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Building reinforced concrete slab on ground

October 26, 2020

Reinforced concrete is capable of accepting both compressive and tensile loadings and therefore is an ideal material for building reinforced concrete slabs on ground for a wide range of applications in modern home construction, if used properly. The reinforced concrete slab on ground has been a practice for many years in Australian residential building industry, replacing the traditional limestone footings and suspended timber systems. The current Australian Standards throughout all Australian states are AS 2870 Residential Slabs and Footings and AS 3600 Concrete Structures.

Step 1: Planning the concrete placement

The concrete must be placed fast and simple, directly from mixer truck. The excavated soil and stacks of materials must be relocated in order to give the truck sufficient space to move. On sites with limited access, superpasticised concrete is used which flows easily. This type of concrete will impose higher loads on the formwork and can move steel reinforcement as it flows into place. When the truck can’t access close to the slab, it is necessary to include concrete pump with hydraulic booms, tipper or dumper for concrete transportation. Concrete pumps can push concrete over 200m in line, but in case the line rises or bends, the pumping distance is less.

Step 2: Ground Preparation

Once the planning of the concrete placement is done, the next phase is the ground preparation.

Remove the top soil with grass on it and level and compact the sub-soil which has been uncovered. Sloping sites will need to be cut and filled where the chunk needs to be placed. Most of the soils can be used for fill, but clay fill is not recommended. The Building Code of Australia sets requirements for ground preparation, excavation and filling under concrete slabs. Dig out the shape of the pillars for the slab in the arranged ground.

Step 3: Fixing the edge formwork

The next step is to form the edge of the slab. The formwork must be well stalked in place, commonly spacing is at a maximum of 1 meter.  The formwork also needs to be rigid and thick enough so it doesn’t bend under the load of fresh concrete placed against it. It is strongly recommended that the level dimensions and shape of the formed area is double checked before placing concrete.

Step 4: Service pipes installation

In this stage the waste and water supply pipes that will be covered by the slab must be installed by a plumber. Where the slab is used as a boundary against termite attack, termite collars must be fitted to all pipes going through the slab. The Basement Construction contractor must care not to move these drainage pipes once they have been set in place.   

Step 5: Laying concrete underlay

The underlay needs to be placed over the prepared ground and lapped over the edge of the framework. It is recommended, a wide underlay to be used so that just a few joints are needed. It is very important to make sure that the underlay folds down into the beam trenches and laps over the top of the formwork. Free edges of underlay must be firmly secured before the concrete is placed. The underlay should be lapped at least 20mm at joints and held in situ with small pieces of tape at about 1m centres.

Step 6: Fixing reinforcement in the beams 

The ground slab has thickened edges called edge beams. Sometimes the panels also have internal beams that serve as rigid beams or wall support. All of these beams require reinforcement of steel that is attached close to the ground – known as bottom steel. Trench mesh is the most common type of ground steel. Reinforcement requires a concrete cover of at least 40mm to up to 75mm for aggressive groundwater. Depending on the area, some sites require greater depth and reinforcement. In order to be effective, the steel reinforcement must be placed in the correct position.  

Step 7: Fixing reinforcement in the slabs

Cover steel is required for the entire area of the reinforced concrete slab. The reason for this is to control the inevitable cracks when the concrete dries. Fabric sheets are typically used as cover steel which are placed on a bar stool before the concrete hardens, leaving at least 20mm of concrete on the steel reinforcement. At this point, the underfloor heating or the electrical wiring, should be secured in the slab. This is usually the phase in which bolts for wind bracing and other ancillary fixtures are placed.

Reinforcement

Step 8: Concrete placing

No water should be added to concrete during this placement and finishing operations because the concrete will be weaker, have a poor surface finish and crack more. 

Place each batch of concrete next to the previous batch. Start with one end and work across the tray, making sure each new load is well mixed with the load. It is important to not let the concrete fall more than a meter free from the gutter, bucket or pipe. Level the concrete surface with screed. A mechanical vibrator should be used to compact the concrete. Use the vibrator every half meter along the beam and hole it until the concrete settles and the bubbles stop riding. In this stage it is important not to move the steel reinforcement or damage the liner of the framework.

Placing Concrete

Step 9: Finishing the slab surface

When the concrete compression is complete, the slabs should be roughly floated with a towel to obtain a smooth surface. The deaeration water rises to the surface of the slab after being levelled. Wait till this water dries before final finishing. A mechanical “helicopter” is an excellent tool to get a good finish on the slab. Hand floats or trowels made of wood or steel also work fine when there is no helicopter, but somehow the entire surface has to be worked twice.

Concrete Formwork

Step 10: Cure the reinforced concrete slab

Once the surface is hard enough to withstand spoilage, the concrete should be protected from moisture loss. This process is known as curing. If the concrete does not harden, it dries too quickly and loses strength, which can cause the surface to crack and become dusty. When hardened, the water collects in the concrete, allowing it to exert sufficient strength. One possibility is to cover the reinforced concrete slab with plastic or construction paper and hold the slab in place with a board. Leave the sheets for at least three days and if needed spray a little more water under the sheets after the first day. This method has the advantage of protecting the slab from rain damage.

Here, you can find more information about why steel reinforcement is needed in concrete house slabs.

Only an experienced, qualified engineer can classify the site in order for the design of the concrete slab on ground to be suitable for the bearing conditions. The Engineer and the Basement Construction Contractor have a valuable role to play even with the simplest slab on ground designs.

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