Reinforced concrete

 Reinforced concrete is composed of aggregate, water, and a cement-like material (almost always Portland cement). Chemicals reactions between cement components and water cause the mixture to harden as it dries (known as curing). If the concrete is well-cured, it will posses great compressive strength but little tensile strength. Reinforced concrete is concrete that has been strengthened by other materials, like rebar. The reinforcement in embedded in the concrete before it sets and gives it great tensile strength. This combination of concrete and metal allows the construction of sturdy slabs, beams, and columns in starting in the early 1900s.

History
The first patented reinforced concrete wall was developed by S.T. Fowler in 1860, but the material was not a phenomenon immediately. Thaddeus Hyatt set a principal that iron reinforcement placed in the bottom of a beam would act in conjunction with concrete. Also, Hyatt thought that bent or deformed bars would achieve more control and greater interactions between reinforcement and concrete. Though these theories were well documented, concrete still could not find popularity. Ernest Ransome is credited with the first practical commercial development of reinforced concrete construction in America. He refined the procedures for casting beams and floor slabs as a unit on top of concrete columns as well as demonstrated load-bearing exterior walls could be replaced by expanses of windows. His services were then marketed as a licensor and consultant and created a relatively successful business that influenced the use of reinforced concrete in warehouses and factories. Soon after his success, competitive techniques emerged with large arrangements of mass-produced metal netting to reinforce slabs and various bars or cables for reinforcing beams and columns. The new technology was known as armored concrete, ferro-concrete, concrete steel, steel concrete, and reinforced concrete. During the 20th century, concrete technology had many advances. Small quantities of admixtures (air-entraining agents, superplasticizers, polymer modifiers, fiber reinforcements) dramatically improved it strength and durability.

Uses and Installation
At the conception of reinforced concrete, the structure imitated the style of timber and steel buildings. Reinforced concrete columns were used to support reinforced concrete girders, which supported reinforced concrete joist. New technology made it possible to transfer the weight bearing from beams and girders to the floor slab, which in turn saved in overhead space and reduced floor to floor height and the expense of building formwork. This technological advance is credited in most part to the Swish engineer Robert Maillart and the Americans Orlando W. Norcros and C.A.P Turner where slabs rested directly on columns. Maillart developed a column capitol that curved from shaft to ceiling; the Americans had straight columns that were mildly less expensive to construct. Both groups concentrated reinforcement in areas of high tension. J-shaped stirrups were put in place to counter shearing. Julius Kahn patented a form of reinforcement that joined these features into a single unit. After 1900, reinforced concrete reached recognition as an ideal industrial buildings material. It could be built quickly, was fireproof, and was ideal for manufacturing plants because it could resist vibrations from heavy machinery. In 1894, Josef Melan reduced the amount of steel needed in concrete bridges and soon a forerunner of the concrete deck bridge was born. Builders soon moved from using girders as projected beams to employ more sophisticated cantilevered girders. In the 1890s, two-way bar reinforcing was developed by G.A. Wayss and was popular in grain elevators and storage tanks. Trade associations promoted reinforced for smaller farm projects and homes. Thomas Edison tried to create a system for molding reinforced concrete houses in a single pour but was not successful. Around 1910, the idea of using reinforced concrete for shells or domes emerged. German engineers constructed an airplane hangar in 1922 by spraying concrete on a hemispherical metal frame. Roofing auditoriums and markets soon adopted this style as it was quick and relatively inexpensive. Rib and arch designs in reinforced concrete began to appear just a few years later. Plain concrete buildings were used in only utilitarian buildings, while other buildings made of concrete usually had it disguised in some manner (usually a terra cotta or brick cladding). By the 1950s, plain concrete gained a positive aspect when functionalist aesthetic gained favor. By the mid 1970s, rough reinforced concrete buildings became overwhelmingly popular. Preformed concrete has waned in and out of favoritism over time.

Conservation
Concrete is most popular in building frames, façade elements, parking structures, bridges, dams, sculptures, and monuments. Reinforced concrete is often exposed to weather and is therefore commonly damaged by moisture. The key issue to focus on in the conservation of reinforced concrete is weather the material can be repaired and conserved or should it be replaced. Concrete structures can be investigated to determine their condition and whether there is a need to develop a repair strategy. Laboratory analysis of the reinforced concrete is important in the investigation.

