market, that it is an utter impossibility within the limits of a moderate chapter to review them all; and although it is a somewhat invidious task to select one or two examples for illustration, such a step is inevitable, and must not be understood to convey any depreciation of those machinés whïch are unävoidäbly excluded. The principal features of an efficient concrete mixer are a thorough and intimate incorporation of the ingredients and a rapid and regular discharge of material. Concrete mixers are of two kinds—intermittent and continuous. Tn the former class, charges are mixed separately; in the latter, they follow one another in unbroken sequence. More perfect incorporation of the in- gredients is the particular claim of the intermittent mixers, while the continuous mixers afford greater regularity of supply. In both instances, that machine must be reckoned best in which the churning action is most thorough. Intermittent Mixers—Messent Mixer.—The best known of the earlier types of mixer is that due to the late Mr. P. J. Messent, of Tynemouth, and the following description of it is extracted from the circular of the makers, Messrs. Stothert & Pitt, of Bath:— “It consists of a closed box or chamber, A (fig. 29), revolving on an axle, and of such a form as, when half-filled with the materials, to cause them to be turned over sideways, as well as endways, four times in each revolution of the chamber, so that in from six to twelve revolutions (the number necessary being varied according to the weight and nature of the materials) a more perfect mixture is effected than could possibly be produced by hand, or (except in a much longer time) by any other machine. “ For filling concrete into a trench, or the hearting of a pier, the machine is supported over the opening, on two balks of timber; a waggon containing the gravel (and cement in bags) follows on the same line. The hopper, shown in the figure, suspended from a davit, is made to contain the * Hurtzig on “The Friction of Timber Piles in Clay,” Min. Proc. Inst., G.E., vol. Ixiv.