Condition Assessment and Distress-diagnostic Techniques
This paper discusses the critical requirement of the correct condition assessment of concrete structures and an open minded approach to assess the cause of structural distresses and related symptoms. What is visible to one is only the symptom. This will enable identification of the problem in the first place. This in turn will lead to the correct solution to the distress of the structure. The diagnosis has to be based on what is visible and duly correlated to the test reports and what one perceives. There are many possible causes that can cause a particular symptom and one has to be methodical and logical in one’s approach to narrow down the possibilities to arrive at the right cause. This paper also shows some of the interesting assessments that the author has made which do not fall under the usual ones.
N S Moorthy, DicoTech LLC, Concrete Repairs and Engineering, UAE
And the cracks could be due to many reasons. Shrinkage cracks, plastic shrinkage cracks, plastic settlement cracks, early heat of hydration cracks, long-term thermal shrinkage cracks, excess stresses in flexure, tensile, shear, punching etc. are the nature of cracks to name a few. It could also be due to corrosion of reinforcement or unequal foundation settlement.
Besides cracks, there are other symptoms of leakages, seepages, discoloration and pigmentation of concrete. i.e. stains on concrete could also lead to important conclusions.
During visual inspection, the pattern of the crack and the width of the cracks are to be documented on floor plans of structure. This is called the crack mapping. The crack mapping could be further enlarged for better understanding for critical areas by marking the grid both on the paper and on the cracked structure and transfer the positions of the cracks on to the paper. The width of the cracks is to be measured using a crack width measuring gauge. If both sides of the structure are accessible, it can be checked whether the cracks are through and through. If it is not accessible, then ultrasonic tests are conducted to determine the depth of the cracks. GPR (Ground Penetration Radar) could also be used for determining the depth of the cracks.
It is also important to decide whether the cracks are active or dormant. A glass strip fixed across the crack (Fig.2) shall determine the status. If the glass is broken, then the crack is active, if not it is dormant.
1. If the cracks are at regular intervals, some- times even through and through, such as in a retaining wall – inject the cracks using epoxy resin and provide control joints.
2. If the cracks are of irregular pattern such as shown in Fig.1, it is most probably plastic shrinkage. A simpler surface repair such as grouting will do (Fig.3).
3. If the cracks are wider at the top and extends only up to the reinforcements, and mainly along the reinfor- cement then it could be due to plastic settlement. Again grouting will do the job.
4. If it is early heat of hydration and long-term thermal cracks, the pattern would be the same as that of item 1 and the treatment will be the same as well, but with one important addition which is introduction of control joints.
5. If it is structural crack due to excessive stresses then structural strengthening could have a number of options including using carbon fibres.
6. The treatment for the cracks due to the corrosion of reinforcement is dealt elsewhere in this paper. But never an epoxy injection to such cracks. These need a special treatment.
Make sure that the ponded water, if any, is because of the leaks. Sometimes this could be the water due to washing of the floor!
Check whether it is coming through the cracks in a clean line or else it is a damp patch. Many a times, it could be the construction/cold joint which is the weakest portion for such kind of leaks. Honeycombs could be the reason for the damp patches. Check if there are any expansion joints that could be leaking. The formation of the salt crystals due to honeycombing would give an indication of the duration of the problem. Always touch to check whether the seemingly obvious leaking points are still leaking. At times, it would have healed by itself.
Sometimes, it could be necessary to test the leaking water to find out the source of leakage. It could be a drinking water line or a sewage line that has burst or could be the ground water.
Check the quality of the concrete by visual and hammer sound testing. If the structure is of poor quality and honeycombed, the possibility of the leaks to spring elsewhere, once the existing ones are closed, is very high.
1. If it is a crack, normally pressure injection using Polyurethane or Acrylic resins would solve the problem. When a construction joint is treated, make sure that the entire lengths of the joint are treated even if the water is coming selectively in portions.
2. If it is a damp patch and the concrete is generally good, then the choice could be between pressure injection or negative waterproofing using crystalline technologies.
3. If the concrete is porous and not of a good quality and the possibility of water to leak through elsewhere, then go in for crystalline treatment without resorting to other polyurethane or other resins (fig 5). Once the concrete is contaminated with Polyurethane or other resins, it would be difficult to go in for crystallization treatment to resolve the problem.
4. Treatment to an expansion joint is done by sealing the joint with an acrylic or polyurethane resin (injection yes, but not necessarily a high pressure injection) to a depth of about 10 to 15 cm and the top of the joint is resealed by using hypalon tape.
Look for the size of the patch. Look for any steel or other metal fixtures nearby which could have caused the stain and not necessarily the reinforcement.
Look for any slight bulge around the satin which could have been caused by the expansion of the rust in the reinforcement. This will show whether it is nearer to spall the concrete.
Hammer sounding test will show the delaminated (but not spalled) areas which is clear indication of corrosion of reinforcement.
Measuring the extent of corrosion and its plotting on floor plans is an important task to be done during condition assessment. Half-cell potential test is the most common one which indicates the extent and the probability of corrosion.
Then come the spalls (Fig.7). It is the splitting of concrete due to the increase in volume of the reinforcing steel because of an advanced stage of corrosion. The corroded reinforcement gets exposed.
Visual inspection will show the extent of corrosion spalling. In some cases, one can find only the traces of reinforcement as the reinforcement is lost due to corrosion.
Cores are taken to determine the actual compressive strength of the concrete and for assessment depth of carbonation i.e. the depth of chemically deteriorated concrete).
The powder samples of concrete are taken at various depths by drilling to determine the chloride and Sulphate profiles. Again this will be necessary to determine the repair strategy.
This was a precast framed structure for a shopping mall in the Middle East. The precast elements of columns and beams with facias were designed, produced and erected by an internationally renowned firm. The columns were erected and the beams were placed in position on the corbels. The problem started when the structure was loaded with the façade panels. Almost all the corbels cracked (Fig.8) and the construction was stopped.
The visual inspection showed that the cracks have opened at the top and narrowed as it went further down. So it is a crack caused due to flexure on the corbel. The load transmission was by a kind of point load through the shims kept in position to adjust the levels during erection. The point of transmission of load appeared to be more towards the edge of the corbel. So, this was an error of erection. But that was not the only error that happened. When the precast elements were produced in the factory, a steel plate was embedded in the corbel to indicate the erection crew, the locations of the leveling shim for the beam. The placement of the beams was wrong. So the production could be wrong. But not really. The production follows a shop drawing which indicates clearly where the embed had to be placed. The original mistake has happened with the draftsman who had put in a wrong dimension.
The beams were slightly jacked up to relieve the corbels from the loads. The existing cracks have been sealed by pressure injection. The joint between the corbels and the beams were sealed. An epoxy resin with 100% solids was injected at low pressures to create a solid and uniform bed to distribute the load over a large area. Once the epoxy resin was cured, the beams after its structural retrofitting and shifting the loading point to safer turfs, were allowed to rest. The structure is still standing after 20 years of this operation.
It is a case of a power generating station. The turbines were seated on heavy foundation pedestals. The flue tube which was about a meter dia was also resting on two pedestal foundations. The power station was stopped after 100 hours of commissioning due to cracks developed on the flue tube pedestals and also the grout bedding under the foundation plate had turned black. Despite re-doing the grouting and injecting the crack, the problems again resurfaced after running the turbines for another 100 hours or so. That was when the problem was referred. The second problem of grout bleeding under foundation plate was easier to identify. An epoxy grout had been used under the base plates. But the epoxy grout had a limitation of higher temperature more than 70c. The flue gas temperature which can reach several multiples of this have obviously burnt the epoxy grout. It was suggested that a cementitious grout of high compressive strength shall be used instead of an epoxy grout. But the cracks on the pedestals were a bit challenging. The cracks have started at the anchor bolt points and wider at the top and narrower as it went down. So it was the lateral pressure exerted on the anchor bolts that could have been the reason. This pressure could come only from the movement of the flue tube. But it is designed for movement. There are two base plates on top of each other. The top plate is rigidly fixed to the flue tube while the bottom one is fixed to the pedestal. The interface between the plates is to be greased to facilitate the movement. It was out in the site that it was not so and grease port was even blocked. Hence, it was advised that the ports are greased periodically and the repairs for the cracks were carried out.
Bomb shelters are structures where the fighter planes are parked for protection against the debris due to bombs and its explosion. The structure is parabolic with 50cm of sand encased by concrete on both sides. The thickness of the concrete casing was 1m external and 50cm internal (Fig.10) There was water leaking through the expansion joints like coming out of a tap. So the first checking to be done was all the plumbing lines. But the lines showed no drop in pressure which showed that the plumbing was not the reason.
The roof had an expansion joint which was sealed by a polysulphide mastic sealant. The joint width was about 10cm (!) as against the 20mm designed (Fig.11).
The polysulphide sealant had given way and in many places come off the sides of the concrete. Whenever there had been rain in that area, the rain water has gone in and collected in the voids of the sand cushion in-between the concrete casings. Even after 3 months after the rain has stopped, the collected water was coming out of the expansion joints by the sides. The collected water around the bunker has caused heightened ambient relative humidity. This in turn and in course of time started the corrosion of the reinforcement. The concrete started splitting and the bomb shelter faced the bleak prospect of falling apart without bombs (Fig.12).
The only remedy was sealing of the expansion joints on the top, but this time using a hypalon tape system which is suitable for higher joint widths. The expansion joints at the sides were injected with polyurethane resin to stop the water and the joints sealed with hypalon tape system.
This was a discharge channel in the process area in a petrochemical plant. The discharge was a combination of EDC (Ethelene di Chloride) and Hydrochloric acid. The discharge had started leaking through the damaged joint sealants (Fig.13) and into the soil. There was a pump and compressor area nearby. The EDC which has gone into the soil had eaten away the construction fillings under the pump foundations and the area started to settle down. The situation was alarming and it might cause the shutdown of the plant. The existing sealant was good enough to resist the combination of this chemical. But it was not elastic enough to resist the movements of the channel. The expansion joints have given way and hence the leakage. The requirement of the client was a sealant that shall resist 100% concentrated EDC and a 15% concentration of HCL both combined at a temperature of 100oC. The material suitable to resist EDC, would not resist HCL and vice versa. Even with a material able to resist both then, the temperature was a problem. If all these conditions were satisfied then it was not flexible enough which could be laid in the channel to remain in good shape. The finding and the subsequent thought process made the team to invent a joint sealant using non-elastomeric HDPE to take the expansion and contraction and also to not to give way.
But why on one and why not on the other? One side of the car park had a mosque, which was of low height and plenty of space around. But on the other side, there were high rise buildings. The Sun was shining on the car park on the mosque side and was shaded constantly on the other side. The result: the ramp on the mosque side was subjected to heightened temperature variations, whereas the other side was relatively stable.
Having solved the puzzle, the remedy given was to inject the cracks with rigid epoxy resins but cutting out control joints to take care of the movements.
N S Moorthy, DicoTech LLC, Concrete Repairs and Engineering, UAE
Condition Assessment
What is condition assessment? It is systematic and logical examination of a structure to identify the cause of the distress. The examination would cover visual inspection, checking of documents; such as drawings, construction records, previous investigation reports (if any), determination of the time of appearance of the defect and analyzing the test reports. Many times one would come across that many of these may not be available! But still one has to find ways and means of overcoming this.Nature of Distresses
Various kinds of distresses that we come across normally largely cover cracks Fig 1. |
| Figure 1: Cracks |
And the cracks could be due to many reasons. Shrinkage cracks, plastic shrinkage cracks, plastic settlement cracks, early heat of hydration cracks, long-term thermal shrinkage cracks, excess stresses in flexure, tensile, shear, punching etc. are the nature of cracks to name a few. It could also be due to corrosion of reinforcement or unequal foundation settlement.
Besides cracks, there are other symptoms of leakages, seepages, discoloration and pigmentation of concrete. i.e. stains on concrete could also lead to important conclusions.
Cracks
The first inspection to be carried out is the visual inspection which is the case for all the distresses explained.Visual inspection
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| Figure 2: Crack Width Gauge |
 |
| Figure 3: Repaired Surface |
 |
| Figure 4: Leakage of Water |
It is also important to decide whether the cracks are active or dormant. A glass strip fixed across the crack (Fig.2) shall determine the status. If the glass is broken, then the crack is active, if not it is dormant.
