By Ben du Preez
All successful concrete repair projects are underpinned by in-depth knowledge of the main causes of degradation. Only once this is fully understood can a suitable repair system be deployed. Proper concrete repairs should not only restore structures to their original state. If undertaken by a professional, concrete repair will prolong the lifecycle of your assets.
There are many factors that lead to concrete deterioration. The most common include:
- Environmental factors
- Materials and concrete placement procedures
- Structural design defects
- Temperature
- Use of concrete structures beyond their original design purposes
Seasoned concrete repair contractors
Seasoned concrete repair contractors will know how to identify these causes and the best way forward their clients. The analysis of the damage and its causes is usually done while working closely with asset owners and their engineering teams. Skilled and experienced contractors may also often involve participants in their large supply chain. This includes concrete repair materials manufacturers and suppliers. These companies have a wealth of information to share to help address all of these causes of concrete degradation. Working with reputable concrete repair contractors, they know that their products will be applied correctly.
This includes to address damage caused by aggressive environmental factors.
Over the many years that we have been operating, Hindle Mason Projects has encountered degradation due to sea water and salt spray. This is in addition to the use of de-icing salts; raw sewage; acids; groundwater; acid rain; and condensation. These lead to various defects, such as staining; reinforcement corrosion; cracking; and spalling.
Salt water is a major concern. Sea water contains about 3,5% of soluble salts. Meanwhile, sodium and chloride ions concentrations are between 11 000 and 20 000mg/l. As chloride content in concrete increases, it alters the anodic curve which maintains passivity. This results in the production of iron which reacts with chloride ions in moisture to form ferrous chloride and oxychloride. The ferrous chloride then reacts with water to create ferrous hydroxide and hydrochloric acid. This, together with the availability of balance chloride ions, perpetuates a cycle of continuous corrosion.
The latter is the lead cause of structural failures. When reinforcing steel corrodes, the resulting rust occupies a greater volume than the steel. This expansion creates tensile stresses in the concrete, which cause cracking, delamination and spalling. Refer to https://www.researchgate.net/figure/Failure-of-concrete-due-to-structural-corrosion-as-a-result-of-aggressive-environment_fig1_321722413. Volume changes due to reinforcement corrosion can be up to six times.
Concrete repairs due to carbonation
A seasoned contractor also has experience executing concrete repairs due to carbonation. Carbonation refers to the formation of calcium carbonate in concrete.
The concentration of carbon may vary from 0,03% by volume in non-industrialised areas to about 0,3% in industrialised nodes. Carbon enters concrete via the capillary pore structures that are formed when it cures. As bleed water exits, it creates these inter-connected structures. Refer to https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1124&context=icdcs.
Carbonic acid is formed when carbon dissolves in the cement pore solution that neutralises the alkaline. It defuses the alkaline calcium hydroxide. This reduces pH levels to less than 8,3 which destroys the passive thin ferrous oxide film around the reinforcement that protects it from corrosion. The continuous process has now been initiated that can result in the complete corrosion and deterioration of structural members.
Sulphates also attack concrete. They are often present in the water used for cooling towers; sewage; and effluents.
Sodium sulphate reacts with free calcium hydroxide to form sulphate. In turn, it reacts with aluminates to form calcium sulphoaluminate. This ettringite and calcium sulphate each have double the volume compared to those of the original matter. Therefore, they apply tremendous tensile strength on the surrounding concrete which then also leads to cracking.
Concrete repairs due to construction
Concrete repairs due to substandard construction workmanship are also common.
A case in point is improper concreting procedures that produce pervious concrete. These include poor vibration; inadequate concrete cover; and leaking formwork. The result is honeycombing and voids in concrete. Moisture in the forms of gas, vapour or liquid can then enter concrete leading to reinforcement corrosion. According to this article, https://www.crown.co.za/construction-world/marketplace/27805-the-benefits-of-imperishable-concrete-reservoirs, there has been a notable decline in construction practices. This is leading to the premature failure of concrete structures. The practice is rife on municipal infrastructure projects.
