20 REINFORCED CONCRETE CONSTRUCTION 



sion of concrete occurs from two causes: (1) temperature, and 

 (2) hardening action. Like most building material, concrete 

 contracts as the temperature falls, and expands as it rises. Its 

 volume is also affected when it hardens, a shrinking action taking 

 place when hardened in air and some expansion when hardened 

 in water. 



Experiments show an average value of the coefficient of expan- 

 sion of concrete, due to temperature changes, to be about 

 0.000006 per degree Fahr. which, it should be noted, is very 

 nearly that of steel. Thus, when steel is embedded in concrete, 

 the two materials will be but slightly stressed because of any 

 difference in their rates of expansion. 



The amount of change in volume of concrete due to hardening 

 increases with the proportion of cement used in the mixture. 

 When hardened in air, concrete contracts about 0.0005 of 

 its length; while the expansion, when hardening takes place- 

 under water, is only about 0.0001 of the length. The whole 

 change is usually not accomplished for some months, but about 

 half the whole is effected in about a week. 



14. Fireproofing Qualities. The value of concrete as a 

 material to be used in the construction of the walls, columns, and 

 floors of buildings is largely dependent upon its fireproofing 

 qualities. 



Concrete has a low conductivity of heat due to two causes: 

 (1) the presence of combined water, and (2) porosity. The 

 water of crystallization, being chemically combined, is not 

 given off at the boiling-point. A part of this water goes off at 

 about 500 Fahr., but the dehydration is not complete until 

 900 Fahr. is reached. This vaporization of the water at the 

 surface absorbs heat and protects the interior. The layer of 

 changed material is then a better nonconductor than before, so 

 the process goes on very slowly. The porosity of concrete also 

 offers considerable resistance to the passage of heat since it is 

 well known that air spaces are a most efficient protection against 

 conduction. 



Steel is said to lose 10 per cent of its strength at 600 Fahr. 

 and 50 per cent at about 750 Fahr. A fire in a building may 

 reach a temperature of 2000 to 2300 Fahr., and the importance 

 of sufficiently protecting the reinforcing steel from loss of strength 

 due to overheating is therefore evident. The surface of a mass of 

 concrete exposed to the action of flames for some time may be 



