Reinforced Concrete: The Portico at Drayton Hall Amber Anderson Conservation Report Fall 2014 HP 810 / HSPV 810 Conservation Lab
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Table of Contents 1. Table of Contents............................................................................................2 2. List of Figures.................................................................................................3 3. Introduction & Methodology........................................................................5 4. Literary Review..............................................................................................7 5. History...........................................................................................................18 6. Physical Descriptions...................................................................................20 A. Building & Site Description...............................................................20 B. Description of the Portico & Reinforced Concrete..........................22 7. Condition Assessment.................................................................................29 8. Conditions Summary.... ..............................................................................39 9. Mitigation Recommendations....................................................................42 10. Conclusion..................................................................................................49 11. Bibliography................................................................................................50
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List of Figures Figure 1: Drayton Hall in disrepair after the Civil War. Figure 2: Map of Drayton Hall’s location in relation the greater Charleston area. Figure 3: Aerial view of Drayton Hall property and nearby Ashley River. Figure 4: Georgian features include jack arches, modillions, a pediment, belt courses, etc. Figure 5: Drayton Hall’s partially recessed portico. Figure 6: The portico’s second floor is supported by wooden joists. Figure 7: Bessie Drayton standing on the portico prior to its reconstruction. Figure 8: Concrete slab let into brick course. Figure 9: Two of the five beams sit appropriately atop columns. Figure 10: Basement plan indicating concrete beam locations. Figure 11: First floor plan indicating beams below. Figure 12: Second floor plan. Figure 13: Slab thickness and rebar size and placement. Figure 14: Portland cement concrete used to set and point stone pavers. Figure 15: Reinforced concrete beams die into masonry walls. Figure 16: Impressions from wooden formwork. Figure 17: Remnants of concrete applied over brickwork on eastern wall. Figure 18: Evidence of north to south beam at western end of portico. Figure 19: Empty joist pockets. Figure 20: Joist plates from prior flooring system. Figure 21: Evidence of lower floor level. Figure 22: Settlement of one pier. Other issues of interest during the assessment were severe oxide jacking and cracked lintels. Figure 23: Ten jacks were installed, two per beam, in order to remove the stress placed by the portico and concrete on the adjacent masonry walls. Figure 24: Brick deterioration near let-in slab. Evidence of brick delamination, spalling, cracking, soiling, and water staining. Figure 25: Stone paver breakage and adjacent brickwork damage due to concrete removal. Page 3
Figure 26: Discoloration of the concrete used to set the stone pavers. Figure 27: Extensive loss of mortar and water staining of brickwork near let-in slab. Figure 28: Small rebar with mild corrosion in slab. Figure 29: Severely corroded rebar and concrete spalling. Figure 30: Brick deterioration and displacement near and behind removed concrete layer. Figure 31: Brick displacement at empty joist pockets. Figure 32: Brick deterioration and water staining near slab to masonry wall juncture. Figure 33: Brick deterioration and water staining near slab to masonry wall juncture. Figure 34: Stucco on the interior of the western wall shows signs of water staining and related damage. Figure 35: Water intrusion and related damage where beams meet western masonry wall. Figure 36: Mildew, water intrusion and related deterioration. Figure 37: Water damaged stucco near base of western wall. Figure 38: Rebar corrosion on western end of portico. Figure 39: Recurring pattern in all beams of lower corners spalling where they meet the rest of the house. Figure 40: Detail of spalling in lower corner where beams meet the house and related corrosion. Figure 41: Evidence of prior patchwork in lower beam corner. Figure 42: Deformation of brickwork as a result of column settlement. Figure 43: A pattern of corrosion and spalling has formed in the bottom corners of the beam where they meet the house.
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Introduction & Methodology
This project was conducted for the Conservation Lab course, HP 810 / HSPV 810,
offered in the Fall of 2014 through the Master of Science in Historic Preservation Program through Clemson University and the College of Charleston. The assignment was to complete an in-depth study of a material or building system facing conservation issues. The topic of this report is the reinforced concrete in the first floor portion of the portico at Drayton Hall. A detailed history and description of Drayton Hall and the portico can be found in the sections entitled “History,” “Building Description,” and “Description of the Portico & Reinforced Concrete.” In summary, the first floor of this portico was largely reconstructed with reinforced concrete in the first half of the twentieth century. At this time five beams and a slab were introduced at the floor level. Through the years this system has suffered and created serious structural problems. While the system itself is undergoing widespread failure in the form of corrosion and spalling, it is also creating undo stresses on the rest of the eighteenth century house.
After Charleston firm Bennett Preservation Engineering completed an assessment
of the portico in 2012 it was determined to be structurally unsound. The specific conditions that have been observed in the structure will be discussed in depth in the “Condition Assessment” portion of this report. However, the portico was immediately closed to the public and stabilized by way of installing ten jacks, two per beam, to relieve stress on the adjacent masonry walls. While the final plan is still being investigated, Drayton Hall will most likely have the beams and slab removed and replaced with a more historically Page 5
accurate system.1 However, the removal of the system has proven difficult thus far. As the concrete mixture is stronger than its adjacent materials, such as brick and stone, breakage of these weaker materials has become a serious problem.
Despite Drayton Hall’s inclination to remove the reinforced concrete, and perhaps
in light of this increasingly difficult process, the objective of this report is to document the concrete system’s existing conditions, analyze the cause of said conditions, and explore potential mitigation recommendations. The conditions were observed by eye and with the help of a digital SLR camera.* Notes were taken on site to accompany the photographs. After the notes and photographs were carefully analyzed, conclusions were formulated as to probable causes of the various portico conditions. Adjacent materials, such as brickwork and stone pavers, were also assessed during this process. The “Conditions Assessment” portion of this report outlines the findings of the above observation and analysis. The findings are divided horizontally by exterior/top side of slab, slab itself, and interior/bottom side of slab. Following this portion, a section titled “Mitigation Recommendations” discusses the options for conserving the reinforced concrete at Drayton Hall. These options range in terms of invasiveness, life span, and cost. The specific processes of these options, such as how to perform a concrete patch, are also outlined in this section.
*All photographs in this report were taken by the author unless otherwise noted.