Concrete repairs due to aggregates
Concrete repairs have also had to be initiated due to the use of poor aggregates for concrete production.
Concrete produced with porous aggregates are especially problematic. These aggregates are unable to withstand the service loads of structures and they absorb water. A water absorption factor of less than 1% seldom results in concrete creep and shrinkage. However, when it exceeds 3%, the water absorption factor can lead to concrete shrinkage. Refer to https://www.concrete.org/topicsinconcrete/topicdetail/shrinkage%20of%20concrete.
Then, there are those concretes that have been produced with aggregates from saline sources or with saline water. This allows chloride ions to react with the various concrete components which lowers pH levels. As a result, alkalinity levels are reduced, exposing reinforcement steel to corrosion.
Meanwhile, aggregates that contain siliceous materials can react with sodium and potassium hydroxides in concrete to form a gel. As it absorbs water, the gel swells excessively creating tremendous pressure and tensile stresses that lead to cracking. In turn, these defects allow chlorides and sulphates to enter concrete. This action is also referred to as the “alkali aggregate reaction” (AAR). The most common AAR types are alkali silica reaction (ASR) and alkali carbonate reaction (ACR). ASR occurs when the alkalis in concrete react with the fine grained argillaceous dolomitic limestone aggregates containing calcite and clay. The reactive silica present in the aggregates reacts with the alkali hydroxides of concrete generated during the hydration of cement. A gel is then formed that swells as it absorbs water from the surrounding cement paste and the environment. This gel can induce sufficient expansive pressure to damage concrete. ACR is less frequent occurrence. Refer to https://www.sciencedirect.com/topics/engineering/aggregate-reaction#:~:text=Alkali%E2%80%93aggregate%20reaction%20(AAR),alkali%E2%80%93carbonate%20reaction%20(ACR).
Concrete repairs due to design
There are many examples of concrete repairs that have had to be executed due to structural design defects.
Overloading a structure that has been under-designed or beyond its original design parameter causes it to deflect and bend. This will eventually lead to cracking of concrete elements.
Inadequate foundation design and construction is also a problem. In these instances, structures yield due to the settlement of soils as a result of consolidation. Settlement also occurs due to evaporation of moisture from the foundation soil and fluctuations in water table levels. Refer to https://trid.trb.org/view/139768.
Inappropriate reinforcement detailing and placement also leads to cracking of structures during their service period. Refer to https://www.researchgate.net/figure/Inappropriate-placement-of-reinforcing-steel-in-concrete-members-and-corrosion_fig3_268799588.
A reputable contractor will also have experiencing concrete repairs due to damage caused by extreme temperatures.
Extreme temperatures create freezing and thawing cycles. When water in the inter-particle voids freeze, it expands by up to 9%. This is even more of a concern when concrete is saturated over the critical level. In these instances, moisture expansion produces osmotic and hydraulic pressures greater than the tensile strength of paste or aggregates. This accelerates the deterioration of pervious concrete.
Concrete that has been exposed to fire has also been compromised. The construction material can resist temperatures of up to about 650oC. However, its strength declines by 80% when exposed to temperatures of 450oC and about 50% at 650oC. At 650oC, steel reinforcement expands. This results in an increase in volume; loss in bond; and decrease in tensile strength, leading to spalling. When this concrete is cooled, it starts to disintegrate because the oxides formed due to heat transform into lime.
Concrete repairs due to loading
Concrete repairs due to incorrect use of and loading on structures is also common. The mechanical effects of improper use of structures include abrasion, erosion, impact and cavitations. Abrasion occurs when harder material exerts friction on concrete surfaces. Overtime, these forces wear out the substrate and, thereby, expose concrete to external agencies. Meanwhile, impacts over a particular duration damage and reduce the strength of concrete. Erosion eventually removes the concrete exposing reinforcing bar to the elements. Erosive forces include water, wind and ice. Cavitation is caused by water that is flowing at speeds greater than 12m/s. This creates turbulence that generates low pressure areas with vortexes eroding concrete surfaces.
Ben du Preez is a Director of Hindle Mason Projects.
