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know How to Repair Reinforced concrete slabs with bottom falling and Lack bond?

know How to Repair Reinforced concrete Old slabs with the bottom falling off due to steel corrosion and New Reinforced Slabs with bottom plaster falling off due to lack of bond

There are many cases where the part below the steel layer of the RC roof or floor slab has collapsed. We can divide the reasons for this failure into two headings.

Case 1: Is the falling off the bottom of the slab after many years due to corrosion of steel.

Case 2: Is that of the bottom layer falling off soon after plastering during the construction of the slab due to lack of bond.

In this article, we explain the repair of these two cases.


Case 1: Old Slabs with the Bottom Falling Off Due to Steel Corrosion

Corrosion of steel and cracking of the underside of older RC roofs is very common. Usually, in the case of roof slabs and slabs under the bathroom, corrosion of steel occurs due to leakage and water seepage. The RC slab below fig shows corroded steel below. When this occurs in other intermediate floor layers, the steel corrosion can be determined by a phenolphthalein test. Then it can happen again. Also, old twisted high-yield bars (Tor steel) usually rust faster compared to modern hot-rolled high-strength steel bars. So, if we add any steel, let it be hot rolled bars. Before starting work, prop the RC slab in some places. It's good to support.


know How to Repair Reinforced concrete slabs

Figure: RC slab with bottom steel corroded.

The main method of repair is to first remove and clean the loose concrete, clean the steel bars, and apply a bonding coat to clear the steel and concrete. We then apply micro concrete made of cement, sand, and small-size coarse aggregates (below 10 mm) along with superplasticizers or polymers to make the concrete workable. We may add additional steel if the corrosion has consumed a lot of steel, say more than 15%. This is done by spot welding the new steel to the old steel or by clamping to concrete.

For filling up the gaps in the concrete, we use less costly plasticizers for large areas and in the case of small areas, we may use more costly and efficient polymer-modified cement and mortar or concrete.


Procedure for repairing the corroded slab by the following steps:

Step 1: Support the slab with props to relieve some load from the slab.


Step 2: Remove all the defective concrete and expose the corroded reinforcement to its full circumference. A clear gap of 15 mm below the steel is recommended by CPWD. As the slab is supported, it becomes more stable.


Step 3: If possible, conduct a carbonation test to find the depth of concrete up to which the carbonation has occurred. Remove concrete from that depth.


Step 4: Clean the reinforcement thoroughly and add extra reinforcement, if required, by using U clamps or by welding it to the old reinforcement.


Step 5: Thoroughly clean the concrete surface.


Step 6: Apply an anticorrosive protective passivating coat to all the steel including ties.


Step 7: Apply the bonding coat to the old concrete surface also.  


Step 8: Apply polymer-modified mortar, as it needs less curing and attains very good strength. This should be applied before the bond coat gets dry  


Step 9: Water cure for 7 days by brushing with water or other means.


Step 10: Painting with cement paint or carbonation-resistant paint, if needed, can be carried out after the surface gets dry.


Slab concrete Repair scheme

Figure: Repair scheme showing applications of (a) corrosion inhibiting cum bond coat, (b) polymer modified repair mortar, and (c) carbonation resistance flexible coating.


Case 2: Falling Off of Bottom Plaster of New RC Slabs Due to Lack of Bond

The bottom falling off new RC slab plastering is mainly due to a lack of bond between the finishing plaster and the concrete. Another major reason is the lack of proper notching or hacking at the base of the slab as per standard practice. As today's cement with high ash content gives a very smooth surface when finished, it is essential to strictly follow the old hacking practice or use modern construction chemicals to bond concrete surfaces.


The old specification called for hacking with a sharp tool so that the depression was not less than 3 mm deep and spaced not more than 50 mm apart. Nowadays, it is difficult to get hackers, thus, plastering under the slab is reduced due to the lack of bond.


In such cases, the solution is to use modern construction chemicals. We must first apply a bond coat of polymer latex or cement polymer-modified cement slurry, then, before the bond coat dries, plaster is applied with polymer-modified 1:3 cement sand mortar as specified by the manufacturers of chemicals. Water purification should be carried out for at least seven days, after which the desired painting can be carried out on the surface.



RC slab affected by corrosion of steel reinforcement can now be successfully repaired using concrete chemicals. If you dry out the repaired slab and keep out the main enemy, moisture, it will look good for a very long time. The use of bonding chemicals can prevent plastering from falling off the bottom of newly constructed slabs due to a lack of bond with the concrete slab.

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