Precast Concrete
Production of Precast Concrete

Environmental and manufacturing conditions at a precast concrete plant are easily monitored. The production of precast concrete elements takes place under controlled conditions in enclosed factories. This makes the control of manufacturing, waste, emissions, noise levels, etc. easy compared with the same processes at a building site. The raw material consumption is similar for similar qualities of concrete, whether the production takes place in a factory, at a ready-mix plant or at a building site. The raw material waste in precast concrete production is very small.

The use of new technologies such as self-consolidating concrete (SCC) can significantly reduce noise and vibration in the production process. The use of high-performance concrete (HPC) enables the design and production of more slender, reliable and more durable structures with function-optimized shapes. Cost and material savings in structural material consumption and natural resources can be realized through controlled production.

Precast forms are normally made of either steel or plywood. A large number of castings in forms is typical in the production of precast concrete elements in plants with savings of raw materials.

Plywood form use is limited to about 20 to 50 castings depending upon the complexity, maintenance and shape of the form. Standardized elements cast in steel forms are one step towards sustainable production. An unlimited number of castings can be made by precasting using steel forms. Standardization of precast products will save cost. Attempts by the precast industry to standardize precast cross sections are designed to save costs and increase market share by getting the maximum number of casts out of every form.

Form Oil
Mineral oil release agents can affect the health of workers, pollute the air, water and soil, be flammable, have poor biodegradability and result in hazardous waste. New form oils have been developed, containing of a mix of marine ester, medical white oil and vegetable oil. These oils are easily biodegradable and are non-toxic. Precasters use approximately 0.5 kg form oil per m3 of concrete. Mineral oils are still used, but as prices come down, precasters will change to the new form oils.

Silicone is frequently used to seal joints in forms. No negative scientifically based ecological findings have been found regarding the life cycle of silicone sealants.

Admixtures can be divided into four main groups: water-reducing, air-entraining, retarders and accelerators. In addition, there are several other types of admixtures for special applications.

The purpose of admixtures is to improve concrete quality both in the fresh and hardened state.

The use of admixtures often has a beneficial effect on the environment: better durability of concrete, less consumption of cement, less energy consumption during casting, etc.

Water-reducing admixtures are normally synthetic organic compounds; sulphonated naphthalene/melamine formaldehyde condensates, sulphonic acid esters, carbohydrate esters etc. The content of water reducing admixture is typically 0.02% – 0.1% by the weight of concrete. Water-reducing admixtures do not contribute to emissions.

Air-entraining agents are used to improve the frost resistance of concrete. These admixtures are surface-active organic compounds. Their amount in concrete is very small - 0.002% – 0.02% by the weight of concrete.

Retarders are used to control the setting of the cement. Contents are typically less than 0.1% by the weight of concrete.

Admixture quantities in precast concrete are very low, normally less than 0.1% by weight of concrete. The quantities of admixtures used are too small to have any significant environmental impact. Most admixtures used in prefabrication are non-toxic, organic compounds.

The process of preparing mild steel reinforcement may be the same for a precaster as for a contractor at a building site, except that precasters will usually have less waste. This results in better utilization of the steel and less consumption of natural resources. Mesh reinforcing in thin members is a very efficient use of materials. Precast elements may require some extra reinforcement to resist lifting and handling stresses. A precast structure may need some additional reinforcement to facilitate the force transfer in building connections. This may reduce some of the advantages gained during production.

Fibre reinforced polymer (FRP)
Infrastructure components that corrode and deteriorate over time, usually force early rehabilitation. Fibre reinforced polymer (FRP) composite materials can be used to reinforce these sections to allow the infrastructure to achieve its full lifetime – see CSA S806 for design and production procedures.

Glass, aramid and carbon FRPs are up to 6 times stronger than steel, one-fifth the weight, non-corrosive and non-magnetic. FRPs are high strength, light weight, and are available in the form of thin sheets make them an attractive alternative and economical solution for strengthening existing concrete bridges and structures. The use of FRP bars and tendons is considered to be one of the most promising solutions to overcome the deterioration aggravated by corroding steel reinforcement in structures and bridges.

Colour Pigments
Colour pigments are commonly used to manufacture coloured concrete. Pigments are normally iron oxides (e.g. red and brown), chrome oxides (green) or cobalt oxides (blue). Pigment content varies normally between 2% and 4% of the cement by weight. Choose pigments that are insoluble and non-toxic.

Sandblasting, Retarders and Acid Etching
Sandblasting is commonly used to create a coarse texture on the surface of concrete panels. The sand can be collected and reused as road base.

Retarders are applied to the forms to slow down the hydration process of the concrete at the surface and allow washing of the concrete afterwards. This is a common method used to create facades with exposed aggregate surfaces.

Acid etching is also used in facade production, mainly to wash a panel surface to give a coarse texture. Wash water containing acid residue is neutralized before disposal.

