How to Repair Rising Dampness in walls of Ground Floors in Buildings Constructed without DPC? and How to Repair Efflorescence in Buildings?

Introduction

The plinth is the portion of the wall of a building immediately above ground level to the ground floor level. This height usually ranges from 45 cm to 100 cm. Nowadays, in building construction, we provide a beam at the lower or upper level of the plinth, called the plinth beam. It is usually 10 cm thick with at least 6 mm rods spaced at 10 cm at the top and the bottom and 6 mm stirrups at the rate of 23 cm. (This is in addition to the grade beam or ground beam we provide at the foundation level for isolated footings, under reamed piles, etc.)

In all buildings, nowadays, we build a damp-proof course (DPC) to prevent dampness from going up the walls from the foundation due to the capillary action of groundwater. The damp-proof course can be on the plinth beam or on a beam built separately always above the ground level. If it is built separately, it should be at least 40 mm to 50 mm thick and should have 1:2:4 concrete with a waterproofing compound. Over this, we paint a thick layer of bitumen to prevent water from rising from the wall from the foundation. A much cheaper way for low-cost buildings is to put only a 1:3 cement mortar layer with bituminous paint (or a bituminous membrane placed over it) in a portion of the wall above the ground. This should be provided for all walls, i.e., external, and internal walls. Thus, DPC prevents water from rising from the wall from the foundation by capillary action. Even though in all the new buildings, nowadays, we place the DPC, in old buildings like old assembly halls, old church buildings, and old residences, these are absent. How we prevent moisture migration in these old buildings where DPC has not been provided is the major problem dealt with in this article.

Methods to Rectify Dampness

The following methods are usually recommended:

1. Construct a new DPC (Note: Conventional type consists of 40 mm to 50 mm thick cement concrete in the proportion of 1:2:4 with water-proofing compound.)

2. Pressure injection or gravity feeding of a suitable chemical solution within the plinth (Water soluble silicon solutions are commonly used.)

3. Pressure injection of resin mortar in boring holes

Construction of New DPC

For installing a DPC in an old building that has been built without DPC, we cut the mortar bed joint of two brick courses above ground level in stages of about one meter in length at a time. No two adjacent lengths should be repaired consecutively. A new DPC with a waterproofing compound can be inserted with the rebuilding of the removed brick course. This method is too slow and may lead to structural settlement and cracking of walls if the walls are weak. Hence, the methods described in the subsequent section are usually recommended.

Method of Injection of Chemicals (Silicon)

Another method of repair is the injection of chemicals as a liquid. The most common method used for placing DPC is based on injecting water-soluble silicon (which has the moisture-resisting property) into the brickwork, as shown in Figure below.

In this article, we will explain the repair process for Reinforced concrete beams and columns damaged by steel corrosion where the depth of the affected concrete has reached the reinforcement level. As the amount of steel rust exceeds that of the original steel, the concrete around the steel cracks. In some cases, the cover concrete may fall off. Corrosion can be caused by carbonation or chloride.

The following procedure is followed to repair these reinforced concrete members. The main difference between slabs from columns and beams is that for the latter, a more elaborate formwork must be used to place the concrete. Otherwise, we can place concrete with shotcrete

Repair of Cracks in Beams and Columns when Corrosion has Reached Reinforcement Level

Step 1: As these members carry heavy loads, first support the beam and column using supports to relieve part of its load. Then, remove the damaged concrete to expose the steel.

Step 2: Investigate the type of corrosion with phenolphthalein tests to determine whether the corrosion is due to carbonation or chloride effect. Also, determine the depth to which erosion has continued. If it's due to carbonation, it's worth fixing. But if it is due to chloride corrosion, the same corrosion can occur elsewhere if the concrete near the steel is not completely replaced. Here, we assume that carbonation is the only cause of corrosion.

Step 3: If the steel is completely corroded, remove 15mm to 25mm of concrete from the entire steel.

Step 4: Clean the concrete surface and steel surface thoroughly with water. A rust removal solution can also be used for steel. It dissolves the rust and adheres to the steel as a coating.

