Pre-cast Kerbing v/s Cast-in-situ Kerbing

There has been a lot of debate regarding the use of precast kerbing as opposed to cast in situ. This is in fact unnecessary as the two types of kerbing have their own applications. Generally figure 3 and 7 kerbs are precast as they require only a thin layer of bedding, between 0 and 20 mm, to place them to level. Because of this bedding layer’s size, it will not cause problems of vertical movement in the life of the kerb. Further and most importantly the strength requirements of SABS 1200 can be met as the bedding layer has no bearing on that of the kerb.

The problem has arisen with precast figure 8 kerbs being used in applications that require them to be placed on bedding layers of up to 100 mm This bedding material by its nature is required to be dry enough to support the precast kerb which has little or no MPA strength. The issue now is the fact that the kerb, or at least half of it, does not comply with the strength requirements of the specification. More importantly however is the fact that the kerb can fail at any time due to heavy vehicles driving over them, or even water eroding the bedding material causing the kerb to subside.

Cast in situ kerbing however does not have this problem as the kerb by its nature or method of casting is the same from top to bottom. A further advantage is that the in situ product is thrown continuously and therefore has a far greater longitudinal strength. Whilst not pertinent to this discussion the cost and time implications can- not be overlooked.

Pre-cast kerbing v/s Cast-in-situ Kerbing

1. Pre-cast kerbs start at a premium of up to 50% of -situ costs.
   
2. Pre-cast kerbs take from 5-10 times longer to place which also impacts on the price.
3. Pre-cast kerbs are placed on bedding that probably has no MPA strength whatsoever, and this bedding often makes up more than 50% of the kerb. Cast-in-situ kerbs offer strengths upward of 25 MPA depending on site requirements and mix design.
   
4. Pre-cast blocks due to their size show road imperfections, whereas due to the extrusion process this imperfection is not as visible with cast-in-situ kerbs.
5. Pre-cast kerbs only utilise local labour in so far as the placement thereof is concerned. Cast-in-situ kerbs utilise some local labour and also offer training in various building disciplines, such as the mixing of bulk concrete and plastering. Furthermore, sand and stone from local quarries is used, thus uplifting the entire community.