CHECK THE FOLLOWING BEFORE CASTING YOUR MAXPAN SLAB

Maxpan slabs are a form of floor diaphragm system that falls in a wider category called Ribbed slabs. Ribbed slabs are a kind in which some of the concrete in the tension zone is removed and replaced with hollow blocks or voids. This reduction of volume of concrete assumes that the tensile strength of concrete is zero and hence the tensile stress is borne by the reinforcements in the tensile zone. This results in a lighter form of construction compared to an equivalent concrete section of slab. In addition, it results in a cheaper structural concept because the cost per square meter of maxpan slab is cheaper than the cost per square meter of solid slab.

It is key to note that even in buildings where maxpan slabs have been provided, localised solid slabs are provided in ‘wet areas’ for durability. Wet areas are areas where water is likely to stagnate and seep into the depth of the slab which would lead to corrosion of reinforcement embedded within the slab. These areas include balconies, roof terraces, toilets, and bathrooms.

In most cases, a contractor on site will call for inspection from the supervising Structural Engineer to ensure not only that all the components of the slab like spacers, reinforcement and maxpan have been placed correctly but also to arrest any design flaws and oversights that might not have been anticipated at the time of design. The following are areas we commonly look out for as Structural Engineers while inspecting a maxpan slab:

1.?????Size of the Maxpan

Because the depth of a maxpan slab is mostly determined by the size of the maxpan used (as the slab topping is fixed to 50mm). This is the most important check we make. Because maxpan slabs are normally manufactured as 300x300mm in plan, the major check we do is on the height/thickness of the maxpan.

2.?????Checking the width of the Rib

A rib is the section of the maxpan slab in which reinforcement is placed and concrete cast. Structurally speaking, this strip of slab section acts as a flanged beam. Depending on the design of the maxpan slab, we would sample a few ribs and make sure that the space provided to receive the rib concrete is the same as that provided in the design and on the Structural drawings. Most structural Engineers provide for a 150mm rib even though the 125mm rib is what I normally go for.?

3.?????Checking for Reinforcement provided in the Ribs

Because the maxpan ribs are the actual load carrying elements of the slab, the most important check I do is on the size and number of bars provided in the rib. Although the reinforcement specification in the rib is based on the area of steel required (and hence some structural engineers provide 1 No. bar within the Rib), it is often advisable to provide for 2No. bars to cater for any possibility of failure of one of the bars. Key to note is that the 2 bars should not be tied together but rather span separately from one support to the next (for the bottoms) or over either end of the support beam(tops).?

4.?????Plugging/Enclosing of Maxpan ends

Because maxpan slabs are hollow slabs, the work of the maxpan is to act as formwork or a void filler. This is means that while concrete is being cast, it shouldn’t get into the holes of the maxpan because that would increase the dead weight of the slab. On site therefore, the contractor always needs to block (plug the holes of the maxpan at the end of the line to ensure that when concrete is cast, the maxpan remains hollow.?

5.?????Provision of Spacer Blocks

Spacer blocks are normally none structural square blocks of mortar placed between the slab soffit and the reinforcement to ensure that the slab has an adequate cover once the concrete is cast. During the inspection therefore, we ensure that an adequate number of spacer blocks is provided within the ribs prior to casting so that the reinforcement isn’t exposed at the time of de-shuttering the slab.

6.?????Ensuring that solid bands have been provided in areas with ducts

Because maxpan slabs are one-way slabs, any areas that require voids or ducts within the slabs require the provision of solid bands. Solid bands are essentially slab beams around the duct or void that allow for the floor load to be redistributed around the void into the adjacent ribs or floor beams.?

7.?????Ensuring that floor beam reinforcement has been placed correctly

As part of the maxpan floor inspection, we always check that the floor beam reinforcement has been provided as shown in the structural design drawings. Any contentious beams can also be rechecked at this point before the contractor is given a green light to cast.?

8.?????Ensuring the Provision of BRC Mesh

Maxpan slabs are designed to have a 50mm topping at the slab top. This topping is normally reinforced with a BRC Mesh to avoid any thermal shrinkage cracking. Although the norm in the industry is to provide a BRC A142 Mesh, the nominal reinforcement requirement calculation (0.0013x1000x50 = 65mm2/m) proves than a BRC A98 is still adequate in the 50mm topping. Propper lapping of this mesh is a must and the contractor must always ensure this prior to casting.?

Once we have made all the above checks and ensured that the contractor has done the right thing, the maxpan slab is ready for casting!