CABRILLO Historic Structures Report

REPAIR PROCEDURES (continued)

Figure 31. Concrete cracks at Army Radio Station headers.	Figure 32. Concrete cracks at Base of Battery Calef & Wilkeson Base-End Station hatch.

REPAIR PROCEDURES

Prior to implementing any repair program, the cause of the problem must be thoroughly identified and understood. Our recommendations therefore begin with survey and diagnoses. This approach will assure that causes as well as symptoms are addressed, and that the problems are less likely to recur. Refer to the Material Matrix located in the Appendix for information on material existing at each structure. Refer to the Ultimate Treatments section, below, for structure-specific recommendations.

Concrete

Description

The majority of the structures surveyed are of reinforced, cast-in-place concrete construction. Finishes on these structures vary, and include the following:

• Smooth finish, no visible formwork lines. Examples: Army Radio Station

• Smooth finish, visible formwork lines. Examples: Battery Commander and Base-End Station for Battery Ashburn, Searchlight Shelters No. 18 and No. 19

• Camouflage finishes, scored or left rough, often with stones embedded into the concrete during the curing process. This finish is common to bunker roofs. Examples: Base-End Station, Batteries Woodward and Grant, Battery Commander Station, Battery Humphreys, Battery Commander and Base-End Station, Battery Ashburn.

Smooth-finished structures are typically painted, while the camouflage-finished surfaces are unfinished. Camouflage roof surfaces typically have a soil-and-vegetation cover.

Some of the concrete may have inherent characteristics that promote deterioration. Many of the structures were constructed quickly, which could result in flaws in workmanship. A large number of structures appear to have inadequate depth of concrete covering the steel reinforcing, leading to corrosion jacking, the expansion of steel reinforcing rod as it rusts. The common use of very large aggregate, especially at the surface, promotes concrete deterioration: aggregate stones get dislodged, creating recesses that collect water. In addition, inherent flaws in the mix can cause long-term problems. Laboratory testing will be essential in determining the concrete's chemical and physical properties.

Figure 33. Spalled concrete at Searchlight Shelter No. 15.	Figure 34. Missing corner and exposed reinforcing rod at lower Billy Goat Point Base-End Station.	Figure 35. Exposed reinforcing rod and post spalls at Calef & Wilkeson.

Conditions

Cracks

Cracks are a common condition in concrete. The cracks at Cabrillo vary in depth, width, and direction. Causes of these cracks may be equally varied, and may include curing-induced shrinkage, seismic movement, structural overload, differential settlement, thermal stresses, and corrosion jacking. Cracks may be active or dormant; dormant hairline cracks may not require repair.

The Army Radio Station, Searchlight Station No. 18, and the Generator Station have cracks common at reentrant corners, and window and door headers; while the base-end stations have cracks around rough-tooled areas at the connections of roofs with shutters. Curing-induced cracks typically occur at rough-tooled or stone-embedded camouflage concrete: variation in mass during curing causes cracking. Once this occurs, water infiltration is more likely, creating additional problems.

Spalls

Spalls are loss of surface material in patches of varying size. Often, reinforcing rods are visible and are the primary cause of the failure. As reinforcing rods corrode, they expand, creating high stresses within the concrete. Surrounding concrete will then become loose and eventually fall off. Incipient spalls are portions of loose concrete that have not yet detached from the building. Indicative signs of spalls or potential spalls are missing material, visible cracks around the edges of a spall, and concrete that bulges beyond the wall plane.

Headers and sills show extensive spalling, often a result of reinforcing steel placed too close to the surface. Critical spalls occur at the header at Battery Commander and Base-End Station, Battery Ashburn. Impact spalls occur at exposed corners at the Army Radio Station. Impact spalls, possibly from an explosive device, were noted at the interior of the Generator Station.

Erosion

Concrete erosion is the weathering of a concrete surface by wind, rain, and salt air or spray. Most of the Cabrillo structures appear to be undergoing this process, particularly where water is not allowed to drain or where paint finishes are absent or worn away.

Stains

Two types of stains were observed on the Cabrillo structures: corrosion and efflorescence. Corroded reinforcing rod and ferrous metal attachments, including shutters and hardware, have caused corrosion staining at almost all structures. The stains occur below or adjacent to exposed metal. Efflorescence, a powdery white surface stain, was also observed on many structures. Efflorescence is often produced by the leaching of lime from Portland cement, or by the pre-World War II practice of adding lime to the mix to whiten the concrete.

Failed Coatings

Flaking and loss of adhesion characterize failed paint coatings. This is a common condition on many of the painted concrete surfaces. The coatings protect the concrete surfaces from eroding, and are also important in retaining the historic appearance of many of the structures. Common colors here include gray and drab green, allowing the structures to blend in with their environment (refer to Paint, below, for more information).

Recommendations

In general, repairs should duplicate, as closely as possible the original construction to assure that the repair is physically and aesthetically compatible with the existing material. For example, formwork lines, where visible in the existing wall, should be duplicated in the repair. This will require imprinting lumber marks on fresh concrete patches and avoiding the use of plywood formwork. Other original surface textures, such as rough camouflage finishes, should be duplicated as closely as possible in the repair. Of course, original details and mix components that may have had deleterious effects should be avoided. The concrete analysis tests will determine the appropriateness of the original concrete mix.

General Concrete Repair Methodology:

1. Begin with a field survey to identify and locate all problems. Map cracks, spalls, stains and other conditions on elevation, floor plan and roof plan drawings.