Cracks Remedies in brief
Let us look at the remedial measures.1. If the cracks are at regular intervals, some- times even through and through, such as in a retaining wall – inject the cracks using epoxy resin and provide control joints.
2. If the cracks are of irregular pattern such as shown in Fig.1, it is most probably plastic shrinkage. A simpler surface repair such as grouting will do (Fig.3).
3. If the cracks are wider at the top and extends only up to the reinforcements, and mainly along the reinfor- cement then it could be due to plastic settlement. Again grouting will do the job.
4. If it is early heat of hydration and long-term thermal cracks, the pattern would be the same as that of item 1 and the treatment will be the same as well, but with one important addition which is introduction of control joints.
5. If it is structural crack due to excessive stresses then structural strengthening could have a number of options including using carbon fibres.
6. The treatment for the cracks due to the corrosion of reinforcement is dealt elsewhere in this paper. But never an epoxy injection to such cracks. These need a special treatment.
Leakage and Seepage
The second most common one is the leak (Fig.4). Mainly in the roofs and many other water retaining structures such as tanks, swimming pools, podiums, dams etc.Make sure that the ponded water, if any, is because of the leaks. Sometimes this could be the water due to washing of the floor!
Check whether it is coming through the cracks in a clean line or else it is a damp patch. Many a times, it could be the construction/cold joint which is the weakest portion for such kind of leaks. Honeycombs could be the reason for the damp patches. Check if there are any expansion joints that could be leaking. The formation of the salt crystals due to honeycombing would give an indication of the duration of the problem. Always touch to check whether the seemingly obvious leaking points are still leaking. At times, it would have healed by itself.
Sometimes, it could be necessary to test the leaking water to find out the source of leakage. It could be a drinking water line or a sewage line that has burst or could be the ground water.
Check the quality of the concrete by visual and hammer sound testing. If the structure is of poor quality and honeycombed, the possibility of the leaks to spring elsewhere, once the existing ones are closed, is very high.
Leakage Remedies in brief
 |
 |
| Figure 5: Repair of leakage by crystalline treatment | Figure 6: Strains on surface |
1. If it is a crack, normally pressure injection using Polyurethane or Acrylic resins would solve the problem. When a construction joint is treated, make sure that the entire lengths of the joint are treated even if the water is coming selectively in portions.
2. If it is a damp patch and the concrete is generally good, then the choice could be between pressure injection or negative waterproofing using crystalline technologies.
3. If the concrete is porous and not of a good quality and the possibility of water to leak through elsewhere, then go in for crystalline treatment without resorting to other polyurethane or other resins (fig 5). Once the concrete is contaminated with Polyurethane or other resins, it would be difficult to go in for crystallization treatment to resolve the problem.
4. Treatment to an expansion joint is done by sealing the joint with an acrylic or polyurethane resin (injection yes, but not necessarily a high pressure injection) to a depth of about 10 to 15 cm and the top of the joint is resealed by using hypalon tape.
Stains, Corrosion Cracks and Spalling of Concrete
Another common problem noticed is corrosion stains on the concrete (Fig 6). The stains are the alarm bells to indicate that reinforcement has started corroding. The question would remain as to at what stage of corrosion, it is.Look for the size of the patch. Look for any steel or other metal fixtures nearby which could have caused the stain and not necessarily the reinforcement.
Look for any slight bulge around the satin which could have been caused by the expansion of the rust in the reinforcement. This will show whether it is nearer to spall the concrete.
Hammer sounding test will show the delaminated (but not spalled) areas which is clear indication of corrosion of reinforcement.
Measuring the extent of corrosion and its plotting on floor plans is an important task to be done during condition assessment. Half-cell potential test is the most common one which indicates the extent and the probability of corrosion.
Spalls
 |
| Figure 7: Spall of Concrete |
Then come the spalls (Fig.7). It is the splitting of concrete due to the increase in volume of the reinforcing steel because of an advanced stage of corrosion. The corroded reinforcement gets exposed.
Visual inspection will show the extent of corrosion spalling. In some cases, one can find only the traces of reinforcement as the reinforcement is lost due to corrosion.
