Architectural Concrete Construction and Finishes

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Architectural concrete construction needs special care in the selection of materials, forming, placing, and finishing in order to achieve the designated architectural appearance. Architectural concrete stays permanently exposed to view on surfaces of completed structures making important for the contractor to work with precision. There are various architectural concrete surface finishes, for example- exposed aggregate which can be produced using a technique like abrasive blasting.

The formwork used for architectural concrete construction needs to be of high density and should conform to the specification of applicable codes such as ACI 117. Great care and attention shall be practised during planning and execution of concrete pouring process in order to lay architectural concrete that conforms 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 continuous, true, and smooth architectural finishes are used.

2. Tolerances

• Formwork tolerances shall conform to ACI 117
• All formwork shall have 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.

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.
• An adequate supply of special cement used for architectural concrete construction shall be ensured.

Admixtures

It is allowed to employ admixtures to impart 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, wheelbarrows, 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. The minimum distance between the 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 the vibrator fails to penetrate concrete under its own weight, concrete placement shall be ended, and level lift and set construction joint.

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 to designated architectural concrete.
• Cure by covering with curing seal or edge-taped waterproof cover.
• Finish tie holes using accepted techniques which are commonly determined during field mockup.

Treatment Types for Architectural Concrete

Table 1 - Types of Architectural Concrete Finishes and their application mechanism

Treatment Types	Application 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 the 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 the 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 the 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 the accepted design reference standard. Defer water blasting of vertical surfaces until compressive strength exceeds 30 MPa.