WPTC photo Historic Lighthouse Preservation: CONCRETE · Historic Lighthouse Preservation Handbook Part IV. D, Page 1 ... Part IV. D, Page 6 CONCRETE between the alkalis in a cement

Historic Lighthouse Preservation Handbook Part IV. D, Page 1

Concrete1 is the fourth most commonlighthouse construction material. The 1908Point Arena Lighthouse, the first reinforcedconcrete lighthouse in the United Stateswas constructed in response to the searchfor an earthquake-proof constructionmaterial. (The Point Arena Lighthouse islocated within 10 miles of the San AndreasFault.) The Point Arena Lighthouse,however, was not the first lighthouse to useconcrete in its construction. Caisson-style

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Historic Lighthouse Preservation:

CONCRETE

Figure 1. Anacapa Island Lighthouse is a reinforcedconcrete lighthouse located on a highly exposed island offthe California coast. Concrete was chosen because of itsability to withstand the high winds, harsh climate, andseismic activity of this exposed location.

lighthouses typically used concrete as filland ballast to help sink the caisson,however, this type of concrete was notreinforced.

Concrete features (such as cast moldingsand brackets, poured in-place foundations)as well as concrete surfaces (modeling andtooling, texture and color) are usuallyimportant in defining the historic characterof a lighthouse and should be retainedduring any treatment. While concrete isamong the most durable of historic buildingmaterials, it is still susceptible to damage byimproper maintenance or repair techniquesand by harsh or abrasive cleaning methods.When performing any treatments on aconcrete lighthouse, use the gentlest meanspossible.

1�Concrete� is a name applied to any of a number ofcompositions consisting of sand, gravel, crushed stone,or other coarse material, bound together with variouskinds of cementitious materials, such as lime or cements.When water is added, the mix undergoes a chemicalreaction and hardens. Traditionally, concrete is notreferred to as masonry.

Part IV. D, Page 2 CONCRETE

Historical Overview

The Romans found that the mixture of limeputty with pozzolana, a fine volcanic ash,would harden under water. The result waspossibly the first hydraulic cement. Itbecame a major feature of Roman buildingpractice, and was used in many buildingsand engineering projects such as bridgesand aqueducts. Concrete technology waskept alive during the Middle Ages in Spainand Africa, with the Spanish introducing aform of concrete to the New World in thefirst decades of the 16th century. It wasused by both the Spanish and English incoastal areas stretching from Florida toSouth Carolina. Called �tapia�, or �tabby�,the substance was a creamy-white,monolithic masonry material composed oflime, sand, and an aggregate of shells,gravel, or stone mixed with water.

Reinforced concrete in the United Statesdates from 1860, when S. T. Fowlerobtained a patent for a reinforced concretewall. In the early 1870s William E. Wardbuilt his own house in Port Chester, NewYork, using concrete reinforced with ironrods for all structural elements. Despitethese developments, such constructionremained a novelty until after 1880, wheninnovations introduced by Ernest L.Ransome made reinforced concrete morepractical. The invention of the horizontalrotary kiln allowed production of a cheaper,more uniform and reliable cement, and ledto the greatly increased acceptance ofconcrete after 1900. These developmentsin concrete technology and manufactureultimately led to the use of reinforcedconcrete in the construction of lighthouses.

As mentioned in the introduction, the firstreinforced concrete lighthouse wasconstructed at Point Arena, California in1908 (see Figure 5). The construction ofthis lighthouse established a trend of usingreinforced concrete for lighthouses on the

Figure 3. For the construction of this caisson-stylelighthouse, unreinforced concrete was used as ballast tosink the cast-iron caisson foundation deep into the mud onthe bottom of the Chesapeake Bay in Maryland.

Figure 2. View of the Alcatraz Island Lighthouse.Reinforced concrete was chosen for this lighthouse locationfor its ability to withstand earthquakes.

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West Coast and Alaska. This trendcontinued into the 1920s and 1930s. Theconstruction of reinforced concretelighthouses was not limited to the WestCoast and Alaska; concrete lighthouseswere constructed on the Eastern Seaboardas well. The last one, St. John's RiverLighthouse, was constructed in 1954 inMayport, Florida (Figure 5). The reason forconcrete�s popularity in lighthouseconstruction was that it was durable, wascheap compared to traditional masonryconstruction, and required minimal initialmaintenance.

Types of Concrete� Unreinforced concrete is a composite material

containing aggregates (sand, gravel, crushedshell, or rock) held together by a cementcombined with water to form a paste. It gets itsname from the fact that it does not have anyiron or steel reinforcing bars. It was the earliestform of concrete. The ingredients become aplastic mass that hardens as the concretehydrates, or �cures�. This form of concrete istypically used in caisson-style lighthouseconstruction as ballast to weight the caissonfoundation (see Figure 3). Unreinforcedconcrete, however, is relatively weak, and sincethe turn of the century has largely been replacedby reinforced concrete.

� Reinforced concrete is concrete strengthenedby the inclusion of metal bars, which increasethe tensile strength of concrete. Bothunreinforced and reinforced concrete can beeither cast in place or precast.

� Cast-in-place concrete is poured onsite into apreviously erected form that is removed after theconcrete has set. Lighthouses are typicallyconstructed using cast-in-place constructionmethods. The advantage to this method ofconstruction is that once the concrete has cured,the lighthouse is a monolithic structure.

� Precast concrete is molded offsite into buildingcomponents. This method of construction isseldom used for lighthouses.

Figure 4. The first reinforced concrete lighthouse, PointArena Light Station in California.

Figure 5. St. John’s River Lighthouse, Mayport,Florida, was constructed of reinforced concrete for itseconomy and low maintenance.

