Precast concrete materials, manufacture, properties and usage - Chapter 1 pdf

1

MOULDS AND MATERIALS

With the exception of admixtures and fly ash, all moulds and materials

are discussed in this chapter. None of the factors listed can be considered

in isolation since variation in one will often affect another. Mix design

for various forms of precast manufacture is dealt with in Chapter 6. The

purpose of this chapter is to acquaint the reader with all the starting

variables. The background picture will then be fully understood before

one proceeds to put these variables into a process, in order to produce a

precast concrete product.

1.1 MOULDS

Moulds are basically means by which:

(a) concrete is kept to a required shape until it is strong enough to be

demoulded, or

(b) concrete is moulded on a machine and retains that shape on virtually

instant demoulding, or

(c) concrete is shaped immediately after casting using an additional or

secondary mould acting on previously un-moulded surfaces.

In the sections that follow are outlined the types of moulding materials

available and how they should be selected. Due to geographical and/or

economic reasons one might be forced to a second or third choice, and

this is acceptable provided that the persons responsible for this choice

appreciate the limitations in use.

Notwithstanding all other factors, the one thing that all moulding

techniques and moulds have in common are dimensions. Whether these be

critical for structural, architectural and/or contractual reasons is a matter

Copyright Applied Science Publishers Ltd 1982

that causes quite a lot of argument. It is imperative that one appreciates

the reasons for dimensions and what tolerances are permissible when

combining the two fields of manufacture and installation.

The specification for the product should state strictly what is required,

bearing in mind what is practical and how the product is to fit into the

main construction. All too often precast products such as cladding are

specified on a dimension such as:

where A is the target dimension often called the work size.

Two important points need to be borne in mind:

(a) Tolerance is an easy thing to find during construction but is a very

difficult thing to lose. By this is meant that a product that is too large

will generally cause more problems than a product that is too small,

i.e. a joint can be filled with mortar, sealant, etc., when the product

is nearer A-y but needs cutting back when there is too much A+x.

(b) Moulds tend to grow in size with continuous usage.

What all this means is that there are a large range of products where

tolerances for a dimension of A are best specified as A-y.

Figure 1.1 shows how a joint can be designed to cater for resistance to

arris damage and give apparent uniform joint thickness.

Fig. 1.1. Chamfered joint to cater for tolerances and arris damage.

Mould construction as well as mould materials play important roles in

shape control. It cannot be stressed too strongly that any parts of the

mould designed to be dismantled should be rigidly fixed at all times

during the setting-out, casting and hardening process. Only in the case of

products such as window-in-panel, culvert units, etc., should the internal

moulding be slackened as soon as practicable in order to avoid the setting

shrinkage of the concrete causing stress round the internal opening.

Dismantleable mould parts should fit snugly together otherwise grout

Copyright Applied Science Publishers Ltd 1982

leakage will occur with subsequent risk of concrete flashings and

honeycombing.

Sealant tapes and compressible seals are often ideal solutions to such

problems. Sealant tapes are generally adhesive PVC tapes 10–25 mm

wide which may be stuck along the joint. The compressible seals are

adhesive-backed expanded soft plastics tape that may be placed inside

the joint at corners, etc.

1.1.1 Steel moulds

Steel moulds, die-head and extruders are used in virtually all large

production processes, whether machine-intensive or vibrated wet-cast

labour-intensive large-scale production. Obviously the strength and

abrasion resistance of steel makes it the best choice. However, no matter

how resistant steel is to abrasion it does wear with use and a time comes

when either refurbishing or replacement becomes necessary. It is up to

the precaster to initiate a scheme for regularly checking the dimensions of

the moulding system and to decide when action needs to be taken and the

form it will take.

Concerning the shrinkage onto openings in a mould mentioned earlier,

Fig. 1.2 illustrates a steel window-in-wall unit where the braces across the

Fig. 1.2. Steel mould with collapsible internal moulding

Copyright Applied Science Publishers Ltd 1982