2. Conduct in-situ tests as appropriate. These include sounding the concrete to identify voids and loose material; using a calibrated metal detector to locate the position, depth and direction of reinforcing bar; and using moisture meters to identify water infiltration and migration patterns.

3. Collect samples for laboratory tests. Recommended tests include petrographic analysis, strength tests, and chemical tests for chlorides and other components. Laboratory testing is essential not only to determine the characteristics and composition of the original concrete mix formulations, but also in identifying the nature and underlying causes of many of the observed problems.

4. On structures where repair work is not immediately scheduled, monitor the deficiencies. For instance, apply calibrated crack monitors to selected cracks to gauge their activity level.

5. Make sure any patch material is physically and visually compatible with surrounding existing material. Repair material should match the composition of the original material as closely as possible.

Concrete Stabilization

The following recommendations are appropriate for structures with designated ultimate treatments of stabilization, preservation, restoration and rehabilitation. For restoration treatments, additional work may be required to return the structure to its appearance during the period of significance.

Cracks

1. Remove any loose material. Test with wooden mallet to identify loose or unstable areas.

2. Repair cracks less than 1/16 inches wide with a mix of cement and water.

3. Repair cracks greater than 1/16 inches with a mixture of cement, sand and water. Field test crack prior to patching to determine whether the crack should be routed (widened and deepened) minimally prior to patching. Patch material must be compatible with surrounding material as determined in laboratory tests described above.

4. Apply coating to match existing or as determined by paint analysis (see below). Coating must be vapor permeable to avoid trapping moisture within walls.

Spalls

1. Remove loose material.

2. Clean corrosion from rusted reinforcing rod by wire brushing or other approved method. Immediately apply an epoxy coating to the clean reinforcing rod to discourage future corrosion. Severely corroded reinforcing rod may need to be supplemented with or, if determined non-essential by a structural engineer, removed entirely.

3. Prepare area to be patched by roughening the surface with a hammer or chisel. Wet area to be patched, and keep moist for at least one hour prior to patching.

4. Encourage bond between patch and substrate by scrubbing substrate with cement paste, or by applying a liquid bonding agent.

5. Patch the area with approved compatible material, matching the original in strength, aggregate, color, and texture. Match surface to surrounding texture.

6. For structures that were originally painted, coat with vapor permeable paint matched to original paint color. If original color is unknown, match existing, leaving earlier paint layers intact.

Camouflage Roof Repairs

1. Survey location and size of voids.

2. Locate compatible stones to fill voids.

3. Clean voids of dust or debris.

4. Clean any exposed rusted reinforcing rod and coat with rust inhibitive coating system. Cut out reinforcing rod if not treatable and insert new reinforcing.

5. Match new concrete mix to existing in strength, aggregate, color, and texture unless testing proves original mix to be unacceptable. Match new surface to surrounding texture.

6. Install new stones in voids, back-bedding with mortar, and pinning the larger stones with stainless steel pins as determined by structural engineer.

Concrete Erosion

1. Diagnose cause of erosion and correct if possible. If cause is coursing water, consider installing drip grooves to undersides of overhanging edges.

2. If erosion is substantial, over one-and-one-half inches of lost surface material, replace lost surface material with a compatible patch as described above.

3. Apply non-staining, vapor permeable water-repellent to horizontal concrete surfaces.

Stains

1. Determine type and source of stain. If the stain is ferrous metal corrosion, locate the metal and determine the cause of the corrosion. Staining may be the first clue that reinforcing rod within the wall is corroding. If the stain is efflorescence, determine and eliminate the source of water.

2. Remove non-historic, non-functional metal attachments. Patch subsequent holes as described above under Spalls. See Ferrous Metals below for recommendations on attachments. Replace functional attachments with non-corrosive attachments, if problem continues to persist.

3. Remove stain using the gentlest means possible. Test the area first to make sure the base material is not harmed and that significant paint materials are not impacted. Use gentlest cleaning method possible, beginning with water and a bristle brush. Mild detergent or tri-sodium-phosphate solutions should be tried next. Use proprietary chemical cleaners designed for concrete as a last resort only if necessary. Non-liquid products such as "Peel-Away" may be preferred, since they would have fewer environmental impacts.

Failed Coatings

1. Conduct paint analyses to determine original paint colors and presence of lead-based paint. At this time, original colors are believed to be olive drab and gray-green. Where paint analysis cannot occur, match existing color but do not remove existing paint layers.

2. Select a breathable coating material such as latex paint to avoid trapping water or water vapor within the concrete walls.

Recommended Tests:

1. In Situ Tests

1. Sounding
2. Mapping reinforcing rod with calibrated metal detectors
3. Moisture meter mapping

2. Laboratory Tests

1. Petrographic analysis
2. Strength testing
3. Chemical analysis

Concrete References:

Coney, William B., A.I.A. Preservation of Historic Concrete: Problems and General Approaches. Preservation Brief No. 15.

Concrete Repair and Restoration. ACI Compilation No. 5. Detroit: American Concrete Institute, 1980. Reprint of Concrete International: Design & Construction. Vol. 2, No. 9 (September 1980)

Condit, Carl W. American Building: Materials and Techniques from the First Colonial Settlements to the Present, Chicago: University of Chicago Press, 1968.

Guide for Evaluation of Concrete Structures Prior to Rehabilitation. ACI Committee 364, ACI 364, 1R-94.

Scott, Gary. Historic Concrete Preservation Problems at Fort Washington, Maryland. APT Vol. X No. 2 1978.