Introduction and Nomenclature
Loose filling
This method of jointing flexible stone pavements has been used for traditional setts and cubes in continental Europe for as long as there have been pavements while in Britain and Ireland it is experiencing wider use as sawn setts and calibrated flagstones become more reasonably priced and readily available for commercial projects as well as private driveways, forecourts and patios. Loose fill jointing is much more than a matter of simply filling the gaps between paving elements: it is a critical component of the overall pavement and plays a major role in the load-bearing ability of the completed structure.
Loose filling relies on the same principle as the jointing used for block paving, in that a dry fine aggregate of a specific range of grain sizes is packed into the joints and thereby generates sufficient vertical interlock to maintain the paving elements in place. The loose material is normally brushed into the joints of the pavement and vibrated into place by means of a plate compactor.
One technique more commonly employed in Europe, particularly during damp weather or the winter months, uses a hose or sprinkler system to wash the fine aggregate into the empty joints. This 'wet jointing' relies on the laying course, and the other sub-layers, being exceptionally free-draining. Should the bedding, for example, retain wash-in water for a period of time, it may result in liquefaction and subsequent movement or settlement of the paving, particularly if a vibrating plate compactor is used to consolidate the paving elements following joint filling.
The grain size of the jointing material is determined by the type of paving elements and the width of joint between them. For relatively narrow joints (2-5mm) such as those found with block paving or calibrated setts, a kiln-dried selected sand (2mm down) is normally used. This is NOT just any old sand that happens to be available, and it is definitely not building sand. Both the grain sizes and shape are specifically selected to maximise interlock and therefore an inappropriate sand may actually weaken the designed strength of the pavement.
For slightly wider joints (4-10mm) a crushed hard-rock aggregate known as “splitt” is normally used. This material is approximately 4mm down and often made from a crushed granite, basalt or diorite, rather than from rock of a sedimentary origin. On some projects, splitt or grit jointing may be topped-up using a limestone dust, as this will loosely bind together the uppermost few millimetres of the jointing and help minimise loss to scour during the early life of the pavement.
Actual coverage for the jointing material varies enormously depending on joint width and depth. Tight-jointed sawn setts require significantly less jointing material per unit area than cropped setts or fettled-edge flagstones.
[jointing quantity calculator]
It is common for jointing material to settle over the first few months, regardless of how well vibrated it might have been when placed. Further, before the loose material effects a natural 'seal' of detritus, it is vulnerable to scour from wind, surface water, and sweeping/cleaning, which can remove some of the loose material and, in extreme cases, completely empty the joint.
Consequently, it is essential that the pavement is inspected every 2-4 weeks during the initial 3-6 months following construction and that the joints be topped up as and when required. In areas experiencing heavy surface water flows or on civic pavements subjected to vacuum/brush street cleaning equipment, it may be beneficial to use a joint stabilising product or even a paving sealant to bond together the sand/aggregate and give it more of a chance to resist the scouring action found in these unfavourable conditions.
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