Repair of Cracks in Concrete Structures | Concrete Technology

In this article we will discuss about:- 1. Concrete Repair by Polymer Impregnation 2. Resin Based Repairs in Concrete 3. General Use of Epoxies in Concrete Construction 4. Repair of Cracks in Massive Structures 5. Common Types of Repairs.

Concrete Repair by Polymer Impregnation:

In this system of repair, the damaged concrete surface is cleaned thoroughly and dried. After cleaning and drying the cracked concrete surface is flooded with a monomer, which is then polymerized in place, filling and structurally repairing the crack.

A monomer system is a liquid consisting of small organic molecules capable of combining to form a solid plastic. The monomer system used for impregnation or injecting into the concrete contains a catalyst or initiator and the basic monomer. They also contain a cross linking agent. On heating the monomers they join together. The process of joining together is known as polymerization. After polymerization the product on cooling becomes tough, strong and durable, plastic that greatly enhances a number of properties of the concrete.

Monomers do not mix with water. They have varying degree of toxicity, flammability and volatility. These are low viscosity fluids which are soaked into the dry concrete filling the cracks in the same way as water. However if the cracks contain moisture, the monomer will not be soaked into the concrete at each crack face, resulting in un-satisfactory repair. Further if a volatile monomer evaporates before the polymeri­zation the repair will be ineffective.

Repair of Fractured Elements:

For repairing fractured elements, polymer impregnation method may be used. In this case the fracture is dried first, and then it is temporarily encased in a water tight sheet metal. Now the fracture is soaked with monomer and polymerized. On polymerization of the monomer the fractured element becomes a solid mass with enhanced strength and properties. Large voids in compression zones may be filled with fine and coarse aggregates before flooding them with monomers, providing a polymer concrete repair.

Treatment of Concrete Surfaces with Large Number of Cracks:

In this situation vacuum impregnation may be used.

Process:

The part of the structure to be repaired is enclosed with in an air tight plastic cover and then the air from all cracks with in the cover is sucked by applying vacuum. After exhausting the air from all cracks, the monomer or resin grout is forced under one atmospheric pressure in cracks and pores of the concrete surface. On completion of impregnation, the cover is removed before the impregnate hardens.

The selection of appropriate impregnate and degree of vacuum depends on experience. This process is extensively used to reduce the permeability of weak concrete or masonry. This process may also be used to improve the abrasive resistance of industrial concrete floor slabs. However polymer impregnation has not been found successful for the repair of fine cracks.

Resin Based Repairs in Concrete:

In reinforced concrete cracks wider than approximately 0.3 mm, require sealing to check the entry of moisture, oxygen and other harmful gases etc. The choice of the method and materials will depend upon the cause of cracking, and whether there is a need of permanent structural filling of cracks to carry out any other required strengthening.

For restoring the structure to its original strength, the use of low viscosity epoxy resin has been found very effective. With epoxy resin system it is possible to fill completely cracks finer than 5 mm by using pressure injection technique. However work should be carried out skillfully to avoid the blowing off the surface seals due to back pressure that may develop in case of very fine cracks. To fill the fine cracks fully sustained pressure for several minutes may be required.

Generally the resin and hardener are in liquid form and each by itself is stable for an indefinite period. On mixing these substances together, a chemical reaction takes place and their liquid system is converted to a tough plastic solid at ambient temperature. They develop excellent strength and adhesive properties. They are resistant to many chemicals. They have been found to possess good chemical and physical stability. They harden rapidly and resist water penetration. Thus they provide durability and resistance to cracking. Resin mortars may be obtained by adding coarse sand as filler.

The epoxy based compounds are invariably formulated with plasticizers, extenders, diluents and fillers to produce a large number of products having a wide range of properties. The excellent adhesion characteri­stics, fast setting properties, high strength and chemical stability has led their extensive use in the concrete construction.

