PORTLAND CEMENT CONCRETE PAVEMENTS. 33 



coating on the particles of stone. The particles of stone shall be graded in 

 size between those retained on a screen having circular openings three-eighths 

 (|) inch in diameter (or a one-fourth (i) inch mesh screen) and those passing 

 a screen having circular openings one and one-half (1A) inches in diameter. 

 Not less than twenty (20) per cent shall be retained on and not less than 

 twenty (20) per cent shall pass a screen having circular openings three-fourths 

 (|) inch in diameter. The gravel shall be free from particles of soft sand- 

 stone, shale, slate, coal, or other material which may readily disintegrate. 



Crushed stone. — Crushed stone for use in the concrete shall be composed of 

 particles of clean, sound, durable stone, crushed to such sizes that all will be 

 retained on a screen having circular openings three-eighths (§) inch in di- 

 ameter (or a one-fourth (J) inch mesh screen) and will pass a screen having 

 circular openings one and one-half (li) inches in diameter.. Not less than 

 twenty-five (25) per cent shall be retained on and not less than twenty-five (25) 

 per. cent shall pass a screen having circular openings three-fourths (i) inch in 

 diameter. 



Samples of the stone when subjected to the hardness, toughness, and abrasion 

 tests, as described in United States Office of Public Roads Bulletin No. 44, 

 shall satisfactorily meet the following requirements: 



Hardness, not less than ten (10) ; toughness, not less than eight (8) ; and 

 per cent of wear, not more than four (4 J. 1 



Water. — The water used in mixing the concrete shall be free from oil, acid, 

 alkali, and vegetable matter, and fairly free from clay or silt. 



CONSTRUCTION. 



Mixing and placing concrete. — Upon the subgrade, prepared as herein speci- 

 fied, shall be laid a concrete surface of the width, thickness, and cross section 

 shown on the plans. The subgrade shall be wet but not muddy when the con- 

 crete is placed upon it. The concrete shall be composed of the following mate- 

 rials proportioned by volume: One (1) part of cement, one and one-half (1J) 

 parts of sand, three (3) parts of gravel, and sufficient water to form a quaky 

 mass; or one (1) part of cement, one and three-quarters (If) parts of sand, 

 three (3) parts of crushed stone, and sufficient water to form a quaky mass. 

 The materials shall be thoroughly mixed in a machine mixer of the batch 

 type, so designed, constructed, and operated that the thorough mixing of the 

 materials is assured and that the consistency of all batches is the same. The 

 operations of transporting the concrete from the mixer to its proper place in 

 the road and of spreading and tamping it in place shall be so conducted as not 

 to cause or permit any separation of the materials of the concrete. The con- 

 crete shall be placed between the forms, hereinafter described, and the surface 

 shall then be shaped, true to grade and to a cross section having one-fourth (1) 

 inch more crown than that shown on the cross-section drawings by means of a 

 well-constructed " strike board." When the concrete is thus shaped, it shall 

 be tamped until mortar flushes to the surface in such quantity as to fill com- 

 pletely all the voids between the particles of the coarse aggregate. The tamp- 

 ing shall be done with a template having a face not less than six (6) inches in 

 width and conforming with the crown shown on the cross-section drawings, or 

 by some other device equally as satisfactory to the engineer. When the tamp- 



1 Stone of only fair quality will meet the above requirements for hardness, toughness, 

 and per cent of wear, and if better stone is available these requirements should be such 

 as to insure its use. It is also desirable to list the available varieties of stone which 

 would be acceptable. 