Other Materials
In precast concrete production, special materials are occasionally needed such as epoxy, lacquer, varnish and paint. Although some of these materials are slightly hazardous, they are normally used in very small amounts and their effects can be controlled during prefabrication. Effective ventilation can be provided in areas where these materials are used. Waste is easily collected and disposed of in an environmentally sound manner.

Quality Assurance
The production of precast concrete elements, using repetitious operations under controlled climatic conditions, allows for accurate control of the results (e.g. tolerances and concrete quality). This facilitates better use of materials and less material consumption. The controllability of the production process allows for strict tolerances and the optimal use of materials.

CSA A23.3 allows a reduced material factor, for precast concrete members that are certified in accordance with A23.4 in recognition of the good quality control and accurate placement of forms and reinforcement. The measures required to obtain accurate results and to check these results are part of the manufacturing process.

CPCI Certification
The purpose of this audit based program is to provide owners and designers with the confidence that certified precast concrete manufacturers are qualified and competent to manufacture quality products they supply to the marketplace. The CPCI Certification program will certify only those precast manufacturers who demonstrate strict conformance to current standards, and are committed to continually improving the quality of their products and process.

What are the program requirements?
The manufacturing of precast concrete products must conform to all the requirements of:

  • CSA Standard A23.4 (latest edition) Precast Concrete –Materials and Construction
  • PCI Quality Control Manual, MNL-116 – Manual for Quality Control for Plants and Production of Structural Precast Concrete Products
  • PCI Quality Control Manual, MNL-117 – Manual for Quality Control for Plants and Production of Architectural Precast Concrete Products

The more stringent requirements of these specifications become the governing criteria. Structural, Architectural and Specialty Precast

Most sources of pollution can be identified and controlled when precasting takes place in a plant. Pollution handlers include exhaust fans at welding sites, with the air passing through filters; and closed systems for the transport of cement, aggregates and fresh concrete.

Concrete production can create pollution mainly in the form of dust and noise. The fabrication of hollow core slabs requires sawing the hardened concrete, producing dust and noise levels that can exceed 85 dB. Saw blades can be cooled with water that can be filtered and reused. Saws can be encased in noise absorbing containers.

Solid waste from precast plants is very low, about 2.5% of the mass of concrete used in the production. About 95% of this waste can be further beneficially reused through crushing and recycling of hardened concrete, leaving approximately 3 kg/m³ of actual waste.

The Workplace Environment
Enclosed precast plants make it easier to control the manufacturing environment and conditions. The environment for workers is can be kept clean, quiet, dust-free and free of toxic fumes. Enclosed plants provide workers with a comfortable work environment (controlled temperature and humidity) protected from adverse weather conditions.

Nearly, but not all, precast plants are enclosed factories. Open-air production facilities are typically located away from populated areas to provide a noise and dust pollution buffer zone.

Production Waste
Surplus materials are generated during the production of precast elements. Much surplus material is recyclable, and consists mainly of:

  • hardened concrete with or without reinforcement
  • steel reinforcement and pieces of structural steel
  • plywood and other wooden materials
  • fresh concrete (from production and washing of equipment)
  • slurry from the sawing of concrete
  • insulating materials (mineral wool and polystyrene)
  • oil etc. from machinery
  • paper and other packaging materials.

The amount of surplus material varies between factories and different types of production. Studies in the Scandinavian countries have shown that the magnitude is typically about 100 kg of surplus material per m3 of concrete produced. About 40% of the surplus material is fresh and hardened concrete and about 45% is wastewater from washing equipment and sawing slurry generated during hollow core slab production.

Source: fib Environmental issues in prefabrication

It is possible to collect and sort different types of surplus materials in precast plants. Excess materials that can be recycled and reused include steel, wood, insulating materials, oil, paper and other packaging materials. Wood can be sorted out, cut and used as industrial firewood, or used for other construction purposes.

Fresh concrete
Most excess fresh concrete comes from the washing of equipment, cut-outs, leftover in the buckets etc. The sand and gravel can be separated from fresh concrete and reused in the production of new concrete. The cement slurry left after the removal of the sand and gravel can be left in basins for evaporation. Concrete leftovers from production are often left to harden.

Hardened Concrete
Surplus concrete can be crushed into fragments that can either be used as road base, fill material or occasionally as aggregates in new concrete. The advantage for the precaster is the known quality of the material being crushed so it can be used with full confidence as recycled aggregate.

Recycling surplus hardened concrete costs about the same as for new crushed natural aggregates. Recycled aggregates can be used in reinforced and prestressed concrete. The amount of recycled aggregate is about 5% of the total amount of the aggregate in some precast plants. Tests have been made with 100% recycled crushed hollow core slab concrete in hollow core production with no detrimental effects on the new hollow core slabs. Removal and recycling of any embedded steel is easily carried out during the crushing operation.

Canadian Precast/Prestressed Concrete Institute
PO Box 24058 Hazeldean, Ottawa Ontario, Canada
Tel: 613.232.2619
Toll Free: 1.877.937.2724
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