General Use of Epoxies in Concrete Construction:

In concrete construction, epoxies have been used for the following purposes:

(a) For providing skid resistant overlays and wearing surfaces on concrete floors.

(b) As water proof membrane.

(c) Most extensively used in the repair of potholes.

(d) To seal cracks in the structural members.

The clean and dry surface is painted with epoxy compounds before placing the repair materials. The cracks may be sealed with epoxy compounds, epoxy mortar or a port-land cement mortar after priming the surface with epoxy compound. For early and quick use polymer or resin overlays may be constructed. They can be put to early use due to faster curing. As they are joint-less, they are more hygienic and chemical resistant.

Materials:

Epoxy, polyester and acrylic resins as a class are known as thermosetting materials due to the fact that on curing their molecular chains are locked together permanently. These materials do not melt on heating like thermoplastic materials. These materials loose strength with the increase in the temperature. Generally they are supplied as two or three component system as resin, hardener and filler.

Classification of Resins:

Broadly resins are classified as follows:

(a) Epoxy resins

(b) Un-saturated reactive polyester resins

(c) Un-saturated acrylic resins.

Acrylic resin system form high strength materials. They are based on monomers of very low viscosity or blend of monomers and methyl-methacrylate monomers.

Polyester and acrylic resins contain volatile constituents which are inflammable. Most acrylic resins are highly inflammable with a flash point below 10°C. Their vapours also cause toxic reaction.

Properties of Commonly Used Resins:

(a) Epoxy Resins:

Following are the properties of epoxy resins:

(i) Their strength is very high

(ii) Their bonding characteristics are good.

(iii) They have high impact resistance

(iv) They have high chemical resistance

They may be used to provide a non-slippery finish:

(b) Polymer Resins:

These resins have a better resistance to heat and thus can be laid over wider tem­perature range. They are mixed with cement and fine hard aggregate and can be laid in thickness upto 15 mm.

(c) Polyvinyl Acetate (PVAC):

When these mortar overlays are laid over the existing concrete, they are used as bonding aid. The liquid can be applied directly on a clean, sound surface and allowed to dry. The slight re-emulsification of the film on being re wetted by the application of fresh mortar topping provides a good bond.

(d) Natural Rubber Latex:

This admixture has excellent adhesive properties, but difficult to mix with ordinary port-land cement. Generally it is used with less alkaline high alumina cement for patch work or for laying floors on which vinyl tiles are to be laid.

(e) Styrene-Butadiene Rubber (SBR):

It is a good and effective alternative to polyvinyl acetate (PVAC). It is highly water resistant. However the dried film does not develop good bond on re wetting. Thus it will act as a de-bonding layer if allowed to dry out. Hence the mortar mix should be applied while the tack coat of SBR is still wet.

(f) Acrylic Resins:

These admixtures when mixed with mortars develop excellent water resistance and improved bond strength. With this type of resins seamless, non-dusting thin floor over lays can be readily produced.

(g) Styrene Acrylic Resins:

A mixture of tough styrene with acrylic resin using 1:3 cement sand mortar can be used to produce hard wearing floor overlays at a moderate cost.

Repair Procedure:

(a) Use of Resin Mortar:

The surface preparation requirements are same as that for cement based repairs. The constituents of resin based material must be mixed together thoroughly in mechanical mixers or stirrers. Most of the failures of the resin based repairs have been repeated due to either inadequate mixing or improper proportioning. For smaller repair works to get proper proportioning, the constituents normally are available in pre batched packs.

After the preparation of surface, a primer or tack coat of un-filled resin is applied to the freshly exposed surface of concrete and reinforcement. In general one coat is sufficient, but if the sub strata are porous two coats will be needed. In case two coats are applied, the second coat should be applied while the first coat still is tacky.

