Waterproofing Basements ~ basements can be waterproofed by one
of three basic methods namely:-
1. Use of dense monolithic concrete walls and floor- Tanking techniques (see pages 274 & 275)
- Drained cavity system (see page 276)
Dense Monolithic Concrete † the main objective is to form a watertight
basement using dense high quality reinforced or prestressed concrete by
a combination of good materials, good workmanship, attention to
design detail and on site construction methods. If strict control of all
aspects is employed a sound watertight structure can be produced but it
should be noted that such structures are not always water
vapourproof. If the latter is desirable some waterproof coating, lining or
tanking should be used. The watertightness of dense concrete mixes
depends primarily upon two factors:- - Water/cement ratio.
- Degree of compaction.
The hydration of cement during the hardening process produces heat
therefore to prevent early stage cracking the temperature changes
within the hardening concrete should be kept to a minimum. The greater
the cement content the more is the evolution of heat therefore the mix
should contain no more cement than is necessary to fulfil design
requirements. Concrete with a free water/cement ratio of 0.5 is
watertight and although the permeability is three time more at a ratio
of 0.6 it is for practical purposes still watertight but above this ratio
the concrete becomes progressively less watertight. For lower water/
cement ratios the workability of the mix would have to be increased,
usually by adding more cement, to enable the concrete to be fully
compacted.
Admixtures † if the ingredients of good design, materials and
workmanship are present watertight concrete can be produced
without the use of admixtures. If admixtures are used they should
be carefully chosen and used to obtain a specific objective:-
1. Water-reducing admixtures † used to improve workability- Retarding admixtures † slow down rate of hardening
- Accelerating admixtures † increase rate of hardening † useful for
low temperatures † calcium chloride not suitable for reinforced
concrete. - Water-repelling admixtures † effective only with low water
head, will not improve poor quality or porous mixes. - Air-entraining admixtures † increases workability † lowers water
content.
Waterproofing Basements