1. Trish Smith. “Conserving Drayton Hall’s Iconic Portico.” Preservation Leadership Forum Blog. May 6th, 2014. http://blog.preservationleadershipforum.org/2014/05/06/drayton-halls-iconic-portico/
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Literary Review
Constructed circa 1738, Drayton Hall is one of the best examples of Georgian
Palladian architecture in the United States. Throughout its life, this plantation has witnessed numerous wars and periods of ownership within the Drayton family. While much of the structure retains its original material and design integrity, there are a handful of changes present throughout. While late eighteenth century fireplace surrounds can be credited to Charles Drayton, hardware and shutter alterations were introduced during the phosphate mining era of the mid-nineteenth century. Among these numerous changes were different attempts to strengthen the building’s grand portico which adorns the land-facing side of the house. A recent report by Drayton Hall’s Curator of Historic Architectural Resources, Trish Smith, indicates that numerous attempts have been made to stabilize the portico throughout its life –including the replacement of limestone columns and the inclusion of the concrete beams and a reinforced slab that support the first floor of the portico today.1 The reinforced concrete intended to support the portico has failed in numerous ways as has been revealed by structural stresses and oxide jacking.2 While structural engineers are assessing the stability and future of the portico and house, the subject of this literature review is the conservation of the reinforced concrete itself.
While reinforced concrete is a subject that received widespread attention in the
second half of the twentieth-century, it is not been the topic of much literature within the past ten years. Thus, this review will take into account the analysis and recommendations 1. Trish Smith. “Conserving Drayton Hall’s Iconic Portico.” Preservation Leadership Forum Blog. May 6th, 2014. http://blog.preservationleadershipforum.org/2014/05/06/drayton-halls-iconic-portico/ 2. Ibid.
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of only the following four recent resources: the National Park Service’s “Preservation Brief 15: Preservation of Historic Concrete,” Theodore Prudon’s Preservation of Modern Architecture, Robert Young’s Historic Preservation Technology, and David Farrell’s and Kevin Davies’ “Repair and Conservation of Reinforced Concrete.” Additional sources which predate these will also be used in following sections of this report.
While the NPS’s Preservation Briefs were not first written within the last ten
years specifically, it is understood that as they are meant to provide guidance they must regularly be reviewed for appropriateness and accuracy and thus will be employed in this project. “Preservation Brief 15” describes in detail the process by which one conserves reinforced concrete. The Brief recommends that thorough research and observation be used to produce a condition assessment. This step is pivotal to understanding the problem’s cause, eventual effects, and potential repair approaches.3 The next step is to assess whether or not the concrete structure needs cleaning. If it is deemed so, cleaning may be the only form of maintenance required or it may be used as a preparatory step prior to repair work.
“Preservation Brief 15” also urges the implementation of a maintenance plan. This
plan should include regular assessment of the concrete’s condition, keeping adjacent systems such as roof and surface drains in working order, and protecting the concrete from harsh materials and products such as deicing salts.4
A plan can also provide a
“baseline” for evaluating the material and its related systems in all conditions. For
3. “Preservation Brief 15: Preservation of Historic Concrete.” Technical Preservation Services. National Park Service. https://www.nps.gov/tps/how-to-preserve/briefs/15-concrete.htm. 4. Ibid.
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example, the brief recommends monitoring protection systems such as sealants and expansion joints as well as evolving conditions like cracking and delamination.5
The Brief then outlines the general steps and options for conserving reinforced
concrete. The first step is adequate surface preparation. Reinforced concrete’s failure is often revealed by spalling concrete and corroded steel reinforcement. In order to prepare these surfaces for repair, the failing material must be removed. Failed concrete can be removed by hand, with hammer and chisels, or with a pneumatic needle-scaling device. However, the Brief underscores the notion that a sufficient amount of substrate must be left intact in order to properly repair the area.6 Additionally, the affected reinforcement should be made as exposed as possible by removing failed concrete from all sides of its surface. This is so that the repair material can “encapsulate the reinforced steel, which provides mechanical attachment for the repair.”7
This is an important step because
applying a new patch of concrete to an old one inevitably creates a cold joint that is more susceptible to water infiltration. By encapsulating the steel as much as possible in the new material, the amount of steel which is exposed to the cold joint will be minimized. Furthermore, the newly-exposed steel reinforcement must be cleaned of oxidized metal, primed, and “painted with a corrosion-inhibiting coating.”8
The Brief also advocates for the proper selection of repair materials and mix
design. Understanding the properties of the original concrete mix is pertinent to creating a cohesive and successful repair. Laboratory testing may be needed in order to find the appropriate water, sand, and aggregate ratio. The Brief recommends selecting a material
5. 6. 7. 8.
“Preservation Brief 15.” Ibid. Ibid. Ibid.
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which addresses the situation’s “durability, workability, strength gain, compressive strength, and other needs.”9 It is also recommended that test applications are made prior to undertaking the entire patch. This will ensure that the design will match with the historic fabric. Where possible the NPS also argues that formed applications are preferred for durability in contrast to trowel-applied. This is due to the increased flexibility of mix and consolidation capabilities of formed concrete.10
The NPS also discusses the possibility of integrating three protection systems into
a concrete repair. These include coatings and penetrating sealers, cathodic protection, and re-alkalization. The use of coatings and sealers can provide protection from the elements but should be thoroughly considered prior to application on an historic structure. The Brief states that this consideration is important because of the aesthetic changes created by these products. For similar reasons, new waterproofing membranes should be out of the line of sight or should match previous systems.11 Cathodic protection can be accomplished in a variety of ways, but the most common is through the use of a sacrificial anode. In this process, water and an electrical current essentially create galvanic corrosion between the sacrificial anode and the reinforcement which serves as the cathode. This helps reduce the rate of corrosion according to the NPS.12 Lastly, re-alkalization addresses the problem of carbonated concrete. When the alkalinity in concrete falls below a certain point, the reinforcement becomes more susceptible to corrosion. In order to re-alkalize the system, it is soaked in an alkaline solution. The process can be strengthened and quickened by forcing the solution deep into the
9. “Preservation Brief 15.” 10. Ibid. 11. Ibid. 12. Ibid.
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concrete via an electrical current.13 This raises the alkalinity in the concrete and helps to continue protecting the reinforcement from corrosion.
Robert Young’s 2008 textbook, Historic Preservation Technology, identifies a
series of a “Remediation Methods” in relation to reinforced concrete: stabilization and conservation, coatings, infill repairs, structural reinforcement, and replication/ replacement. In terms of the first category, Young encourages conservators to properly inspect the situation prior to doing any work. If it is deemed that the concrete is a threat to the adjoining structure or to human life, then stabilization should be the first approach to remediation. This might involve “erecting netting to prevent loose pieces from falling to the ground” and the construction of “secondary support systems” to help brace the structure and support it temporarily or permanently.14 As will be discussed in the portico description portion of this report, stabilization efforts have been made in the form of ten jacks which support the portico’s five concrete beams.
