Learn About Kerbs

Bosun is the only manufacturer in South Africa using the “dry cast” method to produce precast concrete kerbs. In Europe, this is the preferred method of manufacture and we firmly believe that kerbs produced according to this method are of consistent better quality than those produced by other means. Herewith a helpful list of short articles to consult when researching the purchase of kerbs, the installation of kerbs as well as when potential problems arise after installation.

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Dry Cast Kerbs versus Wet Cast Kerbs

Wet cast kerb was installed next to a dry cast kerb. Two years later the dry cast kerbs are darker, the surface doesn’t abrade as easily and withstands acid attack better in the long term. Wet cast kerbs are manufactured in the same way as cobbles. Dry cast kerbs are manufactured in the same way as interlocking pavers.

Bosun kerbs are designed consistently with high quality in mind, ensuring a durable and resilient form. Our kerbs, being manufactured out of concrete, are fire resistant; and due to their solid form, are robust in nature.

Bosun dry cast kerbs were used on the left-hand side of this traffic island. Wet cast kerbs were used on the right-hand side. It is clear to see that the dry cast kerbs are dimensionally more accurate than the wet cast kerbs.

Dry Cast

Wet Cast

SANS Specifications for Concrete Kerbs Explained

The South African National standard for precast concrete kerbs, edgings and channels SANS 927:2013:

Bosun kerbs adhere to SANS specifications and are all manufactured according to the required specifications and product guarantees are based on the same rules.
Kerbs are made up of high quality materials.
The kerbs will be free from cracks other than hairline cracks. They will also be free from visible twists and dents.
Kerbs dimensions stay within +3mm of length and straightness.
Kerbs comply with strength specifications.
Kerbs are sampled and tested in a lab.

Kerb Installation Guidelines

The content of this guideline is largely based on SANS 1200MK, the official South African National Standard for the installation of precast concrete kerbs. As SANS 1200MK has last updated in 1983, we have also included key points from the British standard for the installation of precast concrete kerbs.

What SANS 1200MK says in brief:

Precast concrete kerbs shall be laid and bedded on a 50mm thick layer of bedding material, placed on the compacted subbase or other formation layer to suit the designated levels of the finished road. Bedding material shall consist of crushed stone, cinders, slag, sand, or other approved porous material having a maximum particle size of 13.2mm.

The kerbs and channels shall be laid with close joints (of thickness not exceeding 10 mm) of 3:1 sand:cement mortar on the ends, and neatly pointed with a pointing trowel. Provision shall he made for expansion joints of width at least 12mm at intervals not exceeding 20m.

After the grouting of kerb joints has been completed, the kerbs shall be backed with well-punned concrete of specified strength 15 MPa and the space behind the backing shall be backfilled.

From this, we see that the specification is not clear and sufficient. This is also echoed by all the broken kerbs and failed installations we see in South Africa. Kerbs are supposed to be infrastructure and should last a lifetime. It must therefore also be installed to last a lifetime. For this reason, Bosun consulted various other specifications for the installation of concrete kerbs. Of these, the British Standard have the most synergy with our construction methods.

What the British standard says in brief:

The kerb foundation must be supported, either on an extension to the underlying pavement sub-layers or, for thin pavements (e.g., edgings on pedestrian footways), directly on compacted subgrade. The depth of the unit and that of the pavement construction will determine on which pavement layer the kerb foundation will sit.

A good concrete foundation to support the kerb is essential and must be sufficient for the intended pedestrian or vehicular trafficking. It must be able to spread and transmit the applied vehicular wheel loads to the underlying support layer without overstressing the layer. The foundation must also be capable of resisting any shear forces induced by overrun.

Kerbs require bedding to provide even support and prevent vehicular overrun from damaging the unit by inducing bending stresses. The units may be bedded directly on a freshly mixed concrete kerb foundation, or on mortar on top of a hardened concrete foundation.

Units should be haunched with backing concrete to support them and prevent horizontal displacement.

Bosun recommends the following when it comes to the installation of kerbs:

  1. Compacted layer works should extend at least 100mm behind the kerb.
  2. Construct a 150mm thick concrete foundation on top of the prepared layer works. This foundation spreads and transmits the applied vehicular wheel loads to the underlying support layers without overstressing the foundation.
  3. Laying the kerbs – good adhesion of kerbs to the concrete foundation is crucial. There are two possible methods to ensure this:

Option A (recommended) – Install the kerbs on a 40mm thick bedding layer of fresh mortar (1:3 cement, sand ratio) on top of the hardened concrete foundation. Kerbs should be laid before the mortar starts to harden.

Precast Concrete Kerb Installed on Mortar Bed

Option B – Install the kerbs directly on the concrete foundation. In this instance, the concrete used for the foundation, should not contain stones larger than 13mm. Furthermore, ensure the concrete is sufficiently fluid (wet) to ensure the kerb beds properly.

Precast Concrete Kerb Installed on Concrete Foundation
  1. Kerbs must then be haunched with concrete on the joints at least, but preferably all along the back of kerbs, especially if they are intended to carry heavy vehicular traffic.
  1. Kerbs should then be jointed with fresh mortar. Provision should be made for 12mm expansion joints every 20 metres. Should block paving be installed behind the kerbs, expansion joints must be filled with approved expandable material. When gaps are left between kerbs, the paving layer works will seep through the gaps, causing settlement and other failures of the pavement.
  1. After the work specified above has been completed, spaces and voids must be backfilled with approved material to pavement or shoulder level, as applicable. It should be backfilled in layers not exceeding 100 mm and each layer should be watered and thoroughly compacted to at least 90% of modified AASHTO maximum density, before the next layer is placed. The compacted backfill must extend for a distance of 0.5m behind the kerb face.

Handling Equipment

Kerbs generally weigh in excess of 100kg per unit. Bosun actively promotes the use of specialist handling equipment to protect workers on site and improve efficiency. These tools are available from Cretesol.

Kerb Innovation

Bosun manufactures a range of precast kerbs with innovative castellated bottoms and with spacer nibs. The patented Castle Bottom design ensures better adhesion in all directions, while the spacer nibs ensure even spacing between units. Click below to view more: