In-situ Concrete, also known as insitu concrete, is an outstanding landscaping construction technique. In terms of long terms long term quality, robustness and longevity very few landscape construction techniques come close. To many people in-situ concrete is better know as landscaping architectural concrete and it is the point where landscaping, architecture and civil engineering all meet in landscape design. For the landscape design of public spaces, architectural concrete or in-situ concrete is a great choice for the severe use of our landscaped parks, gardens or commercial buildings. Some other names for in-situ concrete include “poured in place” concrete or “cast in place” concrete”.
What is in-situ concrete?
In-Situ Concrete cast in place.
In-situ is concrete that as poured into formwork at the building site. When compared to pre-cast concrete, in-situ requires no crane or forklift to get it into position. In comparison with concrete block and rendered retaining walls, the very low porosity of sealed and vibed in-situ walls will ensure a quality finish for decades of use. With concrete block retaining walls, the blocks themselves will have some porosity which will allow moisture to creep under the rendered surface and may cause it to lift in a few years. In-situ concrete has the colour mixed in and the texture is provided by the formwork but it can be rendered if required.
In-Situ Concrete – strong connection to footings.
An additional advantage with in-situ is it has a very strong and stable attachment to the very solid footings underneath. This is due to the reinforcement bars coming up from the footings and overlapping with the in-situ concrete steel reinforcement. When compared with concrete block retaining walls, although the steel reinforcement runs up through the centre of the blocks, any voids in the concrete around the reinforcement can lead to corrosion of the reinforcement bars and concrete cancer. The homogeneity of the of the vibed in-situ walls also ensures a good attachment to the steel reinforcing bars and does not suffer the risk of a poor attachment of the core concrete to the inner void of the concrete blocks. Pre-cast concrete will usually require mechanical fixings to the footing. These fixings can corrode over time or can be ground off by vandals. Being securely attached to the footings is a big advantage of in-situ concrete. Newly constructed public spaces such as parks and gardens will often rely on these quality advantages of in-situ concrete.
Precast concrete versus in-situ in landscaping concrete architecture.
One of the advantages of precast concrete is that it is manufactured under controlled conditions in a factory. This ensures a consistent quality product. There can be a risk however of coloured concrete coming from different batches resulting in slightly different colours close together. The quality advantages of precast concrete can be matched by in-situ concrete with close attention to the details and quality control of the processes.
Quality in-situ concrete retaining walls, stairs and seating.
Commercial Retaining Walls Melbourne
Good quality in-situ concreting requires a lot of attention to the materials and the processes. In-situ concrete wall also need to consider the safety as well as the aesthetics of a public space or residential landscape construction project. A well constructed, quality, curved in-situ concrete wall will achieve both of these aims.
Quality In-situ concreting footings.
Concrete sets by a chemical reaction and not by drying. The chemical reaction is ecothermic and water is part of the reaction. If there is a difference in temperature within the concrete or if the water evaporates quickly from the top surface, then cracking can be the result. Concrete footings should not be poured in freezing conditions or below 5 degrees. Fortunately, in Melbourne there are very few days when it is too cold to pour. In hot weather the concrete should not be poured during the hottest part of the day or in extremely hot weather. The top part of the footing can be kept moist with hessian or a light sprinkling of water to prevent the top layer of the footing being weaker.
Concrete reinforcement bars in the footings.
It is essential that the vertical bars coming out of the footing are located accurately. When the in-situ walls are poured, these bars need to be closed to the centre of the retaining wall with good overlap with the wall reinforcement. The concrete reinforcement within the footing should be encased within the concrete as much as possible to minimise the paths for moisture to get into the reinforcement.
Concrete footing design
If designing for in-situ concrete retaining walls, the concrete footing should be a suitable size for the wall, keeping in mind all of the loads on the wall including hydrostatic and mass of the wall. The design of the footings must be to Australian Standards.
Formwork for in-situ concrete retaining walls.
Quality in-situ walls require formwork that is smooth strong and flexible. Any defect, imperfection of inaccuracy in the formwork will show up in the surface of the wall. The formwork needs to be strong enough to resist the weight of the wet concrete without bulging or deforming. Deflections in timer formwork will show up as ripples in the finished wall. A bulge in the formwork due to the hydrostatic load of the concrete will be a disaster when removing the formwork from the wall. The wall should be designed with a slight draft angle to make the removal easy without causing any damage to the retaining wall.
In-situ concrete steel reinforcement
The steel reinforcement bars must have a good overlap with the footing reinforcement bars and well encased within the concrete wall. Reinforcement to close to the surface may result in water ingress and concrete cancer.
Pouring the in-situ concrete.
The concrete walls should be poured in one go and definitely from the same batch of concrete. Any interruption of the pour may show up as a line in the finished wall. The freshly poured concrete should be thoroughly vibed to ensure there are no voids within the concrete especially on the outer surfaces or at the interface with the reinforcement.
The steel reinforcement within tin-situ also helps to prevent surface cracking. To minimise the surface cracks, we leave the formwork in place a little longer to keep the moisture in during curing. The concrete should then be given a light sprinkle of water once the formwork is removed as it is still curing. The concrete will not be fully cured for a few weeks so consider this before applying any excessive loads.
Landscape and Concrete Design
Landscape Design with in-situ walls.
In-situ walls give the landscape designer or the landscape architect enormous freedom to design shapes for retaining walls, seating or concrete stairs that would be very difficult, if not impossible, using precast of other landscape construction techniques. In-situ walls can be designed into three dimensional shapes that would be impossible or expensive to do as Pre-cast. Curved retaining walls on an uneven landscape would be very difficult to achieve with any other landscape construction technique. Another advantage is the wide range of colours available.
As the concrete is pumped in, damage to other landscaping structures can be avoided, making it easier for the landscape project planner to schedule the construction. These are important factors to consider when landscaping Melbourne public spaces.
See also Concrete Architecture in Landscaping