The next step advised by Young mimics the NPS’s in the consideration of
protective coatings. Young similarly warns against the use of coatings in all but the most appropriate cases. He states that using coatings can lead to the trapping of water, which can accelerate deterioration.15 Young also shares the NPS’s concern regarding the varying aesthetics that are creating by using coatings. While the NPS warns against color alterations, Young discusses the differences in matte versus glossy finish and how they affect the visual integrity of the concrete.16
13. 14. 15. 16.
“Preservation Brief 15.” Robert Young. Historic Preservation Technology. (New Jersey: John Wiley & Sons, 2008) p. 127. Ibid. Ibid, p. 128.
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In terms of conducting infill repairs, Young advises an approach similar to that of the
NPS. He advocates for an accurate mix and design to match both the physical properties of the new and existing material and an aesthetic match to retain visual continuity. Young also shares the NPS’s instruction to keep an adequate amount of existing material in order to form the substrate for the patching material. If the new patch does not have a sufficient amount and properly tooled surface the adhesive properties will be diminished and subsequently allow for more deterioration.17 While Young also states that proper surface treatment of the rebar is necessary prior to repairing, he neglects to echo the NPS’s advice in removing failing concrete from as much of the rebar’s surface as possible.
The final two approaches that Young discusses relate to altering the original
design intent and the removal of original material. The concept of adding structural reinforcement must be considered if the structure is no longer performing its required functions. Young explains that either internal or external reinforcement may be added when deemed necessary. Both of these approaches have the potential to alter the visual integrity of the concrete, so they need to be carefully considered. While Young states that adding additional rebar within the concrete can lead to an increased covering requirement, and thus increase the dimensions, he notes that this is the least noticeable of the two approaches and that it doesn’t necessarily have to alter the appearance. However, Young also argues that creating external reinforcement is less invasive and is more easily removed in the long run.18
The final method explained by Young is replication or replacement. In some cases,
it is vital to the function of the concrete structure that the material be replaced in whole 17. Young. Historic Preservation Technology, p. 128. 18. Ibid, p. 129.
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or in part. This is an action that requires deep contemplation in order to determine if it is the best route for the integrity of the building. Young also explains that this strategy is dangerous as removal of structural materials can compromise the safety of the adjoining building. However, if it is deemed necessary to take this route, best attempts should be made to match the original material in aesthetic.19
Theodore Prudon is an expert on the topic of Modern architecture and its
preservation. His 2008 textbook, Preservation of Modern Architecture, discusses the numerous technical and philosophical issues regarding the Modern topic. As reinforced concrete plays a large role in the composition of Modern architecture, he devotes several pages to a discussion of the mechanisms of decay and necessary treatments that plague reinforced concrete. Prudon begins by stressing the importance of maintaining the visual integrity of concrete when undertaking conservation work.20 While he admits that structural integrity is of utmost importance, his bias towards the aesthetic necessities of Modern architecture leads to his advocation of maintaining visual aesthetic above all else. Prudon briefly runs through the traditional steps of concrete repair. He lists them as: removal of failing concrete, rust scraped from rebar, rebar and concrete repaired, and final product painted. He notes that “various bonding agents� can be used to hold the new material to the old, but does not explain what types are available for this use.21
Again, Prudon’s main concern is matching the new material with the old for visual
continuity. In order to do this, he recommends taking color, texture, and surface finish into consideration when choosing a concrete patch mix. To ensure that the mix will work,
19. Young. Historic Preservation Technology, p. 129. 20. Theodore Prudon. Preservation of Modern Architecture. (New Jersey: John Wiley & Sons, 2008) p. 97. 21. Ibid, p. 99.
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Prudon suggests that laboratory testing be done of existing material samples.22 Similarly to Young and the NPS, Prudon warns against using coatings which might alter the visual appearance of the concrete. However, he notes that they may need to be tested in order to correctly match stained portions of concrete with new patch materials. His ultimate conclusion is that thorough research and testing must be completed prior to wholesale repair work.23
The final piece of literature reviewed on the topic of reinforced concrete was an
excerpt from Historic Churches by David Farrell and Kevin Davies. The chapter entitled “Repair and Conservation of Reinforced Concrete” takes a more scientific approach to the initial investigative process in concrete conservation. In order to determine the degree of work done, Farrell and Davis recommend hammer testing, electrical potential, a cover meter survey, and carbonation testing. Essentially, these four tests will provide the conservator with an accurate reading of the physical and chemical makeup of the concrete and its condition. They will help to determine if there is failing concrete, an adequate concrete cover, and a healthy chemical environment for the rebar within the concrete itself.24 In addition to these tests, the authors recommend the following four scientific tests: petrographic analysis to determine varying levels of damage, chemical analysis to determine concrete make-up, chemical analysis of drill samples to “determine concentration and depth of sulphate or chloride in the concrete,” and mechanical analysis to determine compressive strength.25
22. Prudon, Preservation of Modern Architecture, p. 99. 23. Ibid. 24. David Farrell and Kevin Davis. “Repair and Conservation of Reinforced Concrete.” Historic Churches. 2005. Accessed electronically: http://www.buildingconservation.com/articles/concrete-repair/concreterepair.htm. 25. Ibid.
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Once appropriate methods have been determined via the above tests, Farrell
and Davies discuss potential repair techniques through the use of church building case studies. They first advocate for proper testing of repairs before undertaking an entire project. Next, they follow a trajectory which both mimics and deviates from the previously discussed approaches. While they stress that a sufficient amount of concrete must be removed and subsequently tooled to create a prime surface for adhesion, as well as that rebar must be wire-brushed to remove corrosion, they do not advocate for the rebar to be primed.26 Farrell and Davis argue that the new concrete mix will be sufficient to protect the rebar from subsequent corrosion. This approach does not take into account, however, the time in which the rebar will be exposed to the elements while the concrete is being repaired. Nonetheless, the authors also indicate that rebar may be relocated if a sufficient concrete cover does not exist. This is much more invasive than the NPS’s recommendation to increase the concrete cover externally, but will retain more visual integrity. Lastly, Farrell and Davis agree with the NPS that using form-work and pouring new concrete patching material into place is preferable over hand-troweling.27
Farrell and Davis then discuss the concept of using less traditional methods in
repairing reinforced concrete. They assert that in certain situations, using modern proprietary systems that include polymer-based additives may be the best and least invasive solution. These systems allow for repair work that can be applied in thin nondestructive layers rather than the removal of historic fabric required in traditional repairs. These materials are also designed to be more resistive to carbonation and chloride attack.28 Additionally, after these products are used an “anti-carbonation coating” can 26. Farrell and Davis. “Repair and Conservation.” 27. Ibid. 28. Ibid.