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Causes of Concrete Deterioration

Deterioration of concrete lighthouses canbe caused by environmental factors, inferiormaterials, poor workmanship, inherentstructural design defects, and inadequatemaintenance.

� Environmental factors are a principal cause ofconcrete deterioration. Concrete absorbsmoisture readily if not coated; this is particularlytroublesome in regions of recurrent freeze-thawcycles. Freezing water produces expansivepressure in the cement paste or in nondurableaggregates. Carbon dioxide, anotheratmospheric component, can cause the concrete

to deteriorate by reacting with the cement pasteat the surface.

� Materials and workmanship in the constructionof early concrete buildings are potential sourcesof problems. For example, aggregates used inearly concrete, such as cinders from burnedcoal and certain crushed brick, absorb waterand produce a weak and porous concrete.Alkali-aggregate reactions within the concretecan result in cracking and white surfacestaining. Aggregates were not always properlygraded by size to ensure an even distribution ofelements from small to large. The use ofaggregates with similarly sized particlesnormally produced a poorly consolidated andtherefore weaker concrete.

Figure 6. Image showing the placement of reinforcing bars in concrete lighthouse construction. (Image courtesy of CEUOakland)


Early lighthouse builders sometimesinadvertently compromised concrete byusing seawater or beach sand in the mix. Acommon practice, until recently, was to addsalt to strengthen concrete or to lower thefreezing point during cold-weatherconstruction. These practices causeproblems over the long term.

In addition, early concrete was not vibratedwhen poured into forms as it is today.More often it was tamped or rodded forconsolidation; on floor slabs, it was oftenrolled with increasingly heavier rollersfilled with water. These practices tended toleave voids (areas of no concrete) atcongested areas, such as at reinforcing barsat column heads and other critical structurallocations. Areas of connecting voids seenwhen concrete forms are removed areknown as �honeycombs� and can reduce theprotective cover over the reinforcing bars.

Other problems caused by poorworkmanship are not unknown in laterconcrete work. If the first layer of concreteis allowed to harden before the next one ispoured next to or on top of it, joints canform at the interface of the layers. In somecases, these �cold joints� visibly detract fromthe architecture, but are otherwiseharmless. In other cases, �cold joints� canpermit water to infiltrate, and subsequentfreeze-thaw action can cause the joints tomove. Dirt packed in the joints allowsweeds to grow, further opening paths forwater to enter. Inadequate curing can alsolead to problems. If moisture leaves newlypoured concrete too rapidly because of lowhumidity, excessive exposure to sun orwind, or use of too porous a substrate, theconcrete will develop shrinkage cracks andwill not reach its full potential strength.

� Structural design defects in historic concretestructures can be an important cause ofdeterioration. For example, the amount ofprotective concrete cover around reinforcingbars was often insufficient. Another design

problem in early concrete buildings is related tothe absence of standards for expansion-contraction joints to prevent stresses caused bythermal movements, which may result incracking.

� Improper maintenance of historic lighthousescan cause long-term deterioration of concrete.Water is a principal source of damage to historicconcrete (as to almost every other material), andprolonged exposure to it can cause seriousproblems. Unrepaired roof and plumbing leaks,leaks through exterior cladding, and uncheckedabsorption of water from damp earth arepotential problems. Deferred repair of cracksallowing water penetration and freeze-thawattacks can even cause a structure to collapse.In some cases the application of waterproofsurface coatings can aggravate moisture-relatedproblems by trapping water vapor within theunderlying material.

Major Signs of ConcreteDeterioration� Cracking occurs over time in virtually all

concrete. Cracks vary in depth, width,direction, pattern, location, and cause. Crackscan be either active or dormant (inactive).Active cracks widen, deepen, or migratethrough the concrete. Dormant cracks remainunchanged. Some dormant cracks, such asthose caused by shrinkage during the curingprocess, pose no danger, but if left unrepaired,they can provide convenient channels formoisture penetration, which normally causesfurther damage.

Structural cracks can result from temporary orcontinued overloads, uneven foundationsettling, or original design inadequacies.Structural cracks are active if the overload iscontinued or if settlement is ongoing; they aredormant if the temporary overloads have beenremoved, or if differential settlement hasstabilized. Thermally-induced cracks resultfrom stresses produced by temperature changes.They frequently occur at the ends or corners ofolder concrete structures built withoutexpansion joints capable of relieving suchstresses. Random surface cracks (also called�map� cracks because of their resemblance tothe lines on a road map) that deepen over timeand exude a white gel that hardens on thesurface are caused by an adverse reaction


between the alkalis in a cement and someaggregates.

Since superficial repairs that do not eliminateunderlying causes will only tend to aggravateproblems, professional consultation isrecommended in almost every instance wherenoticeable cracking occurs.

� Spalling is the loss of surface material in patchesof varying size. It occurs when reinforcing barscorrode, thus creating high stresses within theconcrete. As a result, chunks of concrete popoff from the surface. Similar damage can occurwhen water absorbed by porous aggregatesfreezes. Vapor-proof paints or sealants, whichtrap moisture beneath the surface of theimpermeable barrier, can also cause spalling.Spalling may also result from the improperconsolidation of concrete during construction.In this case, water-rich cement paste rises to thesurface (a condition known as laitance). Thesurface weakness encourages scaling, which isspalling in thin layers.

� Deflection is the bending or sagging of concretebeams, columns, joists, or slabs, and canseriously affect both the strength and structuralsoundness of concrete. It can be produced byoverloading, by corrosion, by inadequateconstruction techniques (use of low strengthconcrete or undersized reinforcing bars, forexample), or by concrete creep (long-termshrinkage). Corrosion may cause deflection byweakening and ultimately destroying the bondbetween the rebar and the concrete, and finallyby destroying the reinforcing bars themselves.Deflection of this type is preceded by significantcracking at the bottom of the beams or atcolumn supports. Deflection in a structurewithout widespread cracking, sparing, orcorrosion is frequently caused by concretecreep.

