Hydraulic Structures: Fourth Edition

immersion vibrators mounted on suitable tracked plant running on the

surface of the concrete pour.

Planning of concreting operations is a vital but difficult task. Details

of planning methods and production, placing and compacting techniques

are given in Thomas (1976) and Jansen (1988).

It should be noted that the primary objective in concrete production

and placing over the period of construction is to ensure uniformityand

consistency of qualityin the mature finished product.

3.7 The roller-compacted concrete gravity dam

3.7.1 General

The concrete gravity dam shares with the embankment the central attributes

of simplicity of concept and adaptability, but conventional mass concrete

construction rates, unlike those for embankment construction, remain essen-

tially as they were in the 1950s. The volume instability of mass concrete due

to thermal effects imposes severe limitations on the size and rate of concrete

pour, causing delay and disruption through the need to provide contraction

joints and similar design features. Progressive reductions in cement content

and partial replacement of cement with PFA have served only to contain the

problem. Mass concrete construction remains a semi-continuous and labour-

intensive operation of low overall productivity and efficiency.

In some circumstances the technical merits of the gravity dam and

the embankment may be evenly balanced, selection resting on estimated

construction cost. Economic advantage will almost invariably favour the

embankment, particularly if constructed in compacted rockfill. In some

instances, however, factors such as locating a spillway of sufficient capacity

etc. may indicate the concrete gravity dam as being a preferable design

solution, provided that the cost differential lies within acceptable limits.

Despite advances in embankment dam engineering, therefore, there

remains a strong incentive to develop a cheaper concrete gravity dam.

The problem of optimizing concrete dam construction and reducing

costs can be approached in several ways. In the absence of progress

towards an ideal cement and a dimensionally stable concrete the most

promising lines of approach may be classified as follows:

1. a reappraisal of design criteria, particularly with regard to accepting
   modest tensile stresses;


2. the development of improved mass concretes through the use of
   admixtures to enhance tensile strength and to modify stress–strain
   response, and/or the use of modified cements with reduced thermal
   activity;

174 CONCRETE DAM ENGINEERING