Masonry Magazine June 1977 Page. 15
Laying Course and Sub-base
The laying course consists of clean, sharp sand placed to a thickness such that when the blocks are vibrated into place the resulting thickness of the laying course is about 1-1/2 to 2 inches thick. Many opinions exist concerning the maximum size, gradation, and moisture content of the sand used for the laying course. This suggests that the performance of the block pavement is not sensitive to these variables over a fairly wide range. To achieve uniformity, however, the sand should be obtained from one source and before use protected so as to maintain uniform moisture content.
As with any pavement, the sub-base for paving block is of primary importance. Inasmuch as the block pavement can be considered as a flexible pavement, the sub-base should be that required for conventional asphalt pavements; it could consist of old pavements, unjointed concrete slabs or most any old road. They are all suitable sub-bases for concrete block paving. The required thickness of a sub-base will depend upon both the expected service loads and the soil conditions; the need for a substantial sub-base is greater in low, wet areas, Table 1. Also, sub-base materials and standards must comply with the requirements of the local road authority.
In many instances, a sub-base will not be required for light duty residential constructions, such as, patios, walkways around the swimming pool, and garden walks. Under the driveway, however, one would be well advised to place a compacted sub-base of from three to eight inches, depending upon how well the driveway area drains after a rain. The material used for the sub-base should be a well graded gravel or limestone having a maximum topsize of about 1-1/2 inches and containing about 10 percent sand sizes. A thick sub-base should be placed in layers of about four inches, each layer leveled and compacted or vibrated, prior to placing the next layer. Having the sub-base well compacted is important to the durability of the finished construction.
Construction
After the sub-base is in place at the correct surface level and is free of debris and mud, the sand laying course is spread evenly between curbs. This is followed by screeding the sand to the desired level. For roads up to about 16 feet wide the sand course is usually screeded from the curb as shown in Fig. 1; but for wide areas temporary screed rails should be used. As the final road surface is largely governed by the uniformity of screeding, it is important that care is taken during the screeding operation. A cambered screed board is used for pavements with a cambered cross-section. To avoid uneveness caused by pre-compaction, the sand must not be walked on or disturbed once it has been screeded.
Figure 2 shows a typical cross-section of a concrete block pavement consisting of the sub-base, the laying course of sand and the blocks themselves. To help keep the blocks from being displaced horizontally, precast or cast-in-place curbs are used for medium and heavy duty installations.
TABLE 1. Suggested Sub-Base Thickness
| Application | Thickness of Sub-Base, Inches |
|---|---|
| | Well-Drained Dry Areas | Low Wet Areas |
| LIGHT DUTY: Residential: Driveways Patios Pool Decks Walkways Parking Bicycle Path | 0 to 3 inches | 4 to 8 inches. |
| MEDIUM DUTY: Sidewalks Shopping Malls Residential Streets Public Parking Bus Stops Service Roads Parking Lots | 4 to 6 inches | 10 inches |
| HEAVY DUTY: City Streets Intersections Gas Stations Loading Docks Loading Ramps Industrial Floors Stables | 8 inches | 12 inches |
Figure 1
Figure 2