� Stains can be produced by alkali-aggregatereaction, which forms a white gel exudingthrough cracks and hardening as a white stainon the surface. Efflorescence is a white,powdery stain produced by the leaching of limefrom Portland cement, or by the pre-World WarII practice of adding lime to whiten theconcrete. Discoloration can also result frommetals inserted into the concrete, or fromcorrosion products dripping onto the surface.

� Erosion is the weathering of the concretesurface by wind, rain, snow, and salt air orspray. Erosion can also be caused by themechanical action of water channeled overconcrete, by the lack of drip grooves in beltcourses and sills, and by inadequate drainage.

� Corrosion, the rusting of reinforcing bars inconcrete, can be a most serious problem.Normally, embedded reinforcing bars areprotected against corrosion by being buriedwithin the mass of the concrete and by the highalkalinity of the concrete itself. This protection,however, can be destroyed in two ways. First,by carbonation, which occurs when carbondioxide in the air reacts chemically with cementpaste at the surface and reduces the alkalinity ofthe concrete. Second, chloride ions from saltscombine with moisture to produce anelectrolyte that effectively corrodes thereinforcing bars. Chlorides may come fromseawater additives in the original mix, or fromprolonged contact with salt spray, or de-icingsalts. Regardless of the cause, corrosion ofreinforcing bars produces rust, which occupiessignificantly more space than the original metal,and causes expansive forces within theconcrete. Cracking and spalling are frequentresults. In addition, the load-carrying capacityof the structure can be diminished by the loss ofconcrete, by the loss of bond betweenreinforcing bars and concrete, and by thedecrease in thickness of the reinforcing barsthemselves. Rust stains on the surface of theconcrete are an indication that internalcorrosion is taking place.


Inspecting for Concrete Problems

In order to develop an effective treatment plan for concrete problems, an in-depthinspection must be made of the lighthouse and its immediate surroundings. The followingchart is a listing of locations that should be inspected regularly. Associated with theselocations are the possible problems to look for during the inspection.

Inspection Chart for Concrete Lighthouses

THE SITE

Look For: Possible Problems:

Environment

Typical climatic conditions, including averagetemperatures, wind speeds and directions,humidity levels, average snow accumulation, ice,wave action, salt spray, and blown sand

Severe conditions can lead to concretedeterioration, including cracking, spalling, surfaceerosion, and efflorescence. Concrete lighthousefeatures can be broken or damaged by the weightof excessive ice buildup or by the impact ofviolent wave action or wave-carried debris.

Number of freeze-thaw cycles Severe cycles can produce damage from frostaction if moisture is trapped in concrete walls orthere is a lack of total structure ventilation. Dailyfreeze-thaw cycles may also cause excessivecondensation buildup within the tower, promotingfungal growth and rot as well as causing ironcomponents such as stairs, landings, and hatchesto rust and deteriorate.

Location near sea Salt in the air can lead to efflorescence forming onthe concrete.

Acid rain in the region or from nearby industry orfrom automobile exhaust

Acid rain can cause damage to concrete.

Location in the flood plain of a river, lake, or sea Prolonged immersion in floodwater can bringdamaging moisture to foundations and walls.

Exposed or sheltered locations/elements of alighthouse

Exposure to the sun and elements affects moistureevaporation and rain penetration. Portions of thelighthouse that do not receive sunlight aresusceptible to mildew and other forms ofbiological attack.

Terrain

Soil type�clay, sand, rock The type of soil influences water drainage aroundthe structure. Excessive water in the soil can causerising damp, leading to structural problems.

Slope away from lighthouse on all sides If no slope exists, puddles will form at the base ofthe lighthouse during heavy rains. This may leadto localized ground saturation and to waterpenetration. Localized ground saturation maycause soil around the lighthouse to shift, possiblyresulting in uneven settlement.


Look for: Possible Problems:

Earth covering part of a concrete wall orfoundation

Moisture accumulation or penetration is possible.

Asphalt or other impervious paving touching thelighthouse foundation (if exposed) or walls

Detrimental water accumulation and rain splash-back onto the walls can result. Splash-back cansaturate the concrete, which may cause prematurefailure of exterior coatings (if present).

Trees and Vegetation

Species of trees within 50 feet of lighthouse Elms and some poplars dry up clay soil, possiblyleading to foundation failure.

Branches rubbing against a wall or roof Branches may abrade surfaces.

Ivy or creepers on walls Leaves prevent proper drying of the concretesurface.

THE LIGHTHOUSE

Exterior Walls

General state of maintenance and repair A well maintained lighthouse should require fewermajor repairs.

Evidence of previous fire or flooding Such damage may have weakened structuremembers or caused excessive moisture infiltration.

Construction method�solid or cavity wall Knowing how a tower wall is constructed willhelp in analyzing problems and selectingappropriate treatments.

Embedded iron (steel) anchors, structuralmembers, reinforcement bars, etc.

As iron (steel) rusts, it expands; this expansion candamage the surrounding concrete.

Evidence that parts of the lighthouse wereconstructed at different times or of differentmaterials

Similar problems with various parts may needdifferent treatments because of different materials.

Attached antennas, range finders, auxiliary orreplacement lights, etc.

Heavy devices which are cantilevered off the sideof the tower wall may cause eccentric loading. Ifthis load is improperly distributed, severe crackingand possible localized failure may result.

Cracks Cracks indicate that movement has occurredwithin the wall. Small cracks may be patched;large cracks may require reconstruction of theaffected area.

