Architectural Concrete Construction and Finishes

Architectural Concrete Construction and Finishes
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Architectural
concrete construction needs special care in selection of the concrete
materials, forming, placing, and finishing in order to achieve the designated
architectural appearance. Architectural concrete is permanently exposed to view
on surfaces of completed structures or building that is why its construction
required such care and attention. There are various architectural concrete
surface finishes for example exposed aggregate which can be produced using different
techniques like abrasive blasting.

The formwork used for architectural concrete construction need to be of high density and conform to the specification of applicable codes such as ACI 117. Great care and attention shall be practiced during planning and execution of concrete pouring process in order to construction an architectural concrete that is conform to the design requirements.

Formwork for Architectural Concrete

1. Materials

Steel and glass fiber reinforced
plastic, exterior-grade plywood panel, or other non-absorptive materials that
provide continues, true, and smooth architectural finishes

2. Tolerances

  • Formwork tolerances shall conform to ACI 117
  • All formwork shall have a high density, non-vapor transmitting form face.
  • Form face, form liner and molds shall produce a concrete surface matching the Design Reference Standard

3. Design Criteria

  • Face sheet deflection shall not exceed 1/400 of its span.
  • Concrete ties and bolts shall be sized to withstand form design pressures.
Formwork for Architectural Concrete Construction
Fig. 1: Formwork for Architectural Concrete Construction
Formwork Facing
Fig. 2: Formwork Facing

Cement for Construction of
Architectural Concrete

  • One source of cement is recommended to be used.
  • It should conform with ASTM C 150, C595, C845, or C1157.
  • Adequate supply of special cement used for architectural concrete construction shall be ensured.

Admixtures

It is allowed to employ admixtures
to provide certain properties to architectural concrete apart from those
containing calcium chloride.

Mixing and Transporting
Architectural Concrete

  • The equipment used to transport concrete should be clean and used for architectural concrete exclusively.
  • Transport colored concrete in full loads
  • Deliver concrete of uniform proportion and slump in order to achieve the designated architectural concrete.

Conveying and Placement
Architectural Concrete

  1. Provide supports for concrete transporter, pump lines, wheel barrows, and foot traffic to avoid disturbance of reinforcements and concrete placement operations.
  2. Engineer or architect shall accept the formwork and reinforcement placement prior to concrete placement.
  3. Delays in concrete placement need to be prevented. This can be done through adequate scheduling.
  4. Avoid splatter of mortar on formwork faces.
  5. Any splatter on formwork faces shall be cleaned properly.
  6. Place concrete in its final position without segregation.
  7. Avoid horizontal movement of concrete.
  8. Pour concrete in horizontal layers of not more than 0.9m for consolidation.
  9. The rate of concrete placement shall not exceed the placement rate used in the formwork design.
  10. Vibrate placed concrete to achieve maximum consolidation.
  11. Minimum distance between formwork face and internal vibrator shall be 50mm.
  12. Re-vibrate 150mm layer of concrete lift during delays of up to 30minutes as long as the vibrator is penetrated under its own weight.
  13. If vibrator failed to penetrate concrete under its own weight, concrete placement shall be ended, and level lift and set construction joint.
Architectural Concrete Placement
Fig. 3: Architectural Concrete Placement
Architectural Concrete Construction
Fig. 4: Architectural Concrete Construction

Formwork Stripping

Strip formwork from vertical
architectural concrete surfaces when the concrete achieved a compressive
strength of 7 MPa or 24 hours after placement, whichever is later.

Schedule formwork stripping to
maintain surface appearance matching accepted Field Mockup. Break off
fiberglass ties flush with the surface of the concrete and texture to match the
adjacent concrete surface and color

Curing Architectural Concrete

  • Mist concrete surface with water before applying curing compounds.
  • Apply curing compounds manufacturer instructions.
  • Maintain complete contact when using cotton mat curing.

Repairs and Cleanup

  • Repair defects on surfaces immediately to conform designated architectural concrete.
  • Cure by covering with curing seal or edge-taped waterproof cover
  • Finish tie holes using accepted techniques which is commonly determined during field mockup.

Treatment
Types for Architectural Concrete

Table 1 Types of Architectural Concrete Finishes and their application
mechanism

Treatment
Types
Applications
Mechanism
Aggregate
exposed by abrasive blast
It is used when the concrete obtains a minimum compressive strength of 14 MPa.
Aggregate exposed by removing retarded surface paste on vertical
surfaces
Commonly, surface retarder is used for this type of architectural concrete surface treatment. After form removal, remove the retarded outer layer of cement paste Schedule procedures and adjust timing for weather conditions to achieve uniform aggregate exposure.
Aggregate exposed by removing retarded surface paste on
horizontal surfaces
Spray retarder on fresh cast horizontal surfaces after concrete consolidation.Remove the chemically retarded surface cement paste after the mortar retaining th aggregate is set sufficiently to prevent dislodgment of the aggregate.
Acid Etch Acid etch is used only for exposure of aggregate on horizontal surfaces. Defer acid etch until concrete is at least 14 days old and compressive strength exceeds 30 MPa. Wet concrete before acid application. Continue to apply acid while actively brushing or spraying until predetermined depth of reveal is obtained.
Mechanical tooling (bush-hammering) Mechanical tooling is employed to expose aggregate. Defer mechanical tooling until concrete is at least 14 days old and compressive strength exceeds 30 MPa.  
Water blast Water blast is utilized to expose aggregate to match accepted design reference standard.Defer water blasting of vertical surfaces until compressive strength exceeds 30 MPa.



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