Bedding Sand Material and its purpose.
Bedding Material (Types)
When selecting a bedding sand material, there are four general characteristics that should be considered.
1. Gradation of material
2. Particle Shape
4. Moisture content.
The proper gradation of bedding sand ensures the sands ability to allow for proper drainage and will become dense under compaction. Bedding material under all paver application should conform to ASTM C 33 or CSA A23.1 specifications.
The bedding sand particles should be sharp and symmetrical. Non-symmetrical particles will not compact properly and may have a tendency to cause uneven settling over time. Bedding material particles should also be free of any foreign matter. Such as larger pieces of rock and dirt / organic material.
Pavement failures can result from a bedding material that is to soft. Bedding sand material should contain durable particles that are derived from crushed aggregates / rock and will not break down under repeated traffic loads. Soft materials, such as stone dust or mason sand, should never be used as a bedding material. They often contain soft particles that will deteriorate over time and do not compact uniformly. The hardest bedding material that is available should be used as your bedding material. So check with your local suppliers, to see which bedding material products they carry. Limestone screens can be used, but may contribute to possible efflorescence issues later with your paving products.
Frozen or saturated bedding sand should never be installed. Bedding material that is too dry will not compact properly. If your bedding material is dry, then small amounts of water added to the material will help with the overall compaction.
As noted in Section 2, Bedding material should never be used to fill in any larger variations in the base course. Variations in the base course should be properly filled with a suitable base material and compacted.
Bedding Material Purpose
The Bedding Material generally has 3 functions:
1. Bedding material can ensure a consistent final elevation for the paver surface.
2. It helps effective load transfer between pavers.
3. It can act as a drainage medium.
When bedding material is installed, it can accommodate minor / subtle variations in the granular base course. The standard construction tolerances allow for maximum base deviations of 3/8 in ( 10mm ) over a 10 ft run. When the bedding sand material is properly installed, it can accommodate for these minor variations.
The effective load transfer between the pavers is a very critical factor in the overall success of your project. The applied loads are distributed from the individual pavers to the surrounding pavers through the sand between the pavers. During the compaction of the pavers, the bedding sand material will fill the bottom portion of the joints between the pavers. With the final application of jointing sand to the surface of the pavers, the sand will fill in joints from the top down and will ensure the proper structural integrity of the paver system.
After installation of the pavers, a fair amount of water can drain down through the joints and filter into the bedding sand material. Over time the joints between the pavers will fill in and become sealed and only allowing smaller amounts of water to filter through to the bedding sand. The ability of the bedding sand to compact tightly and still allow water to drain during the early stages of the paver application is very important to the overall success of your project.
Bedding Material Thickness
Bedding sand material should be spread and screeded to a consistent thickness of 1 in to 1 1/2 in ( 25mm to 40mm ) after final compaction.
Frozen and saturated bedding material should never be installed. Excessive bedding sand material over 1-1/2 in can and will create the risk of inconsistent compaction and an uneven paver surface over time.
Setting your final grades
Before you start to spread and screed your bedding sand material, you should set your final elevations / grades on your project. This can be achieved by attaching a string line to your grade stakes that you set prior to your base installation. The grade stakes should already be set with your final paver elevations. The top of the string line will represent the top of the paver surface and the finished grade on your project. This will also provide you with a reference line to inspect the positive pitch / slope for surface drainage. Remember that a minimum of 1 in per 10 ft run is recommended for effective drainage.
Screed Rails and Screeding Bedding Materials.
Once you have your final elevations set, you can set your screed rails and begin screeding your bedding material. Screeding bedding sand material is typically done by using 1-1/4in to 1-3/4 in ( 30mm to 40mm ) pipes that are set on top of the compacted gravel base. Set the initial screed rail on top of the compacted base material just below the string line so that the screed rail runs parallel with the string. The top edge of the screed rail will represent the finished elevation of the bedding sand material. By taking a paver and resting it on top of the screed rail it should match your finished paver elevation. With the paver on top of the screed rail, it is best to adjust your screed rail so the paver elevation sits a little higher then the finished elevation of the paver. This is to compensate for compaction and settling. With your first screed rail set at the proper elevation you can now set your second screed rail at the same elevation. Set the second screed rail parallel 10 ft apart from the first rail. Once you have both your screed rails set, you can place a straight edge or 2 x 4 strike board across both rails. Make sure you use a level on top of the straight edge or 2 x 4 strike board to ensure that your second screed rail is at the same elevation as the first rail. On areas that are typically wider then 10 ft, an additional screed rail can be set parallel to the existing rails. This will help prevent the straight edge or 2 x 4 strike board from bowing in the middle.
With the screed rails set, you can begin to fill the area between the rails with your bedding sand material. The preferable thickness of the bedding material should be at 1-1/4 in (30mm). This thickness should be consistent throughout the whole project. With the bedding sand material between the rails and in front of the straight edge / strike board, you can now begin to screed your bedding material by pulling the material down the length of the screed rails. The bedding sand material after screeding should be free of any irregularities and voids. If any do exist, they should be filled and screeded a second time. The finished layer of bedding material should be smooth and consistent.
With the bedding material screeded, do an overall inspection of the area before you move the screed rail. After your inspection, the screed rails can be moved and their depressions should be filled with additional bedding sand material. Use a trowel to finish off the surfae so it blends into the existing bedding sand material. Always screed a large enough area to stay well ahead of the paver installers. After the sand has been screeded it should not be disturbed, even pedestrian traffic can do damage to the screeded bedding material. Only screed enough material for the amount of pavers that you will install in a days time.
Paver that are sand set should always be set on a loose, un-compacted layer of bedding material. The pavers once installed can be compacted with a vibratory tamper. this will set the pavers into the bedding material.
Paver installation and Edging to follow soon.