Cores are taken to determine the actual compressive strength of the concrete and for assessment depth of carbonation i.e. the depth of chemically deteriorated concrete).
The powder samples of concrete are taken at various depths by drilling to determine the chloride and Sulphate profiles. Again this will be necessary to determine the repair strategy.
Remedies in brief
- Identify the areas of corrosion stained delaminated and spalled concrete. Chip off the loose and deteriorated concrete. The chipping shall continue to 25mm behind the reinforcement and 50mm beyond the sign of corrosion. Care shall be taken to cut the edges of repair by at least 5mm to avoid feather edging when the re-profilation is done.
- Grit blast the corroded reinforcement to near white metal condition.
- Apply neat polymer modified cement slurry over the reinforcement to protect from the onslaught of elements.
- Make sure sacrificial anodes are fixed on the reinforcement such as Galvashield to avoid incipient anodic effect.
- Soak the concrete surface to a fully saturated but no standing water condition.
- Carry out the repair and make up the area with micro concreting or shotcreting for the re-profilation. The curing is a must even it is for a limited number of days.
Case Study 1: Eccentric Loading on the Corbels
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| Figure 8: Repair of cracked corbels |
The visual inspection showed that the cracks have opened at the top and narrowed as it went further down. So it is a crack caused due to flexure on the corbel. The load transmission was by a kind of point load through the shims kept in position to adjust the levels during erection. The point of transmission of load appeared to be more towards the edge of the corbel. So, this was an error of erection. But that was not the only error that happened. When the precast elements were produced in the factory, a steel plate was embedded in the corbel to indicate the erection crew, the locations of the leveling shim for the beam. The placement of the beams was wrong. So the production could be wrong. But not really. The production follows a shop drawing which indicates clearly where the embed had to be placed. The original mistake has happened with the draftsman who had put in a wrong dimension.
Remedy
The owner wanted the structure to be rebuilt by the contracting company as he did not want to take any chances with the safety of the public who would be using the shopping mall. The contractor was reluctant to go in for reconstruction. So a solution had to be found out.The beams were slightly jacked up to relieve the corbels from the loads. The existing cracks have been sealed by pressure injection. The joint between the corbels and the beams were sealed. An epoxy resin with 100% solids was injected at low pressures to create a solid and uniform bed to distribute the load over a large area. Once the epoxy resin was cured, the beams after its structural retrofitting and shifting the loading point to safer turfs, were allowed to rest. The structure is still standing after 20 years of this operation.
Case Study 2: Cracks in The Pedestals of Flue Tube
 |
| Figure 9: Schematic layout |
It is a case of a power generating station. The turbines were seated on heavy foundation pedestals. The flue tube which was about a meter dia was also resting on two pedestal foundations. The power station was stopped after 100 hours of commissioning due to cracks developed on the flue tube pedestals and also the grout bedding under the foundation plate had turned black. Despite re-doing the grouting and injecting the crack, the problems again resurfaced after running the turbines for another 100 hours or so. That was when the problem was referred. The second problem of grout bleeding under foundation plate was easier to identify. An epoxy grout had been used under the base plates. But the epoxy grout had a limitation of higher temperature more than 70c. The flue gas temperature which can reach several multiples of this have obviously burnt the epoxy grout. It was suggested that a cementitious grout of high compressive strength shall be used instead of an epoxy grout. But the cracks on the pedestals were a bit challenging. The cracks have started at the anchor bolt points and wider at the top and narrower as it went down. So it was the lateral pressure exerted on the anchor bolts that could have been the reason. This pressure could come only from the movement of the flue tube. But it is designed for movement. There are two base plates on top of each other. The top plate is rigidly fixed to the flue tube while the bottom one is fixed to the pedestal. The interface between the plates is to be greased to facilitate the movement. It was out in the site that it was not so and grease port was even blocked. Hence, it was advised that the ports are greased periodically and the repairs for the cracks were carried out.
