38 ANNUAL REPORT 
to blame, and one or two of these said that the troweling brought an 
excess of cement to the surface. Three said that it was caused by trowel- 
ing after initial set. Two that rapid evaporation or drying out of the 
surface caused the cracks. One claimed that hydration of free lime and 
the absorption of carbon dioxide caused the fine rupturing of the surface. 
One man said that “if the surface be troweled hard just at beginning of 
initial set, until all the water is taken up, there will be no hair-cracks.”’ 
Fig. 3.—Parapet of the Y Bridge at Zanesville, Ohio, Showing Hair Cracks and 
Surface Corrosion. 
A translation from L. Gollinellis* says: “Hair-cracks appear as fine 
lines on cement work which has stood some time. They are especially to 
be noticed on cement which has lain in the open air, and are due to fre- 
quent changes between wet and dry conditions. Hair-cracks and shrink- 
age cracks occur chiefly when pure cement, or mortar too rich in cement, 
is used. They may be certainly avoided by the use of sufficient sand and 
suitable treatment of the work.” 
Mr. W. D. Lober, Secretary of the Vulcanite Portland Cement Com- 
pany of Philadelphia, says of smooth finished concrete work : “The trowel- 
ing brings the water to the surface and also the fine particles of cement 
which are carried with the water and makes the surface practically neat 
*Das Kleine Cement-Buch. Eng Record, May 12 and June 2, 1900. 
