Why does concrete have strength classes?
To ask why concrete should be classified into strength classes, one should first know what concrete strength classes are.The strength grade of concrete is artificially classified according to the cubic compressive strength value of concrete, which is the characteristic strength of concrete.According to the current relevant standards and specifications, the cubic compressive strength of concrete is a standard cubic specimen with 150_edge length made by the standard method. The cubic compressive strength of concrete measured by the standard test method is in a standard maintenance room with a temperature of 20.2C and a humidity of more than 95% and a curing age of 28 days.
According to the relevant standards, the strength grade of building material shall be expressed by the name of the material plus its strength standard value.Therefore, the strength class of concrete is classified by the standard values of the symbol C (abbreviation for Concrete in English) and the cubic compressive strength after it: C10, C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C65, C70, C75, C80.
Because in general, the concrete in different parts of the project is subjected to different loads, some of which are large and some of which are small, so that one strength grade concrete can not be used completely.High-strength grade concrete is required for parts under high pressure and low-strength concrete is required for parts under low pressure. Therefore, concrete is divided into different strength grades to meet the needs of different projects and selected through design calculation.
Why does concrete require 28d strength as standard strength?
Concrete is gradually hardened by cementing of cement to improve its compressive strength.Because cement hardening is not done in one go, but gradually improves over time.Under normal maintenance conditions, the compressive strength increases rapidly in the first seven days, but slowly between 7d and 14d, and slowly after 28d.That is to say, the compressive strength after 28d is the standard strength as the quality standard for design and construction inspection.Obviously, if the strength less than 28d is taken as the standard strength, the concrete performance will not be fully developed.If the strength greater than 28d is taken as the standard strength, although the performance of concrete can be fully exerted, the construction progress is affected because the time to reach the standard strength is too long.
Why are three concrete blocks in a group?
Concrete test block is the standard to measure the strength of concrete members, i.e. the strength of compressive failure of the test block as a member can have strength.Therefore, in addition to the blocks which should be made as members, they must also have a certain number.Because although we try our best to make the concrete of the test block is the same as that of the member, there are certain differences after all. It is difficult to be reliable if only one test block is used to determine the strength of the member.Therefore, in three groups, the average strength of the three blocks is used as the strength of the group (in particular, some values need to be removed), i.e. the strength of the member is defined as it.
Why do concrete have natural and steam curing?
Natural curing refers to concrete curing under natural conditions (temperature not less than +5 C, humidity 90-100%).As mentioned before, the strength increases very slowly under the natural curing temperature, the age of 7 days can only get 28 days (strength of concrete after 28 days) 30-70%, and it will take a long time to guarantee the demoulding strength and ex-works strength.This will extend the cycle time of the entire production process, and require a large number of template equipment and production area, increasing capital investment.Steam curing can be used to speed up the growth of concrete strength. Steam is used to heat concrete so that it hardens rapidly at high temperatures (70-90 C) and high humidity (over 90%).However, in areas with a brighter climate and warmer days, it is still suitable for natural conservation.This saves fuel and the corresponding equipment investment and reduces costs.
What is the workability of concrete?
Concrete workability refers to whether the composition of concrete mixture can be kept uniform and whether it is easy to pour and vibrate during production and operation.
The workability of concrete is a comprehensive indicator.It includes the fluidity, cohesiveness and water retention of concrete.
The fluidity of concrete refers to the properties of concrete mixtures which are easy to flow, convey and fill with concrete formwork under the action of self-gravity or mechanical vibration force.
The cohesiveness of concrete refers to the cohesiveness between the materials that make up the concrete mixture during the production process without stratification and segregation.
Water retention of concrete means that during the production process, the concrete mixture will not produce serious water leakage and has certain water retention capacity.
How to determine the workability of concrete?
At present, there is no comprehensive method to determine the workability and cohesiveness of concrete mixtures, usually by determining their fluidity and then judging their cohesiveness and water retention by experience.
The most commonly used method for determining the fluidity of concrete is the slump method.When measuring, the concrete mixture is divided into three layers into a standard-sized conical slump cylinder, each of which is inserted vertically and evenly from outside to inside with a 16-foot diameter ram. After three layers of ramming, the mix at the cylinder mouth is scraped flat, and then the cylinder is lifted vertically to one side. At this time, the mix collapses downwards due to self-weight, and the dimension () is measured.Call slump.The greater the slump, the greater the fluidity of concrete.
After the slump test, the cohesiveness and water retention of concrete can be observed at the same time.If concrete does not surface
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