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be applied to the surface to prevent further corrosion within the concrete. While this method worked well for the authors in a certain situation, the details of the project needs to be carefully considered prior to undertaking such a nontraditional approach.
Despite slight differences in bias and approach, the four sources discussed above
generally recommend a similar approach to reinforced concrete conservation. The first step of the process is to do thorough background research and an analytical condition assessment. These two steps will provide the conservator with an accurate understanding of all the factors at play. These factors include material make-up and compatibilities, structural integrity, design integrity, historic significance, involved parties, and feasible remediation methods. After an adequate understanding of the situation is created, the first physical step in conserving reinforced concrete is either cleaning or the removal of failed material. Failed concrete can be removed by hand, with hammers and chisels, or with small pneumatic needle-scaling devices. A sufficient amount of concrete should be removed to created an appropriate surface for the patching material to be applied. Thought not discussed by all four of the sources, it is important to expose all sides of the affected rebar when possible in order to create maximum bonding potential of the two eras of concrete. This also reduces the chances of water infiltration reaching the rebar via a cold joint.
When the concrete is removed to a sufficient level, oxidized steel must be removed
from the rebar by scraping or wire-brushing. As this metal is exposed tot he elements, it needs to be coated with a corrosion-inhibiting primer immediately after exposure. Then, an appropriately designed concrete patch can be applied as determined by testing and visual matching. While only discussed by the National Park Service’s “Preservation Brief Page 16
15� and authors Farrell and Davies in Historic Churches, pouring a patch into place via form-work is an easier method to do with integrity than applying patch material by hand with a trowel. However, the specific situation will need to be considered and addressed as necessary. In general, it is of great importance to ensure that the new concrete mixture matches the old in both physical properties and visual appearance. These two factors will create the best bond between new and old material as well as retain visual continuity.
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History
John Drayton was born at Magnolia Plantation in 1715 to the prosperous English
family, the Draytons. As he was not the first born son he understood that it would be on his own shoulders to create a homestead for himself.1 By 1738 John had purchased the three-hundred and fifty acre plantation now known as Drayton Hall. Though the property was purchased with several buildings already in place, within a few years of John’s purchase he started construction of a new house. This was the grand house now known as Drayton Hall. Though the exact dates of construction are unknown, they are estimated to between 1738 and 1742. During the early days of Drayton ownership the property served as a working rice plantation. This rice production molded the landscape surrounding Drayton Hall and its remnants can still be seen today in the form of ditches and marsh fields.2
Drayton Hall derives its significance from its unmatched involvement in several layers of history including the aforementioned colonial plantation era under John Drayton, century
the
nineteenth
ownership
under
Charles Drayton, and the Figure 1: Drayton Hall in disrepair after the Civil War. Photograph courtesy of Drayton Hall.
phosphate mining era of the
1. “Historic Timeline.” Drayton Hall. Accessed electronically on November 15, 2014. http://www.draytonhall.org/overview/historic_timeline.html. 2. Ibid.
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late nineteenth century. Drayton Hall is also a unique survivor of warfare. The house and property served as British headquarters both before and after the Siege of Charleston following the American Revolution. Much to historians dismay, it is also one of the only three Ashley River plantations to survive the Civil War (Figure 1). After a financial lull during the post-war period, Drayton Hall experienced a boom when leased out for phosphate mining during the late nineteenth century. This resulted in numerous changes to the house and landscape. The landscape was fitted with waterways and mining facilities, whereas the house received numerous upgrades that will be discussed in the “Building & Site Description” section.
Throughout its life Drayton Hall has endured not only wars but also natural
disasters, vandalism, and numerous changes of ownership.
Beginning with John
Drayton, an eventual member of the Royal Governor’s Council, the property was passed down through seven generations before ending with Charles Henry and Francis Beatty Drayton’s sale to the National Trust for Historic Preservation in 1974.3 These ownerships have lead to the implementation of various changes to the property and house. Some of these changes were for personal reasons , such as the subdividing of rooms on the second floor. Others were in an attempt to keep up with the fashions of the time. These changes will be discussed later on in further detail. Regardless of these minimal alterations, the house and property has remained relatively intact. It is known as the oldest surviving unrestored plantation house in America4 The house now serves as an important educational tool with public tours. Visitors are enabled to experience the plethora of history and beauty of the nearly three-hundred year old architectural gem. 3. “Seventh Generation.” Drayton Hall. Accessed electronically on November 15, 2014. http://www.draytonhall.org/research/people/drayton_7.html 4 “Historic Timeline.” Drayton Hall.
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Building & Site Description Drayton Hall and its surrounding property lie between the Ashley River and Ashley River Road in Charleston, South Carolina (Figures 2 and 3). With close detail to symmetry and proportion, the house is Georgian-Palladian in style. Figure 2: Map of Drayton Hall’s location in relation to the greater Charleston area. Courtesy On the exterior this style manifests in the of Bing.com. form of a partially recessed pedimented ionic portico (to be discussed in the following section), belt courses, jack arches above the windows and doors, modillions, and other high style features (Figure 4). A double stair adorns both Figure 3: Aerial view of Drayton Hall property and nearby Ashley River. Courtesy of Bing.com.
the front of the portico and the rear of the house. Masonry walls and wood details predominate the material usage of both the exterior and interior of this seven-bay wide and six-bay deep house. The structure is composed of two stories
Figure 4: Georgian features include jack arches, modillions, a pediment, belt courses, etc.
with a raised English basement. Page 20
The interior of the house is clad in raised-panel walls, plaster and beadboard
ceilings, and wooden floorboards. An ornate two story staircase connects the two main floors near the back of the house. Great Halls adjoin the porticos on both the first and second floor. These halls serve as center passageways for the four additional rooms per story. Secondary passageways connect these outer rooms on the north and south sides. The northeastern room on the second floor has been subdivided into three smaller rooms. The rooms range in decor from simplistic moldings and finishings to higher style architraves, fireplaces, and plasterwork in the most public spaces.
