Concrete for Precasting small items
1. Introduction
Precasting is an economical means of producing high-quality concrete items.
This leaflet gives information for small-scale precasting of relatively
small unreinforced or reinforced items such as paving slabs, planks and
elements for post-and-panel walling.
It covers: choice of materials; mix proportions; and manu-facture of concrete.
Concrete masonry units (bricks and blocks), prestressed concrete and heat-accelerated
curing are outside the scope of the leaflet.
2. Important points
- Surface damage and breakages must be avoided if the precasting operation is to be economically successful. Precast items are especially prone to damage and breakage when they are demoulded and when they are handled. Concrete should therefore have a relatively high strength which should develop as rapidly as possible after casting.
- Maximum stone size should be compatible with the minimum dimension of the precast element.
3. Choice of materials
3.1 Cement
Cement should comply with SANS 50197. Strength class should preferably
be 42,5N or higher because the concrete must develop strength as rapidly
as possible. Strength class 32,5N cement may be used, but in this case,
mixes may need to have a high proportion of cement than indicated in Table
2 to ensure adequate early strength. Note that it is illegal to sell cement
which does not bear the SABS mark.
3.2 Aggregates
Sand
Fairly coarse concrete sands are suitable. Either crusher sands or natural
sands obtained from pits or river banks can be used. Replacing about a
quarter of the coarse sand with a clean plaster sand tends to make the
fresh concrete less harsh and more cohesive.
Stone
Any type of stone sold as concrete aggregate may be used. In South Africa
stone is likely to be crushed rock. Natural pebbles, which are available
in some areas, are also
suitable.
Stone size is important. The largest particles should not exceed about
one quarter of the thickness of the concrete item being precast. Table
1 gives minimum concrete thick-ness for various standard stone sizes.
Table 1: Minimum concrete thickness for various stone sizes
3.3 Water
Use water that is fit for drinking. Water that is unfit for drinking should
be tested and approved by a competent laboratory before being used.
3.4 Pigments
Use only best quality synthetic pigments based on metallic (usually iron)
oxides.
Natural pigments may be cheaper, but the colour they produce is variable
and less intense.
4. Mix proportions
4.1 Cement, sand and stone
In this section, trial mix proportions of cement, sand and stone are given
for various stone sizes. Advice is also given on the assessment of proportions.
Trial mix proportions are given in Table 2.
Table 2:Trial mix proportions by volume for concrete for precasting
*The mix porpotions in this table have been calculated using 32,5 R cement
Notes:
- Use sufficient water in the mix to give a plastic mix that can be compacted with the means available. The use of additional water results in weaker concreate. Slump (measured according to SANS 5862-1) suitable for various means of compaction is shown in Table 3.
- 1 bag of cement has a net mass of 50 kg.
- For small batches use the same size container for each material.
For example if the container volume is 6 l, the mix for 9,5 mm stone
is
cement: 6 l
sand: 2 x 6 = 12 l
stone: 1¼ x 6 = 7,5 l - Measure all materials by pouring loosely into the container and striking off flush with the brim.
Measure sand in the damp state.
The stone content of the mix influences the workability of the fresh concrete
and the quality of the hardened concrete. Excessive stone makes the fresh
concrete harsh and
difficult to compact. Insufficient stone content causes the hardened concrete
to be weaker and have greater shrinkage.
Table 3: Slump for means of compaction
Stone content should be assessed as follows:
Make up a batch of concrete according to the proportions given in Table
2. Compact the concrete in the mould using the means available (ie mechanical
vibration or hand
tamping). Examine the top surface of the concrete. If stones protrude,
the stone content of the mix is too high.
If not, scrape the concrete with the point of a nail.
If the stone content of the concrete is right, stone particles should
be found about 2 mm below the surface. If the shallowest particles are
deeper than this, the stone content is too low.
If stone content is too high, reduce it by say 10% and increase the sand
content by the same amount (same absolute volume). Then reassess.
If stone content is too low, increase it by 10% and reduce the sand content
by the same amount. Then reassess.
4.2 Pigment dosage
Pigment dosage depends on the colour and on the intensity of the pigment.
Dosage is normally between 3% and 5% of the mass of the cement. Note that
the final colour of the dry concrete is determined by the combination
of cement colour, sand colour and pigment colour and dosage. It is therefore
advisable to make up trial mixes beforehand using the specific cement
and sand with different pigment dosages. Wait for the concrete to harden
and dry out before assessing the colour because the colour of wet concrete
is darker and more intense than that of dry concrete.
5. Spacer blocks
If precast units are reinforced, the reinforcement should be kept in the
correct position in the mould while the concrete is placed and compacted.
