PORTLAND CEMENT CONCRETE PAVEMENTS. 13 



being less slippery when the pavement is first constructed and is pre- 

 ferred by some engineers on that account. Smooth surfaces are more 

 generally preferred, except on very steep grades, where it is some- 

 times desirable to provide grooves or other comparatively deep mark- 

 ings at right angles to the direction of traffic in order to afford a bet- 

 ter foothold for horses. Such grooves, however, will cause rapid 

 deterioration of the pavement under heavy traffic. 



A satisfactory method of finishing the surface is to use a wooden 

 float for smoothing out all template markings (PL III, fig. 1) and 

 evening up other slight irregularities. This method of finishing 

 produces a surface sufficiently rough for all ordinary grades and pos- 

 sesses the advantage of being extremely simple. In using the float 

 special care must be exercised to keep the pressure of the hand uni- 

 form, in order not to produce irregularities in the surface. Wherever 

 a depression occurs it should be filled by adding concrete, and not by 

 raking mortar into it with the float. The workmen who do the float- 

 ing should be provided with one or more light bridges, which span 

 the pavement and which can be easily moved as the work progresses. 

 Various sizes of floats are used, and provided they are handled by 

 skilled workmen the size is not important. The long float shown in 

 Plate X, figure 5, requires less skill on the part of the operators than 

 short floats. A suitable design for a finishing bridge is shown in 

 Plate XI, figure 1. 



JOINTS. 



It is customary to provide transverse joints at regular intervals 

 in concrete pavements, to prevent irregular cracks from being pro- 

 duced ; and if the width of the pavement exceeds 20 feet, longitudinal 

 joints are also usually provided. Concrete contracts and expands 

 with changes in temperature and also with changes in its moisture 

 content. It also shrinks or contracts upon setting; and since the 

 strength of the concrete is then comparatively low, the tensile stresses 

 developed are much more likely to produce cracks than equivalent 

 stresses developed in older concrete. It is evident that the greatest 

 longitudinal stress which can be developed at any section of the 

 pavement, due to contraction, is equal to the weight of the pavement, 

 included between the section under consideration and the nearest 

 free end, multiplied by the coefficient of friction between the pave- 

 ment and the subgrade. Therefore, if contraction joints are spaced 

 sufficiently close together to prevent this stress from exceeding the 

 tensile strength of the concrete, no cracks should occur. 



If no transverse joints are constructed in the pavement, the length 

 of the sections between cracks, judging from such limited data as are 

 at present available, will vary from 20 to 150 feet, and depends upon 

 the kind of aggregate used, the relative richness of the concrete, the 



