Precast Concrete and Production Process

Precast Concrete and Production Process

1. Advantages

   • Speed
   • Low Manpower
   • Economical
   • No plastering
   • Durable Buildings
   • Better Design
   • Low Wastage
   • Eco friendly
   • Ability to standardize

2. Facts

   • 2 times faster than conventional construction
   • 3 times less labour than conventional construction
   • 5 years payback period for machinery and mould CAPEX
   • up to 25% less consumption of raw material
   • 100+ years in the effective building life as precast concrete gains strength with time
   • Over 5000 repetitions with precast casting moulds

3. Precast concrete structural elements:

   • solid flat slab
   • hollow core slab
   • double tee
   • single tee

4. Precast Beams and girders

   • rectangular beams
   • L shaped beam
   • Inverted Tee beam
   • AASHTO Beam

5. Panels and Walls

   • Sandwich Panels
   • Cladding Panels
   • Solid Panels
   • Solid Slabs
   • Extruder: Hollow-core slabs
   • Slipformer: Hollow-core slabs
   • Slipformer: RIbbed slabs, half slabs
   • Slipformer: T-beams
   • Acotec Panels

6. Concrete Strengths

   • Where the manufacturer wishes to use a higher strength concrete, they must obtain prior approval from the designer as higher strength concrete can have an adverse effect on a building's performance during an earthquake
   • An optimum strength for different purposes of demoulding, handling, lifting, propping etc must be considered.
   • With approval from the building designer, higher strength concrete can be used: to allow early removal from moulds, to meet handling requirements and to accommodate construction loads
   • Concrete strength at different stages:
     • Concrete strength increases over time and is affected by curing conditions, environment and temperature
     • The concrete strength required for each stage including lifting from moulds, de-stressing, factory handling transport, site handling, temporary fixing and more need to be considered.

7. Production

   • Documentation and Check Sheets
     • Manufacturing Tolerances
     • Mould friction or suction
     • Tilting Moulds and Vertical Moulds
     • Confirmation of compliance with this guide
     • Curing compounds and release agents

8. Precast concrete technology

   • Materials used for pre-casting of concrete elements include concrete mix and its components - cement, aggregates, reinforcing steel and various admixtures
   • Preparation of concrete mix
   • Concrete Curing
   • Production of main concrete elements - the process and equipment used for this purpose
   • Erection and jointing of elements on site

9. Cement (main tpoes by their function, used in precast concrete production)

   • Ordinary Portland Cement
   • Rapid Hardening Cements
   • High Strength Portland Cements
   • Sulphate-Resisting Portland Cement
   • White Portland Cement
   • Blended Portland Cements
   • Other types of cement: blast furnace, pozzolanic, low heat, expansive - rarely used in precasting

10. Aggregates:

    • Lightweight - below 1100kg/m3 - the average bulk density of lightweight aggregates used for structural precast concrete production is 800-1100kg/m3 and the density of concrete produced with them is 1400-1900kg/m. LIghtweights can also be natural; includes pumice and scoria or artificial: includes: expanded clay, expanded shale, foamed slag, fly ash and polystyrene beads
    • Normal weight - 1100 - 1750 kg/m3. Normal weight aggregates (average density 1400 - 1600 kg/m3) are sand and gravel, which are found on riverbeds and the seashore(sand) and crushed rock
    • Heavyweight - above 2100 kg/m3
    • fine - below 5mm and coarse is 5mm and above

11. Admixtures: to improve concrete performance:

    • air entraining admixtures
    • Accelerating admixtures
    • Retarders: slows down the hardening of concrete used to attain an exposed aggregate surface in architectural concrete precast elements; allowing washing away cement from element surface
    • Water reducers (plasticizers): Water reduces improve workability of concrete without increasing their water/cement ratio
    • Microsilica - enhances concrete strength and density
    • Pigments to produce concrete of a desired colour

12. Reinforcing Steel

    • Round Bars
    • Wire Fabric
    • Pre-stressing wire
    • Fibres of steel, glass, polypropylene and other materials

13. Concrete mix basic requirements:

    • Strength: major design parameter for their eventual service under imposed loads and also for handling during the production, transportation and erection. Target strength for design is the 28 day compressive strength of precast concrete (Determined by water/cement (w/c) ratio in the mix)
      • 20MPa for regular bearing non-pre-stressed elements, 40-60MPa for prestressed concrete slabs, beams and columns and in special elements, concrete of up to 120MPa strength is used.
      • Under a temperature of 20 degrees, concrete is expected to attain 25-30% f its 28 day strength after 24 hours, and 60-70% after 7 days
      • Minimum strength required for demoulding of a non pre-stressed element is about 10MPa
      • Releasing of prestressing tendons requires a strength of 30-35MPa
      • Reduction of the w/c ration from 0.7 to 0.4 increases the strength of the concrete from 20 to 50MPa
    • Workability:
      • Measured through the slump test and compacting factor test (or the V-B test)
      • The recommended slump of a mix for precast concrete elements is between 30mm (stiff mixture) for horizontally cast elements and 90mm for thin vertically cast elements
    • Durability:
    • Density:

14. Parameters of concrete mix

    • 1) Water/cement Ratio (w/c): weight of free water (exclusive of the amount absorbed by aggregates)/ cement weight in the mix
    • 2) Water content: is a workability issue which also depends on aggregate composition (varies typically between 1.2 and 2.0 kN/m3 of concrete mix); and maybe reduced by using plasticisers
    • 3) Cement content; is determined by dividing the water content by the w/c ration. For precast elements, its should not be less than 2.4kN/m3, and not more than 5.5kN/m3
    • 4) Aggregate content: is determined by subtracting from the total density of concrete (2300 - 2400 kg/m3) the weight of cement and water as established in the former steps

15. Aerated concrete precast units:

    • Generally for partitions, walls and roof slabs
    • very low density, 500-800 kg/m3
    • Obtained by applying high-pressure steam curing (or autoclaving) to a slurry of Portland Cement or lime and finely ground siliceous sand.
    • Aluminium powder is added and reacts with slurry to cause air bubbles
    • The elements are usually produced in a modular width of 500-750mm and of varying lengths and thicknesses
    • Compressive strength is 2.5-4.0MPa
    • Can be used for structural purposes - bearing walls, slabs with light loads with reinforcement protected against corrosion
    • Advantages include being low weight, low thermal conductivity and high fire resistance

16. Placing and processing of concrete:Vibration should accomplish full coating of the coarse aggregate and reinforcement

    • 1) Internal Vibrating - vibrator is immersed in the concrete mix
    • 2) Surface Vibrating - vibrator is applied to the exterior surface of the placed concrete and its effect is relayed inward.
    • 3) Mould vibrating - vibrator is attached to the mould and the vibration is transferred through it to the concrete mix
    • 4) Vibrating table, the table and the superimposed mould are separated from the base by springs or other systems

17. The finish

    • Plain
    • Decorative

18. Accelerated curing

    • Purpose: early development of sufficient concrete strength for demoulding of a precast concrete element of for cutting of pre-stressing wires
    • Approach 1: concrete is processed in ambient temperatures and its higher strength is attained by affecting the mix, this can be performed in 3 ways:
      • Using Rapid hardening cement: - after one day, attains 50-60% of its final value. High contents, 400-450kg/m3, of ultra-rapid cements with low water/cement ratios may produce a demoulding strength of 10MPa after 5-7 hours at normal ambient temperatures. Strict control of placing and mechanical processing is required to prevent adverse effects of rapid hydration/
      • Adding accelerating admixtures; admixtures accelerate the concrete hardening process, which may shorten, in precasting, the mould utilization cycle (may have corrosive influence on reinforcing steel eg. calcium chloride)
      • Overdesigning the concrete strength: a higher than required concrete strength after 28 days will also produce higher strength after 16-20 hours
    • Approach 2 - heat curing - this is based on utilization of the relationship between concrete strength and "maturity". The maturity is, in general terms, a product of the curing temperature and its application time.
      • Steam curing of concrete - most widely applied accelerated curing method. The heating is to start 3-5 hours after casting, where the temparture is to rise at a rate of 10-30C/hr and then maintained at a unifrom level of 60 - 80C and finally decreased again at a rate of 10-30C/hr. Total cycle time may vary in the range of 12-18 hours, depending on the maximum uniform temperature applied and its duration
      • Other heating methods - mould heating, raising ambient temp in the casting area to 30-35C, or preheating concrete ingredients like water, cement and aggregates. It can be done by steam injection during the mixing cycle to a temperature of 40-50C and the mix cast into an insulated mould. Portable hot air blower and infrared heating may also be applied. It is very important that a controlled cooling period be applied after the heating of the concrete has stopped.
      • Rational utilization of hydration heat: For large numbers of elements in an insulated area, it can produce sufficient concrete strength, even without additional heating, after 16-20 hours.

19. Production of floor slabs:

    • Table moulds: mostly used for production of room-size slabs and exterior walls, and sometimes also for interior bearing walls and partitions
      • 1) Cleaning of plate and side frame and application of an appropriate release agent
      • 2) Fixing of the side frame to the table plate
      • 3) Placing of reinforcement and fixtures. Reinforcement is positioned on the mold with plastic or steel spacers, which determine the required concrete cover.
      • 4) Pouring of concrete, as required, to attain sufficient strength of side frame
      • 5) Curing of concrete, as required, to attain sufficient strength for demoulding
      • 6) Stripping of side frame
      • 7) Removing of element to storage
    • Stacks: Floor slabs are sometimes produced in stacks, one on top of the other. Although this method saves on investments in moulds, it may introduce distortions and inaccuracies which increase with each additional panel.
    • Pre-stressing beds: for production of hollow core modular slabs. The prestressimg tendons extend between two anchorage blocks on both sides of the bed. Tendons are made of wire strands and wedged to end blocks after pre-stressing.
    • Production of interior walls: Interior are usually cast in vertical moulds to ensure a smooth surface on both sides.
      • Battery Mould Production
      • Simple vertical moulds
    • Production of exterior walls: Exterior walls are produced on horizontal tables which can be tilted to an almost vertical positions for convenient demoulding of the panel (tilted by a crane or more often by its own hydraulic)
      • Layers of exterior walls:
        • Structural Concrete layers
        • Insulation Layer
        • Additional Concrete Layer
        • Exterior finish layer

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