Various alterations have been made to the house’s basic form throughout the
years. The landscape and building stock have also been altered extensively under different ownerships. In the house itself, major changes began with Charles Drayton in the early nineteenth century. Charles swapped three Georgian fireplace mantels for Federal ones and installed several “Rumford fireboxes” around 1802.1 An additional Federal swap was that of the original twelve-by-twelve windows for the current six-bysix.2 The first floor ceiling and second story floor were reconstructed mid-century. As a result of the phosphate mining era, numerous upgrades were undertaken at the house after 1875 including a change in paint color, new balusters, hardware changes, and shutter additions.3 There were also structural issues with the portico in the 1920s or 1930s which lead to replacement of the portico’s first floor which will be discussed further in the following section. Addtionally, a new roof was put on Drayton Hall in 1982 and during the 1970ss and early 2000s numerous conservation projects were also undertaken.4
1. “Historic Timeline.” Drayton Hall. 2. Ibid. 3. Ibid. 4. Ibid.
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Description of the Portico & Reinforced Concrete
The front, road-facing,
side of Drayton Hall is adorned with a two story portico over raised
English
basement.
The portico, which is ionic in order, is three bays wide and lies recessed one bay into the front of the house. As is Figure 5: Drayton Hall’s partially recessed portico. evident in Figure 5, a few feet of the portico project outward from the house, making it only partially recessed.
Ionic capitals
top the second story columns while doric capitals top the first story’s. The non-ionic first story elements are thought to be later alterations whereas the iconic are thought to be original.1 An ornamented pediment sits atop the portico with modillions, an
Figure 6: The portico’s second floor is oculus, and fish-scale shingles. The floors supported by wooden joists. are finished in limestone, sandstone, and white marble pavers. As the second floor is supported by a wooden joist system, the first floor would have also been originally 1. Smith, “Conserving Drayton Hall’s Iconic Portico.”
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(Figure 6). Investigation done by Drayton Hall has determined that at least two wooden flooring systems predated the current concrete system.2
While there have been several occasions of structural issues occurring with the
portico throughout the centuries, the first mention of actual collapse is thought to have taken place around the 1920s when the first floor fell into the basement3 When this floor was reconstructed the reinforced concrete slab and five beams that exist today were introduced. There are debates surrounding the date of this reconstruction, however. Previous thought and an oral interview with former Drayton Hall employee C. Stuart Dawson indicated that it took place during the 1930s. However, Dawson’s interview states that the work was done while a “Miss Bessie,” or Eliza Drayton, was still alive.4 Figure 7 also pictures Eliza on the portico prior to the installation of the current flooring system. Eliza passed away in 1918, however. This means that the probability of the floor collapsing after 1918 is highly unlikely. The Figure 7: Bessie Drayton standing on the earlier date of the collapse, coupled with portico prior to its reconstruction. The photographed portico floor sits lower than Dawon’s recollection of “Miss Bessie,” likely the present day and is not finished with pavers. indicates that the portico was reconstructed prior to or shortly after 1918 as well.
Regardless of the date of installation, the current first floor system is composed
of a reinforced concrete slab supported by five beams. The slab is let one wythe of one course into the four adjacent masonry walls (Figure 8). The beams tie into the walls via 2. Smith, “Conserving Drayton Hall’s Iconic Portico.” 3. “Historic Timeline.” Drayton Hall. 4. C. Stuart Dawson, Sr., interview by C. Stuart Dawson, Jr. Courtesy of Drayton Hall.
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Figure 8: Concrete slab let into brick course. their embedded rebar. Two of the five beams are appropriately placed over the columns of the masonry arches in the front wall of the basement (Figure 9). The other three, however,
Figure 9: Two of the five beams sit appropriately atop columns
are improperly supported atop the height of the arches themselves. Figures 10 through 12 show the three levels of floor plans at Drayton Hall and the locations of the concrete beams.
Figure 10: Basement plan indicating concrete beam locations. Courtesy of Drayton Hall. Page 24
Figure 11: First floor plan indicating location of beams below. Courtesy of Drayton Hall.
Figure 12: Second floor plan. Courtesy of Drayton Hall.
On top of the reinforced concrete slab lay the limestone, sandstone, and white
marble pavers. These pavers are set and pointed in a Portland cement concrete mixture (Figure 13). This mixture was specified in 1977 by Robert A. Shoolbred, Inc., a local engineering firm.1 Additional testing by this firm discovered that the earlier concrete 1. Robert A. Shoolbred, Inc. “Drayton Hall Analysis.� Job No. 7702. Soil Consultants report. March 1, 1977. Courtesy of Drayton Hall.
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slab and beams contained aggregate of up to 1.5” in diameter. This study also determined that the concrete slab is 4.5” thick with #4 rebar placed only an inch from the bottom of Figure 13: Slab thickness and rebar size and placement.
the assembly.2 This rebar is rather thin and infrequently placed in relation to the amount of load placed upon it. Figure 14 shows a rough cut-through of the slab with its thickness and rebar exposed.
Figure 14: Portland cement concrete used to set and point stone pavers.
On the underside of
the reinforced concrete
slab the five beams die into the adjacent east and west walls as can be seen in Figure 15. These beams were poured using wooden formwork as is indicated by impressions from the wood (Figure 16). Horizontal bands of concrete also run from north to south along the base of the slab (Figure 17). After investigation, however, it was found that only some of these areas were poured and treated as beams or girders -primarily in the northwestern end of the portico. The majority of this is merely concrete smeared 2. Shoolbred, “Drayton Hall Analysis.”
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over brickwork. However, rebar corrosion and related concrete spalling indicate that there was an intention for a structural beam or lintel to placed over the front door to the basement Figure 15: Reinforced concrete beams die into masonry walls. level (Figure 18).
Evidence still exists of
previous flooring systems in the portico. When the north to south running concrete was removed during investigation procedures, empty joist Figure 16: Impressions from wooden formwork.
pockets were uncovered in addition to wooden remnants of joist plates (Figures 19 and 20).3 Some of the joists pockets indicated that they had
Figure 17: Remnants of concrete applied over brickwork on eastern wall.
been filled with concrete
and later generations of wooden joists as there were end grains visible in the concrete 3. Smith. “Conserving Drayton Hall’s Iconic Portico.�
Page 27
fill.4 Additionally, when some of the pavers were removed on the first floor of the portico, the level of paint on the door stoops showed that the floor level Figure 18: Evidence of north to south beam at western end of portico.
used to sit an inch or so lower (Figure 21). This evidence is corroborated by the floor height in the Bessie Drayton image discussed in the previous section (Figure 7).
Figure 19: Empty joist pockets.
