CONSTRUCTION OF A SINGLE SAND SEAL

A sand seal comprises a single or double seal of aggregate (river sand, crusher dust or grit) held together with a bituminous binder. For a permanent wearing course at least a double sand seal would normally be required. A single sand seal can be used as a temporary seal until the permanent seal can be applied.

Sand seals are well suited for construction by labour and light plant as:

• The binder, in the form of a bituminous emulsion can be applied using a motorised hand spray; • The sand aggregate for the single seal can be distributed with shovels and brooms;

• Suitable sand aggregate can often be found near the construction site and easily supplied by manual labour • The seal can be compacted using a pedestrian roller

8.12.1 Specifications

Sand is fine aggregate most of which passes a 5mm sieve. It is used as an ingredient in concrete, and bituminous sealing that fills the voids to produce a dense matrix. Sand used in construction works should be clean and free from impurities such as clay, silts, salts, mica and organic matter. Sea sand is generally too fine and its salt content tends also to be detrimental to construction works.

Specifications36 _{for sand seals are provided in various countries as part of standard surfacing specifications.}

8.12.2 Materials

Materials required for the construction of a Sand seal are:

• Surfacing aggregate of the specified size (obtained from a commercial source/quarry or natural source) • Bituminous binder in the form of a bitumen emulsion Cationic37 _{spray grade emulsion (65/35).}

8.12.3 Grading

The grading of sand may vary to a fair degree, but the specifications below should normally be met. However, if sand from commercial sources satisfying these specifications is not available or is too expensive (due to transport costs), good results may be achieved using locally available sources (e.g. river sand or leached sand on the road side) in which case care should be taken to remove excessive dust.

Table 8-13: Sand grading envelope

Sieve Size (mm) Percentage by Mass Passing through Sieve

6.70 100

0.300 0 - 15

0.150 0 -2

8.12.4 Construction plant and equipment

The following are the main tools and equipment used in the construction of Sand Seal: • Motorized hand sprayer

• Lifting frame

• Half drum containers for spotting of aggregate heaps (105 litres) • Wheelbarrows

36 _{In South Africa, for instance sand seal specifications are given in COLTO (1998), Section 4900.}

37 _{Most river sands tend to be acidic and are attracted to cationic emulsifiers, hence the use of cationic emulsion. Anionic emulsion would be more}

• Shovels

• Protective screens • 6 mm rope

• Nails • Hammer

• Reinforced brown paper – to obtain clean trimmed edges of joints • Calibrated Dipstick

8.12.5 Bitumen

Cationic spray grade emulsion (65% bitumen and 35% water) is ideal for natural sand seals. In cool weather it is advisable to heat the emulsion to 50°C. The flow properties of this emulsion are better than that of anionic emulsions. For instance, this grade Cationic emulsion does not flow as easily as anionic emulsion; therefore, the application rate can be higher.

8.12.6 Application of binder

The tack coat is applied at 1.6 litres/m2_.

During spraying the team must:

- Protect kerbs and other road furniture using suitable protective material - Reinforced brown paper should be used to obtain neat joints.

- The binder should be applied evenly with the hand sprayer following the procedure described earlier in this chapter..

8.12.7 Application of sand

- The spread rate of the sand is 0.007m3_/m2_.

- The washing of the sand (if it is required) should be done well ahead of the construction date.

- Stockpile the sand of known quantity along the road at calculated intervals. See procedure below for calculating the spotting distance.

- Apply the aggregate only after the emulsion has partially broken (as an indication, the colour of the emulsion changes from brown to black when it breaks. The duration for this to occur depends on the temperature on that particular day).

- To reduce dust in the sand, flip the shovel during the spreading in order for the sand to be lifted into the air before dropping onto the binder. The wind will thus blow most of the dust away.

- Uneven sand patches can be rectified by the use of hand brooms.

For spotting of the sand, use the following steps to determine the spacing distance along the road: 1. Rate of application of the aggregate = 0.007m3_/m2

2. Width of the road = 5.5m

3. Volume of the ½ drum = 105 litres (0.105m3₎ 4. Volume of aggregates spread from half drum = 5.5m x y x 0.007 m3_/m2 5. Therefore the spacing distance y = 0.105

5.5 0.007 = 2.73m

6. Hence the area covered by each half drum = 5.5 x 2.73 = 15.02m2

The sand from the half drums are spaced at 2.73m intervals and spread evenly over the entire area which has been demarcated with stones beforehand.

Use of ‘leached’ natural sand 8.12.8 Rolling and aftercare

The rolling of the aggregate is carried out with a pedestrian roller, but the use of a tractor or loaded truck would add further compaction, especially if there are undulations in the surfacing.

The sand should be swept back onto the road with brooms periodically after the application of the sand seal. Patches where bleeding occurs are blinded off with more sand.

Crusher dust meeting the grading requirement can be used with anionic emulsion. Example of sand seal using commercial crusher dust is shown below;

Sand Seal with Commercial Crusher Dust Compaction of Natural Sand Sealed Section