The patching material should be applied while the primer is still tacky and each successive layer should be applied before the previous layer has cured too much. The resin based materials cure or harden by che­mical reaction, which starts as soon as constituent materials are mixed. Thus they have little pot life i.e. mixing period is very short. Hence the quantity of materials to be mixed in any one batch should be pre-calculated so that it could be used before it becomes too stiff. The resin based patches should be well compacted so that they become impermeable.

While using resins and hardeners, normal safety measures must be observed, that is gloves should be worn and splashes should be washed off the skin, but solvents should not be used for this purpose Adequate ventilation should be provided and smoking, eating and drinking should be prohibited during the application of resin based materials.

(b) Resin Injections:

The polymer injection under pressure will ensure that the sealing material or sealant penetrates to the full depth of the crack.

Procedure:

The injection holes are drilled at close intervals along the length of the crack and the epoxy is injected under pressure in each hole in turn till the injection material starts to flow out of the next hole. The hole which is injected is then sealed off and the next hole is treated.

Precautions:

Before injecting the epoxy, it should be ensured that the crack at surface is sealed between the holes with rapid curing resin.

Repair of Cracks in Massive Structures:

In this case a series of holes usually 20 mm in diameter and 20 mm deep spaced at 150 to 200 mm interval intercepting the crack at a number of points are drilled and then epoxy injection is injected in these holes to seal the crack. The method has been used successfully in the repair of cracks in dams, piers, buildings and other such structures.

However unless cause of cracks is removed, cracks may reoccur again somewhere in the structure. This method has not been found effective if the cracks are constantly leaking and cannot be dried out. Epoxy injecting is a highly specialised job, which requires a high degree of skill for satisfactory execution of the work.

Steps Involved in Epoxy Injection:

Following steps are involved in the Epoxy injection:

1. Preparation of the Surface:

The cracks are cleaned by removing all dirt, oil and grease, fine parti­cles of concrete etc. These elements may prevent the penetration of epoxy and development of the bond between the filling material and the surface of the cracks. The contaminants should preferably be removed by flushing the surface with water or solvent. The solvent is then blown out using compressed air or by air drying.

To check the leaking out of epoxy before it has gelled or cured, the surface cracks should be sealed. The surface can be sealed by brushing an epoxy along the surface of the cracks and allowing it to harden. If extremely high pressure is needed for injecting the epoxy, then epoxy injection should be sent through a V shaped grove of 12 mm depth and 20 mm width. After filling the epoxy in the grove it should be struck off flush with the surface.

2. Installation of Entry Ports or Device:

The entry port or nipple is an opening to allow the injection of an adhesive directly into the crack without leaking. The spacing of injection ports depends upon many factors such as depth and width of cracks, the variation in crack width with its depth, viscosity of epoxy, injection pressure etc. The choice of spacing actually depends on experience.

In case of V shaped groove of the cracks, a hole of 20 mm diameter upto a depth of 12 to 25 mm below the top of the V grooved section should be drilled into the crack and A tire valve stern is bonded with an epoxy adhesive in the hole. In case of other shaped cracks than V shaped groove, the entry port is provided by bonding a fitting having a hat shaped cross section with an opening at top to fill the adhesive. This fitting is kept flush with the concrete face over the crack.

3. Mixing of the Epoxy:

The mixing can be done either by batch or continuous method. In batch mixing the adhesive components are premixed in specified proportions with a mechanical stirrer in quan­tities which can be used prior to the commencement of the curing of the material. With the curing of the material the pressure injection becomes more difficult. In the continuous mixing system the two liquid adhesive components pass through metering and driving pumps prior to passing through an automatic mixing head. The continuous mixing system allows the use of fast setting adhesives that have short working life.

4. Injection of Epoxy:

In its simplest form, the injection equipment is consisted of a reservoir or funnel attached to a long flexible tube. This system provides a gravity head to the flowing material. For small quantities of repair materials, usually the small hand held guns are the most economical. A steady pressure can be maintained by these small guns which reduces the chances of damage to the surface seal.

