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Concrete exposed to ammonium nitrate and its repair strategy

Abstract: Concrete structure in chemical Industry is exposed to many different severe and aggressive corrosive conditions. Corrosive chemicals contaminate concrete structure & sometimes, such deterioration leads to disaster. Effects of chlorides, sulfates, fluorides & phosphates on concrete have been analyzed in depth till date but very few observations for effects of ammonium nitrate on concrete are analyzed. This paper deals with deterioration observed due to effects of ammonium nitrate on concrete, the deterioration process, principles of structural rehabilitation as a whole & remedial measures for repair & protection of damaged structure due to this effect.

Introduction

Calcium Ammonium Nitrate (CAN) is a physical mixture of ammonium nitrate (AN) & lime mixed at a particular temperature and pressure to form granules. As the mixture is not a chemical reaction, lime & ammonium nitrate are freely available in its granules. Lime is inert & remains in dormant condition as far as the effect on concrete structure is concerned, but ammonium nitrate reacts with hydration products of concrete & deteriorates concrete. Reaction mechanism between concrete & CAN shall be discussed in later part of the paper. CAN also corrodes reinforcement of concrete.

Observations

 

Er. Chirag K Baxi is seen behind reinforcement bars under helmet
Er. Chirag K Baxi is seen behind reinforcement bars under helmet

Distinct observations of damages due to the attack of ammonium nitrate include the following.

– Losing of cement slurry on the surface of concrete element giving honeycomb appearance and loss of integrity of the external surface of the damaged element.
– Cracks on edges of RC member with progressive widening,
– Concrete in cover portion started sounding hollow,
– De-bonding of cover mass from the core mass of concrete.
– Watery droplets containing CAN around damaged elements.
– Spalling of concrete, – Rusting of reinforcement,
– Leaching of liquid, – Progressive reduction in strength measured with NDT.

Causes of Damage

The damages & deterioration observed on RC surface were correlated with process reactions and it was established that the damages were only due to CAN attack.

Being hygroscopic in nature, CAN, accumulated on RC surface attracted moisture from the atmosphere, settled on the external surface of the element, damaged the exterior cement slurry layer and penetrated inside the concrete mass of an element through micro pores of the concrete surface which was no more having sound integrity. Following chemical reaction took place.

CaCO3 + NH4NO3 = CAN
(25 to 28%) (72 to 75%)

CAN reacts further like this,

Ca (OH)2* + 2NH4NO3 = Ca (NO3) + 2NH4OH + Calcium Nitro Aluminate. (* hydration product of concrete)

Thus ingredients of concrete / cement, mainly Ca (OH)2, are converted to Ca (NO3)2 which is a disintegration of cement resulting into loss of properties of binding & strength. This is a reversible reaction which goes on endlessly and deteriorates concrete mass by reducing effects of Alumina which is responsible for providing binding properties to cement. Another cause of decay is the expansive stresses generated due to corrosion of reinforcing steel increasing their volume by approximately 2.5 times.

CAN dust flying in the atmosphere also causes abrasion on the exterior surface of the concrete element and erodes cement slurry which forms a finishing layer on the topmost portion of the concrete member. It leaves coarse aggregates exposed to give honeycomb type appearance. Such situation also allows CAN particles penetrate through the core of concrete.

The entire process can be briefly described as follows.

– CAN dust generation.
– Airborne CAN particles containing Ammonium Nitrate, settling on concrete surface
– Ammonium Nitrate is highly hygroscopic in nature.
– Ammonium Nitrate solution is formed.
– Reaction with lime (liberated during cement hydration).
– Calcium salt + C2A (Calcium Aluminite Hydrate)
– Hydrates of Calcium Nitro Aluminate.
– Deterioration in properties of cement like bonding & strength

Once these damages are observed, they are to be scientifically analyzed to quantify them in measurable mode. Non-Destructive Testing including tests like rebound hammer test, Ultra Sound Pulse Velocity (USPV) test, Windsor probe test & half cell potential test help to know the depth, volume and total quantification of damages.

Method & material for repairs:

Considering the loss of integrity which is one of the major after-effect of said damages, repair method involving polymer concrete is the most suitable for repairing damaged concrete elements under above-mentioned conditions. The bond between the surface of original member and repair mass is an important factor. Instead of relying on the chemical bond like bonding agents, it is advisable to go for the mechanical bond like shear connector placed at calculated distances in both directions of the element under repair. If the repair activity is to be done in the area where CAN dust is flying in the air around the element under repair, then de-shuttering needs to be done only after completion of the curing period to protect the external surface of repaired member from the attack of airborne CAN dust. Protection of external surface of an element after the repair is advisable to be carried out with water-borne coating because humidity would still be there within the elements which has been recently repaired and hence conditions would be conducive for waterborne or water cured coating to be effectively functional.

