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For any concrete repair project, the process of investigation, and mechanically attached to the existing concrete.
laboratory analysis, trial samples, mock-ups, and full-scale Reinforcement materials used in repairs most often
repairs allows ongoing refinement of the repair work as include mild steel, epoxy-coated steel, or stainless steel,
well as implementation of quality-control measures. The depending on existing conditions.
trial repair process provides an opportunity for the owner,
architect, engineer, and contractor to evaluate the concrete Formwork and Molds
mix design and the installation and finishing techniques for
the repairs from both technical and aesthetic standpoints. Special formwork is needed to recreate ornamental
The final repair materials and procedures should match concrete features - which may be complex, in high
the original concrete in appearance while meeting the relief, or architecturally detailed-and to provide special
established criteria for durability. Information gathered surface finishes such as wood form board textures.
through trial repairs and mock-ups is invaluable in refining Construction of the formwork itself requires particular
the construction documents prior to the start of the overall skill and craftsmanship. Reusable forms can be used for
repair project (Fig. 17). concrete ornamentation that is repeated across a building
facade, or precast concrete elements may be used to
Surface Preparation replace missing or unrepairable architectural features.
Formwork for ornamental concrete is often created using
In undertaking surface preparation for historic concrete a four-step process: a casting of the original concrete is
repair, care must be taken to limit removal of existing taken; a plaster replica of the unit is prepared; a mold or
material while still providing an appropriate substrate for form is made from the plaster replica; and a new concrete
repairs. This is particularly important where ornamentation unit is cast. Custom formwork and molds are often the
and fine details are involved. Preparation for localized work of specialty companies, such as precasters and cast
repairs usually begins with removal of the loose concrete stone fabricators.
to determine the general extent of the repair, followed by
saw-cutting the perimeter of the repair area. The repair area The process of forming architectural features or special
should extend beyond the area of concrete deterioration surface textures is particularly challenging if early age
to a sufficient extent to provide a sound substrate. When stripping (removal of formwork early in the concrete
repairing concrete with an exposed aggregate or other curing process) is needed to perform surface treatment
special surface texture, a sawcut edge may be too visually on the concrete. Timing for formwork removal is related
evident. To hide the repair edge, techniques such as lightly to strength gain, which in turn is partly dependent on
hand-chipping the edge of the patch may be used to temperature and weather conditions. Early age removal of
conceal the joint between the original concrete and the new formwork in highly detailed concrete can lead to damage
repair material. The depth to which the concrete needs to of the new concrete that has not yet gained sufficient
be removed may be difficult to determine without invasive strength through curing.
probing in the repair area. Removal of concrete should
typically extend beyond the level of the reinforcing steel, if Selection of Repair Materials and Mix Design
present, so that the patch encapsulates the reinforcing steel,
Selection and design of proper repair materials is a
which provides mechanical attachment for the repair.
critical component of the repair project. This process
If the concrete was originally of lower strength and quality, requires evaluation of the performance, characteristics,
the assessment of present soundness is more difficult. and limitations of the repair materials, and may involve
Deteriorated and unsound concrete is typically removed laboratory testing of proposed materials and trial repairs.
using pneumatic chipping hammers. Removal of concrete The materials should be selected to address the specific
in historic structures is better controlled by using smaller type of repair required and to be compatible with special
chipping hammers or hand tools. The area of the concrete characteristics of the original concrete. Some modern
to be repaired and the exposed reinforcing steel are repair materials are designed to have a high compressive
then cleaned, usually by careful sandblast and air blast strength and to be impermeable. Even though inherently
procedures applied only within the repair area. Adjacent durable, these newer materials may not be appropriate for
original concrete surfaces should be protected during this use in repairing a low strength historic concrete.
work. In some cases, project constraints such as dust control
The concrete's durability, or resistance to deterioration,
may limit the ability to thoroughly clean the concrete and
and the materials and methods selected for repair
steel. For example, it may be necessary to use needle scaling
depend on its composition, design, and quality of
(a small pneumatic impact device) and wire brushing
workmanship. In most cases, a mix design for durable
instead of sandblasting.
replacement concrete should use materials similar to
Supplemental steel may be needed when existing those of the original concrete mix. Prepackaged materials
reinforcing steel is severely deteriorated, or if reinforcing are often not appropriate for repair of historic concrete.
steel is not present in repair areas. Exposed existing The concrete patching material can be air entrained or
reinforcing and other embedded steel elements can be polymer-modified if subject to exterior exposure, and
cleaned, primed, and painted with a corrosion-inhibiting should incorporate an appropriate selection of aggregate
coating. The patching material should be reinforced and cement type, and proper water content and water
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