Concrete (Part 4: Historic Lighthouse Preservation)
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Historic Lighthouse Preservation: WPTC photo CONCRETE Figure 1. Anacapa Island Lighthouse is a reinforced concrete lighthouse located on a highly exposed island off the California coast. Concrete was chosen because of its ability to withstand the high winds, harsh climate, and seismic activity of this exposed location. Concrete1 is the fourth most common lighthouses typically used concrete as fill lighthouse construction material. The 1908 and ballast to help sink the caisson, Point Arena Lighthouse, the first reinforced however, this type of concrete was not concrete lighthouse in the United States reinforced. was constructed in response to the search Concrete features (such as cast moldings for an earthquake-proof construction and brackets, poured in-place foundations) material. (The Point Arena Lighthouse is as well as concrete surfaces (modeling and located within 10 miles of the San Andreas tooling, texture and color) are usually Fault.) The Point Arena Lighthouse, important in defining the historic character however, was not the first lighthouse to use of a lighthouse and should be retained concrete in its construction. Caisson-style during any treatment. While concrete is among the most durable of historic building materials, it is still susceptible to damage by improper maintenance or repair techniques 1Concrete is a name applied to any of a number of compositions consisting of sand, gravel, crushed stone, and by harsh or abrasive cleaning methods. or other coarse material, bound together with various When performing any treatments on a kinds of cementitious materials, such as lime or cements. concrete lighthouse, use the gentlest means When water is added, the mix undergoes a chemical reaction and hardens. Traditionally, concrete is not possible. referred to as masonry. Historic Lighthouse Preservation Handbook Part IV. D, Page 1 Historical Overview The Romans found that the mixture of lime putty with pozzolana, a fine volcanic ash, would harden under water. The result was possibly the first hydraulic cement. It became a major feature of Roman building practice, and was used in many buildings and engineering projects such as bridges and aqueducts. Concrete technology was kept alive during the Middle Ages in Spain and Africa, with the Spanish introducing a form of concrete to the New World in the first decades of the 16th century. It was used by both the Spanish and English in coastal areas stretching from Florida to South Carolina. Called tapia, or tabby, the substance was a creamy-white, monolithic masonry material composed of lime, sand, and an aggregate of shells, gravel, or stone mixed with water. WPTC photo Figure 2. View of the Alcatraz Island Lighthouse. Reinforced concrete in the United States Reinforced concrete was chosen for this lighthouse location for its ability to withstand earthquakes. dates from 1860, when S. T. Fowler obtained a patent for a reinforced concrete wall. In the early 1870s William E. Ward built his own house in Port Chester, New York, using concrete reinforced with iron rods for all structural elements. Despite these developments, such construction remained a novelty until after 1880, when innovations introduced by Ernest L. Ransome made reinforced concrete more practical. The invention of the horizontal rotary kiln allowed production of a cheaper, more uniform and reliable cement, and led to the greatly increased acceptance of concrete after 1900. These developments in concrete technology and manufacture ultimately led to the use of reinforced concrete in the construction of lighthouses. As mentioned in the introduction, the first reinforced concrete lighthouse was constructed at Point Arena, California in WPTC photo 1908 (see Figure 5). The construction of Figure 3. For the construction of this caisson-style lighthouse, unreinforced concrete was used as ballast to this lighthouse established a trend of using sink the cast-iron caisson foundation deep into the mud on reinforced concrete for lighthouses on the the bottom of the Chesapeake Bay in Maryland. Part IV. D, Page 2 CONCRETE West Coast and Alaska. This trend continued into the 1920s and 1930s. The construction of reinforced concrete lighthouses was not limited to the West Coast and Alaska; concrete lighthouses were constructed on the Eastern Seaboard as well. The last one, St. John's River Lighthouse, was constructed in 1954 in Mayport, Florida (Figure 5). The reason for concretes popularity in lighthouse construction was that it was durable, was cheap compared to traditional masonry construction, and required minimal initial maintenance. Types of Concrete Unreinforced concrete is a composite material containing aggregates (sand, gravel, crushed shell, or