Delay detensioning or post-tensioning of prestressed architectural precast concrete units until concrete has reached its indicated minimum design release compressive strength as established by test cylinders cured under the same conditions as concrete member.
Detension pretensioned tendons either by gradually releasing tensioning jacks or by heat- cutting tendons, using a sequence and pattern to prevent shock or unbalanced loading.
If concrete has been heat cured, detension while concrete is still warm and moist to avoid dimensional changes that may cause cracking or undesirable stresses.
Protect strand ends and anchorages with bituminous, zinc-rich or epoxy paint to avoid corrosion and possible rust spots.
Mix concrete according to PCI MNL 117 and requirements in this Section. After concrete batching, no additional water may be added.
Retain paragraph below if a separate face mix is required or is Contractor’s option.
Place face mix to a minimum thickness after consolidation of the greater of 1 inch (25 mm) or 1.5 times the maximum aggregate size, but not less than the minimum reinforcing cover as indicated on contract drawings.
At the fabricator’s option either of the following mix design/casting techniques may be used:
a.A single design mix throughout the entire thickness of panel.
b.Design mixes for facing and backup; using cement and aggregates for each type as indicated, for consecutive placement in the mold. Use cement and aggregate specified for facing mix, use cement and aggregate for backup mix complying with criteria specified as selected by the fabricator.
J.Place concrete in a continuous operation to prevent seams or planes of weakness from forming in precast concrete units. Comply with requirements in PCI MNL 117 for measuring, mixing, transporting, and placing concrete.
Place backup concrete to ensure bond with face mix concrete.
K.Thoroughly consolidate placed concrete by internal and/or external vibration without dislocating or damaging reinforcement and built-in items, and minimize pour lines, honeycombing or entrapped air on surfaces. Use equipment and procedures complying with PCI MNL 117.
1.Place self-consolidating concrete without vibration in accordance with PCI Interim Guidelines for the Use of Self-Consolidating Concrete.
L.Comply with ACI 306.1 procedures for cold-weather concrete placement.
M.Comply with ACI 305R recommendations for hot-weather concrete placement.
Identify pickup points of architectural precast concrete units and orientation in structure with permanent markings, complying with markings indicated on Shop Drawings. Imprint or permanently mark casting date on each architectural precast concrete unit on a surface that will not show in finished structure.
P. Cure concrete, according to requirements in PCI MNL 117, by moisture retention without heat or by