Construction Stages and Formwork

Stages of Building Construction

• The stages building construction are comprised of preliminary works, foundations, ground floors, frames and upper floors, stars, roofs, walls and partitions with doors and windows, mechanical and electrical works, internal and external wall floor and ceiling finishes, and external works.

Formwork Basics

• Formwork for cast in-situ concrete work is a mold or box.
• Concrete is poured and compacted into the box, flowing, and setting to the inner profile of the mold.
• The general requirements for formwork materials include withstanding dead and live loads.
• The formwork should retain its shape, with adequate props and braces, and not warp or distort.
• Joints must be leak-proof, and the removal process should not damage the concrete, while also being reusable and lightweight.

Types of Formwork

• Timber is a suitable material for concrete structures
• It can be of board form, either wrot or unwrot, depending on whether a smooth or rough surface is required.
• Softwood boards used to form panels for foundations, beam, and column sides are joined by cross members at centers not exceeding 24 times the board's thickness.
• Steel formwork is used in large projects or frequent formwork reuses and is suitable for circular or curved structures.
• Steel forms are more resistant, durable, and longer lasting compared to wooden formworks, with more frequent reuse.
• Steel formworks can be installed and removed quickly and easily.
• Steel formworks ensures a better quality of concrete surface and do not require further treatment.
• Steel formworks do not absorb moisture from concrete and do not shrink nor deform.

Steel Reinforcement

• Steel reinforcement to concrete must fulfill certain requirements, including tensile strength and they must be easily bent to any required shape.
• The reinforcement surface must develop adequate bond between the concrete and the reinforcement and needs to be available at a reasonable cost.
• Reinforcement is available in two forms which are steel bars and steel fabrics.
• Mild steel bars have a characteristic strength of 250N/mm2 and range in bar sizes from 6 - 10mm diameter, including 6mm, 8mm, and 10mm.
• High tensile steel bars have a characteristic strength of 460N/mm2 and range in bar sizes from 10 - 40mm diameter, including 10, 12, 16, 20, 25, 32, and 40mm.
• Rebar reference numbers and conversion factors (kg/m) according to BS4449 include:
  • Y10/T10 at 0.616 kg/m
  • Y12/T12 at 0.888 kg/m
  • Y16/T16 at 1.579 kg/m
  • Y20 / T20 at 2.465 kg/m
  • Y25 / T25 at 3.852 kg/m
  • Y32 / T32 at 6.313 kg/m
  • R6 at 0.222 kg/m
  • R8 at 0.394 kg/m
  • R10 at 0.616 kg/m

Steel Bar Naming Conventions

• Reinforcement on construction drawings is represented by a coding system to simplify preparation and reading of the details.
• "2 = number of bars"
• "Y = deformed high tensile steel"
• "R = plain round mild steel"
• "25 or 6 = diameter of bar in mm"
• "200 = spacing of bar centre to centre in mm"
• The code letter Y can also be replaced by the code letter T, which indicates deformed high tensile steel.

Steel Fabrics

• Steel fabric is covered by BS4483 and is classified into four basic types:
  • 'A' is for square mesh used for floor or roof slabs.
  • 'B' is for rectangular mesh also used for floor or roof slabs.
  • 'C' is for long mesh used for roads and pavements.
  • 'D' is for a small diameter wrapping mesh used for binding concrete fireproofing.
• Each type has a letter prefix followed by a reference number, which is the total cross-sectional area of main bars in mm² per metre width.
• Steel fabric (BRC) is supplied in sheets (4.8 x 2.4m) or rolls (48 or 72 x 2.4m).

Concrete Cover

• Reinforcement must have a minimum concrete cover to protect steel from corrosion due to contact with moisture and fire exposure.
• Reinforcement cover to reinforcement should always be equal to or more than the size of the largest bar diameter in a group.

Concrete

• Concrete is a mixture of cement, sand, aggregate, and water, with a water/cement ratio between 0.4 to 0.7
• Concrete mixes are expressed as a ratio, e.g. 1:3:6/20mm, where 1 part cement, 3 parts sand, 6 parts aggregate, with 20mm maximum size of coarse aggregate.
• Concrete grades are numerically related to cube test strength taken at 28 days.
• Concrete grades range from C7.5 to C60, where the "C" indicates compressive strength in N/mm2.
• Grades 7.5, 10, and 15 are used for plain (unreinforced) concrete.
• Grade 20 is used for reinforced concrete with lightweight aggregates.
• Grade 25 is used for reinforced concrete with dense aggregates.
• Grades 30 to 60 are used for prestressed concrete.
• Common concrete mixes in use, including:
  • 1:10 – not a strong mix but suitable as blinding layers filling weak pockets in excavation.
  • 1:8 – slightly better mix suitable for paths and pavings.
  • 1:6 – a strong mix suitable for mass concrete foundations, paths, and pavings.
  • 1:3:6 – the weakest mix of reinforced concrete.
  • 1:2:4 - a strong mix that is practically impervious to water and is in common use especially for reinforced concrete.