Step 5: When the steel is reduced by about 15% due to corrosion, place the necessary additional steel and bond it properly with the old steel or additional support. New steel can also be supported by spot welding to old steel. Place required shear reinforcements for beams and binders for columns.

Step 6: Apply proper bonding coat to the old concrete surface and steel surface.

Step 7: If necessary, attach the formwork to the column for concreting the removed area. For columns, it is easy to place formwork or self-compacting concrete for pouring concrete. For beams, it is more convenient to use expanded wire mesh

Step 8: Before the bond coat dries, compact the void space of the member with cement, sand, small coarse aggregate (10mm and below) and micro concrete with superplasticizer as required. Water-cement ratio should not exceed 0.5. In some cases, we may have to use self-compacting concrete. This is achieved by using concrete chemicals such as superplasticizers. For patch repairs or repairing small areas, we usually use simple polymer-modified cement mortar. For larger repairs, it is better if we use shotcrete to fill the removed area.

Step 9: Apply a 1:3 (finely sanded) coating within 48 hours of completing Step 8.

Step 10: Wet curing should be done for at least 7 days.

Step 11: After complete drying, a coat of waterproof paint is given to protect the member and match the surrounding surfaces. Note: In larger jobs, it is better to place concrete or mortar using shotcrete, also known as gunite.

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.

Repair of Concrete Affected by Steel Corrosion

In this article, we have explained how to repair the corrosion of steel in reinforced concrete members. As we know, steel corrosion not only corrodes steel but also affects the concrete around the steel in the structure and causes its cracking. This is due to the carbonation of concrete. We proceed as follows for repairs:

1.   First, we make a visual observation of the problem.

2.   Then, we determine the nature of corrosion whether it is carbonation or chloride induced.

The nature of corrosion, as we can see with our eyes, also tells us how corrosion has taken place, Therefore, it clarifies whether it is due to carbonation or chloride effect. Now, the treatment of the corroded structure is explained here depending on its nature.

Procedure for Repair of Carbonation-Induced Corrosion

If the cracking of concrete is due to carbonation-induced corrosion of steel, we can have two distinct cases of the state of corrosion, namely,

1.   Where the carbonation has not reached the level of steel

2.   Where it has reached the level of steel

Test for the extent of carbonation can be carried out by spraying 0.2% solution of phenolphthalein indicator on freshly chipped concrete which is in contact with steel and also on the concrete near the reinforcement. The change in colour of the concrete indicates that the concrete is in good condition. When there is no change in colour, it shows that it is carbonation-affected concrete. The pH value can also be found by analysing the samples of mortar collected by drilling, dissolving them in distilled water and conducting titration. There can be two cases of corrosion which are to be treated as follows:

Case 1: (where the carbonation depth has not reached the reinforcement level)

In this case, we are concerned with carbonation for a small depth only. The crack can be sealed by widening it and placing a polymer-modified cement slurry. In important cases, we may use injection grouting with grouting nipples and slurry pumps.

Case 2: (where the carbonation depth has reached reinforcement level)

This is a bad case. In this case, we proceed along the following steps after supporting the member with external supports to carry the load on the member, if necessary.

Step 1: Remove all the defective concrete that has been carbonised and expose the reinforcement bars. If the bars are rusted very much, then it is good to remove the concrete so as to give a clear air gap all around the steel of about 15 mm in slabs and up to 25 mm in beams and columns. (This is a must in chloride corrosion.) Clean the steel thoroughly with a wire brush and manual labour or by shot blasting or other means. We may use a water jet to clean the surface. Rust remover chemicals are also available for easy removal of rust. Additional steel with necessary supports may have to be provided if needed. This addition is compulsory if the rusted steel is around or over 15%. It can be tied or welded to old steel.

Step 2: Apply a corrosion-inhibiting slurry coat to steel using acrylic polymer and cement (with 43-grade cement) in a ratio of 1:5. Alternately, use an epoxy bonding coat with no cement. These chemicals are available in the market. (In the former days, we used to paint the steel with red oxide chromium paint. Two coats one after the other at intervals of not less than four hours were specified.

