To the History of Construction
With the introduction of the railways and steam machinery, transportation and manufacturing costs were considerably reduced and concrete came to be more widely used, but it was still very much a neglected material. Therefore, good concrete was scarce and a great deal of poor concrete was used.
The big break-through was the discovery of Portland cement by Joseph Aspdin in 1824, a worker in an English town.
When he was working an idea came to him as to how to make his work better. He started his experiments. After some time he obtained a powder. When it was mixed with water and allowed to stand it “sets” forming a hard substance. This substance was so much like the building stone from Portland that the powder was named Portland cement. As years passed different materials were found in many countries from which Portland cement could be made.
Portland cement was first used on a large scale in the construction of the Thames tunnel in 1828.
As early as 1830 the first idea of reinforced concrete was mentioned in a publication, which suggested that a lattice of iron rods be embedded in concrete to from a roof.
Patents were taken out for all sorts of systems in all countries. The development of reinforced concrete really got under way in the 1850’s and 60’s.
Lambort, a French contractor, built a concrete boat for the Paris International Exhibition of 1855, with 2 inches sides reinforced with a skeleton of iron rods.
W. Wilkinson, who patented a method of constructing a concrete floor in 1854, is considered by many to be the inventor of reinforced concrete as well.
But many people say that a Frenchman, J. Monier, who took out apatent in 1867 for the construction of plant tubs, tanks, etc., made of concrete reinforced with a mesh of rods or wires, should be credited with the invention. Certainly Monier did a great deal to develop the use of reinforced concrete and his name came to be so closely linked with reinforced concrete that reinforced concrete was known as the Monier System.
Wilkinson, however, certainly appears to have been the first. His patent covered for concrete floor slabs reinforced with a network of flat iron rods placed on edge. One of his main claims was the good fire resistance of the floor. He appears to have understood the principles of reinforced concrete, for he stated that the reinforcement was to be placed in the concrete to take the tension.
A number of buildings were erected, using Wilkinson’s system. He also described method for the construction of pipes, reservoirs, and walls of concrete reinforced with metal sheets, bars and chains.
Freyssinet is known for his work in prestressed concrete for which he had his first ideas before First World War. With the improved materials and the new knowledge available, Freyssinet realized the advantage to be obtained from prestressing, and he used his system in prestressed works.
From now on structures became bigger, better and more exciting, and concrete steadily strengthened its position as a building material. Reinforced concrete was recognized as the best material for all types of structures.
The post-war era has given the biggest boost to concrete, both reinforced and prestressed. After the war steel was short in Europe and many architects had to use either reinforced or prestressed concrete in their structures in order to economize in steel.
Architects were perhaps a little surprised to discover that in many cases reinforced concrete structures, apart from using the minimum of steel, where also cheaper than other forms of construction, and could be erected as quickly. They also discovered that they had more freedom for planning than they had ever before, and a larger number of different solutions to each structural problem were available.
Beams could be eliminated, floor spans could be increased, and shells were available for roofing large areas.
Another big factor, which encouraged the use of concrete, was the introduction of fire regulations, which recognized the superiority of concrete over other structural materials in its fire resistance properties.
The Properties of Concrete
Concrete must be hard, strong, durable, dense, non-porous, fire-resisting and economical.
Concrete has proved to be durable when made of good materials, well mixed, and properly cured. Failures can be found in concrete work, but the trouble is usually caused by poor material, faulty foundations, lack of knowledge of the properties of concrete or poor workmanship. For example, some cements will give better results in sea water than others. This fact had to be established by experience and experiments.
It is more difficult to secure durable reinforced concrete than mass concrete. This is due to the reinforcing steel and the additional water required to make the concrete flow around the steel bars. When moisture reaches the steel, it will rust and the expansion caused by the rust will crack the concrete, resulting in an unsightly structure and necessary repairs. In all structures exposed to the weather the reinforcing steel must be carefully placed and well secured so that it cannot be displaced while concreting. No metal should project to the surfaces. Small wires will soon cause rust spots on the surface of the concrete if they are exposed.
Concrete, to be durable, must be made of good materials, uniform in quality, mixed with a minimum amount of water, and properly placed and protected while curing. Concrete exposed to sea water and the rise and fall of water levels, especially in cold climates where ice forms on the structures, requires specials attention in the selection of the cement, aggregates, mixing, placing and curing.
With the use of dense aggregates the proportions which will produce the densest products are generally those which contain the maximum amount of coarse aggregate and still contain enough fine aggregate to produce a smooth surface. With porous aggregates used in the production of light weight units, the amount of material in the mix passing a 50-mesh sieve is generally limited and in addition more of the coarse aggregate is used to produce a unit of less density and lower weight. This is generally desirable for light weight units except where fire resistance or watertightness are important.
The strength of plain concrete depends upon the quality of the cement, the strength and character of the aggregate, the quantity of cement in a unit of volume, and the density of the concrete. Other things being equal the strongest concrete is that containing the largest amount of cement in a given volume of concrete, the strength of the concrete varying directly as the amount of cement. With a given quantity of cement in a unit of volume, the strongest concrete is that in which the aggregates are proportioned so as to give a concrete of the greatest density that is of the greatest weight per unit of volume. The strength of concrete also depends upon the methods used in mixing, upon the care taken in measuring the ingredients, and in mixing and placing the concrete. Concrete exposed to the air hardens more rapidly than protected concrete. The setting of cement is a chemical change brought about by the addition of water to the cement, the strength increasing very rapidly the first few days, after which the mixture slowly hardens and increases in strength.
Concrete has poor elastic and tensional properties, but it is strong in compression. Its tensile strength is only one-tenth of its compressive strength. The compressive strength of plain concrete varies between wide limits, depending upon the cement, the proportions of cement and aggregates, and the methods of mixing, and depositing, and the age.