Concrete Production and Reinforcing: A Comprehensive Guide

Posted on Aug 9, 2024 in Design and Engineering

Placement of Reinforcing

Concrete is weak in resistance to tensile forces. Reinforcing is used primarily to resist tension and thus prevent cracking or failure of the concrete member under tension.

Tension May Be Induced By:

  • Shrinkage of concrete as it hardens
  • Temperature changes
  • Bending and shear forces

Concrete Cover for Reinforcing Steel

To provide protection of reinforcing steel against corrosion and fire, a minimum cover of concrete must be furnished. Engineer to specify. Recommended cover:

  1. Slabs/walls not exposed to weather/ground: 20mm
  2. Beams/columns not exposed to weather/ground: 40mm
  3. Concrete placed in forms but exposed: 40mm
  4. Concrete placed on ground surfaces: 75mm
  5. Practice: At least 1 bar diameter of cover to be used in any case (Example: Y32 = 32mm)

Bending Schedule for Reinforcing

  • Issued by the civil engineer and designed according to his calculations in terms of tension & compression forces.
  • Issued to the contractor and/or supplier on site to “fix” the steel as per bending schedules: columns, bases, wall panels etc.
  • Engineer to inspect the fixing of the reinforcing as per bending schedule and “sign” it off before concrete may be poured – or issue site instruction to rectify (whether to add/change etc.)

Production of High-Quality Concrete/Asphalt

Requires supply of quality aggregate: gravel, sand & mineral filler meeting specified gradation and other requirements, thus by crushing rock or gravel & blending it with sand & other minerals.

Production:

Excavate, load and transport rock or gravel to processing plant (crushing plant) where the raw material is washed, crushed, screened, sorted & blended if necessary and stored or loaded into loading hauls. Sands are not crushed but washed & de-watered before use.

Rock Crushers

Utilize mechanical action to reduce rock or gravel to a smaller size: Jaw crushers, Impact crushers, Cone/gyratory crushers & Roll crushers.

  1. Jaw crushers: Utilize a fixed plate and a moving plate to crush stone between the 2 jaws – used as primary crushers.
  2. Impact crushers: Use breakers or hammers rotating at high speed to fracture the input stone.
  3. Cone/gyratory crushers: Use an eccentrically rotating head to crush stone between the rotating head and the crusher body.
  4. Roller crushers: Material is passed between 2 or more closely spaced rollers producing fractured stone.

Production of Concrete

  • Concrete produced by: mixture of Portland cement, aggregates and water.
  • A 4th additive may be added – to improve workability/other properties of the concrete mix.

Construction Operations Involved in the Production of Concrete:

  1. Batching – proportioning of quantities
  2. Mixing – of elements by hand, mixer etc.
  3. Transporting – by truck, wheelbarrow, crane etc.
  4. Placing – by hand, truck chute, pump etc.
  5. Consolidating – vibrating to expel air
  6. Finishing – floating with tools example power float
  7. Curing – to reach strengthening point (Mpa)

Concrete Design Requirements

Concrete must meet design requirements (prescribed strength) – Hardened concrete must meet design strength and must be:

  • Uniform
  • Watertight
  • Durable
  • Wear-resistant

All of these properties are influenced by:

  • Concrete components
  • Mix design used
  • Construction techniques employed

Types of Concrete

Classified into categories according to its application & density:

  • Normal-weight concrete: Weighs 2243 – 2563kg/m³
  • Structural lightweight concrete: Weighs 1922kg/m³. Its lightweight is obtained by using lightweight aggregates such as clay, slate, slag etc.
  • Lightweight insulating concrete: Used for its thermal insulating properties. Aggregates used – perlite and vermiculite.
  • Mass concrete: Used in a structure such as a dam in which the weight of the concrete provides most of the strength of the structure. Thus little or no reinforcing steel is used. Its unit weight is usually the same as that of regular concrete.
  • Heavyweight concrete: Made with heavy aggregates such as magnetite and steel punchings – used primarily for nuclear radiation shielding.
  • No-slump concrete: Slump of 2,5cm or less. Slump is a measure of concrete consistency obtained by placing concrete into a test cone following a standard test procedure and measuring the decrease in height (slump) of the sample when the cone is removed: “Slump-test” Used for bedding of pipelines etc.
  • Refractory concrete: Suitable for high-temperature applications such as boilers and furnaces.
  • Pre-cast concrete: Has been cast into desired shape prior to placement into the structure: pipes, edging etc.
  • Architectural concrete: Concrete that will be exposed to view example curtain walls and screens. White or coloured cement may be used in these applications.

Concrete Admixtures/Additives

Another component – admixture/additive is often added to impart certain desirable properties to the concrete mix.

Aggregates in Concrete

Aggregates are used to reduce the cost of the mix and to reduce shrinkage, leading to cracking.

  • Aggregates make up 60 – 80% of concrete volume, their properties strongly influence the properties of the finished concrete.
  • To produce quality concrete, each aggregate particle must be completely coated with cement paste and paste must fill all void spaces between aggregate particles, i.e. aggregate consolidation.
  • The quantity of cement paste required is reduced if the aggregate particle sizes are well distributed and the aggregate particles are rounded or cubical.

Aggregates Must Be:

  • Strong
  • Resistant to freezing/thawing (melting)
  • Chemically stable
  • Free of fine material that would affect the bonding of the cement paste to the aggregate

Water in Concrete

Water is Required for Several Purposes:

  1. Hydration: To provide moisture required for hydration of the cement to take place – hydration is the chemical reaction between cement and water produces hardened cement.
  2. Aggregate Saturation: If aggregates are not in a saturated surface-dry condition they may add or subtract water from the mix.
  3. Workability: The amount of water in a mix affects the plasticity or workability of plastic concrete.
  4. Strength and Durability: Strength, water tightness, durability and wear resistance of concrete is related to the water/cement ratio.
  5. Water/Cement Ratio: The lower the water/cement ratio, the higher concrete strength and durability achieved.
  6. Water Quality: In terms of water quality – any water suitable for human consumption, will do.
  7. Trial Mixes: Where water quality is in doubt – make trial mixes.
  8. Organic Material: Organic material in mix water tends to prevent the cement paste from bonding properly to aggregate surfaces (clay content).
  9. Chemical Reactions: Alkalies or acids in mix water may react with cement and interfere with hydration.
  10. Seawater: Seawater may be used – will result in concrete strengths 10 – 20% weaker than normal. The use of a lower water/cement ratio can compensate for this (avoid use of sea water unless approved by engineer).
  11. Prestressed Concrete: Seawater should not be used for prestressed concrete where steel will come into contact with the concrete (due to the salt content).

Concrete Additives

Principal Types:

  • Air-entraining agents: Increase resistance to freezing.
  • Water-reducing agents: Increase the slump/workability of plastic concrete.
  • Retarders: Slow the rate of hardening of concrete – use in high temp and when pumping over long distances.
  • Accelerators: They decrease setting time and increase early strength (act in opposite manner as retarders).
  • Pozzolans: Finely divided material – used to replace some of the cement in concrete mix. Reduce segregation.
  • Workability agents: Increase workability.

Pre-Cast Concrete

  • Concrete that has been cast into the desired shapes prior to placement in a structure.
  • Done in a central plant where industrial production techniques may be used under controlled environment/procedures.
  • Obtaining better surface quality & quality control than in-situ concrete.

Pre-Stressed Concrete

  • Concrete to which an initial compression load has been applied.
  • With concrete being strong in compression but weak in tension, pre-stressing serves to increase the load that a beam or other flexural member can carry before allowable tensile stresses are reached.
  • Permits a smaller, lighter member to be used in supporting a given load.
  • Pre-stressing also reduces the amount of deflection in a beam.
  • As the member is under compression, cracking will remain closed up and not be apparent.