For big jobs often power driven pumps are used for injection. Pressure to be applied for injecting the material should be selected carefully. The use of excessive pressure may multiply the cracks causing additional damage. The injection pressures governed by the depth and width of the cracks, the viscosity of resins and seldom exceed 0.1 MPa.

Fine cracks preferably are injected under low pressure in order to allow the material to be drawn into the concrete by capillary actin. To increase the injection pressure during the course of work the common practice is to overcome the increased resistance against flow as crack is filled with material. For relatively wider cracks gravity head of few centimeters may be enough.

In case of vertical and inclined surfaces, the injection process should start by injecting epoxy into the entry port at the lowest level until the epoxy level reaches the entry port above. Then the injection tube is removed and the lower entry port is capped. For forcing the epoxy into fine hair line cracks, a pressure upto 0.7 MPa can be applied on the port from which the injection tube has been removed for a period of 1 to 10 minutes using an inert gas. The process is repeated at the successively higher ports until the cracks have been fully filled and all ports capped.

For horizontal cracks, the injection should start from one end of the crack to the other in the same man­ner. If the pressure maintained remains constant, it indicates that the crack is full. In case pressure does not remain constant, it indicates that the epoxy is still flowing into un-filled portions or leaking out of the crack.

5. Removal of Surface Seal:

After curing of the injected epoxy, the surface seal may be removed either by grinding or by other suitable means. Fittings and holes at the entry ports should be painted with an epoxy patching compound.

6. Health and Safety Precautions:

The epoxy materials are toxic and skin irritants. Hence their skin contact, inhalation of their vapours and ingestion must always be avoided.

Following precautions must be observed:

(a) Full face should be covered and goggles should be used during all the mixing and blending operations.

(b) Rubber gloves, polythene or protective overall should be used.

(c) For the protection of skin, skin cream may be used.

(d) Adequate fire protection should be provided.

Common Types of Repairs in Concrete Structures:

i. Sealing of Cracks:

Every concrete structure has joints and cracks hence joints and crack sealers are very important in concrete structures. The crack sealers should ensure the structural integrity and service­ability. They should also provide protection from penetration or ingress of harmful liquids and gases.

ii. Method of Sealing:

For sealing the crack, it should be enlarged along its length on the exposed surface. The process of doing the enlargement of the crack is known as chasing or routing the crack. After doing the enlargement of the crack, it is sealed with a suitable joint sealer as shown in Fig.26.11.

 

Non routing operation may affect the permanency of the repair. The routing operation consists of cutting a sufficiently large groove to receive the sealer on the surface. The groove may be cut by a concrete saw or hand tools. The minimum surface width of routing of 6 mm is sufficient as repair of narrower grooves is difficult. The surface of the routed joint should be cleaned with air jet and allowed to dry before placing the sealer or sealant.

Functions of the Sealant:

Following are the functions of the sealant or scalar:

1. To prevent water from leaking to reinforcement.

2. To prevent the development of hydrostatic pressure within the joint.

3. To check the development of stains on the surface of the concrete.

4. To prevents the development of moisture problems on the far side of the member.

Often epoxy compounds are used as sealant material. When appearance is not important and water tightness of the joint is not required, hot poured joint sealants are used. Urethanes which remains flexible over a wide range of temperature, have been used successfully for sealing the cracks upto 20 mm in width and of considerable depth.

Flexible Scaling:

For repairing an active crack, it is necessary to pro­vide for its continuing move­ment i.e. its movement is not checked. This can be achie­ved by following the crack along its length, the process is known as to rout or chase the crack. The surface of the crack is prepared as discus­sed above and it is filled with a suitable field moulded flexible sealant. The strain capacity of the sealant should be of the same order as that of the crack material.

A wide crack spreads movement over a greater width so that the resulting strain is compatible with sealant to be used. The sealant must adhere to the sides of the chase or rout, but it should not be bonded from its bottom, so that the movement in the crack spreads over the full width of the chase. This can be achieved by providing a bond breaker or de-bonding strips of a material such as polythene or pressure sensitive tape at the bottom of the chase before applying the sealant.

