Civil Engineering Project Management (4th Edition)
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238 <strong>Civil</strong> <strong>Engineering</strong> <strong>Project</strong> <strong>Management</strong><br />
by the engineer. In the material which follows only the ordinary matters which<br />
those on site will be expected to deal with are described.<br />
It should be mentioned here that, in recent years there has been an increasing<br />
trend to use cement blends. These may be either Portland cement/pulverized<br />
fuel ash (PFA), OPC/PFA, or Portland cement/ground granulated blast<br />
furnace slag (GGBS), OPC/GGBS.<br />
PFA is a residue of pulverized coal burnt in the furnaces of many modern<br />
power stations. It can be supplied to the concrete mixer as a component of a<br />
ready-blended cement, or as a separate material with its own storage and handling<br />
facilities. The advantages of the use of PFA in concrete are: overall economy<br />
of materials, improved workability and compactability, reduced water<br />
content, reduction of heat evolution, increased resistance to chemical attack<br />
(sulphate or acid). The advantage of improved workability is of considerable<br />
benefit where concrete pumping is required.<br />
GGBS is a by-product of iron manufacture, where slag issuing from a blast<br />
furnace at a temperature of approximately 1500 degrees centigrade is rapidly<br />
quenched in water. This material is subsequently dried and ground to a fine<br />
powder which again can be supplied to the concrete mixer as a component of a<br />
ready-blended cement, for example, Portland blast furnace cement (PBFC), or<br />
as a separate material with its own storage and handling facilities. The advantages<br />
of the use of GGBS in concrete are: increased strength over the longer term<br />
(slower strength gain at first, then catching up and overtaking normal OPC<br />
concrete at 28 days and beyond), reduction in water content for equivalent cohesion,<br />
flow and compaction particularly when pumping, reduced heat evolution<br />
especially with thick sections and improved resistance to most forms of chemical<br />
attack (sulphate or acid). This is particularly advantageous in foundation<br />
works subject to sulphate attack.<br />
19.2 Standards for concrete quality<br />
The specification should define the mixes or grades of concrete required and<br />
in what parts of the works each mix or grade is to be used. As mentioned in<br />
the preceding section, both BS 5328-2:1997 and BS 8500:2002 describe four<br />
classes of concrete mixes – designed mixes, prescribed mixes, standard mixes,<br />
and designated mixes.<br />
Designed mixes are specified by the purchaser stating the characteristic<br />
strength required, maximum size of aggregate and minimum cement content,<br />
leaving the supplier to design the mix proportions.<br />
Prescribed mixes are specified by the purchaser stating the proportions of<br />
the mix constituents required – cement, aggregate, size and type, etc. – the purchaser<br />
being responsible for the performance of the mix.<br />
Standard mixes are set out in a Section 4 Table 5 of BS 5328-2, and also in BS<br />
8500 where they are called Standardized Prescribed Concretes. They are for concrete<br />
of characteristic strengths from 7.5 to 25 N/mm 2 and BS 5328 gives the