Surveying Property: Concrete Defects

Showing posts with label Concrete Defects. Show all posts

Sunday, November 16, 2014

Concrete – Part 3 – Understanding the basics - Chloride Attack, Alkaline Silica Reaction, Sulphate Attack & Others

In last week’s article I considered carbonation and in the final part of this ‘mini-series’ I will focus on other concrete defects, such as Chloride Attack, Alkaline Silica Reaction and Sulphate Attack amongst other concrete defects

Source: http://theconstructor.org/

Over the last few weeks I have discussed the wide use of concrete as a construction material, considered its many positive attributes and also explained the vulnerability of concrete to certain defects. In last week’s article I considered carbonation (Link) and in this final part of this ‘mini-series’ I will focus on other concrete defects.

Chloride Attack – Chloride finds its way into and through concrete in a similar process to carbonation, due to its porous nature. Chloride ions which are introduced into concrete from de-icing salts or are within or in close proximity to marine environments can attack concrete aggressively resulting in a faster rate of deterioration compared to carbonation. When chloride passes through the concrete and eventually reaches any reinforcement, corrosion will occur. Salt is a mineral substance which consists primarily of sodium chloride. When the sodium chloride is dissolved in water, which may be present in the pores of concrete, a versatile, highly corrosive and mobile solution is formed of sodium ions (Na+) and chloride ions (Cl-). Once this solution comes into contact with any reinforcement it will attach the passive layer which protects it. The reinforcement will then corrode in the presence of air and water, resulting in corrosion. This will result in cracking and spalling which will appear very similar to the effects of carbonation.

The consequence of chloride attack can be seen on the underside of road bridges and buildings and structures in close proximity to the coast. As discussed above, the impact of chloride attack will appear very similar to carbonation, so it will be necessary to not only consider the environment but also undertake testing to confirm the cause.

Source: Source: http://www.adfil.co.uk/

Alkaline Silica Reaction – A good explanation of alkaline silica reaction is defined by www.lmcc.com/ as ‘Alkali-silica reaction takes place between reactive siliceous minerals in certain aggregates and OH- (hydroxide ions) in the cement paste. Alkalis (Na+ and K+) from the cement, mixing water, or environment increase the concentration of OH- ions in the concrete. The OH- ions attack susceptible aggregate minerals. The damaged framework forms a gel that absorbs water from the surrounding concrete. The gel expands, generating pressures that can crack the concrete. The damage may not be visible to the naked eye for years after the concrete has been placed’

Alkaline silica reaction can usually be identified by random cracking on the surface of the concrete and in advanced cases, a gel like substance may be visible or possible spalling of the concrete. Cracking usually appears in areas with a regular supply of moisture, such as close to the watercourses and ground behind retaining walls etc. In order to confirm the presence of alkaline silica reaction it is necessary for core samples of the concrete to be taken and put under a microscope to establish their mineralogical and chemical characteristics, this is something referred to as petrographic testing.

Source: Source: http://www.delftcluster.nl/

Sulphate Attack – Again, the porous nature of concrete makes it vulnerable to sulphate attack in the same manner as previously discussed for other defects. Water containing dissolved sulphates such as sodium sulphate, potassium sulphate or magnesium sulphate can penetrate into the concrete and as it progresses, the composition and microstructure of the concrete will be changed. This can then result in cracking, expansion and loss of bond between the cement paste and the aggregate, which often results in a loss of strength of the concrete.

Possible sources of sulphates include seawater, oxidation of sulphide minerals in clay (such as copper) adjacent to the concrete (this can produce sulphuric acid which reacts with the concrete), bacterial action in sewers (anaerobic bacteria produces sulphur dioxide which dissolves in water and then oxidizes to form sulphuric acid), In masonry, sulphates are present in bricks and can be released over a long period of time, causing sulphate attack of mortar.

In the UK sulphate attack is particularly common in ‘older’ solid concrete ground floors. The use of fill material became very popular for residential solid concrete ground floor construction from the early 1940s. In the early post war years, waste materials such as burnt colliery shale, blast furnace slag and red ash were promoted by the government as appropriate materials to use for this purpose. However, it was later discovered that these types of fill materials contained high levels of sulphates which often resulted in significant problems and associated expensive remedial works to replace affected floors. Sulphates from these fill materials, with the presence of water, would attack the tri-calcium aluminate (one of the components of Portland Cement), within the concrete, which would result in lifting, expansion and cracking of the concrete floor slab. This problem was significantly reduced with the use of appropriate fill that did not contain these high levels of sulphates, together with the introduction of damp proof membranes (to reduce water penetration) and insulation to improve thermal efficiency. Replacement of a solid ground floor with a new floor is the only practical way of dealing with sulphate attack, which as you can see from the image below can be very disruptive as well as expensive.

Source: Source: http://www.mybuilder.com/

Other concrete defects – There are a number of other concrete defects that may be identified including ‘honeycombing’ concrete which occurs where wet concrete has not had all of the air taken out of it due to poor workmanship and lack of quality control. In order to remove any air within wet concrete a vibrating poker is put into the wet concrete which agitates the wet mix and ensures that air voids are removed before the concrete cures (hardens). If care is not taken during the installation process, or the installation is rushed, voids or honeycombs will be visible in the concrete when the formwork is struck (removed), similar to that identified in the image below.