Before proceeding further, some basic principles of structural rehabilitation must be refreshed.

What is structural rehabilitation? – Principles of Structural rehabilitation:

It is the group of activities required for restoring a damaged, deteriorated or under-performing structure. Such a structure which is either damaged or deteriorated and which is not performing as per its functional needs is required to be either restored or to be reconstructed. A reconstruction is an easy option because no expert skills are required here but to rehabilitate a damaged structure is certainly a task which needs a combination of following four aspects in appropriate proportion.

(1) Technical skill,
(2) Theoretical knowledge of designing and detailing,
(3) Practical experience
(4) Courage to implement the restoration scheme.

It is important to know different types of damages a structure can undergo! They are listed below.

– Earth-quake, – Flood, – Tsunami / Hurricane or such disaster
– Chemical attack, – Terrorist attack, – Explosion, – Biological factors, – Natural degradation including weathering effects

There are different methods to restore the damaged structure. Restoration strategy should be based on following:

(A) Safety / environmental / health hazard should never be created while implementing the rehabilitation activity. Finally selected scheme should be very safe and should not invite any hazard or accident. Implementation of the scheme should be made in such a way that at no stage of its execution, an environmental hazard is generated causing environmental imbalance like a generation of dust, generation of fumes, generation of in disposable waste etc. The material and its application scheme should be such that it should not create any health hazard to the workmen who are working for the activity. There are some materials which have to be handled with delicacy and care to see that they do not generate harmful dust or fume which can cause the health hazard. Resin – Hardener mixing should be done very carefully because the mistake in mixing operation sequence can cause burn hazard.
(B) The economy should be carefully considered while finalizing rehabilitation scheme. Sometimes expensive solutions are available but if they are at high cost, they might invite a second thought before they are implemented. All those rehabilitation schemes which are technically excellent need not be the most economical. Hence a balance is to be struck between technical excellence and cost economy. The importance of structure should be appropriately considered with respect to its non-availability due to its repair period and also with respect to its rehabilitation cost. Here an optimum balance decided the viability of executing the scheme.
(C) Availability of materials selected for rehabilitation scheme is an important factor. Once the scheme is conceived, it should be ensured that all the materials to be used for the scheme are available in their envisaged quantity. If some material becomes non-available during the execution; finding out its compatible option is very difficult.
(D It is necessary to have an integrated idea about the deteriorated structure with help of NDT (NonDestructive Testing). This helps to know the actual weaknesses with their location and quantity in the structure which helps in selection of an appropriate method of rehabilitation.
(E) Selection of appropriate construction chemical is very important. The material property, its chemical compatibility with an external and internal environment, its exothermic behavior, its pot-life, its consumption pattern etc are to be closely examined before finally selecting the construction chemical. Selection of one improper material can cause failure of the rehab scheme.
(F) It is always to be ensured that the fresh / additional mass which is used for rehabilitation purpose should have identical physical and thermal properties as the material of member under repair. Fresh / repair mass should expand and contract in the same way as the material of member under repair. The density of repair material also should be same or similar to the member under repair. In short, fresh / repair mass should not become an additional burden on the original mother structure even by virtue of its properties. This is important because the damaged member is already weak and in addition if the repair mass becomes a burden, it may bring further weakness in the originally damaged structure. It is a general belief that repairing is to be done with high-grade material and strong method. This may prove to be fatal if the extra high grade of the material becomes the burden on the original structure.
(G) In a case of a build-up of additional mass around original damage structure, it should be ensured that a load of this additional mass should invariably go to its foundation. The additional mass should not hang on the original structure by creating cantilever effect. This practice further weakens the structure.
(H) Wherever high-pressure applications are to be involved in rehabilitation scheme, they should be critically reviewed before implementing them in practice. Sometimes high-pressure applications can further aggravate internal crevices of the originally damaged structure and can lead to further damage. Though the crevices would be finally filled with the appropriate grout but first making the weak structure further weak and then to strengthen it with high-pressure technique does not really strengthen the structure.
(I) The rehabilitated structure must be protected externally with the suitable protective coating. This ensures the preservation of the actions taken for rehabilitation and finally ensures the functional success of rehabilitation activity. This protective coating which can also be designed as “Sacrificial coating” which can be sacrificed over a period of time depending on its exposure to the respective environment, and it can be again reapplied after regular intervals so that the repaired structure remains protected from external factors and confirms a desired functional life of rehabilitation scheme.

