Page 258 - Civil Engineering Project Management, Fourth Edition

Civil Engineering Project Management
238
by the engineer. In the material which follows only the ordinary matters which
those on site will be expected to deal with are described.
It should be mentioned here that, in recent years there has been an increas-
ing trend to use cement blends. These may be either Portland cement/pulver-
ized fuel ash (PFA), OPC/PFA, or Portland cement/ground granulated blast
furnace slag (GGBS), OPC/GGBS.
PFA is a residue of pulverized coal burnt in the furnaces of many modern
power stations. It can be supplied to the concrete mixer as a component of a
ready-blended cement, or as a separate material with its own storage and hand-
ling facilities. The advantages of the use of PFA in concrete are: overall econ-
omy of materials, improved workability and compactability, reduced water
content, reduction of heat evolution, increased resistance to chemical attack
(sulphate or acid). The advantage of improved workability is of considerable
benefit where concrete pumping is required.
GGBS is a by-product of iron manufacture, where slag issuing from a blast
furnace at a temperature of approximately 1500 degrees centigrade is rapidly
quenched in water. This material is subsequently dried and ground to a fine
powder which again can be supplied to the concrete mixer as a component of a
ready-blended cement, for example, Portland blast furnace cement (PBFC), or
as a separate material with its own storage and handling facilities. The advan-
tages of the use of GGBS in concrete are: increased strength over the longer term
(slower strength gain at first, then catching up and overtaking normal OPC
concrete at 28 days and beyond), reduction in water content for equivalent cohe-
sion, flow and compaction particularly when pumping, reduced heat evolution
especially with thick sections and improved resistance to most forms of chem-
ical attack (sulphate or acid). This is particularly advantageous in foundation
works subject to sulphate attack.

19.2 Standards for concrete quality

The specification should define the mixes or grades of concrete required and
in what parts of the works each mix or grade is to be used. As mentioned in
the preceding section, both BS 5328-2:1997 and BS 8500:2002 describe four
classes of concrete mixes – designed mixes, prescribed mixes, standard mixes,
and designated mixes.
Designed mixes are specified by the purchaser stating the characteristic
strength required, maximum size of aggregate and minimum cement content,
leaving the supplier to design the mix proportions.
Prescribed mixes are specified by the purchaser stating the proportions of
the mix constituents required – cement, aggregate, size and type, etc. – the pur-
chaser being responsible for the performance of the mix.
Standard mixes are set out in a Section 4 Table 5 of BS 5328-2, and also in BS
8500 where they are called Standardized Prescribed Concretes. They are for con-
2
crete of characteristic strengths from 7.5 to 25N/mm and BS 5328 gives the