This de-bonding strip does not bond the sealant during or before the cure or hardening of the sealant and allows the sealant to change its shape without stress concentration at the bottom. A sectional view of a typical movement joint or flexible sealing is shown in Fig. 26.12.With an increase in chase width, the crack movement which induces tension or shear in sealant will exert considerably reduced stress on the adhesive interface with the concrete, enabling the face seal to cope with the extensive movements.

 

Providing Additional Steel:

The cracked reinforced concrete elements, usually bridge decks can be repaired successfully by using epoxy injection and reinforcement bars.

The procedure is as follows:

(a) 1st the crack is sealed by epoxy injection.

(b) After sealing the crack, holes of 20 mm diameter are drilled at 45° to the element surface and crossing the crack plane approximately at right angle.

 

(c) 12 to 16 mm diameter reinforcing bars are placed in the drilled holes. These bars should extend at least 50 cms on each side of the crack. The spacing of these bars may be adopted as per need or as per design.

(d) The holes and crack plane is filled with an epoxy injection under low pressure varying from 0.35 to 0.55 MPa. The epoxy bonds the bars to the sides of the hole and fills the crack plane. In this way the cracked concrete surface becomes to a monolithic form.

For a successful repair, an elastic exterior crack sealant is required. For this purpose gel type epoxy crack sealants have been found use-full. These sealants should be applied in a uniform layer of 1.5 to 2.5mm thickness extending upto approximately 20 mm on each side of the crack.

For local strengthening, resin bonding of flat steel plates to the external surface of the critical structural member of the bridge or building has been found most practical and economical.

Stitching of Cracks:

Stitching of cracks process is used when the tensile strength of the member is to be restored across the major cracks. Stitching does not close the crack, it only prevents it from further spreading. Stitching tends to stiffen the structure which may concentrate the overall structural restraint, developing cracks in concrete somewhere else. Thus it is necessary that the adjacent section to the stitching should be strengthened by using external reinforcement embedded in a suitable overlay.

Procedure of Stitching:

A stitching dog is a U shaped metal unit with short legs.

The procedure is as follows:

(a) The holes are drilled on both sides of the crack in a staggered way.

(b) The holes are fully cleaned by air jet or otherwise.

(c) Now the stitching dogs are placed in the holes, so prep­ared across the crack.

(d) The holes with stitching dogs are filled with either non shrinkable grout or with an epoxy resin based bonding system.

The stitching dog should be variable in length and orientation or both. It should be so located that the tension transmitted across the crack is not concentrated to a single plane with in the section, but spreads over a large area. The spacing of stitch­ing dogs near the ends of the crack should be reduced.

In case of bending members, the stitching should be done on the tension face where the movement is taking place. In case the member is under axial tension then the stitch­ing dogs should be placed symme­trically even if demolition or exca­vation has to be resorted to gain access to opposite side of the section.

In this case the stitching dogs should be relatively thin and long. These dogs cannot take much com­pressive force. In case the crack has a tendency to close as well as to open or spread, the dogs must be strengthened by encasement in an overlay. In case of water retaining structures, the cracks first are made water tight before starting the stitching.

The remedial measures for repairing the structural cracking of a slab and beam are shown in Fig. 26.15. Fig. 26.15 (a) shows the method of repair the cracking in slab while Fig. 26.15 (b) shows the method of correction of cracking beam.

Repair by Jacketing:

This method of repair is useful for the compression member as columns and piles, piers etc. In this case a durable material is fastened over the existing concrete and the gap is filled with a grout. The grout filled provides the needed performance characteristics. Thus jacketing restores or increases the section of an existing member by encasement in a new concrete. This technique is applicable to protect the member against further deterioration as well as for strengthening it. In either case the concrete section is increased beyond the designed value to allow for some future deterioration.