Paths, Patios & Driveways

Choosing the Right Mix

Select the mix according to the job. Drives need a higher quality mix than paths to cope with the heavier loading. Larger pours, such as for complete new driveways, are best suited to the delivery of ready-mixed concrete. Otherwise, mixing your own concrete may be more convenient. The approximate rate of cement usage for mix-your-own concrete corresponds to:240 kg (=6 x 40kg bags) per m3 of concrete for paths, and300 kg (=7 1/2 x 40kg bags) per m3 of concrete for drives.Remember that sand, aggregates, and builders mix are all supplied in a "loose" condition. A decrease in bulk volume will generally occur when these are mixed with cement and water. For each m3 of concrete, allow for around 0.7m3 of sand and 0.7m3 of coarse aggregates or 1.2m3 of all-in builders mix.

Mix Table

End Use DIY MIX Proportions (by volume)
Builder's buckets per 50-litre
batch
Ready-Mix
Order
Paths mowing strips & small
patios
cement : sand : 20mm aggregate=
1 : 4 : 4 or
cement : all-in "builders mix" =
1 : 7
15 MPA
@
100mm slump
Driveways & large patios cement : sand : 20mm aggregate =
1 1/4 : 4 : 4 or
cement : all-in "builders mix" =
1 1/4 : 6 1/2
20 MPa
@
80mm slump



Ground Preparations

Clear the designated area of all plants, roots, etc to at least 1500mm beyond the final edges. To get the best performance (and appearance) out of the final concrete, try to avoid any contamination of the aggregates by leaves, twigs, soil, sawdust, etc. Check your site for buried pipes or cables. If you find any, seek guidance from the relevant authority.Where the ground is of clay or peaty material or if truck or multiple car loadings are expected, it may be necessary to lay a 100mm sub-base of crushed stone or gravel. (Ask around whether the use of a sub-base is normal practice in your local area). Otherwise, a well compacted soil will normally offer sufficient bearing capacity. It is very important here to ensure evenness of surface and proper compaction of the ground (and of the sub-base, if present). Use a heavy garden roller or rammer.The minimum thickness of concrete is commonly recommended as: Paths 75mm, Patios and Drives 100mm.In choosing the actual thickness to be used, make use of local knowledge. If still in doubt, add an extra 25mm for greater security.

Setting Out and fixing of Side forms

For simple garden paths, small pads, etc, the setting out can largely be done "by eye". Where the job involves working to a set of plans, the "paper" measurements are normally transferred to the site through the use of a builders tape (and probably a builders square) together with a system of pegs and stringlines as shown below.



At each corner stringlines are tied to the pegs outside the working area passing above the nails on pegs marking points A, B, C, D.Side-forms can be made from stout timber boards (at least 20mm thick) or framing timber. The depth of the side-forms should match the depth of the slab. A smooth inner surface will minimise bonding to the concrete and make for easier stripping. Fix the side forms with steel or wooden pegs. Keep the spacing of the pegs close enough to restrict flexing of the side forms. Drive the tops of the pegs below the top edge of the formwork. Nail timber side-forms to wooden pegs.Bending of the side-forms to produce curved edges may be eased by sawing a series of cuts to mid-thickness of the timber, on the inside of the curve, and soaking the timber prior to the bending. For particularly "tight" curves use hardboard with closely spaced support pegs.Set up the side-forms with an adequate cross-fall to assist the run-off of rainwater from the finished path, patio or driveway. A fall of at least 1 in 40 (25mm/m) is recommended here. Check the cross-falls using a spirit level set on a straight-edged board spanning between the side-forms (with appropriate packing).


Setting out the boxing

To prevent unsightly shrinkage cracking, sizeable areas of concrete should be divided into sections through the use of control joints. The maximum length of section (spacing of control joints) should be no greater than one and a half times the width or 4 m, whichever is less. For example, in the case of a concrete path, 800mm wide, the maximum recommended section length is 1200 mm. Control joints can be achieved via:

  • Setting full-depth hardboard strips between the side-forms. (These require temporary support during concrete placement).
  • Forming a cut/groove in the concrete with a trowel or special grooving tool after the concrete has first begun to stiffen.
  • Cutting a groove with a concrete saw within a day or so of the concrete having set and begun to harden (gain in strength).