Enlarging cracks Active cracks indicate a continuing problem. Thecause must be dealt with before the crack itself isrepaired.

Spalling of concrete surface Spalling indicates that the internal reinforcing barshave corroded, causing high stresses within theconcrete. As a result, the concrete �pops� off inchunks, exposing the corroded reinforcing bar.



Windows and Doors

Sills sloped to shed water; drips under sills toprevent water from running back underneath;caulking

If any of these is inadequate, water can penetrateinto the lighthouse wall.

Sealed window and door frames If caulking is missing or deteriorated aroundwindow and door frames, moisture can penetrateinto the wall cavity and cause deterioration of thewindow or door frame, as well as of the concrete.

Foundation

Uneven settlement This may cause the leaning tower effect andpossibly result in collapse of the lighthouse.

Composition of foundation walls Stone or brick are more likely than concrete toallow water to infiltrate.

Undermining of the foundation caused by erosion Undermining may cause a catastrophic failure ofthe foundation and may result in total lighthousecollapse. The remedy is to address the cause ofthe erosion. Minor undermining should be back-filled with a stable, graded fill material containinga range of aggregate sizes for good compaction.For major undermining a structural engineershould be consulted.

Interior

Cracked plaster, signs of patching, stairs andlandings askew

These may be signs of lighthouse settlement.

Damp walls, mold or mildew stains on walls,efflorescence, �bubbling� or blistering plaster orpainted finish, rotting wood

These indicate water infiltration or severecondensation or moisture buildup within thetower.

Concrete Components

Materials

Composition, including secondary materials;characteristics�color and color variation;texture�smooth or patterned surfaces

Types of materials indicate the susceptibility orresistance to damage and should be matched if theconcrete component is repaired or replaced.

Areas of delicate moldings or fine castings These sections may need special attention orprotection during rehabilitation.

Missing or spalled patches of concrete Missing material may allow water penetration, aswell as indicate corrosion of the concretereinforcement.

Evidence of previous patches Patches are signs of past damage or deterioration.If the repair was not made properly, the patch maycover a potential source of future deterioration.Any patched areas should be monitored andinspected regularly for signs of deterioration.



Dirt or stains Surface stains usually cause few problems otherthan being unpleasant to look at. Streaking on thesurface of the lighthouse tower, however, may bean indicator of deteriorating materials that are notreadily visible, such as rust streaks from embeddediron anchors or reinforcement rods, etc.

Moisture

Water penetration through joints betweenconcrete and other lighthouse components,through expansion joints or, rarely, through theconcrete itself

Moisture can lead to deterioration of the concreteand other parts of the structure.

Staining or white deposits (efflorescence), moldand mildew stains on walls

White deposits are evidence of excessivedampness.

Location and type of salt deposits on surface;standing water

Deposits can indicate a source of dampness, suchas rainwater or ground water, inside the lighthousematerials.

Coatings

Applied coating type: stucco, lime mortar wash, orpaint

Applied stucco is common lighthouse coating; thesurface should be inspected for cracks that couldallow water infiltration. Lime mortar wash orwhitewash is another common concretelighthouse coating. This coating is considered asacrificial coating. The lime mortar wash protectsthe lighthouse by wearing away over time. Thiscoating is meant to be reapplied periodically likepaint.

Paint; type of paint A paint that does not breathe can trap moisturewithin the concrete and cause the surface to spall.

Blistering, flaking and peeling paint, failure ofplaster or stucco and interior painted finishes

These conditions indicate the paint does notbreathe.

Waterproof or water-repellent coating Such coatings often trap moisture within theconcrete.


PRESERVATION TREATMENTS

WARNING: Many of the maintenance and repair techniques described in this text, particularlythose relating to cleaning and painting, are potentially dangerous and should be carried out onlyby experienced and qualified workmen using protective equipment suitable to the task. It may benecessary to involve a USCG engineer or architect, preservation architect, or buildingconservator familiar with lighthouse preservation to assess the condition of the concrete andprepare contract documents for its treatment.

Many of the maintenance and repair techniques described in this text, if not properlyperformed, can cause potentially irreversible damage to the character-defining features andhistoric fabric of the concrete lighthouse. Therefore, if the tasks to be performed arebeyond the skills of onsite personnel, they should be carried out by experienced andqualified workmen. In the Beyond Basic Preservation section of this Handbook, examplesof treatments that are considered rehabilitation and restoration are illustrated anddiscussed.

Protection and Stabilization (Mothballing)

Despite their inherent durability, concrete lighthouses are still susceptible to accelerateddecay. A historic concrete lighthouse that receives only minimal routine maintenance ishighly vulnerable to decay if it is not protected and stabilized properly. A thoroughinspection and diagnosis should be performed using the inspection chart in the precedingsection as a guide; the results of which can then be used to develop a protection andstabilization plan. The following recommended protection and stabilization guidelines forvacant historic concrete lighthouses are minimum treatment requirements to prevent anyfurther damage from occurring.

Weatherization � Lantern glass: Lantern glass, frames, and roofsmust be weathertight before mothballing. Referto the Lantern section for more informationconcerning the weatherproofing of the lanterncomponents. (See Figures 7 and 8 in theMasonry section which illustrate a broken glasspanel and a temporary repair using a metal plateand caulking.)

A temporary patch repair will prevent waterfrom entering the lantern and therefore helpavoid incurring further damage. This fix shouldbe considered only as an interim treatment untilthe glass can be replaced. For more on lanternglass replacement refer to Lantern section.

� Built-in gutter systems: All rainwater guttersystems (lantern roofs, or other tower roofforms) should be cleaned and checked forholes. All holes and non-functioning guttersystem components should be repaired. Formore information refer to the discussion onroofing in the Lantern section.

