CONCRETES
Most concrete is required to carry some applied load and to have a rea- sonably economic life without excessive maintenance. When we talk about the 'strength of the concrete we normally refer to its compressive strength measured by tests on cube specimens, but other forms of strength, such as the transverse strength, may be important.
as
be
Most ordinary concrete should durable under ordinary conditions of exposure but for a concrete to be dur- able when exposed to aggressive agencies such as various forms of chemical at- tack, or when subjected to heavy abra- sive forces, special precautions have to be taken. When reinforced, the con- crete must also provide a.n effective cover to prevent corrosion of the rein- forcing steel.
Bending strength
There are conditions where the trans- verse or bending strength of the con- crete is of greater importance than the compressive strength, as in road slabs. The shape and surface texture of the aggregate may then be important; con- crete made with a crushed rock has a higher transverse strength than a con- crete made with a sand and gravel ag- gregate having the same compressive strength; if follows that, if a concrete is required basically for its transverse strength, a more economical mix may be possible if a crushed rock is used.
Structural lightweight concrete
Unlike ordinary lightweight concrete, in which large air voids may be desir- able, to improve the thermal insula- tion, structural lightweight concrete is fully compacted so as to provide cover and bond to the reinforcing steel. This type of concrete combines the advantage of a reduction in dead-weight with an adequate structural strength.
Aggregate
Owing to the particle shape and sur- face texture of most lightweight aggre- gates, the mix is not as workable as ordinary concrete and some of the fines may be replaced by sand to improve the workability; this has the effect of in- creasing the density of the concrete (the higher density values in the table relate to sanded mixes).
Resistance to chemical attack
One basic requirement for any con- crete to withstand chemical attack is that it should be thoroughly compacted and dense. It may be necessary to use types of cement other than ordinary Portland cement, such as sulphate- resisting, pozzolanic, super-sulphate or high-alumina cement.
Surface finishes
Special aggregates and casting tech- niques can be used to get a desired colour and texture. Considerable care is needed in the proportioning and mixing of the concrete in order to obtain uniformity.
Concrete for fire protection
The behaviour of concrete when sub- jected to great heat is largely dependent upon the type of aggregate used. Flint gravel and, to a lesser degree, other natural stones expand greatly and cause the concrete to spall.
Some aggregates such as clean crushed clay brick and slag do not causse spall- ing; any weakening of concrete based on these aggregates is due principally to the dehydration of the cement. If there is no spalling, the process of disintegra- tion of the concrete is very slow.
Disintegration beyond the plane of the reinforcement is usually much slower if the disintegrated material is held in position by the steel, since this will delay the rise in temperature of the material in the interior of the structural member.
CONCRETES
Density of Compressive Thermal
strength conductivity lb/ft2 B.t.u./ft2h
°F in.
concrete lb/ft3
10
8
3
2
1
50
100
Density 16/cu ft
150
The accepted classification of aggre- gates in descending order of merit, for fire resistance of the concrete made from them, is as follows:
Class 1-
(a) Pumice and foamed slag. (b) Blastfurnace slag, crushed brick and burnt clay products, well burnt clinker and crushed lime- stone.
-
Class 2 (a) Siliceous aggregates gen- erally; e.g. flint gravel, granite and all crushed natural stones other than limestone.
A particular advantage of lightweight concrete for fire protection is the possible economy in the design of a structure due to the reduced dead load.
(Continued in page 74)
Particular use
64
Density of
aggregate
lb/ft3
Expanded vermiculite and perlite
4-15
25-70
70-500
0.75-2.0
Aerated concrete
25-90
200-1500
0-75-2-5
Pumice
30-55
45-70
200-550
1.50-2.0
For
Foamed slag
30-60
60-95
200-800
1.50-3.0
Sintered p.f.a.
40-60
70-80
400-1000
thermal insulation
Expanded clay or shale
35-65
60-75
800-1200
2.30-3.20
Clinker
45-65
65-95
300-900
2.80-4.0
Foamed slag
30-60
105-130
2000-5000
For fire
Sintered p.f.a.
(structural
40-60
85-110
2000-5000
Expanded clay or shale)
concrete)
35-55
85-115
protection
2000-5000
Crushed brick
70-85
Crushed limestone
85-100
Flint gravel, and crushed stone
85-100
105-135 135-150 3500-5000 140-155
(i) 2000-6000
2000-4000
For load- carrying
and
durability
Fire resistance Class I
(ii) 6000-10000
Fire resistance Class 2
THE HONG KONG & FAR LAST BUILDER-VOLUME 16, NUMBER 4