Repairing concrete balconies
Arnold, Richard C
Whether sipping morning coffee, reading a favorite novel, or soaking in some sun, most people tend to make good use of their balconies without a thought to their durability. So it is a bit disconcerting for them to learn balconies are more susceptible to distress and deterioration than any other exposed concrete building element. Yet, when constructed, most balconies do not receive the protection required to prevent them from deteriorating. Consequently, they often command repairs before the rest of the building. Repairing balconies involves not only correcting the damage but taking measures to prevent further decline. To do this, those designing the repairs must know why balconies deteriorate.
The Culprit: Corrosion
Corrosion of reinforcing steel and hand railing embedments is the main cause of balcony deterioration. Shallow concrete cover over reinforcing steel, placement of carpet or other porous coverings on the balcony, and water infiltration allowed by railing embedments all accelerate the corrosion process of metal construction.
The typical cantilevered-designed balcony requires the majority of reinforcing steel to be placed near the top and edge surfaces of the slab, minimizing the concrete cover. The cover often is reduced further by a number of conditions-tapering the slab to provide drainage; placing rebar closer to the slab edge to accommodate railing embedments; and installing drip notches at the slabs bottom.
Carpet or other porous coverings on balconies often absorb water, holding it against the concrete surface (Figure 1). Improper or insufficient drainage of the balcony surface also causes water to remain in contact with the concrete. When that water freezes, the potential increases for the reinforcement to corrode and concrete to crack and spall.
Hand railings and their attachments also are subject to corrosion and could prove dangerous if they fail. Steel embedment cups often are cast into balcony slabs to make construction easier and are later filled with grout or other setting materials to anchor the railings. These embedded cups provide paths for moisture to penetrate the concrete, causing corrosion of the cups, railing posts, or nearby reinforcing steel. Expansion pressures caused by the corrosion can crack, and eventually spall, the surrounding concrete.
Corrosion is accelerated when different metals come into contact. For example, aluminum railings tend to corrode when in contact with steel embedment cups because of their different electrochemical potentials. Such corrosion diminishes railing strength and prompts slab-edge concrete to spall.
Concrete cracks unrelated to corrosion can occur if an expansive material is used to anchor hand railings. Temperature forces or wind-induced railing movement also can crack concrete.
Perform a Condition Assessment
Before establishing repair procedures, assess the balcony’s condition. The assessment typically includes a review of the original balcony design, material sampling and testing, and a thorough visual inspection. A similar process is involved when repairing parking areas, steps, and walls with railings attached.
Based on initial assessment, sampling and testing of the concrete may include determination of compressive strength, carbonation depth, air and chloride content, and water-cement ratio. Identification of existing coatings and types of railing embedments and setting materials also is often necessary. Measurement of half-cell potentials can help determine which areas of the balconies are corroding. During the visual survey, find and document all deteriorated areas. Corrosion-related cracks, delaminations, and spalls are most common.
These cracks generally occur at the concrete surface directly over rebars or near railing embedments. In addition to leading to further rebar corrosion, cracks may provide paths for water to enter the building, resulting in damage to drywall and other interior building components.
Delaminations are horizontal cracks parallel to the slab surface caused by rebar corrosion near the top surface or below railing embedments. As embedded steel corrodes, the pressure causes planar cracks to develop below the concrete surface, eventually causing the concrete over the bar to spall off. By hammer-sounding the suspect area, delaminations which have not spalled off can be detected. A hollow sound indicates a delaminated area that may not be visible at the surface.
Small spalls, or popouts, typically occur at slab edges where bar ends terminate near the concrete surface or at other localized areas of shallow concrete cover. Larger spalls often are present near railing embedments and where long sections of reinforcing steel with insufficient cover run parallel to the surface (Figure 2).
Distress not caused by corrosion will generally take the form of cracking, which may indicate inadequate design, detailing, or construction. If this distress is left untreated, the cracks allow moisture to reach the embedded reinforcing or interior space, leading to more serious distress or damage.
To complete the assessment, visual examination results, material sampling and testing, and design review should be considered together. This will help determine the appropriate repair approach.
The following repairs can be done to ensure a more durable balcony.
Remove deteriorated concrete. For small spalls caused by corroded reinforcing steel, remove concrete only as deep as the rebar level. For deeper spalls, or where the rebars have debonded from the underlying concrete, remove concrete below them. Deep concrete delaminations often require full-depth slab removal (Figure 3). As the depth of repairs increases, so do labor and material costs. Since the balcony and formwork for larger repairs must be supported from lower balconies (Figure 4), shoring costs also rise.
Remove and replace. Where replacement is impractical, add supplemental support to the railing base to compensate for the loss of section (Figure 5).
Remove corrosion from rebar. Sandblasting or waterblasting are the most effective methods for this procedure. Cleaning with hand-held grinders usually does not produce acceptable results and should be avoided when possible. When the corroded steel extends into areas of sound concrete, the concrete must be removed to clean the rebar and embedments.
Replace or supplement reinforcing steel. Severely corroded reinforcing steel that has suffered from section loss must be replaced or supplemented. Because most slabs are cantilevered, top-of-slab reinforcement length and bar termination are critical. Placing dowel bars into the interior slabs often is required.
Coat rebar and railing embedments. Protecting cleaned or newly embedded steel with an epoxy or other protective coating greatly increases its resistance to corrosion. Be sure to thoroughly coat the embedded steel. Uncoated sections will be a source of further corrosion.
Recast the concrete section. Use air-entrained or polymer-modified structural concrete to recast the section. To achieve a dense, uniform patch, use forms to support patches, rather than troweling on a non-sag material. Where repair areas are small and forming is impractical, consider using polymer patching materials.
Repair cracks. Cracks that have not caused structural distress to the balcony should be routed and sealed to prevent moisture from entering the concrete. Epoxy injection may be used for non-moving structural cracks that will not undergo much thermal movement.
Protect the concrete surface. The amount of surface protection provided for newly placed concrete will greatly influence its longevity. Unprotected patches can allow moisture to enter the concrete at patch edges, railing anchorages, or through shrinkage cracks.
Waterproofing paints help vertical or soffit surfaces shed water, but do not protect surfaces exposed to standing water. Surface sealers penetrate the concrete surface and fill cracks, providing more protection than paints, but do not keep all water out.
Elastomeric membranes protect horizontal surfaces from all water infiltration and provide good wear resistance. However, they cost much more than paints or sealers. Do not apply membranes to both top and bottom balcony surfaces because moisture can be trapped between the membranes, leading to freezethaw damage and accelerated corrosion. Because carpets and other porous surfaces keep moisture in the concrete, apply the membrane before installing finishes.
Access and Safety
All but the most minor repairs must be accessed by suspended scaffolding. The scaffolding must be versatile enough to allow access to all balcony surfaces, including sides and soffits, and room for suspension cables and safety lines. Because of the danger of falling debris, a protective canopy often is required at ground level to supply safe access to the building for pedestrians and motor vehicles, while protecting areas such as sidewalks and storefronts.
Preventing Balcony Failures By reducing the amount of water reaching the embedded steel in concrete, most common types of deterioration can be prevented. The following practices should help prevent most balcony problems:
* Provide more concrete cover over reinforcing bars.
* Use low-water-cement-ratio, latex-modified concrete.
* Epoxy-coat reinforcing steel.
* Seal vertical and soffit surfaces with sealers or waterproofing paints.
* Apply elastomeric membranes on top slab surfaces.
* Provide better detailing of railing embedments and attachments.
As with any repairs, prevention is the key. If left unattended, the deterioration can progress resulting in structural distress and/or unsafe railing conditions. By correcting the problem while it is still minor, high repair costs and dangerous conditions can be avoided.
This article adpated and reprinted with permission from Aberdeen’s Concrete Repair, June/July 1992, The Aberdeen Group, Addison, III.
Richard C. Arnold is a senior engineer for Wiss, Janney, Elstner Associates, Inc., in Northbrook, III. He is a professional engineer who specializes in condition assessment and design of repairs for concrete structures.
Copyright Institute of Real Estate Management Nov/Dec 1998
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