To protect a historic concrete lighthouse,the exterior envelope should be completelyweathertight. When moisture penetratesinto concrete walls and foundations, it canbe exceedingly detrimental to the integrityof the concrete. Moisture in a wall orfoundation may cause various types ofdamage: reinforcement bars may rust,expansion caused by freezing may cracksurrounding concrete, efflorescence (theleaching of salts out of the concrete) mayappear, adjacent wood elements may rot,adjacent metal elements may rust orcorrode, or fungal growth may occur.

To prevent moisture penetration, be surethe following moisture infiltration points areweathertight or functioning properly:


� Gallery decks: In most concrete lighthousesgallery decks are cast iron, stone, or concrete.These decks are generally laid directly on top ofthe concrete wall structure. The decking shouldbe sloped away from the lighthouse to shed thewater away from the structure. If the deckingmaterial is not weathertight, moisture can enterthe interior of the lighthouse or saturate theconcrete wall. See Figure 7 for signs that agallery deck is failing. Refer to the Lanternsection for more information concerning theweatherproofing of gallery decks.

� Door and window frames: The joints along theperimeter of doors and windows where a woodor metal frame is fitted into a concrete openingshould be caulked to prevent moisture fromentering the walls. See the Windows section forthe proper caulk for this application.

� Protective coatings: Lighthouses were oftenpainted as a protective measure and foridentification as a daymark. As part of aprotective treatment, the exterior coating shouldbe checked for loose and flaking paint. Anydeteriorating areas should be scraped andrepainted to match the existing color.

Ultimately, the entire lighthouse should have allloose and flaking paint removed and a newcoating applied according to the manufacturer�sspecifications. If the overall condition of thecoating system is sound and there are only afew bare spots, however, the lighthouse can bespot-painted to provide a weatherproof coating.Either of these actions will result in a coatingsystem that will require minimal interimmaintenance. For more information refer to thediscussion on repainting in the following repairsection.

If the lighthouse was historically protected by astucco coating, the surface should be checkedfor loose or delaminating patches. Any sectionsthat are loose could trap moisture and causepremature deterioration of the concretestructure. The repair of damaged stuccocoatings is covered in the following section onconcrete repair.

� Open cracks in walls: Cracks in exteriorconcrete walls indicate that movement has

Figure 8. Close-up view of a glass-block window in aconcrete lighthouse. The joint where the glass block meetsthe concrete wall should be caulked to prevent moistureinfiltration.

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Figure 7. The concrete gallery deck on this concretelighthouse has been covered with a rubber membrane. Thiscovering should be checked regularly for leaks.


occurred, either movement caused by shrinkage(in the case of stucco) or by settlement ormechanical impact. Cracks should bemonitored to determine if movement is stilloccurring and structural stabilization isnecessary before the crack is filled. Refer to thefollowing repair section for more informationconcerning wall repair.

Stabilization

If funds are insufficient to make repairs,structural stabilization should be performedas a less expensive temporary alternative.Temporary blocking in of window and dooropenings and installation of interior orexterior shoring or bracing are allstabilization methods. The stabilizationtreatment utilized should not permanentlydamage historic character-defining featuresand should be easily reversible so thatwhen the budget allows, the structure canbe properly repaired. Refer to the following

repair section for more information onstructural concrete repairs.

Ventilation

Much like masonry lighthouses, concretelighthouses are difficult to ventilate withoutresorting to extensive louvering and/ormechanical exhaust fan systems. Duringthe summer months concrete lighthouseswill need to have maximum air exchange toeliminate damaging condensation on theinterior walls and woodwork and ironcomponents. In order to achieve this,almost every window opening will need tobe fitted with some type of passive louveredventilation. Installation of window-mounted passive louver systems is coveredin the Windows section of this handbook.For more information on lighthouseventilation refer to the Interiors section.

Figure 10. Cracks like the one here, extending down fromthe belt course of this concrete lighthouse, should bemonitored for movement before any patching is performed.

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Figure 9. Patches of spalled concrete: familiar withlighthouse preservation the one pictured here should berepaired to protect the underlying concrete structure.

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Fire Protection

Despite the fact that concrete lighthousesare constructed of noncombustiblematerials, fire can still be a threat to theirpreservation. The impacts of a fire aredevastating and will often cause seriousirreversible damage and loss to historicinterior fabric. For guidance on theseissues, refer to "Fire Protection andProtection Objectives� under RelatedActivities in Part VI.

Planning for Concrete Repair

Whatever the causes of deterioration,careful analysis, supplemented by testing, isvital to the success of any historic concreterepair project. Undertaken by experiencedengineers or architects, the basic steps in aprogram of testing and analysis aredocument review, field survey, testing, andanalysis.

� Document review: While plans andspecifications for historic concrete lighthousesare rarely extant, they can be an invaluable aid,and every attempt should be made to find them.They may provide information on the intendedcomposition of the concrete mix, or on the typeand location of reinforcing bars. Oldphotographs, records of previous repairs,documents for lighthouses of the same basicconstruction or age, and news reports may also

document original construction or changes overtime.

� Field survey: A thorough visual examinationcan assist in locating and recording the type,extent, and severity of stress, deterioration, anddamage.

� Testing: Two types of testing, onsite andlaboratory, can supplement the field conditionsurvey as necessary. Onsite, nondestructivetesting may include use of a calibrated metaldetector or sonic tests to locate the position,depth, and direction of reinforcing bars. Voidscan frequently be detected by �sounding� with ametal hammer. Chains about 30 inches longattached to a 2-foot-long crossbar, dragged overthe slabs while listening for hollowreverberations, can locate areas of slabs thathave delaminated. In order to find areas ofwalls that allow moisture to penetrate to thelighthouse interior, areas may be tested from theoutside by spraying water at the walls and theninspecting the interior for water. If leaks are notreadily apparent, sophisticated equipment isavailable to measure the water permeability ofconcrete walls.

