Forty year review of the hottest concrete research

2022-08-08
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Forty years' review of concrete research Abstract: the forty years since the founding of the British concrete society almost covered my research career. Over the past 40 years, major changes in the concrete industry have had a great impact on the type and quantity of research work, which can be summarized in two aspects. Preface as far as the concrete industry is concerned, the 1960s was a period full of optimism and enterprising. Although reinforced concrete was used before World War II, it took off in the late 1950s and 1960s. The first reason is the shortage of steel, and the second is the need for major redevelopment of the urban center. This is a golden age for residential and commercial buildings, not to mention highway projects

preface

as far as the concrete industry is concerned, the 1960s was a period full of optimism and progress. Although reinforced concrete was used before World War II, it took off in the late 1950s and 1960s. The first reason is the shortage of steel, and the second is the need for major redevelopment of the urban center. This is a golden age for residential and commercial buildings, not to mention the construction planning of highway projects. At that time, due to the financial constraints, the construction capacity of the construction industry and its pursuit of construction speed, many experimental solutions were transferred to the direction of concrete structures. Perhaps the most memorable is the development of a variety of timber building systems for multi-storey buildings

highs and lows

concrete

in the mid-1970s, some structures built in the golden age of the 1960s had problems. The partial 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 inadequate construction, society, and perhaps maintenance. In the field of concrete, the subsequent research topic is durability. Various problems in concrete structures and the increasing supply of cheap structural steel at that time led to a period of depression and suspicion for concrete structures. It was not until the mid-1990s, that is, in the past ten years, that concrete architecture gradually regained its former self-confidence, which made us see more concrete architectural design and application with exploratory spirit again

reinforcement

the second major change is in the performance of reinforcement and concrete. Although the use of high-strength steel bars has long been allowed, most reinforced concrete structures built before World War II and in the late 1950s used low-carbon steel bars with a yield strength of about 250Mpa. By the time the British Concrete Institute was founded, the situation had changed rapidly. The yield strength of 410 or 460Mpa was helpful to solve the problem of excess PE capacity in the world. Steel has become a conventional material. However, the application experience still stays 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, and the safety factor can be reduced. Therefore, the stress level of reinforcement commonly used at present is twice higher than that based on long-term experience in the early stage. Because the elastic modulus of steel does not change with its strength, the cracks and deformation in a certain size member will be twice as large as its initial stage

cement

cement has also changed significantly. At present, the strength of Portland cement is much higher than that in the initial stage. In addition, in the 1960s, concrete was generally made of ordinary portland cement, aggregate and water. However, in today's concrete, various admixtures, such as powdered fly ash (PFA), micro silica fume or kaolin, and additives, such as superplasticizer, will be added. Concrete has become a completely different material from the past, and has become a material with more "high-tech" content. In the early 1960s, the strength grade of conventional concrete structures was MPa. Nowadays, structural concrete with a strength lower than 30MPa is rarely seen. Generally, the strength of concrete is much higher. Ready mixed concrete suppliers have been able to prepare concrete with a strength of more than 100MPa, which is a completely different material from the early concrete

research work

structural research

in the late 1950s and 1960s, research on structural concrete was quite common. During this period, the research work was characterized by focusing on the behavior characteristics of materials, in which many large-scale experimental research plans were carried out. Now only two research plans are cited as examples for a brief explanation

one of the most ambitious research projects may be the research on the fracture toughness of reinforced concrete organized by Professor Baker of the Royal College of England and sponsored by the European concrete Commission. This study attempts to establish redistribution rules for reinforced concrete, and almost develops a plastic design method for structural steel. 16 laboratories cooperated to carry out this study. The research results come from about 170 reinforced concrete beams. The practical results of this research work have been included in the redistribution rules of cp110 and bs8110 specifications

the second research plan is the investigation and research of the cement and Concrete Association on crack control. The first research report on 105 reinforced concrete beams was published in 1966. Then, several research reports were successively published. More than 200 reinforced concrete beams were investigated and tension tests were carried out. The result of this research plan is that the crack control formula listed in the successively published specifications in the UK is obtained. A by-product of this research work is the deflection calculation method and the effective span height ratio. This British research project is only a microcosm of a large number of crack studies carried out on a series of large-scale projects in Britain, the United States and Europe during this period. In the 1970s, the data of concrete research carried out in this way will be widely adopted by the database used by the specification Drafting Committee, so as to derive effective design methods. The regulations drafted during this period are based on a large number of data obtained from experiments