Figure 20: Joist plates from prior flooring system. Figure 21: Evidence of lower floor level. 4. Ibid.
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Condition Assessment Major Structural Issues - 2012 Assessment
Figure 22: Settlement of one pier. Other issues of interest during the assessment were severe oxide jacking and cracked lintels.
Figure 23: Ten jacks were installed, two per beam, in order to remove the stress placed by the portico and concrete on the adjacent masonry walls. Each jack was raised 1/10th of a millimeter.
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Exterior / Top of Slab & Adjacent Conditions
Figure 24: Brick deterioration near let-in slab. Evidence of brick delamination, spalling, cracking, soiling, and water staining.
Figure 25: Stone paver breakage and adjacent brickwork damage due to concrete removal. Concrete was used as a setting and pointing mortar for the pavers and, due to its unforgiving nature, is causing them to be the sacrificial element during the removal process. Page 30
Figure 26: Discoloration of the concrete used to set the stone pavers. There is evidence not only of standing water (darker areas) but also of mineral leaching and staining from nearby materials (reddish areas).
Figure 27: Extensive loss of mortar and water staining of brickwork near let-in slab.
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Slab Conditions
Figure 28: Small rebar with mild corrosion in slab. Slab depth, which is roughly four inches, can be seen here.
Figure 29: Severely corroded rebar and concrete spalling. Inadequate concrete coverage also evident. Water staining and related damage on adjacent surfaces as well. Page 32
Interior / Bottom Side of Slab & Adjacent Conditions
Figure 30: Brick deterioration and displacement near and behind removed concrete layer.
Figure 31: Brick displacement at empty joist pockets. Joist pockets are remnants of two wooden flooring systems which predate the concrete.
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Figure 32: Brick deterioration and water staining near slab to masonry wall juncture.
Figure 33: Brick deterioration and water staining near slab to masonry wall juncture. Evidence of previous concrete repair work at this location. Page 34
Figure 34: Stucco on the interior of the western wall shows signs of water staining and related damage. Stucco has delaminated in places and is discolored.
Figure 35: Water intrusion and related damage where beams meet western masonry wall. Page 35
Figure 36: Mildew, water intrusion, and related deterioration.
Figure 37: Water damaged stucco near base of western wall. Stucco has delaminated and discolored. Concentration of discoloration near the base suggests periods of standing water. Page 36
Figure 38: Rebar corrosion on western end of portico. Though this portion of concrete does not consistently run across the entire western end, this exposed rebar suggests that at least part of it serves as a structural beam or lintel.
Figure 39: Recurring pattern in all beams of lower corners spalling where they meet the rest of the house. Severe corrosion is evident.
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Figure 40: Detail of spalling in lower corner where beams meet the house and related corrosion. This example is representative of damage on all five of the beams.
Figure 41: Evidence of prior patchwork in lower beam corner.
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Conditions Summary & Analysis
The 2012 assessment done by Bennett Preservation Engineering confirmed that
there was a pronounced settlement, equalling several inches in depth, of the column identified in Figure 22. This settlement has translated upwards throughout the house as is evident from visually sagging brickwork and door and window jambs in line with this column (Figure 42). While the specific cause of this settlement is hard to determine, it is likely the combination of both live and dead loads
created
by
the house itself and
Figure 42: Deformation of brickwork as a result of column differential settlement settlement. of the soil below. Regardless of the cause, the weight of the reinforced concrete, in addition to the system’s rebar insufficiency and problems with oxide jacking which will be discussed below, was only creating more stress on the adjacent masonry walls. While the concrete was undoubtedly placing undue stress on the western most, or front, masonry wall of the portico at the basement level, the most troublesome of the stresses appeared to be placed on the western most wall of the house and thus the location of the measured settlement problems. In order to alleviate these stresses while formulating a more permanent plan, Drayton Hall installed and slightly raised the ten jacks seen in Figure 23. Page 39
The stone pavers which adorn the floor of the first level portico indicate several
problematic conditions. As Drayton Hall begun its investigative work in order to determine a method of concrete removal, several pavers were damaged in the process. The durability and hardness of the setting and pointing concrete means that the stone pavers have become the sacrificial element. This means that they will break before the concrete does during purposeful attempts to remove the concrete, anthropogenic wearing, or even just normal expansion and contraction of the floor. There also seems to have been trapping of water underneath the pavers at several locations as is depicted in Figure 26. The most evident of these locations is near the outer edge of the portico and is thus more exposed to the elements. However, the impermeability of both concrete and stone mean that any water which enters the system will have no way to quickly escape. This can lead to deterioration of the flooring system itself or the eventual passage of water to the route of least resistance. In this case, that route would be at the juncture of the concrete slab to the masonry wall below. This water travel has lead to numerous other water related conditions described below.
Likely because there is no flashing or other mechanism to divert water off of the
portico floor, there is obvious ponding and passage of water where the slab is let into the house’s masonry walls. The impermeability of the stone and concrete also allows water to sit in these locations for prolonged periods of time. On the first floor of the portico this has lead to severe deterioration of the brickwork. As can be seen in Figures 24 and 27, the concentration of water in these areas has lead to brick delamination, cracking, excess soiling, water staining, and extensive mortar loss. In addition to water intrusion, this problem is exacerbated by the concrete’s having been let-in to the masonry wall. This Page 40
means that every time the wall attempts to expand and contract with climate changes, it is met with resistance from the concrete slab. This pressure has contributed to the breakdown of bricks as described above.
The underside of the portico’s concrete slab also shows evidence of water intrusion
around the perimeter. Where the slab enters the four masonry walls enough stucco and concrete have been removed to make the underlying brickwork visible. Figure 32, which is typical of all four sides, shows the brickwork in poor condition. The mortar appears to be in rough shape and there is a distinct discoloration of the brick from water staining. Figure 33 also shows that previous repair work has been undertaken on the bottom side of the concrete slab near the masonry walls. The work was likely a patching job induced by corroded rebar and spalled concrete. This juncture is likely the avenue by which water enters the structure and creates extensive damage to adjacent materials. It is also likely that as the brickwork further deteriorates, whether from water damage or pressureinduced breakdown, more water is allowed to enter. This creates a cyclical problem that grows exponentially.
As water enters through the floor perimeter it streams down the walls and pools
on the ground. The flow of water down and through the walls is made evident by visible deterioration of the stucco which covers the masonry walls. As shown in Figure 34, the stucco has spalled and become discolored as a result. There are also some locations of mildew forming on the stucco (Figure 36). Evidence of standing water can also be found near the base of the stuccoed walls as there is a concentration of water related damage near this area (Figure 37).
