The main reason for the cessation of hardening of concrete mixtures when exposed to low temperatures - the freezing of water in them. It is known that the content of salts in water sharply reduces its freezing temperature. If, during the preparation process, a certain amount of dissolved salts is introduced into the concrete mix, then the hardening process will proceed at a temperature below 00C.
The choice of antifreeze additives and their optimal amount depend on the type of concreted structure, the degree of reinforcement, the presence of aggressive media and stray currents, ambient temperature, etc., and is carried out in accordance with GOST 24211-2003 "Additives for concrete and mortars".
Scope of additives is presented in tab.10.1.
Antifreeze chemical additives should not be used when concreting pre-stressed structures reinforced with thermally hardened steel; in the construction of reinforced concrete structures for electrified railways and industrial enterprises, where the occurrence of stray currents that contribute to the destruction of concrete.
The introduction of chemical additives leads to some slowdown in the curing of concrete compared to the rate of concrete hardening in normal conditions. So when introducing potash, the strength of concrete at the age of 28 days at an ambient temperature of -25 ° C is 50%, and at the age of 90 days it is 60%. At a temperature of -50 ° C, the curing takes place more intensively and by the 28-day age it can be 75%.
Depending on the outdoor temperature, various combinations of additives are possible. Concrete with antifreeze additives are used in cases where a set of critical strength is achieved before they freeze. The speeds of curing with concrete with antifreeze additives are given in tab. 10.12 When choosing additives take into account their cost and the impact on the physico-mechanical and technological properties of concrete and concrete mixtures. So when you make potash, the setting time of the cement is reduced, as a result of which the workability of the mixture deteriorates. The cheapest and most affordable supplements are calcium and sodium chlorides. Additives are introduced in the form of water solutions in the process of preparing concrete mixes in an amount of 3 --- 18% by weight of cement. The use of additives is advisable in combination with additional heating. Solutions containing urea should not be heated above 40 ° C. Solutions of salts of working concentration should not have precipitates of undissolved salts.
Scope of additives.
Table 10.1
Type of structures and their operating conditions
Supplements
NN
HK + XH
HC + NN
NKM, NK + M, NNK + HK
NNK + M, NNHK, NNHK M
P
Reinforced concrete structures with reinforcement diameter, mm:
more than 5
+
-
+
+
+
+
5 or less
+
-
+
+
-
+
Monolithic structures; joints with issues of reinforcement or embedded parts:
without special steel protection
+
-
-
+
-
+
with metal coatings
-
-
-
+
-
-
with combined coatings
+
-
+
+
+
+
Reinforced concrete structures intended for operation:
In water
+
+
+
+
+
+
In a non-aggressive gas environment with a relative humidity of up to 60%
Some additives, such as chloride salts, impair the surface quality of structures erected due to the formation of efflorescence. Therefore, they are used in the construction of structures of small volumes, the quality of the surfaces of which do not impose high requirements (for example, foundations, beams). The process of laying and compaction of mixtures does not differ from the usual methods of concreting.
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