Laying Procedures
Do not lay concrete when rain threatens, nor in very hot, cold or windy conditions.Before pouring any concrete, wet the side-forms and soak the ground/sub-base (but leave no lasting puddles of water).For those producing their own concrete (in a powered concrete mixer), the most common procedure is to:

  • Mix up around half of the required quantity of aggregate and sand or builders mix for a single batch together with the bulk of the water.
  • Add all of the cement
  • Load the remaining aggregates and top up on water as required to give adequate workability. (Use no more water than is really needed. Overwetting will lower the final quality of concrete obtained).
  • Place the concrete within the forms. Distribute the material as evenly as possible in the first instance. Take special care where hardboard strips are being used to form control joints; keep the same level of fresh concrete on both sides of each strip at any one time. Spread and level the concrete with a rake. Use a shovel (or boot!) to work the concrete well into the corners and edge regions. Overfill the forms by 15-25mm to allow for settlement during the tamping and screedling operation which follow on immediately after.
  • Tamp/screed the top surface with a timber beam as shown below. Where the job is less than 1 1/2m wide, a length of 100 x 50mm timber laid on the edge will normally do as a tamping beam.




Screeding the surface

  • Do not scrape the beam across the concrete. The aim here is to impart up-and-down motions, closely overlapped in the direction of the travel. Fill up any "low spots" left behind and rescreed as required. Alternatively, if the level is too high, use the beam with a sawing action to remove surplus material and then rescreed as normal to close up the surface. Try to complete the tamping/screeding operations before too much "bleed" water rises to the top of the concrete. (Continued re-working in the presence of significant quantities of bleed water will tend to produce a poor quality concrete surface).


Finishing Procedures

For garden paths, and some driveway situations, the as-tamped finish may be quite adequate. Note that a whole variety of as-tamped finishes can be obtained depending on how the tamping beam is actually used. If another finish is required, there are several alternatives on offer; see illustrations below.All of these finishes are best achieved when the bleed water has gone from the surface and the concrete has begun to stiffen. The degree of stiffening reached at the time of applying the "secondary" finish will influence the appearance, colour (shade of grey) and texture obtained. So too will the type of finishing tool. (You might wish to experiment with different methods of finishing in an unobstrusive area first). Thus, for example, a wooden or plastic float used fairly early in the piece will give a much coarser texture and lighter colouring than a "late" steel float. It is worth emphasising here that the extreme smoothness of steel-trowelled finishes is generally unsuited to paths, patios and drives (slippery when wet).

Steerowelled finish Shovel back finish Broom finish

     


Brush/broom finishes:
Pulling a brush or broom over the concrete creates a surface with excellent non-slip properties and is the quickest/easiest form of applied finish. Evenness of texture and colour is best achieved if the brushing/brooming process is done with minimum delay between start and completion. For driveway jobs, brush/broom crosswise to assist crossfall drainage.

Exposed aggregate finish Using an edging tool

   


Exposed aggregate finish:
Apply a soft broom finish, allow the surface to stiffen enough such that the coarser aggregate particles become firmly bound, and then remove surface cement and fine sand through gentle brushing/watering/washing. The timing of the various operations and the uniformity of treatment are important factors here as regards determining final appearance.

Rounded edges:
Achieved around the outer perimeter and on/along control joints through the use of edging or arrissing tools available from most builders merchants.Finally, as soon as the finishing operations are complete, steps should be taken to protect the young concrete from any unwanted trafficking (by children and animals in particular) and from the adverse effects of premature drying. If possible, keep the concrete in a moist condition for up to seven days, whether by a regular sprinkling or through the use of a weighted-down plastic (eg polythene sheet) cover which serves to prevent evaporation losses. This early-age moist "curing" greatly enhances the final quality of the concrete surface. Where control joints are to be sawn in the hardened concrete, the retention of moisture becomes all the more important (to prevent the onset of drying shrinkage prior to cutting).

Disclaimer:
While the information presented within this web site is offered in good faith, Holcim (New Zealand) Ltd disclaims any and all responsibility for the application of the principles and procedures discussed.