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The concrete slab itself exhibits numerous problematic conditions. As can be seen
in Figures 28 and 29, the slab is only a few inches thick with one course of thin rebar near the bottom of the slab. It is evident that an adequate amount of concrete coverage over the rebar does not exist. An ideal coverage would be two inches or more in order to combat carbonation and a decreased alkalinity level in the concrete. As this slab has at most a one inch coverage the rebar is much more susceptible to related corrosion. This corrosion has grown increasingly problematic as it has resulted in exposed rebar and spalled concrete caused by the expansion of corroding rebar. This exposure quickens the corrosion process and makes future repair difficult. Additionally, as the rebar itself is too skinny and infrequently placed to adequately support the slab and five connected beams, there is likely an added stress placed upon the areas where the slab and beams anchor in the adjacent masonry walls. As corrosion is accelerated in overstressed members, this may also be a contributing factor in the deterioration of the outer edges of this system.
The most severe rebar corrosion and subsequent concrete spalling is located
where the five concrete beams meet the back wall of the portico. Figure 43 shows that a clear pattern has formed. Rebar near the bottom corners of each beam has corroded and caused large concrete blowouts. While some areas are currently blown out and exposed, there is also evidence of repair work which indicates that this has happened on all of the beams at some point (Figures 40 and 41). This corrosion is likely the result of a number of factors. Due to a lack of flashing around the perimeter of the first floor portico, water is allowed to infiltrate at this location. As the beams create a cold joint where they meet the masonry wall, these areas are more susceptible to water related damage. Additionally, the aforementioned notion of accelerated rebar corrosion when Page 42
under stress is likely in play. The majority of the weight from the portico and front of the house comes down on this masonry wall and the adjoining concrete slab and beams. The settlement of the one pier only compounds the issue as it most likely creates an increased incline in the portico towards the front of the house -which inevitably increases both weight and water flow directed to this area. Furthermore, as there is no rebar placed in the top of the concrete beams and slab, the Figure 43: A pattern of corrosion and spalling has formed in the bottom corners of the beam where they concrete is not likely as supported meet the house. in compression at the edges of the system as it is in tension towards the center. This means that the system may be allowed to bend upwards in the center which in turn places more stress on the outer edges.
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Mitigation Recommendations
There are a number of options regarding the conservation of Drayton Hall’s
portico. The large question, however, is whether or not to retain or replace any of the reinforced concrete assessed in this report. A determining factor in this decision is the expected life span of reinforced concrete. While a large building or bridge built of reinforced concrete may only remain structurally sound, without any intervention, for fifty to seventy-five years, a smaller structure might last much longer. The longevity of reinforced concrete is dependent on a variety of factors. While size and related loading patterns can have an effect on the performance of concrete and embedded reinforcement, the environment and construction details also play a large role. Improper construction techniques, such as the omission of flashing or insufficient execution of tamping, can lead to rapid deterioration of reinforced concrete. Any avenue created for the passage of water into the concrete will increase the likelihood of rebar corrosion and subsequent damage to the concrete. These problems are only exacerbated in marine environments where certain minerals in the air quicken the corrosion process.
While the longevity of reinforced concrete is dependant on the factors listed
above, it often grows diminished from the point of first repair. For example, once a portion of concrete is patched an almost inevitable cold joint is formed between the existing and new materials. This joint allows for faster penetration of chemicals and water, thus accelerating the deterioration process and subsequent needs for repair. Page 44
This is the case at Drayton Hall. On the underside of the first floor of the portico,
the five reinforced concrete beams have exhibited cyclical deterioration as described in the previous section (Figures 39 through 41). These areas exhibit the notion that previous patchwork is susceptible to failure more quickly than original material. While portions of the reinforced concrete have remained undisturbed since the early part of the twentieth century, several of these patches dating to the 1970s have recently blown out again.
These natural deficiencies in the repair process beg the question as to whether
or not reinforced concrete should simply be replaced. This question is one of necessary skill and finances. While patching concrete is a quick and cheap fix, repouring beams involves not only the material costs but those of formwork, mechanical mixers, a higher labor skill in both execution and design, and other unforeseen requirements. The long term treatment and use of a structure must also be considered when determining whether or not these extra efforts are worth the time and money. For example, if Drayton Hall were to be restored, the current reinforced concrete would undoubtedly be removed. In this case, undertaking a major repair or reconstruction would be pointless and it may be decided that patching would suffice for the time being.
In the case of Drayton Hall’s portico it makes sense to return the structure to
an historically and structurally appropriate wooden flooring system. However, the recent challenges faced when trying to remove the existing concrete bring up another question of feasibility. If a successful method can be found for concrete removal, stabilization during the process, and the eventual installation of wooden joists, this process seems the best route for Drayton Hall’s needs. If this is not the case however, Page 45
other alternatives need to be sought. The “Literary Review” at the beginning of this report provided an in-depth description of some of these alternatives. The following sections discuss potential options for Drayton Hall:
Water Intrusion
Before addressing the issues with Drayton Hall’s reinforced concrete, the
source of much of this material’s deteriation needs to be remedied. Water intrusion has caused extensive damage to not only the concrete but the adjacent masonry and stucco as well. As was made evident by the “Condition Assessment” portion of this report, water seems to be entering the portico from the perimeter of its first floor. It also seems to be concentrating near the perimeter, particularly in the northwestern corner. Before the concrete, masonry and stucco can be repaired, the cause of the problem must be removed. In order to do this close observation must occur to determine whether not ponding is taking place on portions of the floor or running into the adjoing walls in any particular fashion. As it appears that no flashing currently exists between the conrete slab and masonry walls, the installation of flashing might be of great help in directing water away from the structure. A permeable membrance could also be placed between the stone pavers and concrete slab in order to help accelerate the drying process. Once the water issue has been addressed, repair work on the adjacent materials can proceed.
Rebar Insufficiencies
In order to determine how effectively the current rebar is performing,
further investigation should be conducted. Xray technology can help identify the Page 46
reinforcement layout and its conditions. Mechanical analysis can also help to determine the strength potential of the concrete and embedded rebar. If it is concluded that the rebar is in fact insufficient in size and number, two potential options would be the installation of external stabilization or the addition of extra rebar. In terms of stabilization, jacks like the ones currently in place could remain in place. A less visually intrusive method may also be sistering additional, sufficiently reinforced, beams alongside the current ones or the installation of a bracing system. While the addition of extra rebar into the system would ultimately be the least visible it is the most intrusive. This would involve the removal of large amounts of historic fabric in order to insert the additional reinforcement. This route would, however, provide the system with sufficient strength. If the addition or replacement of rebar method were chosen, the current material could be replaced with a corrosion-resistance stainless steel. This would help prevent oxide jacking and related deterioration.
