Pressed Concrete Flags – Patios
Patios
Design and Installation Factors and Considerations
Important things to consider when laying concrete flags to a random pattern are, no straight lines over 1800mm, no cross joints, keep the joints even and to manufacturers specifications widths, and don’t lay two slabs the same size adjacent to each other. Cross joints are not random when included in a random pattern. On a driveway they also affect the structural integrity of the project.
When laying in rows such as half bond or 1/3 offset bond. Set out the area so as not to have small cuts of less than 1/3 at the edge of the paved areas: this is important not only for aesthetic reasons but also for the structural integrity of the paving.
When installing smooth surface and riven concrete flags, a minimum fall of 1-60 should be installed to each of the subgrade, i.e. the MOT type 1 base, the bedding mortar and the surface course.
Drainage must also be considered, especially where paving meets a lawn. If there is no drainage at the bottom of the end of the fall this can often cause a wet, boggy lawn. For large patios, drainage should be installed at every 10m of fall.
Concrete flags may also be pre-sealed to reduce maintenance, or sealed in accordance with manufacturers instructions after laying.
Check Points
Upon delivery of your paving slabs paving, it is vital to check that all crates are colour matched on the information given on the crates of paving and visual inspection upon opening the crates. It may be best to select from three or more packs to avoid colour batching in some products.
Prior to installation, ensure the finished level of the paving is 150mm below the damp proof course of the house and preferably falling away from the building. Should there be no option of falling the paving away from the house, the next best practice would be that the first row of paving falls away from the building, then a channel drain installed, and the rest of the paving fall towards the channel drain (see figure ).
Excavation (assuming 30mm slabs to be laid)
The minimum excavation depth is to be 160mm to accommodate 100mm compacted depth of MOT type 1, 25 – 30mm bedding mortar and 30mm for the paving flag. The thickness of the geosynthetic membrane and bonding mortar is approximately 3-4mm.
If you are unsure about the strength of the subgrade conduct the heel test as outlined in British Standard BS7533:102 and adjust the depths of excavation accordingly to meet British Standard BS7533:101.
The fall of the formation level should match that of the surface course.
Compact the subgrade using a suitable heavy-duty vibrating compacting plate or roller.
Base – Sub-base Installation
When installing your patio base, it is best practice to first lay a geosynthetic non-woven membrane to prevent fine particle migration into the subgrade. The geosynthetic membrane should overlap by a minimum of 300mm.
Good practice, especially on poor subgrades, is to install geogrid/biaxial mesh. This is specifically designed for soil stabilisation and reinforcement. Again, geogrid should be overlapped by a minimum of 300mm and suitable zip ties installed to connect them together.
Best practice is to install edge restraints prior to base and sub-base installation so as to provide sufficient load-bearing support and to resist lateral force. The edge restraints should be laid to a concrete bed and haunch and should be below the bedding mortar/laying course.
Upon delivery of your MOT type 1, a close inspection should be carried out to its moisture content. It should be moist but not saturated. If it is too dry, it will not be lubricated sufficiently and will lack interlocking capacity. If it is too wet, it can lose its load-bearing capacity.
Next, install MOT type 1 to a minimum and consistent compacted depth of 100mm, and at the same fall as the subgrade. The MOT type 1 should be installed in layers no more than 70mm in depth which will compact to approximately 50mm depth. Compaction should consist of 5 passes with a suitable heavy-duty vibrating plate, both horizontally and perpendicular. A good indication that sufficient compaction has been achieved is there will be no lines visible in the MOT type 1 surface.
It is good practice to ‘dip’ the base, which is a method of installing to parallel string lines. Usually, the first stringline would be 150mm below the damp proof course, and the second running horizontally parallel at the bottom of the patio 150mm from the subgrade.
The string line is pulled taught every 1m to ensure there is an even distribution of MOT type 1 with no humps or hollows. Should there be low spots they can be filled with MOT type 1 and compacted. High spots may be raked to low spots or levelled.
Laying Course – Bedding Mortar
Set out string lines to levels and falls. The string line should not touch the edge of the pavers and should be 1-2mm away to ensure it is not displaced. String lines should have a tingle or intermediate line pin every 6m to prevent line sag.
