Since it came into use, now more than a century ago, concrete has become an indispensable and the most widely used construction material. It is considered to cover more than 70% of the needs in construction and infrastructure. In addition to being durable, it becomes stronger and better with age. Some of the major advantages of concrete are that it is cost-effective and adaptable to various shapes and working conditions. If some of the available modern additives are added to it, its properties can be further improved.
Fresh Concrete: Properties
The properties of a fresh concrete mix are very important for the properties of hardened concrete, its workability, and the possibility of finishing. In this form, the concrete mix is an elastoplastic viscous system, which allows it to change its behavior depending on the forces acting upon it. Under the influence of vibration, it takes on a form similar to that of a heavy liquid, which is used in practice to achieve concrete compaction through vibration and to ensure that it better fills all voids in the formwork while maintaining optimal compaction of the placed concrete.
Concrete Consistency and Cohesion
A very important property of a fresh concrete mix is consistency, which can be stiff, low-plastic, plastic, or fluid. The workability and ease of handling of fresh concrete depend largely on its consistency, which refers to its ability to fill all cavities and spaces in the formwork as a result of compaction. Several methods can be used to determine this characteristic, depending on the type of concrete being tested. The cohesion of the concrete mix greatly affects the homogeneity and properties of hardened concrete. If cohesion is poor, segregation of the concrete components and bleeding may occur during transport and placement, which is particularly pronounced in concrete with a high water content. The appropriate consistency of fresh concrete is selected in each individual case, depending on the type and dimensions of the structure or concrete elements, reinforcement density, method of placement, and finishing. The greatest influence on the consistency of fresh concrete is exerted by the amount of water it contains, which in practice is determined so that the concrete has appropriate workability and ease of handling.
What Determines the Properties of Hardened Concrete?
As for the physical and mechanical properties of hardened concrete, they depend on numerous factors that influence the resulting structure of the concrete. This structure is formed after the placement of fresh concrete as a result of the cement hydration process, with the water-cement ratio having a very significant influence. The formation of the concrete structure can be divided into three characteristic phases:
- Initial structure formation phase: occurs after the placement and compaction of concrete, the cement begins to set, and the cement paste changes from a liquid to a solid state.
- Gradual structure formation and strength development phase: compressive mechanical strength increases, and this phase can last from several days to several weeks, depending on whether and which additives are used.
- Structure stabilization phase: the strength and structure have been achieved, the concrete has stable porosity and the required mechanical properties.
In order to achieve early concrete strength, technological, chemical, and physical methods can be used. Technological methods include the use of higher-grade cement, reducing the water-cement ratio, revibration of concrete, and similar methods. Chemical methods include the use of various types of additives (accelerators, plasticizers, and superplasticizers), while the most effective physical methods are thermal treatment procedures.
Concrete Placement and the Importance of Quality Molds
Concrete placement includes filling the formwork or concrete molds, compacting the concrete, and finishing the upper surfaces. This process must be completed before the cement begins to set; otherwise, the concrete must not be placed. Depending on weather conditions and the type of concrete, in most cases this period is between one and two hours from the completion of concrete production. Concrete elements and structures can also be produced in appropriate concrete molds that give them the correct and required shape. In this case, the quality of the concrete element depends not only on the quality of the concrete and its placement but also on the quality of the concrete mold itself. The concrete molds must have a surface that will ensure the required surface quality of the cast element, as well as sufficient strength to prevent deformation of the concrete molds during pouring and vibration of the concrete inside them. Before pouring the concrete, it is important to properly prepare the concrete mold and coat it with form-release oil in order to prevent damage to both the concrete mold and the cast element. When placing concrete into formwork and concrete molds, attention should also be paid to the height from which the concrete is poured into the formwork in order to prevent concrete segregation.
How to Work with Concrete in Hot and Cold Weather?
When it comes to placing concrete under special conditions, this refers to temperatures below 5°C and above 30°C. In the case of temperatures below 5°C, a number of methods can be taken, such as using cement with higher hydration temperatures, using additives for accelerated setting and hardening, as well as antifreeze. In addition, it is possible to heat the water, and in exceptional cases, to heat the aggregates as well. In the case of working at high temperatures, the problem is reduced workability and ease of handling of the concrete. This is particularly pronounced in the case of remote locations where a long period of time is required between concrete production and placement. In such cases, the use of cement with the lowest possible hydration temperature and the smallest possible amount of cement is recommended, as well as cooling of formwork or aggregates, and nighttime concreting.
Why Is Proper Concrete Curing So Important?
For the concrete curing process, preventing water evaporation from the placed concrete is essential. In this way, the loss of water from the concrete that is necessary for the cement hydration process is prevented. If such measures are not taken in a timely manner, there is a high probability that cracks caused by concrete shrinkage will appear in young concrete, whose mechanical resistance is low. Such problems can be prevented by covering the concrete and maintaining it in a moist condition until it reaches sufficient strength to resist crack formation. Concrete curing should begin very early, as soon as the cement setting process begins in the concrete. The duration of concrete curing depends on many factors, such as the type and composition of the concrete, the conditions and environmental influences to which the concrete is exposed, temperature, the importance of the structure or element, as well as many others. In the case of large concrete surfaces, special coatings are also used, which are applied to the surface of fresh concrete and create a thin waterproof film that decomposes on its own after a certain period of time.
The Future of Concrete in Modern Construction
In addition to its widespread use in construction, the use of concrete continues to grow and increase. Concrete, its additives, and technologies for its production and placement are constantly evolving. When the possibility of producing prefabricated elements and their significant advantages, both in terms of time and cost, are considered, it is clear that there is still significant potential for further development in this field. All of this confirms that concrete, thanks to its properties, wide range of applications, and the continuous development of technologies, represents one of the key materials in modern construction.