Different methods of rehabilitation are mentioned below.
– Normal plastering, – Jacketing, – Reinstatement
– Guniting, – Pressure Grouting

Normal Plastering

Here the damaged part is removed from the structure and the space created by this removal is then replaced by normal plastering consisting of cement mortar with appropriate construction chemicals. Sometimes this plastering is carried out even with epoxies or special polymers depending upon end requirements. Normally this activity needs average skill. This method is adopted in residential, commercial or recreational areas where load bearing capacity / strength is an insignificant criterion as compared to cosmetic considerations. However, this can’t be generalized. The repair scheme has to be decided based on the findings of a close inspection of the damaged structure and preferably after its testing is done and results are properly analyzed. As such it is always advisable to provide protective treatment suitable to the external environment.

Jacketing

Normally this is adopted where the structure is buckled. When the columns are found tilted or bent from any part of its height, they are required to be protected with a suitable treatment which not only holds them in their original position but also stops them from getting further tilted or buckled. This method consists of providing an additional layer of concrete with suitable construction chemical over the originally damaged surface after making arrangements for properly reinforcing the additional layer and its proper bonding arrangement with original surface. Special care should be taken to see that this additional reinforcement must be connected with the base of the structure under repair to ensure that a load of the self-weight of the additional mass is transferred to the foundation and not on the column or structure under repair. Bond coat of suitable material considering its setting time, curing time, bond strength, reactivity with the surrounding environment etc. is to be applied prior to fixing the reinforcement. Watertight shuttering is essential to confirm that there Is no leakage of cement slurry from the shuttering during vibration of concrete while it is placed in the shuttering. It brings surface porosity which is very dangerous for any structure because it gives an avenue to all foreign contaminant to penetrate inside the element and then its deteriorating effects get activated. Loss of cement slurry means loss of water from water cement ratio. This affects the bonding properties of cement. In short, watertight shuttering is the prime important requirement under any rehabilitation activity.

Guniting

This method has been in practice for a long time. Here the mortar of suitable composition is spread over the surface of the structure under adequate pressure which is to be calculated in advance with proper calculations so that the pressure is just enough to spray the repair mortar under pressure and on the other side, to see that the member is not subjected to unnecessary pressure which can further damage it instead of strengthening it. The composition of the repair mortar needs to be such that it flows and spreads freely over the surface of the element to be repaired. Its setting time should match the sequence of guniting operation i.e. neither it should set during its spraying process nor should it remain unset after the guniting process is completed. The gunited surface is finally to be finished smooth or kept rough as per the site requirement. However, it is also to be coated with the protective coating to confirm desired performance of the guniting activity. This is a good method of rehabilitation with the major limitation about the compatibility of the method for the element to be rehabilitated with respect to its residual strength to sustain the pressure of guniting.

Pressure Grouting

This is widely adopted method of rehabilitation as a part treatment. It is always combined with subsequent treatment to give composite success. This method is normally adopted when the damaged member is found having crevices or honeycombing within the member under NDT. Repairing of these small crevices within the member always requires each smallest gap to be filled with the suitable material which behaves like the mother element. The material selected for this purpose should have (a) elasto-meric property, (b) same density as mother element and (c) compatible with the thermal properties of mother material in addition to the specific properties with respect to the purpose of rehabilitation of that member.

Here appropriate material is injected inside the member to be rehabilitated by providing nipples of adequate diameter and at pre-decided c/c distances. The pressure with which the grout is injected inside the member is very important. It should neither be very high such that the crevices are widened under the pressure nor should be very low such that the grout does not reach the farthest point inside the crevices. The direction from which the injection operation is carried out is also significant.

Reinstatement

This method consists of reinstalling of fresh concrete with suitable construction chemicals. This method is similar to jacketing method but with the difference that jacketing method. All care as discussed for earlier methods is also to be taken here. Special care to be considered here is the workability of concrete or the material which is to be used for reinstatement purpose. The consistency of the material should be good and it should be easily workable i.e. it should be able to flow freely even under the small spaces through which it is poured into shutters.

These are some of the methods for rehabilitation of an underperforming structure. All these methods have specific considerations under which they can be adopted and implemented. In few the cases, only one method may not suffice to achieve the end requirement. In that case, the combination of the different methods in appropriate proportion is to be adopted to achieve the complete success.

Some highly skilled and specialized methods are there to rehabilitate a damaged structure but they are not discussed here because they are tailor-made methods.

Final method of structural rehabilitation for CAN affected members:

Finally implemented repair scheme for CAN affected area is as mentioned below.