Source: http://www.concrete.org/

Source: Source: http://www.zimbio.com/

Plastic Shrinkage – small cracks appear in the surface of freshly laid wet concrete soon after it has been placed, while it is still wet or plastic. Plastic shrinkage cracking is highly likely to occur when high evaporation rates cause the concrete surface to dry out too quickly, before it has the opportunity to set. This is particularly problematic when wet concrete is being laid in high temperatures and highly humidity. In order to slow down or control the curing process, damp or wet hessian may be laid over the newly laid concrete which is repeatedly wet to allow the water within the concrete to evaporate out at a much more natural rate, which will significantly reduce the risk of plastic shrinkage occurring.

Source: Source: http://theconstructor.org/

Over the last few weeks I have discussed some of the common defects that occur in concrete however you may come across other issues relating to expansion joints or insufficient cover, which primarily relate to poor design or poor workmanship. These articles can become very technical and sometimes quite complex, hopefully the information provided has introduced some of the defects and may motivate you to undertake more in depth research for some of the defects considered.

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Information/opinions posted on this site are the personal views of the author and should not be relied upon by any person or any third party without first seeking further professional advice. Also, please scroll down and read the copyright notice at the end of the blog.

Sunday, November 9, 2014

Concrete – Part 2 - Understanding the basics – Carbonation

There are a number of different defects that will influence the durability and ultimately the structural integrity of a concrete component. Concrete defects are difficult to visually identify in their early stages and generally only become evident when staining, cracking or distortion, start to occur

Source: http://sussexropeaccess.co.uk/

The popularity of concrete as a construction material was discussed in last week’s article (Link) due to its many positive attributes, including; it’s strength in compression, it’s flexibility, as it can be poured into infinite shapes/forms and sizes, it can be applied in situ (on site in its wet form), or it can be cast in a factory and delivered site as a complete component (pre-fabricated), it has good fire resistant qualities and is very durable if constructed correctly and maintained well. The imperfections in concrete were also introduced and in particular concrete’s weakness in tension, where steel reinforcement is introduced to address this deficiency.

As also stated in last week’s article, when encapsulated in the very high alkaline environment of concrete, reinforced steel will passivate. This means that the steel will be much less chemically active than it would normally be as the alkaline concrete is effectively protecting it. A particular problem however is that concrete is porous allowing moisture and other contaminants to enter the concrete which can eventually lead to corrosion problems of the steel reinforcement. This will then lead to associated degradation and spalling (pieces of concrete breaking and falling away), which can result in significant serious health & safety implications.

A number of years ago I can particularly remember undertaking an inspection of a large Secondary School in the West Midlands area. One of the largest blocks within the school was a three storey 1970’s exposed concrete frame building with brick infill panels. As with many secondary schools around the UK much of the building stock was poorly maintained due to ongoing funding issues. I noted severe horizontal cracking of the concrete in a number of locations at first and second storey level, to a point where large piece of concrete were hanging precariously over a walkway which was one of the main thoroughfares around the school. Strangely, staff at the school seemed to be oblivious to the danger, possibly because the problem was not at ground level and therefore not obviously visible. Needless to say, I spoke to the Head Teacher and contacted the Local Authority immediately and had the area cordoned off until emergency repairs had been carried out to make the area safe. This emphasises how serious concrete defects can be if left untreated.

There are a number of different defects that will influence the durability and ultimately the structural integrity of a concrete component, similarly to the example discussed above. Concrete defects are difficult to visually identify in their early stages and generally only become evident when staining, cracking or distortion, start to occur. It is therefore worth understanding some of the common defects which affect concrete, one of which is carbonation.

Carbonation – Newly installed untreated concrete especially where located externally is particularly vulnerable to carbonation. Once concrete is exposed to the air, carbon dioxide which is present in the air in concentrations of approximately 0.3% by volume (www.answers.com) will dissolve in water which can be present within the pores of the concrete and will form a mildly carbolic acidic solution. This acidic solution reacts with the alkaline calcium hydroxide (which is one of the compounds in concrete) to form calcium carbonate. This results in a pH value drop from more than 12.5 to approximately 8.5, which significantly reduces the alkalinity of the concrete. The carbonation process progressively moves through the concrete, with the pH drop occurring through the concrete. When the carbonation reaches any reinforcing steel, the passive layer around the steel will deteriorate when the pH value falls below 10.5. The passive protection around the steel therefore disappears so that when the steel is now exposed to moisture and oxygen, it makes it vulnerable to corrosion. The image below (started at the top left hand corner), demonstrates the progress of carbonation through concrete.