Reinforced Concrete Repair / Patching
If it is determined that the concrete should be repaired, certain patching
techniques should be applied. To begin with, the failing concrete material must be removed. This can be done by hand, with a hammer and chisel, or with a small pneumatic needle-scaler. Keeping failed material will only exacerbate the problem. Additionally, as much of the rebar’s circumference should be exposed as possible. This allows for the subsequent patching material to encapsulate the rebar and reduce the number of cold joints created. However, when removing the loose concrete it is also important to retain enough substrate for the patch to adhere to.
Once the rebar is exposed, a wire-brush can be used to remove oxidation, or Page 47
rust, from the surface. The removal of this rust will make the rebar instantly vulnerable to the elements and thus it must be painted with a corrosion-inhibiting primer immediately. The use of zinc-based primers is of growing interesting due to its electrical properties. When these “zinc-rich epoxy resins” are applied to the rebar, they allow for “electrical contact between the bar and the active zinc.”1 Similar to electrolysis, this process makes zinc the sacrificial annode and thus protects the rebar from corrosion.
After the rebar is primed, an appropriate concrete mixture can be applied to
the area. Chemical analysis can be used to determine the proper mixture which will match the existing concrete both physically and visually. Once a mix is selected, it is recommended that the concrete be applied in formwork versus by hand-troweling. Hand-trowleing inevitably creates gap between applied layers. These gaps will eventually acceleration future deterioration.
Cathodic Protection & Re-alkilization
Cathodic protection and re-alkilzation, which are discussed at length in the
“Literary Review,” are two effective ways of protecting reinforcement in concrete. Their complicated application process, high level of skill necessary, and cost do not make them suitable candidates for Drayton Hall’s portico, however. While they are ideal for large structures such as bridges, their involved nature would not be appropriate on such a small scale.
1. “Preventing Further Corrosion in Repaired Concrete.” Concrete Construction. Accessed electronically on November 23, 2014: http://www.concreteconstruction.net/zinc/preventing-further-corrosion-in-repaired-concrete.aspx
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Conclusion
While there exist a variety of options when it comes to conserving the portico
at Drayton Hall, the decision will ultimately have to come down to what is historically appropriate, financially feasible, and in the best interest of Drayton Hall’s longevity. Although technical procedures such as cathodic protection and re-alkalization may prove extremely effective and non-invasive, their high cost and skill requirement may render them unfeasible in the conservation of a system which does not even date from any of Drayton Hall’s numerous periods of significance. Alternately, going the periodic stabilization and repair work route would require the least amount of money and skill up front. However, the frequency with which these treatments, such as bracing and concrete patching, need done may not translate into the best course of action.
The plan that Drayton Hall is currently developing will involve removing most
if not all of the reinforced concrete in the portico. While it may be deemed that this is the most historically and structurally appropriate avenue in the long run, the risks of this option need to be adequately assessed. For example, the inevitable loss of historic fabric during concrete removal must be considered. This plan will also involve the shifting of loading patterns which have been in place for three-quarters of a century. Because of this, the continued guidance of a structural engineer is imperative to the safety of the building and its occupants.
All in all, the unique needs and requirements of Drayton Hall should ultimately
drive the decision making process regarding its future treatment. Page 49
Bibliography Collins, Peter. Concrete: The Vision of a New Architecture, 2nd edition. Montreal : McGill-Queen’s University Press. 2004. Accessed electronically on November 23, 2014: http://eds.a.ebscohost.com.nuncio.cofc.edu/eds/detail?sid=5e70d914-c78541a2-b082-5b41509ae25d@sessionmgr4002&vid=4#db=nlebk&AN=403979 “Comments on Work Done on Drayton Hall.” C. Stuart Dawson, Sr., interview by C. Stuart Dawson, Jr., January 8, 1973. Courtesy of Drayton Hall. Farrell, Dave and Kevin Davies. “Repair and Conservation of Reinforced Concrete.” Historic Churches. 2005. Accessed electronically on October 17, 2014: http://www. buildingconservation.com/articles/concrete-repair/concrete-repair.htm. Forsyth, Michael. “Conservation of Concrete and Reinforced Concrete,” in Structures & Construction in Historic Building Conservation. New Jersey: Wiley and Blackwell, 2007. P192-210. Forsyth, Michael. “Concrete and Reinforced Concrete,” in Materials and Skills for Historic Building Conservation. New Jersey: Wiley and Blackwell, 2008. P92-108. Friedman, Donald. Historic Building Construction: Design, Materials & Technology. New York: W.W. Norton & Company, 1995. “Guide to Nondestructive Testing of Concrete.” U.S. Department of Transportation: Federal Highway Administration. Publication No. FHWA-SA-97-105. September 1997. Accessed electronically on November 23, 2014: http://isddc.dot.gov/OLPFiles/ FHWA/006641.pdf Jester, Thomas C. Twentieth Century Building Materials: History and Conservation. New York: McGraw Hill, 1995. Newby, Frank. Studies in the History of Civil Engineering. Vol. 2, Early Reinforced Concrete. Aldershot, Great Britain: Ashgate Publishing Limited, 2001. “Preservation Brief 15: Preservation of Historic Concrete.” Technical Preservation Services. National Park Service. https://www.nps.gov/tps/how-to-preserve/ briefs/15-concrete.htm. Prudon, Theodore. Preservation of Modern Architecture. New Jersey: John Wiley & Sons, 2008. Shoolbred, Inc., Robert A. “Drayton Hall Analysis.” Job No. 7702. Soil Consultants report. March 1, 1977. Courtesy of Drayton Hall. Slaton, Amy. Reinforced Concrete and the Modernization of American Building, 1900-1930. Baltimore: The John Hopskins University Press, 2001. Smith, Trish. “Conserving Drayton Hall’s Iconic Portico.” Preservation Leadership Forum Blog. May 6th, 2014. Accessed electronically on October 15, 2014: http://blog. preservationleadershipforum.org/2014/05/06/drayton-halls-iconic-portico/ Young, Robert. Historic Preservation Technology. New Jersey: John Wiley & Sons, 2008. Page 50