The concrete setting time is divided into initial setting time and final setting time.
Initial setting: The initial setting time is the time required for the cement to be mixed with water until the cement slurry begins to lose its plasticity.
Final setting: The final setting time is the time required for the cement slurry to completely lose its plasticity and begin to develop strength from when the cement is mixed with water.
Cement setting time is of great significance in construction. The initial setting time should not be too short and the final setting time should not be too long. The initial setting time of Portland cement shall not be earlier than 45min, and the final setting time shall not be later than 390min; the initial setting time of ordinary cement shall not be earlier than 45min, and the final setting time shall not be later than 600min. If the initial setting time of cement does not meet the requirements, the cement is scrapped; if the final setting time does not meet the requirements, it is deemed unqualified.
The initial setting time of concrete is generally determined by the type of cement, and there is basically no uniform time, but a rough range is 2-3 hours. If an early setting agent is added, the initial setting time can be roughly shortened to half an hour; if a retarder is added, the initial setting time can be extended to 5-10 hours. The specific initial setting time is generally determined by the test, and each batch of cement in each factory has to be tested. The initial setting time refers to the time when the cement is mixed with water until the cement slurry begins to lose its plasticity; the final setting time is the time when the cement is mixed with water until the cement slurry completely loses its plasticity and begins to develop strength.
In order to ensure that the cement slurry has enough time to be in a plastic state during the construction of the project for easy operation and use, the national standard specifies the minimum initial setting time of the cement; in order to make the cement slurry that has formed the shape of the engineering structure gain strength as soon as possible, so as to be able to bear the load , The national standard stipulates that the final setting time of cement shall not be later than the prescribed time.
Determine the time of initial setting
From the formation process of the cement paste structure, it is known that the hydration products must be grown and increased enough to initially connect various particles into a network to form a cohesive structure, so that the cement paste can begin to coagulate. From the perspective of the rheological characteristics of cement paste, the external force must be increased to a certain extent, and the resulting shear stress will disassemble the formed network structure in order to make the paste flow. The shear stress required to disassemble the network structure is usually called the "yield value". After the cement is mixed with water, the yield value immediately increases with the progress of hydration, then slows down, and then rises at a faster rate. It is generally believed that the initial increase in yield value is due to the rapid formation of ettringite; if there is hemihydrate gypsum in the cement, there will also be reasons for the formation of dihydrate gypsum. As for the second rapid increase in yield value, it is due to the C-S-H formed by the strong hydration of tricalcium silicate. The so-called "initial setting time" is actually equivalent to the time when the yield value is increased to a certain value, and the second rapid rise is about to begin. It can be shown that the initial setting time is not only determined by the hydration of the tricalcium aluminate and iron phase, but also closely related to the hydration of the tricalcium silicate; and the coagulation stage from the initial setting to the final setting is mainly affected by the tricalcium silicate. Hydration control.
The cement test conditions are specified as follows: the temperature of the test room should be 17～25℃, the relative humidity should be greater than 50%; the temperature of the curing box should be 20±1℃; the temperature of the cement sample, standard sand, mixing water and test mold should be the same as the temperature of the test room The temperature is the same; the test water must be clean fresh water.
(1) National standards stipulate that the initial setting time of cement shall not be earlier than 45 minutes, generally 1 to 3 hours; the final setting time shall not be later than 12 hours, generally 5 to 8 hours.
(2) The test method is to add water of standard consistency to the cement to make a pure slurry test model. From the time of adding water to the setting time, the test needle of the tester sinks into the pure slurry 0.5～1.0mm from the bottom plate. It is the initial setting time, and the time until the test needle sinks into the pure slurry does not exceed 1.0mm is the final setting time.
The initial setting time of concrete is generally 2 to 4 hours, and it can reach 6 to 10 hours with the addition of retarder. However, due to the continuous mixing movement of concrete during transportation, the initial setting time of concrete will also be prolonged. The high temperature in summer also has a great influence on the initial setting of concrete.
The initial setting and final setting time of the concrete are divided into initial setting and final setting. When the concrete just loses its plasticity, it is called initial setting, and when the concrete completely loses its plasticity, it is called final setting.
Generally speaking, the setting time of concrete is related to the setting time of cement. For ordinary cement, the initial setting is not less than 45min and the final setting is no later than 10h. Concrete is similar.
However, current concrete is often mixed with some mixed materials and additives, which will affect the normal setting time, especially the additives. There are many varieties of concrete admixtures. The setting time includes coagulants and accelerators, etc., which can prolong or shorten the setting time.
Generally speaking, if the setting time is too long, the effect on the later strength is not great. The strength of concrete is mainly related to the water-cement ratio and the amount of cement. However, if the setting time is too long, and the concrete is accidentally damaged during this period, it is hard to say that this will not reduce the strength of the concrete.
Determine the final setting time
Concrete solidification time is generally called curing time, and it is also called age in days. The strength of concrete increases continuously with the increase of curing time, showing a curve relationship. Before 14 days, the curve was steep, after 14 days, the curve began to become flat, and after 28 days, the curve became more flat. That is to say, the strength of the mixed concrete increases continuously with the extension of the curing time, starting faster, and then gradually slowing down. After about 2 to 3 years, the strength stops increasing. The increase in concrete strength is not related to the curing time, but also to the type of cement, curing conditions, and environmental temperature. For example, the concrete prepared with No. 425 ordinary Portland cement is cured under natural conditions. When the ambient temperature is 20℃, it can reach 60% of the design strength in 7 days and 95-100% of the design strength in 28 days; and at ambient temperature It can only reach about 45% of the design strength in 7 days at 10℃, and only about 80% of the design strength in 28 days. Under negative temperature conditions, as long as the strength of the concrete before freezing has reached more than 30% of the design strength, the strength of the concrete can also increase, but the increase is slower. From 45 minutes to 1 hour after the cement is mixed with water, the cement gel begins to set, which is referred to as initial set; to 12 hours after mixing, the formation of cement gel is almost over, this period of time is called final set. However, the cement gel formed at this time is still in a soft plastic state, and it will take several hours to gradually harden and become a solid state. Generally, the period of time during which cement is mixed from a fluid state and loses its plasticity to a solid state is called the "coagulation process", and the time during which the strength gradually develops is called the "hardening process".
The cement slurry has a certain degree of fluidity before the initial setting, and during this time it should be transported, poured, and tamped. From the initial setting to the final setting, its fluidity gradually disappears. If it is vibrated again, the coagulated colloid can still be closed, but after 6 hours (close to the final setting) to 8 hours after self-mixing, it has been lost Fluidity, without strength, cannot close by itself when damaged, so it cannot withstand external forces. During this time, it is necessary to strengthen the maintenance to ensure the stable development of its strength. Condensation = fluid state-loss of plasticity->solid = initial setting + final setting; Initial setting: gradually lose plasticity and get preliminary hardening->Construction time limit; Final setting: completely lose plasticity and begin to have strength.
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