Bedding mortar should consist of 1-4 Ordinary Portland Cement (OPC) and one of the following aggregates: 2-6mm, 1-4mm or 2-8mm crushed rock. The mortar mix should have sufficient water added to hydrate the cement and form a ball that holds its shape in your hand when compressed with your fingers. A wet or semi-dry mix is more than acceptable, but not a dry mix.
The mortar beds should be prepared flat (see figure ) with no visible trowel marks (see figure ), as this will provide a full contact mortar bed. Full contact mortar beds are essential to provide full surface area adhesion between the back of the slab and the mortar bed. Voids may cause reflective staining in natural stone and occasionally in concrete slabs, and may also cause weak spots. Under no circumstances should paving be laid on ‘dot-and-dab’ or spots or mortar.
The thickness of a mortar bed should be a minimum of 25 mm and no more 40mm. As an approximate guide, mostly depending on the moisture content of your mortar mix, 35-40mm will compress to 30mm. It is important to compress the mortar when using your rubber maul to achieve sufficient strength.
A good practice is to install geogrid/biaxial mesh to the mortar bed, especially on poor subgrades. This is in addition to any mesh that that may have been installed beneath the MOT Type 1 base.
Mortar in ambient temperatures is workable for approximately 2 hours, after that a new mix is required. It is considered bad practice to keep adding water to rehydrate the mortar as the mortar loses strength. (In fact, water acts as an accelerant when added to cement)
There should be no organic matter such as leaves or soil on the type 1 base prior to laying mortar.
Laying Paving Slabs
Weather check. Paving should not be laid below 3 degrees. Temperatures should be 3 degrees and rising above 5 degrees. All materials should be covered, as should the laying face. Overnight night temperatures should also be considered.
Wash the dust, loose debris off the back paving slab. In some cases a brush or sponge may also be necessary to remove thicker layers. If dust and dirt are not removed it affect adhesion. It is important to also remove any wrapping or packaging material from the slabs. Note: On many occasions the paper backing is left on the back of the paving flag which more often than not leads to the paving slab debonding.
Paving slabs should always be stacked vertically on timber skids to avoid chipping or damage.
The back of the paving slabs should be fully coated with a British Standard BS7533 certified bonding mortar/slurry primer, applied with a grout flat at 1-2mm thickness (see figure ).
The British Standard BS7533:102 tolerance for level variation between paving units is plus or minus 2mm.
As there are occasional dimensional tolerances of +/- 3mm, it is good practice to use a string line to check, periodically, that joint lines are straight horizontally and perpendicularly. Measuring back to the base line, to make sure the paving is square or parallel is also good practice. Usually, checking for accuracy would involve measuring at either end and also in the middle as a minimum.
Using a Washboy, clean the paving of mortar or bonding mortar as you lay.
Rake the joints out of mortar as you lay the paving, so a full joint of grout may be installed. Do not wait until the end of the day before cleaning operations, as the mortar is likely to have hardened.
The paving should not be walked on or trafficked for a minimum of 24 hours. Clients should be made aware of this and paved areas cordoned off.
Jointing
Allow the mortar bed to cure sufficiently prior to grout installation: this can be up to 28 days in some instances. Read the manufacturer’s specifications or contact their helpline for guidance as needed. If you grout before the mortar bed has sufficiently cured you may get efflorescence staining or water staining appearing in the grout.
The use of single part vacuum packed brush-in grouts is not recommended.
Check the weather forecast. If rain or freezing temperatures are imminent then delay grouting.
Check the joints are of sufficient depth to receive the grout, usually a minimum 20mm depth.
Clean the surface of the paving to ensure there is no contamination of the grout.
Using a leaf blower remove excess water from the joints and surface of the paving.
Install the chosen grout as per manufacturers’ specifications, ensuring the joints are full with no voids.
Aftercare
Regular maintenance is key, not only to the aesthetics but also to the longevity of the patio. A light power washing, using proprietary cleaners is recommended. Regular brushing/sweeping to remove dirt and debris build up and proprietary cleaners are also recommended. Should any deteriorations in the surface arise, they should be repaired immediately.
Breathable sealants may be applied to the paving post-laying if not already pre-sealed.
Notes compiled by Gareth Wilson / Paving Site Limited.