Case Study 3: Leakage in Bomb Shelter
 |
| Figure 10: Schematic View of Bomb Shelter |
Bomb shelters are structures where the fighter planes are parked for protection against the debris due to bombs and its explosion. The structure is parabolic with 50cm of sand encased by concrete on both sides. The thickness of the concrete casing was 1m external and 50cm internal (Fig.10) There was water leaking through the expansion joints like coming out of a tap. So the first checking to be done was all the plumbing lines. But the lines showed no drop in pressure which showed that the plumbing was not the reason.
The roof had an expansion joint which was sealed by a polysulphide mastic sealant. The joint width was about 10cm (!) as against the 20mm designed (Fig.11).
 |
| Figure 11: Schematic View of failed expansion joint |
The polysulphide sealant had given way and in many places come off the sides of the concrete. Whenever there had been rain in that area, the rain water has gone in and collected in the voids of the sand cushion in-between the concrete casings. Even after 3 months after the rain has stopped, the collected water was coming out of the expansion joints by the sides. The collected water around the bunker has caused heightened ambient relative humidity. This in turn and in course of time started the corrosion of the reinforcement. The concrete started splitting and the bomb shelter faced the bleak prospect of falling apart without bombs (Fig.12).
 |
| Figure 12: Schematic View of damaged concrete |
The only remedy was sealing of the expansion joints on the top, but this time using a hypalon tape system which is suitable for higher joint widths. The expansion joints at the sides were injected with polyurethane resin to stop the water and the joints sealed with hypalon tape system.
Case Study 4: Leakage Through Joints
 |
| Figure 13: Schematic View of Channel and failed expansion joints |
This was a discharge channel in the process area in a petrochemical plant. The discharge was a combination of EDC (Ethelene di Chloride) and Hydrochloric acid. The discharge had started leaking through the damaged joint sealants (Fig.13) and into the soil. There was a pump and compressor area nearby. The EDC which has gone into the soil had eaten away the construction fillings under the pump foundations and the area started to settle down. The situation was alarming and it might cause the shutdown of the plant. The existing sealant was good enough to resist the combination of this chemical. But it was not elastic enough to resist the movements of the channel. The expansion joints have given way and hence the leakage. The requirement of the client was a sealant that shall resist 100% concentrated EDC and a 15% concentration of HCL both combined at a temperature of 100oC. The material suitable to resist EDC, would not resist HCL and vice versa. Even with a material able to resist both then, the temperature was a problem. If all these conditions were satisfied then it was not flexible enough which could be laid in the channel to remain in good shape. The finding and the subsequent thought process made the team to invent a joint sealant using non-elastomeric HDPE to take the expansion and contraction and also to not to give way.
Case Study 5: Cracks in Car Park
The car park was of 4 levels and the main contractor was in the process of handing over. But was not able to, because there were cracks developed in the ramps (Fig.14). When checked, something peculiar was noticed. The cracks were in the ramps in a kind of regular intervals. But the ramps in the opposite side of the car park floor did not have any crack! All these were done by the same company and at the same time. |
| Figure 14: Schematic view of Cracked Locations |
But why on one and why not on the other? One side of the car park had a mosque, which was of low height and plenty of space around. But on the other side, there were high rise buildings. The Sun was shining on the car park on the mosque side and was shaded constantly on the other side. The result: the ramp on the mosque side was subjected to heightened temperature variations, whereas the other side was relatively stable.
Having solved the puzzle, the remedy given was to inject the cracks with rigid epoxy resins but cutting out control joints to take care of the movements.
Conclusion
An assessor would find that many times the cause of distress could be found by methodical and logical derivations. But some times what one sees and assumes is not what it is. That was why an emphasis has been made to keep an open mind when one goes for a condition survey. It is always good to remember not only to look at the structure but look its surroundings. One has to look for the basic reasons that might have caused the distress. Such as when a roof is leaking, just unblock the drains. This will save the problem of roof leakage and save lot of money to the owner.Reference:
N.S.Moorthy, condition assement and distress diagnoshe techniques, National conf. on repair rehab of cons. strs, 6 & 7 May 2011, Noida, India.Acknowledgment
This article has been reproduced from the proceeding of 'National Conference on Repair & Rehabilitation of Concrete Structures' organized by ICI western U.P Gaziabad, IA Sructural Engg, and Association of Structural Rehabilitation, with the kind permission of the organisers.NBMCW June 2011
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