In the past forty years, great changes have taken place in the concrete industry, which have great influence on the types and quantities of the research work. There are two major aspects. Preface to the concrete industry, the 1960s of last century is a period full of optimism and enterprising. Although reinforced concrete was used before World War II, the actual takeoff took place in the late 1950s and 1960s of the last century. The first reason is the shortage of steel, and the two is the need for a major redevelopment of the city center. It’s a golden age for residential and commercial buildings, not to mention road projects……
As far as the concrete industry is concerned, the 1960s of last century was a period full of optimism and aggressiveness. Although reinforced concrete was used before World War II, the actual takeoff took place in the late 1950s and 1960s of the last century. The first reason is the shortage of steel, and the two is the need for a major redevelopment of the city center. This is a golden period for residential and commercial buildings, not to mention the construction planning of highway projects. At that time, because of fiscal austerity, the construction industry construction ability and the pursuit of the pace of construction, promote the solution of many experimental, are transferred to the concrete structure in the direction of one of the most memorable, perhaps is to develop a variety of products for the wooden building system of multi-storey buildings.
In the middle of the 70s, some of the structures that were built during the golden age of the 60s saw problems. The collapse of the Ronan Point building in 1968 marked the beginning of disillusionment with high-rise buildings. Although many problems are not structural problems, they are caused by problems such as architecture, society, and perhaps inadequate maintenance. In the field of concrete, the following research topic is durability. A variety of problems arising from concrete structures, as well as the increasing supply of cheap structural steel at that time, led to concrete structures entering into a period of depression and doubt. Until the middle of the 90s, nearly ten years, the concrete building has gradually restored its past self-confidence, let us once again see more exploration of the concrete design and application of the spirit of architecture.
The second major change is in the performance of steel and concrete. Although high-strength steel has been allowed to use, reinforced concrete structures built before World War II and the late 50s are mostly made of low carbon steel bars with a yield strength of about 250Mpa. With the establishment of the British concrete society, the situation has changed rapidly, yielding strength of 410 or 460Mpa of steel, has become a conventional material. However, the use of experience remains in low carbon steel. Over the years, the strength of steel has been steadily improved, at present, the general strength of steel can reach 500Mpa, but also reduce the safety factor. Therefore, the current values of the stress levels of steel bars are two times higher than those in the initial stage, which are based on long-term experience. Because the elastic modulus of steel does not change with its strength, the cracks and deformations produced in a certain size component will be two times as early as that in the initial stage.
Cement has also undergone significant changes. At present, the strength of Portland cement is much higher than that of the initial stage. In addition, in 60s, concrete was usually made of ordinary Portland cement, aggregate and water. However, today’s concrete will be added a variety of admixtures, such as: powdered fly ash (PFA), micro silica fume or kaolin, as well as admixtures, such as: superplasticizer. Concrete has become a kind of material which is completely different from the past, and has become a more high-tech content of materials. In the early 60s, the conventional concrete structure strength grade was 21-28Mpa. Nowadays, structural concrete with strength less than 30Mpa has been seldom seen. Usually, the strength of concrete is much higher. Ready mix concrete vendors have been able to mix concrete with strength over 100Mpa, which is a completely different material than early concrete.
In the late 50s and 60s, the study of structural concrete was quite common. In this period, the characteristics of the research work are: to focus on solving the behavioral characteristics of materials, which have carried out many very large scale experimental research projects, and now only two research projects as an example, a brief explanation. One of the most ambitious research projects is probably the Royal academy.