Step 3: Make the exposed concrete damp (but not saturated) with water. Apply a bond coat of latex or acrylic-modified cement slurry (using commercially available chemicals) over the concrete surface. This coat is applied to bond the old concrete with the new concrete or mortar (as described in step 4). If this bond coat is not applied, the old concrete and new concrete will not stick together. (These bond coats are conventional cement slurry to which we add chemicals like synthetic latex.)

Step 4: Replace the concrete that we have removed with latex-modified concrete (with small aggregates) or with latex-modified mortar. This work should be carried out before the bonding coat dries up. Compact this concrete wall.

Step 5: Apply to cure paint on the finished surface for curing without watering or use the conventional method of curing the concrete (or mortar) with wet gunny bags for 7 days.

Step 6: Apply decorative paint after curing to match the surrounding surfaces.

Note: There is also ready-to-use mineral-based one-component material which acts as a combined corrosion inhibitor and bond coat. One such chemical is Zentrifix KMH of MC Bauchemie available in the market. A 100 parts by weight of this material is slowly added to 20–25 parts by weight of water and mixed thoroughly. This is applied to the prepared steel and prewetted concrete as corrosion inhibitors and bonding coats.

Repair of Chloride Induced Corrosion

There can be two cases under this head that are as follows:

Case 1: Corrosion due to chlorides in the concrete used for construction

This is the worst case. The sand, water or aggregate used to make the concrete may have contained chlorides. shows the formation of corrosion by voltaic action. This case is very difficult to cure, as the cause is internal. It is better to rebuild the structure. For temporary relief, we may proceed as follows: In this case, for repair, we have to completely remove the concrete around the steel, leaving a clear gap. We then carry out the operations of cleaning both steel as well as concrete, applying bonding coats and filling the space with very good concrete and small aggregates mortar. As corrosion can occur at any place of the structure, it is better to rebuild the member rather than make frequent repairs.

Defects in Paintworks

The common defects that should be avoided in the painting are as follows:

General Preparation

Let us assume that the wall has been built and cured. After the wall has dried and matured for a few months, we first apply a coat of cement primer (white). Irregularities of the surfaces can be rectified by applying putty before painting is started. For example, in ordinary house construction, we can use plaster of Paris as putty. For superior work as in showrooms, special copolymer emulsion putty which is suitable for exterior and interior and also contains the primer coat is also available in the market. They are, however, costly. It is more convenient to first apply white primer on cement-plastered walls before applying putty.

The white background obtained after applying white primer shows up the irregularities and then, the putty can be applied more effectively. This procedure also reduces the amount of putty required. The putty coat is rubbed with sandpaper to smoothen the surface. After that, one more coat of primer can be applied, and it is allowed to dry for 10 to 12 hours before applying the chosen paint as the undercoat. The following points are important in this case:

1. Oil and acrylic paints, and distempers do not fare well if there is alkalinity in the surface to be plastered or if the wall has been previously whitewashed with lime. Hence, it is essential to apply alkali-resistant primer on these whitewashed surfaces before applying the prescribed paints.

2. While painting the exterior walls, all molds, algae, moss, etc. should be removed

3. When efflorescence is seen in the brick wall, water should not be used to wash out the efflorescence. If water is used, the salt may be absorbed back into the pores and precipitated again on the outside when it dries out. Hence, the deposit should be brushed and cleaned in the dry state itself. We may use acid etching

Procedure for Painting New Buildings

As we have seen in the last section, fresh concrete, cement, and lime plaster have moisture in them. They are highly alkaline. Hence, it is preferable to paint these surfaces only after 3 to 6 months when the plaster work is completed. Even then for new works, it is considered better to use primer paint to start with. These primers are special paints for new works.

In addition, there may be pores to be filled. Hence, we use a filling-in compound or a filler coat. Thus, in all new painting works, we have the following five operations:

1.  Cleaning the surface by proper sandpapering (The sandpaper we use for different surfaces is different.)

2.  Application of primer coat

3.  Filling the small dents with a filler coat

4.  Application of the first undercoat of the paint

5.  Application of finishing coats

Thus, in addition to the main paint, we must use the primer coat and the filler coat in the new works.

Primer Coats

The names of some of the primer coat paints used for building works are given in Table