Source: http://theconstructor.org/

Testing is necessary to confirm that carbonation is occurring, which is usually carried out by applying a phenolphthalein solution to the surface of a freshly fractured or freshly cut piece of concrete. When the solution is applied non-carbonated areas will turn red or purple while carbonated areas remain colourless. Phenolphthalein will change colour at a pH of 9.0 to 9.5. Non-carbonated concrete without any admixtures will achieve a pH value of 12.5 or slightly higher. The image below shows that carbonation has occurred at the left hand side of the sample taken, whereas the right hand side remains un-carbonated.

Source: http://www.expin.it/

I have made reference the pH scale throughout this article. It is therefore worthwhile adding the image below for completeness;

Source: http://alissasohlovechemistry.wikispaces.com/

In next week’s article I will discuss a number of other defects that can occur in concrete including chloride attack, alkaline silica reaction, sulphate attack amongst others.

Please feel free to share this article and other articles on this site with friends, family and colleagues who you think would be interested

Sunday, November 2, 2014

Concrete – Part 1 - Understanding the basics

If installed correctly, newly installed reinforced concrete should last for many years with a minimal amount of maintenance; however it can be vulnerable in certain locations/uses to a number of possible defects, especially where used externally

Source: https://www.flickr.com

Concrete is an extremely popular material for construction and can be found in most parts of the World in one form or another. In many countries, along with the use of steel, concrete is the primary material used for buildings/structures of all shapes and sizes, because of its many positive attributes. These include; being extremely strong in compression, which means that it can sustain large loads which are applied, before it will start to deteriorate or fail. It is extremely flexible as it can be poured into infinite shapes/forms and sizes, it can be applied in situ (on site in its wet form), or it can be cast in a factory and delivered site as a complete component (pre-fabricated), it has very good fire resistant qualities and is durable if constructed correctly and maintained well. A significant disadvantage however is that concrete is extremely weak in tension, which in basic terms means that it will break up very easily when forces are applied that try to push or pull it apart.

Compressive and Tensile forces - To understand compression and tension forces let us think about a spring as an example. If an even ‘push’ force is applied to each end of the spring at the same time then the spring will compress and shorten. This action is adding compressive forces into the spring. When a load is applied to the top of a concrete component in a building then exactly the same forces are being introduced, however concrete has the ability to withstand high levels of compressive forces, particularly when certain mixes are used or when steel reinforcement is added. Tension or tensile forces are effectively the opposite of what is described for compression. Using our spring again as an example, instead of applying an equal ‘push’ force at either end, let us now apply a ‘pull’ force. This will lengthen the spring and add tensile forces within it. This is what will happen toward the bottom of a concrete beam. Whereas the compressive forces applied at the top of a concrete beam will apply load in a downward direction and compress the beam, this can also result in tensile forces appearing near the bottom of the beam which will have a tendency to want to ‘pull apart’, as identified in the image below:

Source: http://www.concretecountertopinstitute.com/

Concrete’s weakness in tension is therefore mitigated by introducing steel reinforcement (which is strong in tension) at the position in the concrete which is weakest in tension, which is near the bottom of the beam. The result is a complete component which is strong in both compression and tension and capable of withstanding extremely large loads/forces, which is ideal for building and construction.

If installed with the correct materials/mix and good workmanship, newly installed reinforced concrete should last for many years with a minimal amount of maintenance; however it can be vulnerable in certain locations/uses to a number of possible defects, especially where used externally. Some of the more serious concrete defects are a result of deterioration of the concrete which results in the reinforcement being exposed and starting to corrode. Concrete is a very alkaline material, typically 12.5 to 13 on the PH scale. When encapsulated in the very high alkaline environment of concrete, reinforced steel will passivate. This means that the steel will be much less chemically active than it would normally be as the alkaline concrete is effectively protecting it. A particular problem however is that concrete is porous allowing moisture and other contaminants to enter the concrete which can eventually lead to corrosion problems of the steel reinforcement. If corrosion to the reinforcing steel occurs this will result in the build-up of corrosion generating internal stresses and subsequent cracking and spalling (breaking and falling away) of the concrete. This is demonstrated in the image below.

Source: http://www.corrosion-club.com/

As explained above, when first installed the reinforcement in the concrete does not corrode because the concrete provides a protective alkaline environment due to the presence of large quantities of calcium hydroxide which is produced as Portland Cement hydrates and cures (hardens). (Portland Cement is the most common form of cement used in concrete for general purposes, which is produced from firing a mixture of clay or shale, and limestone or chalk. The clinker that is produced in the kiln, as a result of the firing process is ground to the fine light grey powder which most people will be familiar with). However, when moisture and other contaminants enter the concrete an environment for a range of different concrete defects is created.

Over the next two weeks I will consider a number of different concrete defects including Carbonation, Chloride Attack, Alkaline Silica Reaction and Sulphate Attack. Specific concrete defects are difficult to identify from a purely visual inspection, however, armed with the information discussed above and a little knowledge of what to look for it is possible arrive at a reasonable prognosis, which can be later confirmed with sampling and testing of the concrete.

Concrete is a very dense/heavy material and when it starts to exhibit defects that can result in cracking and spalling it can be very serious from a structural perspective as well as a health & safety perspective. The images below provide some examples of what can happen when concrete starts to exhibit defects, some of which I will discuss in more detail next week.