Spacers of plastic or high-strength mortar may be used to support the
steel and ensure that the minimum concrete cover is provided.
Plastic spacers are commercially available from specialist suppliers.
Mortar spacer blocks may be made with a 1:1 ratio of cement and concrete
sand and sufficient water to make a workable consistence. Details of a
spacer block are shown in
Figure 1. Cover the blocks to prevent drying out after casting and place
them in water the next day. Keep the blocks in water for at least a week
before using them.
Figure 1: Details of mortar spacer block
6. Manufacture
6.1 Preparing the mould
Surfaces of the mould that will come into contact with the concrete should be cleaned and then given a very thin coating of mould-release oil. (Rubber moulds do not normally require a release agent.)
Mould-release oils are formulated specifically for concrete casting and are obtainable from oil companies and reputable manufacturers of concrete admixtures. The use of straight lubricating oils is not recommended.
6.2 Batching
Containers used for batching should be robust. Builder's wheelbarrows (which have a capacity of 65 l) and steel buckets and drums are suitable. Batching should be done by pouring material loosely into the container to overfill it, and striking off level with the brim. Partial filling of containers leads to non-uniform batching. Containers should therefore be sized accordingly.
To avoid errors, there should be sufficient containers to measure out a complete batch without having to use any container more than once.
6.3 Mixing
The concrete must be thoroughly mixed.
Although small quantities may be mixed by hand on a concrete floor or steel sheet, machine mixing is preferable. The best type of mixer is a pan mixer which has a forced mixing action.
The amount of water added to the mix depends on the workability required.
The workability of the concrete should be compatible with the means of compaction. Relatively stiff, unworkable mixes can be compacted using high-energy mechanical vibration; if this type of equipment is not available then more workable mixes will be needed. See Table 3. Do not use more water than is required, as this reduces the strength of the concrete.
6.4 Transporting
Transport the concrete from the mixer to the mould in such a way that it does not segregate. If segregation (separation of stone and mortar) is unavoidable, remix the concrete before placing it in the mould.
6.5 Placing
Distribute the concrete evenly in the mould. Do not place the concrete in one position and rely on the vibration to distribute it because this causes segregation.
6.6 Compacting
Precast concrete is usually compacted by means of mechanical vibrators attached to the mould or by placing the mould on a vibrating table. Continue to vibrate the concrete until a water sheen appears on the surface and air bubbles no longer break through the surface.
Hand compacting, which should be used only for limited numbers of small items, may be done by: tamping the concrete with the end of a conveniently sized piece of timber or steel; jigging or jolting the mould; or a combination of tamping and jigging.
6.7 Striking off and floating
Use the edge of a screedboard (a planed wooden plank used on edge) which rests on the top edges of the mould to strike off the concrete flush with the top of the mould. The concrete may be woodfloated if necessary.
6.8 Protecting
Newly cast concrete must be protected from rain which could damage and soften the surface.
It is also most important to protect the concrete against loss of moisture from the exposed surface. If moisture is lost the fresh concrete shrinks and this shrinkage can cause serious cracking. Possible ways of preventing moisture loss are:
- Work indoors or at least in shade.
- Place screens around the working area to keep out drying winds.
- Once the concrete has been compacted, struck off and floated, maintain a water sheen on the surface by fog spraying with water or cover with plastic sheeting.
6.9 Demoulding
Precast items must be demoulded carefully so as to avoid damage to the concrete. Once they are demoulded they should be stacked or supported in such a way that they do not warp or twist.
6.10 Curing
Concrete is cured by ensuring that there is sufficient moisture available
and that the temperature is suitable for the cement to hydrate. Note that
concrete gains strength as the cement hydrates (combines chemically with
water). It does not gain strength by drying out and if allowed to dry
prematurely, will fail to achieve its full strength. The optimum temperature
for cement hydration is between 20°C and 25°C. At lower temperatures
hydration takes place more slowly and minimum mould turn-around time increases.
At higher temperatures hydration is more rapid but the quality of the
mature product may be inferior.
The following methods may be used to wet-cure concrete:
- Immerse in water.
- Spray continuously with water.
- Wet the concrete and then wrap or cover with plastic sheeting.
- Apply a membrane-forming curing compound of good quality to all surfaces of the concrete.
Curing should be continued for at least seven days in normal
weather (20°C to 25°C) and longer at lower
Cement & Concrete Institute
PO Box 168, Halfway House, 1685
Tel (011) 315-0300 Fax (011) 315-0584 e-mail info@cnci.org.za
website http://www.cnci.org.za
Published by the Cement & Concrete Institute, Midrand, 1997, reprinted
1999, 2001, 2003, 2007
©Cement & Concrete Institute