If more detailed examinations are required,nondestructive instruments are available thatcan assist in determining the presence of voidsor internal cracks, the location and size ofrebars, and the strength of the concrete.Laboratory testing can be invaluable indetermining the composition and characteristicsof historic concrete and in formulating acompatible design mix for repair materials.These tests, however, are expensive. A well-equipped concrete laboratory can analyzeconcrete samples for strength, alkalinity,carbonation, porosity, alkali-aggregate reaction,presence of chlorides, and past composition.

� Analysis: Analysis is probably the mostimportant step in the process of evaluation. Assurvey and test results are revised inconjunction with available documentation, theanalysis should focus on determining the natureand causes of the concrete problems, onassessing both the short-term and long-termeffects of the deterioration, and on formulatingproper remedial measures.

Figure 11. These replacement lighthouse doors havescreened louvers that cover approximately 25% of the dooropening to maximize air exchange.


RepairRepairs should be undertaken only after the planning measures outlined above have beenfollowed. Principal concrete repair treatments are discussed below. While they arepresented separately here, in practice, preservation projects typically incorporate multipletreatments. When performing any of the following treatments, the gentlest means possibleshould be used.

following guidelines to prevent soiling andto determine the most effective cleaningmethods.

� Cover areas where pigeons roost with speciallymanufactured and sensitively installed birdnetting.

� Conduct concrete cleaning tests if cleaning isappropriate. Tests should be observed over asufficient period of time to assess both theimmediate and the long-range effects ofcleaning. Clean concrete surfaces with thegentlest method possible, such as a low-pressurewater rinse using a mild detergent applied withnatural bristle brushes. Chemicals applied as apoultice may be necessary to remove tenaciousstains without abrading surface texture or detail.After treatment, thoroughly rinse the surface ofall residual chemicals.

� Do not use a cleaning method that involveswater or liquid chemical solutions when there isany possibility of freezing temperatures.

� Do not clean with chemical products that willdamage concrete. When using chemicalcleaning products, be sure to rinse the surfaceclean of chemicals.

� Do not apply high-pressure water cleaningmethods that will damage historic surfacetreatment or coating and drive water into thewall, causing corrosion of the steel reinforcingbars.

Coatings

As a protective measure and foridentification as a daymark, most concretelighthouses had an external coating. Theexternal coating was the first line of defenseagainst the elements. Typically this waseither a paint, stucco, or in some caseswhitewash or lime-mortar-wash coating. Aspart of preserving the lighthouse, allcoatings must be maintained.

The following general guidelines should befollowed when performing any treatmenton a concrete lighthouse.

� Identify, retain, and preserve concrete featuresthat are important in defining the overall historiccharacter of the lighthouse.

� Identify finished surface texture, color, andcoatings. Some walls bear the impression ofwooden form boards used during construction.Any repairs made to surfaces with suchimpressions should reproduce the same finish todisguise the repaired area.

� When determining the best treatment forcoating removal, coating application, or appliedpatches, test panels should be prepared usingthe proposed treatments in inconspicuouslocations.

� Identify the age and potential inherentpreservation problems in original materials orconstruction methods, which may requirelaboratory analysis. Any rehabilitation planmust be based on a thorough knowledge of theproperties of the original materials.

� Identify type and location of reinforcing bars.

� Be sure to evaluate and treat the various causesof deterioration, such as leaking roofs or gutters,differential settlement of the lighthouse,capillary action (such as rising damp), orchloride contamination.

� Do not remove or radically change concretefeatures that are important in defining theoverall historic character of the lighthouse.

Cleaning

Clean concrete only when necessary to haltdeterioration. Heavy soiling, bird debris,ferrous stains, graffiti, and biological growthcan trap moisture and damaging chemicalsagainst the surface of the concrete, initiatingand sustaining deterioration. Consider the


Each type of coating protected thelighthouse in a slightly different manner.Paint provides a film over the concrete thatprevents water from penetrating. Stucco isa three-layer mortar-and-sand shell thatbonds to the concrete to prevent water frompenetrating. Whitewash and lime mortarwash are lime-and-water-based �sacrificial�coatings that protect the lighthouse byslowly deteriorating as they weather.

The key to the preservation of an externalcoating system, especially a lighthousecoating that is subjected to severe marineenvironment conditions, is a completeunderstanding of the mechanics of thesystem. Whether simply touching up thecoating or following through with acomplete restoration of the externalcoatings, it is wise to seek the advice ofpaint manufacturers' technicalrepresentatives.

All external coatings, especially paintswhich may date from the early 20thcentury, should be tested for lead andasbestos content. If lead or asbestos ispresent, local codes on health, life safety,and environmental requirements must bemet. Lead or asbestos found in otherwisesound paint layers, does not dictate theremoval of that paint. In most cases it is farsafer and more cost-effective to leave intactpaint areas in place. For further informationrefer to NPS Preservation Briefs 37:Appropriate Methods of Reducing Lead-Paint Hazards in Historic Housing.

Refer to the Masonry section for moreinformation on the repair of stucco andlime-mortar-wash coatings.

When performing coating removal andreapplication on a historic concretelighthouse, consider the following generalguidelines:

� Inspect painted and stuccoed concrete surfacesto determine whether recoating is necessary.

Failed coatings are characterized by flaking orloss of adhesion.

� Locate areas of delaminated stucco andincipient concrete spalls by sounding. Spalledconcrete or delaminated stucco will reverberatewith a distinctly hollow sound.