1. Removal of loose and damaged concrete including loose lattices and remains of damaged concrete.
2. Cleaning the surface with a wire brush and water jetting to ensure thorough cleaning till a hard, sound and original surface is reached. However, care is to be taken to see that at no time, more than half of the depth of that element was not removed by this dismantling.
3. Rust from the corroded reinforcement bars is to be removed & the bars are to be thoroughly cleaned followed by application of appropriate rust converter / primer / passivating coat (to improve alkalinity around the bars and exposed surface) is done on reinforcement for their protection against corrosion.
4. Pressure grouting is carried out at pre-decided locations, by drilling holes of 12 mm diameter, fixing nipples and pumping the grout with 5 to 6 Kg pressure. Low viscosity high molecular weight polymer grout material is preferred for such applications where damage is high. The grouting is to be continued till droplets of residual grout liquid start coming out at the other end. Once the grouting is complete, the open ends of nipples are to be plugged.
5. Shear connectors are then provided on the exposed surface by drilling holes at 150 mm c/c locations in both horizontal and vertical directions (it can be at suitable distance depending on the structural requirement of bond between old material and fresh repair mass). Then 12 mm diameter bar is fixed with high strength anticorrosive grout in every hole.
6. Additional reinforcement bars (if existing reinforcement bars are corroded and thinned out such that they are no more capable of carrying any tensile load) are to be provided as per structural design. These bars shall also be coated with rust converter cum primer as applied in step No. 02.
7. Then a perfect watertight and well supported shuttering around the element under repair to give finally desired shape of that element is to be provided. Care is specifically taken to see that no slurry drops out during pouring of repair concrete.
8. Next activity is to lay high performance free flow polymer concrete having following properties inside the shutters. Polymer concrete with water as specified by manufacturer is to be prepared and laid within shutters. Care is to be taken to properly vibrate and compact it to achieve its maximum density as specified by the manufacturer.
– Polymer concrete should have flowable consistency.
– Cured mass should not contain any metallic aggregates or chlorides.
– It possesses shrinkage compensation in plastic stage.
– It has coefficient of thermal expansion similar to that of the mother concrete.
– Physical properties of the same are as mentioned below.
– Compressive strength at 3 days : 30 N/mm^2
– Tensile strength at 28 days : 02 N/mm^2
– Flexural strength at 28 days : 05 N/mm^2
– Wet density {fresh} : 2100 to 2200 kg/cm^2.
9. Once the concreting is completed, de-shuttering is done after its curing period mentioned by manufacturer is over. After de-shuttering, entire surface is wiped, cleaned and made ready for receiving protective coating.
10. Finally water based anti-carbonation protective coating is to be applied in requisite coats on the surface after proper surface preparation as specified by manufacturer. The coated surface should give uniform look to avoid marks of brush application.

Repair of damaged concrete elements due to Ammonium Nitrate has been carried out with the above mentioned repair method. It has successfully performed for a period of 15 years. No sign of any deterioration in rehabilitated members is observed on the surface. NDTs at regular intervals have been carried out to ensure the successful performance of the repair method and have yielded positive and confirmatory results.

Probable precautions which can prevent damage

Prevention is always better than cure & hence the precautions which can prevent the concrete members from getting affected by CAN attack as mentioned below.

Coating of external surface

External surface of the RC member should be coated with preventive or sacrificial coating to protect it from entry of CAN / AN particles in it.

Coating top of RC floor

Top of production floor should also be coated with impervious monolithic lining to stop penetration of CAN into the floor.

Slope on RC floor

Concrete floor should be provided with gentle slope to allow water accumulated on the floor by way of hygroscopicity of CAN or by way of washing of equipment / piping / floor; to flow away. By doing so, water coming on the floor due to such activities would not stagnate on the floor for long time and penetrate into floor & damage it.

Reduced cut outs in RC slab

Concrete floor should have minimum number of cut outs for allowing routing of piping and ducts. This is because cut outs become a major source of leaching or leakage of liquid. It would penetrate into surface & travel in horizontal direction which is extremely dangerous.

Conclusion

Concrete is a boon for Civil Engineering Industry. However its contamination – often spoils it & makes it unusable. Chlorides & Sulphates are well known contaminants to deteriorate concrete but ammonium nitrate (AN) is several times more corrosive as compared to chlorides & sulphates due to depolarizing abilities of NO3 ions present in AN. Though AN is a comparatively less known contaminant for concrete, it possesses high contaminating effects.

 

chirag-k-baxi

Er. Chirag K Baxi
Gujarat Narmada valley Fertilizers and Chemicals Limited.
Bharuch, Gujarat, India.

1 COMMENT

  1. I need a successful solution on concrete and wall plaster repair in AN Stoe. and where Ammonium Nitrate solution has been contaminated/ingress.

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