Civil Engineering Project Management (4th Edition)

254 Civil Engineering Project Management
drawals from stock without his permission. If the contractor does not pay
sufficient attention to this and, for example, lets various steelfixers pick what
steel they think is right, the resident engineer should forewarn the contractor
this is a recipe for ultimate chaos and delay.
Properly designed and bent bars can, in the hands of a good steelfixer, be as
accurately placed as formwork. Crossings of reinforcement have to be wired
together so that a rigid cage is built, able to withstand concrete placing without
displacement. To ensure that the correct cover is given to bars, the contractor
will need to prepare many small spacer blocks of concrete of the
requisite cover thickness and about 25 mm square, which are wired onto the
outside of reinforcement, keeping it the required distance from the formwork
to give the specified cover. All wire ties should be snipped off close to the reinforcement
so that their ends do not penetrate the concrete cover and form a
path for corrosion of the reinforcement. The steelfixer will need to make and
position spacer bars, generally U-shaped, which keep reinforcement layers
the correct distance apart in slabs and walls. He may need many of these.
They are not included in the bar-bending schedules and the cost to the contractor
of supplying and fixing them is usually included in the price for
steelfixing. Fig. 19.5 shows some points to watch when formwork and reinforcement
is being erected.
Steel reinforcement stored on site rusts, but provided the rust is not so
advanced that rust scales are formed, the rust does not appear to affect the
bonding of the reinforcement to the concrete. A problem more likely to arise
is the contamination of steel reinforcement with oil, grease, or bitumen. If the
contractor wishes to oil or grease formwork to prevent it sticking to concrete,
he should do so before the formwork is erected and not after it has been put in
place. If the latter is attempted it will be almost impossible to prevent some oil
or grease getting onto the reinforcement. Similarly, if contraction joints are to
be bitumen painted, care must be taken not to get bitumen on bars passing
through such a joint.
The proper design and detailing of reinforcement makes a major contribution
to the quality and durability of reinforced concrete. The designer must
choose diameters, spacings and lengths of bars which not only meet the theoretical
design requirements but which make a practical system for erection
and concreting. Reinforcement to slabs must either be strong enough for the
steel fixer to stand on, or spaced far enough apart for him to get a foot
between bars onto the formwork below. Wall and column reinforcement must
be large enough diameter that it does not tend to sag under its own weight.
Beam reinforcement should not be so congested that it will be difficult to get
concrete to surround the bars without using a mix with too high a water content.
The designer should consider options of design available to avoid heavy
congestion of bars. An experienced designer who understands site erection
problems will make as much use as possible of the four most commonly used
bar diameters – 10, 12, 20 and 25 mm. He will appreciate that a 5 m long bar
25 mm diameter weighs about 20 kg, so that larger diameter or longer bars can