The construction industry is one of the most developed, and therefore it uses a large number of various building materials with a variety of characteristics. And for some substances, such as concrete mixtures, for example, a number of requirements are immediately imposed. One of the important properties that each grade of mortar should have is the mobility of concrete. Let's consider it in the article.
General information
There is such a thing as workability. This term describes how the concrete mix will fill the formwork with the chosen tamping method and at the same time will form a compacted and homogeneous mass. Characteristics such as connectivity, stiffness, and mobility are used to describe this property. A cone for the mobility of concrete (cone sediment) is the property of a substance to spread over an area only due to its own weight. This parameter is the main one if the admission of the mixture to use at a particular construction site is evaluated.
Mobility Categories
It is important to note here that the usability of this solution lies precisely in the mobility of concrete. In addition, this parameter has several established yield levels. The dependence is approximately the following: the higher this characteristic, the better it will fill the formwork and flow around the volumetric reinforcement, as well as better spread over formwork of complex configurations.
All concrete mixtures can be divided into two categories - these are inactive and highly mobile. All solutions belonging to the first category are not used in construction without prior mixing with plasticizers, as well as without going through a preliminary vibrocompression procedure. Initially, those grades that contain the aforementioned plasticizers in small quantities belong to this category.
Mobility dependence
Generally speaking, the mobility of concrete depends on factors such as quality and quantity, as well as on the constituent elements of the mixture.
If we consider the issue in more detail, this parameter will depend on such properties as the grade of cement, the density of the cement paste, the ratio of water to cement, as well as on the fraction and shape of the filler grain (sand and gravel).
It is worth noting that this factor will also change depending on the method of pouring the mixture into the formwork. For example, if a substance is poured into a dense and voluminous reinforcing cage, then it is necessary to choose a mixture whose mobility will be quite high. This is justified by the fact that it will be very difficult to apply vibration ramming under such conditions.
If in such conditions a solution with a low mobility rate is used, then after the concrete compaction operation, it will not meet all the necessary standards, such as porosity or sinks.
For this reason, during the selection of the grade of the composition, it is necessary to understand and know what requirements will be imposed on the supporting structure of the object, especially relevant if the foundation is poured, as well as know the exact conditions for pouring the substance into the formwork. It is also necessary to take into account such characteristics as connectivity and rigidity.
Designation
In order to briefly indicate the indicator of the mobility of the element, use the letter "P". Depending on a factor such as gradation, I add an index to this notation. The higher the index value, the higher the fluidity of the composition. There are 5 grades of concrete mobility. Thus, all compositions from P1 to P3 are considered inactive, and P4 and P5 are classified as highly mobile.
For example, solution P1 is used for such purposes as the construction of stairs. Although it is worth noting that such concrete is rarely used, and at the same time it always passes through the mechanical seal of the composition. Almost all standard buildings are constructed using such moving concrete mixtures as P2 and P3.
Stamps with the designation P4 are used in cases of construction of columns or a high foundation. This category of work relates to dense reinforcement. The most fluid solution P5 is intended for pouring only into practically sealed formwork.
Cone draft
There are several methods by which you can define this parameter in a numerical value. The difference between these methods is the difficulty in obtaining the final result.
The fastest method is cone draft. This operation will allow you to determine how quickly concrete will settle under the influence of only its own weight. It is important to note that the calculations are carried out provided that the solution is poured into a pre-prepared cone.
To determine the mobility class of concrete, therefore, it is necessary to use a metal form of a conical type. Dimensions of this form will depend on which fraction of crushed stone is selected. Suppose the height of the cone is 300 mm, its small diameter will be 100 mm, and its large diameter will be 300 mm. With such indicators, the cone will have a volume of 7 liters.
Class definition
In order to determine the index of mobility of concrete in this way, it is necessary to perform the following manipulations. A concrete solution is placed in three portions into a cone-shaped type from its wide side. Each of these layers must be densified by applying bayonet. It is necessary to make 8-9 movements per layer, using smooth reinforcement for these purposes.
If an excess mixture is formed, then it must be removed. After this, the form must be turned over, like a baby cake. Thus, it will be possible to free the entire mixture available inside. After this, some time is given for the concrete to settle and carry out the process of checking the value of mobility.
To do this, calculate the reduced height of the solution relative to the upper edge of the form. In order to get a more accurate result or the arithmetic mean, you need to repeat the procedure several times. The difference between the height of the cone is 300 mm and how much concrete has settled, and will be the mobility of the substance.
If there is no difference at all, then the mixture is attributed to the most rigid composition. If in the process of precipitation the difference reached up to 150 mm, then such a composition is considered to be inactive. If the difference reached 150 mm or more, then the brand is considered highly mobile.
Second method
One of the methods for checking the composition for mobility is a test with a viscometer. This method is resorted to if the fraction of crushed stone in the solution is in the range from 0.5 to 4 cm.
For the experiment, it is necessary to form a conical shape and pour concrete in the same way as in the previous experiment. After that, it is placed on a vibrating table. Next, a tripod is inserted into the form, on which there are divisions. A metal disk is put on top of it. After these operations, the vibrating table and at the same time a stopwatch are turned on. After this, it is necessary to detect the time during which the disk will go down to a certain point. The resulting coefficient must be multiplied by a constant of 0.45. The numerical result of this action will determine the mobility of concrete.
Third method
Another of the methods used is experiments in forms. For this experiment, you must have a cube with one open side. Dimensions of capacity, for example, 200x200x200 mm. Such a cube can be used for all fractions of the mixture with crushed stone, up to 7 cm. Inside this device, it is necessary to place a conical mass of concrete.
After completion of these processes, the cube is transferred to the vibrating plate. Here it is also necessary to simultaneously turn on the stove and the stopwatch. In this experiment, it is necessary to detect the time during which the solution will fill all the corners of the cube, and the surface of the mixture will be completely smooth.
The time that was obtained as a result must be multiplied by a constant coefficient of 0.7. The resulting number after multiplication will be an indicator of the mobility of concrete.
Concrete Mobility Table
In order to make it convenient to use concrete mixtures with different mobility indicators, they were systematized by this feature. Other workability properties — connectivity and rigidity — were structured along the same lines. All this data was placed in a table.
According to her, if the cone shrinks by 1-5 cm, the substance refers to hard or severe mobility. Concrete with this characteristic is marked P1. Grades P2 and P3 are characterized by shrinkage of the cone of 5-10 cm and 10-15 cm, respectively. The designation P4 indicates that the shrinkage is in the region of 15 to 20 cm. The remaining solutions, whose mobility exceeds 20 cm, belong to the P5 group.
There is also GOST for the mobility of concrete, which governs the division of all types of mix into several categories according to their main indicators. So, this state standard establishes the separation of all solutions into two categories - these are ready-to-use mixtures (BSG) and dry mixtures (BSS). It should further be noted that there is a division into several groups according to the workability of each substance. The first group is superhard (SJ), the second group is hard (G) and the third group is mobile (P).
In order to determine the quality of any concrete brand, it is necessary to check its basic qualities: average density, workability, delamination and the amount of air involved.