428 THE HANDBOOK FOE PEACTICAL FAKMEES 



extra barrels of cement if the dealer is at considerable dis- 

 tance, as this avoids any possible trouble that a shortage might 

 cause. 



In case a natural mixture of bank sand and gravel is used, the 

 following table should be consulted for the quantities of the 

 mixture : 



Quantities of Materials and the Resulting Amount of Concrete for a Two- 

 bag Batch, Using a Natural Mixture of Bank Sand and Gravel 



PRACTICAL HINTS 



Use care in selecting the right kind and proportions of sand, 

 gravel and crushed stone. 



Cement usually hardens thirty minutes after it is wet if 

 allowed to stand. 



Do not expose newly set concrete to the sun for four to five 

 days. Protect newly set concrete from frost. 



Keep cement in a dry place on an elevated wooden floor. 



If cement is left out of doors over night cover with canvas. 



Screen sand at the pit and save time, hauling and labor. 



Sand of various sized grains is most desirable, giving more 

 strength. 



Never use sand that softens in water. 



Large pebbles and stone may be used m constructing heavy 

 foundations and abutments. 



For reinforced concrete work, material that passes through 

 one-inch rings is recommended. 



Mix thoroughly. Concrete having a gray streaky appearance 

 is not well mixed. 



Mix sand and cement thorougly before applying the water.. 



