A Treatise On The Principles And Practice Of Harbour Engineering

STONE: NATURAL AND ARTIFICIAL. IOI by making the concrète as impervious as possible, so that the bulk of it may be inaccessible to external influences. Concrète made from sound Portland cement, mixed in proper proportions and thoroughly incorporated, is sufficiently impermeable for all practical purposes. It may not be absolutely water-tight—this is by no means essential1 and can only be attained by the exercise of cousiderable trouble,— but it will display amply serviceable resistance to infiltration, which will prove little more than superficial. Even in those cases where chemical changes have taken place, the evidence simply points to the deposition of magnesian salts in the outer pores of concrete from which the calcic hydrate has been removed. The magnesia is an inert substance, and while, in itself, an evidence of decomposition, its presence is attended by no additional ill effects ; in fact, it may even be claimed that it exerts a beneficial action in closing up pores which would otherwise remain open for the further penetration of sea- water into the interior of the mass. In order to secure the highest degree of impermeability, a sufficiency of water must be used for mixing the concrete. An excess, of course, is objec- tionable, chiefly on the ground that it forms an incompressible volume in the fluid concrete, which passes away in evaporation, tending to leave the concrete porous. But, on the other hand, an insufficiency is attended by the evil that particles of cement may escape hydration, and this is a more vital considéra- tion, in that there is a conséquent lack of present cohesion and a source of future disturbance. It is better, on the whole, to water the concrete weil rather than sparsely ; some proportion of moisture will be absorbed by the environment, the foundation, and adjacent work, and unless the mass be allowed to harden without undue abstraction of moisture, its strength will become impaired. Speaking from long experience of a wide range of concrete work deposited in a tidal estuary, where the fluctuations of level are very great and where the circumstances are most propitious to the exercise of decomposing influences, the writer is convinced that the dangers attending the use of concrete work in maritime situations are often greatly and needlessly exaggerated. Ordinary care and discretion in the processes of mixing and deposition will prevent any evil consequences, provided, of course, the cement be of unassailable character, conforming in all respects to the requirements of the standard spécification. Another point affecting the use of Portland cement concrete in maritime work is the influence exerted by sea temperature upon its setting properties. The crystallisation or setting of cement is favoured by warmth and retarded by cold. The presence, therefore, of cold or warm currents in the sea exercises a corresponding effect upon the setting time, so that it is not a matter for surprise to find considérable variation at different places, aud even at the same place at different seasons, in the period during which concrete work 1 The remark, needless to say, applies to block work and not to reinforced concrete, the special treatment of which is described elsewhere (p. 76).