F. 14. Read the text Tower Cranes and then translate it using a dictionary

Tower cranes are a comparatively recent innovation and have already proved their worth. They consist of a vertical mast structure mounted on a wheeled chassis, the mast being built up in sections to suit individual requirements and rotatable through 360°. The wheeled chassis is self-propelled on railway track and the whole base is suitably ballasted for stability. The design of the jib varies, there being two main types: a fixed horizontal jib and a raisable jib.

One or more control cabins are located in the mast section and in one design the cabin may be moved up or down inside the mast like a lift to enable the operator to select the best inspection point. Tower cranes are electrically operated from an external power source and incorporate the usual safety devices. They are best suited to restricted sites where derrick cranes would be difficult to accommodate.

Investigation has shown that, if properly used, tower cranes will effect considerable savings in building costs by avoiding the necessity for double handling of materials and by enabling large pieces of structure to be prefabricated on the ground and then hoisted into position. To obtain full value of the machine, however, it is essential that the job be large enough and the work planned in advance to ensure that the crane is kept fully employed.

F. 15. Read the text and then discuss it

Chemically resistant concrete may be sometimes used in the construction of structures attacked by chemically active media, i.e. industrial, hydraulic and underground structures. It has been proposed to prepare a chemically resistant concrete using a binder a vitreous sodium silicate. When such a vitreous silicate is dissolved in water, liquid glass is obtained. In order to assist in the solidification of the liquid glass and increase its water resistance certain elements are added to the concrete composition. They serve to neutralize the alkali in the liquid glass and convert it into a water-insoluble compound. Thus, during the course of the neutralization of the alkali, free silica is evolved from the liquid glass in the form of a gel which serves as a binder.

Notes: free silica is evolved выделяется кварц chemically resistant кислоупорный

a vitreous sodium silicate стекловидный силикат натрия solidification затвердение