by the late 1970s and 1980s, the way of large-scale research on structural concrete basically stopped. Its original change angle θ There are many reasons, but the main factors are:

the funding of universities and other research institutions is increasingly tight

the concrete industry believes that all the behavior characteristics of concrete structures they need have been clearly studied

the investment direction of the research fund has shifted from structural research to durability research, which should be given more priority

this does not mean that the research on structural concrete has been completely stopped. For example, the research on shear force is still a continuous hot research topic. From a historical point of view, an interesting phenomenon is that resilience will again become an interesting topic. Early research organized by Professor Baker proved that the final damage of concrete structures is often due to the impact of concrete structures, and the structural damage caused by the fracture of reinforcement can be ignored. However, recently, Professor eligehausen and his collaborators in Stuttgart, Germany, jointly carried out a study, mainly based on the nonlinear computer analysis of the plastic hinge zone. According to the research results, the above conclusions are no longer correct. Some modern processing technologies produce more brittle steel, which leads to the damage of concrete structure caused by steel fracture. In addition, compared with the concrete of Professor Baker's research era, the current concrete with higher strength is often more brittle. Therefore, due to the changes of concrete production and steel production process, the data collected by Professor Baker during the main research plan period has become obsolete. If you want to update the design rules, you need to repeat similar research work

in modern circumstances, it is unrealistic to organize an international cooperative research project like Professor Baker. However, such research can be carried out in a number of countries, including the UK. A panel organized by the European concrete Commission and led by Professor eligehausen has provided an academic exchange forum to discuss the research results, and published a research report with advanced technology in 1997. The report led to the revision of the design rules for toughness and redistribution limits in the European code, which need to consider the fracture of steel and the increasing higher strength when dealing with the brittleness of concrete

recently, another major research area that has been successfully developed is the design of fiber reinforced composites and more unique maintenance work

durability

in the past 40 years, the main change in research direction is to pay more attention to the durability research of concrete, and the reason is well known. From the late 1950s to the early 1970s, in this golden period of construction, the problem of durability has not been recognized by customary forces. Until the mid-1970s, some building structures built during that period had problems, and a series of obvious problems were exposed. They were:

the conversion reaction of high alumina cement (HAC) led to a sharp decline in strength. Although this reaction is well known, it has not been recognized that it will occur under British environmental conditions. According to the study of the collapsed roof beam of Camden college, this may lead to all buildings containing high alumina cement concrete components requiring a major assessment of the degree of impact

using early strength agent containing calcium chloride will cause reinforcement corrosion

the use of deicing salts on roads will lead to reinforcement corrosion. The corrosive effect of chloride has long been known, but the large amount of salt on roads is actually a common phenomenon in the late 1960s, because a large number of transportation shifted from railways to highways. In addition, due to political reasons, the main roads must be kept unblocked all year round. The consequences of this policy were not recognized for the time being. So there was a period of about ten years, that is, the use of deicing salt was increased on the highway until a large number of steel corrosion problems occurred

there are a lot of steel corrosion problems in the Middle East, which is the area where British designers and contractors are very active

alkali silica reaction - this is a reaction between strong alkali in cement and some kinds of silicate aggregate. In the past, people did not believe that British aggregates would cause such problems. However, in the late 1970s, such reaction cases also appeared in several parts of the UK

mosi2 also has some fatal shortcomings. Various attempts to solve these problems reflect how superficial the understanding of concrete durability is. Moreover, it is the design formulas and construction procedures that lack rational basis, especially the research on the reinforcement corrosion, which is recognized as the most serious practical problem, that has led to a major transfer of research funds to topics related to durability. In the past 25 years, durability is probably the most active research topic of concrete

discussion

I have briefly reviewed some research fields of concrete in the past 40 years. What lessons can we learn from this history

research work can provide various answers to some specific problems. However, the existence of problems is the most important. It is a kind of training for experts, so that experts can obtain

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