Experiences in obtaining concrete are very old. Even the ancient Egyptians, Greeks, and Romans were familiar with the properties of a mixture of pozzolana, burnt clay, and lime.
Modern scientific experiences began in 1818 when Vicat discovered the causes of the hydraulic properties of some types of binders.
The first Portland cement (PC) was produced in 1824 by John Aspdin, but of insufficiently good quality because it was not baked enough. In 1845, Isaac Johnson succeeded in obtaining Portland cement with the same properties for which it is known today. In 1850, the Frenchman Lambot made a boat from a wire mesh, which he coated with a cement mortar on both sides.
The end of the 19th century can be considered the beginning of the first stage of the development of reinforced concrete, in which various systems of reinforced concrete structures emerged (a system of ribbed ceilings - Hennebique).
CONCRETE is a complex building material made by mixing binders (cement), water, and aggregates (sand, gravel, crushed stone).
Concrete has the great advantage that it can be shaped at will in its fresh state and retains that shape after hardening. It is characterized by high compressive strength (10 - 60 N / mm2), and its tensile strength is very low (it is 1/11 - 1/8 of the compressive strength). This disadvantage is eliminated in reinforced concrete. Although it is quite resistant to atmospheric influences, concrete is, due to the basic composition of the cement, not resistant to acids, and it is also a poor sound and thermal insulator.
The classification of concrete is done either by weight per m3 of the finished mixture or by the type of binder. After mixing in mixers, the fresh concrete is transported in wheelbarrows, closed containers, pumps, or flows through troughs.
Better quality is achieved by compacting, vibrating, or centrifuging. If it is desired to achieve greater plasticity, strength, or resistance to frost, lower water permeability, or some other special property, some components (e.g., plasticizers, setting regulators, thickeners, aerators, anti-freeze agents, surface protection agents, organic materials, steel shot) are added to the fresh concrete mass.
For the evaluation of concrete, the concrete brand is usually taken. The concrete brand (MB) indicates the compressive strength (to failure) of test cubes with a size of 20x20x20 cm, made from the amounts of cement needed for 1 m3 of finished or installed concrete of the mentioned masses. The strength of concrete of a particular brand does not depend only on the amount of cement and its class but also on the quality of the aggregates, the amount of water, the method of installation, and the maintenance of the concrete.
Depending on the amount of water added when making concrete, we get different densities of fresh concrete mass:
- humid concrete - can be compacted well and is used for massive constructions (walls, foundations, etc.)
- plastic concrete - is used for thinner and almost all reinforced concrete constructions
- thin or fluid concrete - is used for very narrow constructions and constructions with dense reinforcement
The chemical process of binding normal slow-binding cement begins about 1 ½ hours after pouring the dry mixture with water. When the cement starts to bind, the concrete must rest and cannot be shaken anymore.
The main types of concrete are:
- ordinary or unreinforced concrete is obtained by mixing Portland cement and stone aggregate. It is used for elements and structures loaded with pressure (pressure)
- economical concrete is used for massive constructions of ordinary concrete, into which pieces of natural stone (with a maximum size of 30 cm) can be added during installation.
- concretes with metallurgical and pozzolanic cement are used for constructions exposed to chemical influences and aggressive waters.
- concrete with aluminous cement must not be mixed with Portland cement due to special chemical reactions
- lightweight concrete is divided into two groups: -
- concrete mixture in which the aggregate is of lightweight porous material normally connected with cement
- concrete mixture with finer lightweight aggregate with the addition of a chemical agent that forms gases or foam in the fresh mixture (gas and foam concrete)
- reinforced concrete is ordinary concrete into which steel bars or a mesh are embedded to imparttensilestresstothestructuralelement.
Concrete has the property of adhering firmly to steel as it hardens, so that when external forces act, both materials work together. The advantages of reinforced concrete include: non-flammability, durability, low maintenance costs, the possibility of creating a wide range of shapes, resistance to earthquakes, and certain mechanical properties.
Concrete protects steel from corrosion. The steel bars that take the main stresses are called main reinforcement. The main reinforcement bars are arranged according to certain rules and are connected to each other by thinner bars, which are called distributive reinforcement, and if they take some of the stresses, then they are auxiliary reinforcement. In columns and beams, such reinforcement is called stirrups or ties.
7. Pre-stressed concrete. If before loading a structural element, its reinforcement is stretched to a greater extent than it would be under full load, and if the concrete encompasses and solidifies it in that state, then the reinforcement will try to return to its original (unstretched) state. In the area where the stretched rods are located in the concrete, compressive stress will occur. When the structural element is loaded, the compressive stress achieved in the pre-stressedconcrete will counteract the tensile stress. In pre-stressedconcrete, the reinforcement does not take tensile stresses, but allows the concrete to counteract the tensile forces caused by the load through counter-stress (compressive stress).
Concrete installation can be done using molds, formwork, or without formwork. Installation without formwork is usually done for foundations in excavated trenches.