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					                               CONCRETE WORK

CLASSIFICATION

The classification of concrete work in the standard method of Measurement of Building Works
can be seen to cover a wide variety of work, such as:

   in-situ concrete
   reinforcement
   form-work
   pre-cast concrete
   composite construction

Concrete work often forms the largest single item of work for a main contractor, on construction
other than traditional housing, and thus merits the closest attention if the most economic methods
are to be selected. In practice several different methods may be considered. They thus need
pricing, before a decision can be made as to which is the most economical.

IN-SITU CONCRETE

Work operations:

Concreting involves a chain of events; mixing, transporting, placing - and all these are inter-
related so that the speed of the slowest operation governs the rest. Since the demand for concrete
is often intermittent, it is important to determine the maximum output required and the total
concreting period. A detailed programme is thus necessary. This programme will give the size
of mixer, or mixers and the time required on site.

The cost of the mixer and ancillary equipment can be calculated, and allocated in either the
project overhead or in the bill quantities for “maintaining on site all plant required for
concreting”

Transporting Concrete

Concrete may require to be transported horizontally, or vertically and horizontally, and any
machine used for this purpose should, if possible, be capable of handling the mixer discharge in
one load. Many machines are in use for transporting concrete, and examples are given below:

   Wheelbarrows are satiable for small quantities only, or in situations where it is impracticable
    to use mechanical means.

   Dumpers in the smaller range, 500 - 750kg, are capable of carrying 0.20m3 of concrete, and
    are suitable for housing sites or other situations where the concrete has to be transported over
    fair distances on site.

   Platform hoists, where one or two wheelbarrows are placed on platform, are used when
    relatively small quantities of concrete require to be transported vertically.
   Skip hoists are suitable for larger quantities and where high production rates are required.

   Cranes of many types are now in use, such as truck cranes, crawler crane and tower cranes.
    These cranes range in lifting capacity, lift height and radius to suit most circumstances. The
    type chosen depends on many factors, such as the total quantities, maximum and minimum
    pour requirements, distance to be moved horizontally and vertically. The skip should be
    capable of carrying a complete batch of concrete.

   Concrete pumps are now in increasing use, either in static or mobile form, and         are
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    capable of very large output, e.g. 45m per hour. The concrete must be specially designed for
    concrete pouring. This usually requires a higher cement content, and the material cost may
    therefore increase.

Placing Concrete

Placing concrete in foundations, i.e. shovelling it into position and then levelling it will take
about one man-hour per cubic metre. Concrete in other locations and categories will usually take
longer to place due to restrictions in placing, etc.

Materials Costs

The following information is now required in order to calculate the cost per cubic metre of
concrete: the specified mix; the prices of cement and aggregates; and the relative weights of the
materials.

Concrete mixes

There are three methods in use for specifying concrete mixes:

   Specification by volume e.g. 1 : 2 : 4 mix being one part cement, two parts fine aggregate and
    four parts course aggregates by volume. This method has been widely used in the past and is
    widely understood, but has been superseded to a great extent by the following two
    specification methods.

   Specification by weight - called “prescribed mixes”. The most common method now in use
    being defined in CP110: part 1 1972 and BS5328:1976.

   Specification by performance - called “designed mixes”. The specifications may call for
    concrete of a certain minimum strength, e.g. 31.5N/mm2, with a minimum specified cement
    content to ensure durability.

READY-MIXED CONCRETE

More than half the total amount of in-situ concrete placed by contractors is now supplied by the
ready-mixed concrete industry. So it is usual to consider the supply of ready-mixed concrete for
most contracts. Ready-mixed concrete can be supplied by truck mixers in one of several ways:-

   the concrete is mixed at the depot and is agitated during transit;
   the concrete is mixed during transit, the material being dry-batched and water added at the
    depot;

   the concrete is mixed at the site, the materials being dry-batched at the depot and water added
    on arrival at the site;

The last method is necessary when travelling from the depot to the site will take a considerable
time.

Points to Watch

The usual load is 5m3 - smaller loads incur a surcharge.

Extra cost can be incurred due to waiting and excess time in off-loading; anything over 30
minutes on site carries on extra charge in most instances.

Good access is required on site for the very heavy trucks.

Careful timing of deliveries is required, since the Supply Company has no liability for losses
arising out of delay. Sufficient labour must be available to distribute and place concrete quickly
before initial set. Waste should be less than when mixing on site, since stockpiling waste is not
incurred. However, waste will arise during transporting and placing operations on site; a 2%
allowance is suggested, but this may vary according to circumstances.

Cost comparison with mix on site

Convenience may be a factor in the decision to use ready-mixed concrete, but price is of course
usually paramount.

A price comparison should be based on the whole process of mixing, transporting and placing the
concrete. Furthermore, the comparison should take into account the influence on other dependent
operations such as formwork and reinforcement, and most importantly the construction
programme. It is usually not sufficient to consider just the relative unit rates per cubic metre,
since the project overhead costs may be altered considerably.

Additional costs

It is necessary to study the specification and placing requirements carefully to allow for the cost
of construction joints in beds, suspended slabs and walls. Where these joints are required for
expansion or contraction purposes, they will be measured under clause F7 of the SMM7. They
can be very expensive to form, particularly, if as in usual, the reinforcement is continuous
necessitating complicated “split” formwork. Where the joints occur due to the contractors
methods of placing - usually because the end of a day’s work has been reached - these joints will
have to be measured by the estimator, priced as usual and added to the cost of the relevant
concrete items. Curing concrete slabs will be priced on a superficial basis.

				
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Description: Concrete work. Being classified in various forms, e.g in-situ, precast, this doc covers how it can be transported the cost of materials it is composed of.