Deterioration
Two principal reasons reinforced concrete would deteriorate is: corrosion of the embedded steel and degradation of the concrete. Concrete typically protects the embedded steel through alkalinity. When embedded reinforcing steel is not protected by the concretes normal alkaline environment and the steel is exposed to water or high humidity. Essentially, corrosion of the reinforcing steel was believed to be a reaction to electric currents that were straying from nearby railroads and elevators. In the 1950s, however, the major causes of corrosion were identified the loss of passivation in the alkaline concrete due to the presence of chloride ions from sea water or deicing salts; and the loss of the concrete’s alkalinity due to penetration of atmospheric carbon dioxide and the consequent conversion of alkaline components to less alkaline carbonates. When steel corrodes, it expands significantly, causing adjacent concrete to crack and spall. Concrete deterioration due to embedded steel usually will crack and will form rust stains of the steel. Carbonation normally occurs only on the exposed surfaces of concrete but may extend to the steel level and begin to corrode the metal. Calcium chloride initiates and accelerates corrosion. Sea water, boasting large quantities of chloride, also can heighten the speed of deterioration of concrete’s reinforcing steel. Typical weathering is observed as erosion of the cement paste. In highly acidic regions, exposure has resulted in significant erosion of the paste. Cycle freezing causes damage when concrete is saturated with water and appears as surface degradation (severe scaling and cracking). By accident, portland cement was found to be resistant to freezing and thawing damage because of microscopic air bubbles, which act as relief valves. Air-entraining agents are now commonly added to mortar or concrete that is used in exposed applications. Alkali aggregate reactions are caused when alkalis normally present in cement react with siliceous aggregates in concrete that is exposed to moisture. A toothpaste like gel is produced and will develop over years until the forces created expand and cark the concrete. Most of these can be detected by experience and testing. Low alkali cements can be used in new construction to prevent significant reactions. Sulfates attack through a reaction of excessive amounts of sulfate salts with cement components that are exposed to moisture. This reaction leads to the development of expansive forces that will also crack the concrete. Sulfate salts can come from the environment or more concrete constituents. Harmful porous aggregates can lead to popouts on the surface of pavements. It will also deteriorate if exposed to acidic soils or chemicals. Petrographic microscopy is often used to determine the reinforced concrete’s condition.

Conservation Techniques
Condition assessment of the concrete should begin with a review of all construction documents. With historic structures, it is imperative to review historic photographs carefully to understand its past performance. A visual condition survey should always be done to inspect the extent, type, and pattern of distress or deterioration. Next, non destructive field testing methods should be used to evaluate its concealed conditions. Sounding with a hand-held hammer or chain helps to identify areas of delamination. Impact-echo techniques can help locate voids and delamination within the concrete while embedded reinforced steel can be located with a magnetic detection instrument that can identify the depth and size of the reinforcement. To further evaluate the condition, samples may be removed for laboratory analysis including petrographic evaluations of the sample. A concrete’s durability to deterioration depends on its composition, design, and quality of workmanship. To replace damaged pieces, replacements should be of the same basic composition. Good workmanship should address proper mix, placement, and curing procedures. When repairing existing concrete, parameters based on visual evaluation and lab studies must be established to define the project’s goal. Selecting repair interventions that retain as much of the original material is imperative. Also, removing an adequate amount of the distressed concrete will lead to a more durable repair. Proper preparation in necessary of the concrete that will receive the material. Sandblasting, air blasting, and mechanical scarification are appropriate ways to obtain an ideal clean work place that will encourage bonding between the new and old pieces. Bonding agents are commonly used on the substrate surface to enhance the bonding and cleaning. Regular steel, epoxy coated steel, or stainless steel make for ideal replacement reinforcement materials. Proper placement and finishing is also of the utmost importance. Curing in the same fashion as the replaced piece is essential for a durable repair. Wet curing is generally recommended to reduce the curing time and to minimize the potential of cracking and shrinkage. Trial and mock-ups are ideal to repair designs and evaluate the procedures. Also, they allow for evaluation of the aesthetics’ acceptability. A decorative surface coating or a clear coating may be necessary if the concrete had been damaged by an excess of water, and should be breathable and alkali resistant. Several repair methods are available today to reduce the rate of corrosion of embedded reinforcing and associated concrete deterioration. The cathodic protection method requires an auxiliary anode so that the entire reinforcing bar is a cathode. Corrosion is an electrochemical process and an auxiliary anode should prevent corrosion in the embedded steel. Another technique is realkalization, which involves returning the natural alkalinity to the concrete by soaking it with an alkaline solution. Flooding with a corrosion inhibitor or removing the concrete, cleaning the exposed steel coating with an epoxy and recovering and sealing it are other solutions. Cathodic protection is effective but may not be correct for every project.

Replacement
If the vital components of the reinforced concrete are beyond repair, replacement can be cast to match historic ones. Placement and finishing will dictate how well replacement concrete will match its predecessor. Mock-ups to ensure compatibility with adjacent areas is important to the uniformity of the area.