rock) held together by a cement combined with water to form a paste. It gets its name from the fact that it does not have any USCG photo iron or steel reinforcing bars. It was the earliest Figure 4. The first reinforced concrete lighthouse, Point form of concrete. The ingredients become a Arena Light Station in California. plastic mass that hardens as the concrete hydrates, or cures. This form of concrete is typically used in caisson-style lighthouse construction as ballast to weight the caisson foundation (see Figure 3). Unreinforced concrete, however, is relatively weak, and since the turn of the century has largely been replaced by reinforced concrete. Reinforced concrete is concrete strengthened by the inclusion of metal bars, which increase the tensile strength of concrete. Both unreinforced and reinforced concrete can be either cast in place or precast. Cast-in-place concrete is poured onsite into a previously erected form that is removed after the concrete has set. Lighthouses are typically constructed using cast-in-place construction methods. The advantage to this method of construction is that once the concrete has cured, the lighthouse is a monolithic structure. Precast concrete is molded offsite into building components. This method of construction is seldom used for lighthouses. WPTC photo Figure 5. St. John’s River Lighthouse, Mayport, Florida, was constructed of reinforced concrete for its economy and low maintenance. Historic Lighthouse Preservation Handbook Part IV. D, Page 3 Figure 6. Image showing the placement of reinforcing bars in concrete lighthouse construction. (Image courtesy of CEU Oakland) Causes of Concrete Deterioration to deteriorate by reacting with the cement paste at the surface. Deterioration of concrete lighthouses can be caused by environmental factors, inferior Materials and workmanship in the construction materials, poor workmanship, inherent of early concrete buildings are potential sources of problems. For example, aggregates used in structural design defects, and inadequate early concrete, such as cinders from burned maintenance. coal and certain crushed brick, absorb water Environmental factors are a principal cause of and produce a weak and porous concrete. concrete deterioration. Concrete absorbs Alkali-aggregate reactions within the concrete moisture readily if not coated; this is particularly can result in cracking and white surface troublesome in regions of recurrent freeze-thaw staining. Aggregates were not always properly cycles. Freezing water produces expansive graded by size to ensure an even distribution of pressure in the cement paste or in nondurable elements from small to large. The use of aggregates. Carbon dioxide, another aggregates with similarly sized particles atmospheric component, can cause the concrete normally produced a poorly consolidated and therefore weaker concrete. Part IV. D, Page 4 CONCRETE Early lighthouse builders sometimes problem in early concrete buildings is related to inadvertently compromised concrete by the absence of standards for expansion- using seawater or beach sand in the mix. A contraction joints to prevent stresses caused by thermal movements, which may result in common practice, until recently, was to add cracking. salt to strengthen concrete or to lower the freezing point during cold-weather Improper maintenance of historic lighthouses can cause long-term deterioration of concrete. construction. These practices cause Water is a principal source of damage to historic problems over the long term. concrete (as to almost every other material), and prolonged exposure to it can cause serious In addition, early concrete was not vibrated problems. Unrepaired roof and plumbing leaks, when poured into forms as it is today. leaks through exterior cladding, and unchecked More often it was tamped or rodded for absorption of water from damp earth are consolidation; on floor slabs, it was often potential problems. Deferred repair of cracks rolled with increasingly heavier rollers allowing water penetration and freeze-thaw filled with water. These practices tended to attacks can even cause a structure to collapse. In some cases the application of waterproof leave voids (areas of no concrete) at surface coatings can aggravate moisture-related congested areas, such as at reinforcing bars problems by trapping water vapor within the at column heads and other critical structural underlying material. locations. Areas of connecting voids seen when concrete forms are removed are Major Signs of Concrete known as honeycombs and can reduce the Deterioration protective cover over the reinforcing bars. Cracking occurs over time in virtually all Other problems caused by poor concrete. Cracks vary in depth, width, workmanship are not unknown in later direction, pattern, location, and cause. Cracks concrete