Sidewalks, curbs and combined curbs and gutters shall be constructed in two courses.
A. 
The base course shall consist of a 1:2:4 mix by volume of air-entraining portland cement, sand and gravel, mixed with water.
B. 
The top course shall consist of a 1:3 mix by volume of air-entraining portland cement and sand, mixed with water, and shall be applied within 25 minutes after placing the base course. The top course shall be one inch thick and cover all exposed surfaces, including the face of curbs.
Sand and gravel shall be screened, graded and washed free of injurious amounts of clay, loam and dirt. Sand shall be sharp, coarse, natural sand. Gravel shall be No. 1 size, crushed gravel or broken stone. Mixing water shall be suitable to drink. All materials shall be kept separated to prevent intermixing and the intrusion of foreign matter. Cement shall be protected from damage by the elements.
All materials shall be accurately measured by volume. Batch boxes must be used for measuring the correct amount of sand and gravel when charging the mixer. Only full sacks of cement shall be used. No fractional bag mixes will be allowed. All concrete shall be mixed for at least 1 1/2 minutes in a clean, approved mechanical mixer until the color is uniform and there is a uniform distribution of materials.
A. 
Concrete shall be handled in such a manner as to prevent the segregation of materials and the intrusion of foreign matter.
B. 
Concrete shall only be placed in clean, oiled forms set true to line, grade and dimensions, and only on well-compacted, damp subgrades free from loose or objectionable materials and frost.
C. 
The base course shall be thoroughly tamped with a heavy hand tamper to form a dense homogeneous mass. The base course shall contain just enough water to the mix so that the concrete may be tamped without excessive displacement or splashing.
D. 
The top course must be placed before the base course receives its initial set. The top course shall be floated to a true, even surface without bringing free water to the surface and shall then be given a "broom" finish.
E. 
Immediately after finishing, the work shall be cured by covering with canvas, burlap or other approved material, which shall be kept moistened for three days. Vaporproof membranes used for curing will not require wetting.
F. 
Protection. Concrete shall only be placed between April 15 and October 15 unless specifically approved by the Sidewalk Inspector. Precautions shall be taken to prevent freezing and to prevent the placing of concrete on frozen ground.