� Remove damaged or deteriorated coating onlyto the next sound layer using the gentlestmethod possible (e.g., hand scraping) beforerecoating.

� Evaluate the overall condition of the concrete todetermine if protection and maintenance aresufficient, or if material analysis and repairs arenecessary.

Coating Removal

When there is extensive failure of theprotective coating, most or all of the paintmust be removed to prepare the surfaces fornew protective coatings. The selection ofan appropriate technique depends uponhow much paint failure and concretedeterioration has occurred. Localenvironmental regulations may restrict theoptions for cleaning and paint removalmethods, as well as the disposal ofmaterials.

Many of these techniques are potentiallydangerous and should be carried out onlyby experienced and qualified workers usingproper personal protective equipment suchas full-face respirators, eye protection,protective clothing, and optimumworkplace safety conditions. Beforeselecting a process, test panels should beprepared on the concrete to be cleaned todetermine the relative effectiveness ofvarious techniques. The cleaning processwill very likely expose additional coatingdefects, cracks, and deterioration that maynot have been obvious before.

The following are guidelines to considerwhen removing coatings from historicconcrete lighthouses:

� Proven paint removal methods include: waterbased paint strippers designed for concrete use,low pressure needle guns, and hand scraping.


(For more information on the use of needle gunsrefer to the case study on the rehabilitation workperformed at Anacapa Island Lighthouse in PartV., Beyond Basic Preservation.)

� Do not sandblast concrete surfaces using dry orwet abrasives. This treatment will permanentlyerode the surface of the material and acceleratedeterioration.

� Do not remove paint or stucco from ahistorically coated concrete lighthouse and notrecoat, thus changing the appearance.

� Do not remove sound stucco, then recoat theentire lighthouse only to achieve a uniformappearance.

� Do not remove paint or stucco by methods thatdestroy concrete, such as sandblasting,application of caustic solutions, or high-pressurewater blasting.

RecoatingA thorough study of materials isrecommended before starting any coatingprogram. An understanding of thesubstrate, or base material, must also behad. This can best be achieved by athorough inspection of both the substrateand the existing coating system. Any areasof deteriorated substrate should beexamined and repaired before recoating.

Coatings applied to masonry surfacesshould �breathe�. This means the coatingshould allow the transpiration of moistureat the microscopic level. Modern paintcoatings are able to do this. A successfulcoating system for exterior concretesurfaces is to use an acrylic primer with anelastomeric acrylic top coat with mold andfungus inhibitors, mineral oxide pigments,and freeze-thaw stabilizers. A successfulcoating system for interior concrete is to usean acrylic primer and a 100% acrylicemulsion top coat with a minimum 55%permeability rating. This coating systemwill allow the concrete to breathe, thusallowing moisture in the concrete toescape.

� Apply compatible coating systems followingproper surface preparation. Testing is

mandatory to ensure that replacement materialis compatible with the aesthetic and physicalproperties of the existing fabric and todetermine short- and long-term adverse effects.

� Recoat with materials, textures, and colors thatare historically appropriate to the lighthouse.

� Follow the manufacturer�s specifications forsurface preparation and application of paint.This will ensure that the coating will perform asdesigned. For more information on types ofmasonry paints currently being used in the field,refer to the case study on Anacapa IslandLighthouse in Part V., Beyond BasicPreservation.

� Use brushes to apply coatings. Brushapplication will provide the best coverage aswell as be historically accurate. The use ofbrushes will also eliminate the need to containoverspray that is associated with sprayapplications.

� Do not apply paint or other coatings such asstucco to concrete in a manner that creates anew appearance.

Damaged Concrete Repair

Repair of historic concrete may consist ofeither patching the historic material orfilling in with new material worked tomatch the historic material. If replacementis necessary, duplication of historicmaterials and detailing should be as exactas possible to assure a repair that isfunctionally and aesthetically acceptable.The correction and elimination of concreteproblems can be difficult, time-consuming,and costly. Yet the temptation to resort totemporary solutions should be avoided,since their failure can expose a lighthouseto further and more serious deterioration,and in some cases can mask underlyingstructural problems that could lead toserious safety hazards.

The following are guidelines to considerwhen repairing damaged or deterioratedhistoric concrete.


� Repair of Cracking

Hairline, nonstructural cracks that show no signof worsening normally need not be repaired.Cracks larger than hairline cracks, but less thanapproximately one-sixteenth of an inch, can berepaired with a mix of cement and water. If thecrack is wider than one-sixteenth of an inch,fine sand should be added to the mix to allowfor greater compatibility, and to reduceshrinkage during drying. Field trials willdetermine whether the crack should be routed(widened and deepened) minimally beforepatching to allow sufficient penetration of thepatching material. To ensure a long-term repair,the patching materials should be carefullyselected to be compatible with the existingconcrete as well as with subsequent surfacetreatments such as paint or stucco.

When it is desirable to reestablish the structuralintegrity of a concrete lighthouse involvingdormant cracks, epoxy injection repair shouldbe considered. An epoxy injection repair ismade by sealing the crack on both sides of awall or a structural member with an epoxymortar, leaving small holes, or 'ports' to receivethe epoxy resin. After the surface mortar hashardened, epoxy is pumped into the ports.Once the epoxy in the crack has hardened, thesurface mortar can be ground off, but the repairmay be visually noticeable. (It is possible toinject epoxy without leaving noticeable patches,but the procedure is much more complex and isbeyond the scope of this text.)

Other cracks are active, changing their widthand length. Active structural cracks will moveas loads are added or removed. Thermal crackswill move as temperatures fluctuate. Thus,expansion-contraction joints may have to beintroduced before repair is undertaken. Activecracks should be filled with sealants that willadhere to the sides of the cracks and willcompress or expand during crack movement.The design, detailing, and execution of sealant-filled cracks require considerable attention, orelse they will detract from the appearance of thehistoric lighthouse.

