BUREAU OF PUBLIC ROADS. 423 



also made of the pressure exerted by concrete against forms when 

 the concrete was poured at different rates. An elaborate series of 

 tests has been planned on an investigation of blast-furnace slag for 

 use in road and bridge construction, and a large number of samples 

 of slag have been collected from all of the principal slag producing 

 plants in the country. A new instrument has been designed and built 

 in the division for measuring the wear of concrete roads under traffic. 

 This instrument will be described in a coming report in Public Roads. 

 Wear measurements were made on the Camp A. A. Humphreys con- 

 crete road with this instrument. 



The work of testing concrete slabs w-as temporarily discontinued 

 during the year but will be resumed as soon as possible. A paper 

 based on past tests appeared in the September, 1918, issue of Public 

 Eoads. An investigation on the " Bonding of new cement mortar 

 and concrete to old " was completed, and a paper on this subject was 

 published in Public Roads of June, 1919. 



ROAD AND BRIDGE FOUNDATION TESTS. 



The field tests conducted upon earth fills in the vicinity of Wash- 

 ington were continued, an effort ]:)eing made to determine accurately 

 the pressures exerted by earth fills against retaining walls. Tests 

 were also continued on the distribution of pressures through earth 

 fills. Specimens have been prepared for conducting impact tests on 

 road slabs. The slabs are laid directly on the subgrade and will be 

 tested under the impact of a machine designed to give the same 

 effect as the rear wheel of a heavy motor truck. A large number of 

 tests have also been made to measure the impact exerted by trucks 

 on roads and a preliminarly paper has been prepared. Specimens 

 and apparatus have also been built for a new series of tests on the 

 wear-resisting cjualities of paving sections. These sections are laid 

 in narrow strips to a total length of 400 feet and will be subjected 

 to rapid wear by means of a special truck designed to travel con- 

 tinuously back and forth over a narrow portion of the test section. 

 Preliminary tests have also been started on the bearing power of 

 soils with particular reference to the subgrades of roads. A field test 

 was made on the Camp Humphreys concrete road in order to de- 

 termine the distribution of concentrated loads through the road to 

 the subgrade. A paper on this test appeared in the April issue of 

 Public Roads. The above tests were outlined to throw light upon the 

 theoretical design of higher t3'pes of roads. 



FEDERAL-AID MATERIALS TESTS. 



During the year 2,191 test reports submitted by laboratories 

 throughout the country on samples of material used in Federal-aid 

 road construction were examined and comments prepared regarding 

 them. In addition, 92 specifications proposed for use in Federal-aid 

 construction were examined and recommendations were made regard- 

 ing those parts of the specifications dealing with materials. 



INSTRUMENT MAKING AND REPAIRING. 



Since March 5 the machine shop has been included under the direc- 

 tion of the Division of Tests, and during that period 28 jobs have 

 been completed. 