Random (map) cracks throughout a structure aredifficult to correct, and may be unrepairable.Repair, if undertaken, requires removing thecracked concrete. A compatible concrete patchto replace the removed concrete is theninstalled. For some lighthouses withoutsignificant historic finishes, an effective and

economical repair material is probably asprayed concrete coating, troweled or brushedsmooth. Because the original concrete willultimately contaminate new concrete,lighthouses with map cracks will presentcontinuing maintenance problems.

� Repair of Spalling

Repair of spalling entails removing the loose,deteriorated concrete and installing acompatible patch that dovetails into the existingsound concrete. In order to prevent future crackdevelopment after the spall has been patchedand to ensure that the patch matches the historicconcrete, great attention must be paid to thetreatment of rebars, the preparation of theexisting concrete substrate, the selection ofcompatible patch material, the development ofgood contact between patch and substrate, andthe curing of the patch.

Once the deteriorated concrete in a spalled areahas been removed, rust on the exposed rebarsmust be removed by wire brush or sandblasting.An epoxy coating applied immediately over thecleaned rebars will diminish the possibility offurther corrosion. As a general rule, if the rebarsare so corroded that a structural engineerdetermines they should be replaced, newsupplemental reinforcing bars will normally berequired, assuming that the rebar is important tothe strength of the concrete. If not, it is possibleto cut away the rebar.

Proper preparation of the substrate will ensure agood bond between the patch and the existingconcrete. If a large, clean break or othersmooth surface is to be patched, the contactarea should be roughened with a hammer andchisel. In all cases, the substrate should be keptmoist with wet rags, sponges, or running waterfor at least an hour before placement of thepatch. Bonding between the patch andsubstrate can be encouraged by scrubbing thesubstrate with cement paste, or by applying aliquid bonding agent to the surface of thesubstrate. Admixtures such as epoxy resins,latexes, and acrylics in the patch may also beused to increase bonding, but this may causeproblems with color matching if the surfaces areto be left unpainted.

Compatible matching of patch material to theexisting concrete is critical for both appearanceand durability. In general, repair materialshould match the composition of the originalmaterial (as revealed by laboratory analysis) as


closely as possible so that the properties of thetwo materials, such as coefficient of thermalexpansion and strength, are compatible.Matching the color and texture of the existingconcrete requires special care. Several testbatches of patching material should be mixedby adding carefully selected mineral pigmentsthat vary slightly in color. After the sampleshave cured, they can be compared to thehistoric concrete and the closest match selected.

Contact between the patch and the existingconcrete can be enhanced through the use ofanchors, preferably stainless-steel hooked pins,placed in holes drilled into the structure andsecured in place with epoxy. Good compactionof the patch material will encourage the contact.Compaction is difficult when the patch is �laid-up� with a trowel without the use of forms; bybuilding up thin layers of concrete, however,each layer can be worked with a trowel toachieve compaction. Board forms will benecessary for large patches. In cases where theexisting concrete has a significant finish, caremust be taken to pin the form to the existingconcrete without marring the surface. Thepatch in the form can be consolidated byrodding or vibration.

Because formed concrete surfaces normallydevelop a sheen that does not match the surfacetexture of most historic concrete, the forms mustbe removed before the patch has fully set. Thesurface of the patch must then be finished tomatch the historic concrete. A brush or wetsponge is particularly useful in achievingmatching textures. It may be difficult to matchhistoric concrete surfaces that were textured, asa result of exposed aggregate for example, but itis important that these visual qualities bematched. Once the forms are removed, holesfrom the bolts must also be patched andfinished to match adjacent surfaces.

Regardless of size, a patch containing cementbinder (especially Portland cement) will tend toshrink during drying. Adequate curing of thepatch may be achieved by keeping it wet forseveral days with damp burlap bags. Note thatalthough greater amounts of sand will reduceoverall shrinkage, patches with a high sandcontent normally will not bond well to thesubstrate.

� Repair of Deflection

Deflection can indicate significant structuralproblems and often requires the strengthening

or replacement of structural members. Becausedeflection can lead to structural failure andserious safety hazards, its repair should be left toengineering professionals.

� Repair of Erosion

Repair of eroded concrete will normally requirereplacing lost surface material with acompatible patching material (as outlinedabove) and then applying an appropriate finishto match the historic appearance. Theelimination of water coursing over concretesurfaces should be accomplished to preventfurther erosion. If necessary, drip grooves at theunderside of overhanging edges of sills, beltcourses, cornices, and projecting slabs shouldbe installed.

Limited Replacement In Kind

If repair by stabilization, consolidation, andconservation proves inadequate, the nextlevel of intervention involves the limitedreplacement in kind of extensivelydeteriorated or missing parts of featureswhen there are surviving prototypes (forexample, concrete cornices and doorpediments, window architraves, gallerybrackets, etc). The replacement materialneeds to match the old both physically andvisually, i.e., gray, portland-cement-basedconcrete needs to be replaced with gray,portland-cement-based concrete. Thus,with the exception of hidden structuralreinforcement and new mechanical systemcomponents, substitute materials are notappropriate in the treatment preservation.Again, it is important that all new materialbe identified and properly documented forfuture research.

If prominent features are missing, then arehabilitation or restoration treatment maybe more appropriate.

Documents

WPTC photo Historic Lighthouse Preservation: CONCRETE · Historic Lighthouse Preservation Handbook Part IV. D, Page 1 ... Part IV. D, Page 6 CONCRETE between the alkalis in a cement