Concrete Deterioration

Concrete structures can be affected by a range of defects and deterioration, often due to a combination of factors. Their structures can deteriorate with time, typically as a result of the environment they are in, which impacts on both the safety and service life of structures.

Deterioration results in cracking, loss of strength and material coming loose from the structure.

There are a range of tests that can assist to determine the full extent of degradation of a concrete structure. These are defined in this blog.

Depth of Carbonation Analysis

Understanding the action of carbon dioxide

Carbonation is the result of the dissolution of carbon dioxide from atmosphere with hydrated cement compounds causing significant reduction in the alkalinity of the concrete and its quality.

Concrete has a high alkali condition which protects steel Reinforcement embedded in the concrete from corrosion. The reduced pH level reduces the protective layer surrounding the reinforcement, potentially causing the steel to corrode once Carbonation has reached it, given the right environmental conditions.

The rate of Carbonation

The rate of Carbonation within concrete depends on environmental conditions and quality of the concrete itself. Carbonation depths can range from 1mm to 5mm depth per year given the right conditions and durability of the concrete. Carbonation does however slow down after the initial onset as the concrete becomes less exposed to any environmental conditions. If hairline cracks are found within the concrete, the chance of carbonation is greatly increased due to the reduced density of the concrete.

It must be noted that carbonation depth will only play a role in the corrosion of steel if sufficient environmental conditions are present such as humidity and heat. If no moisture is present, the carbonation process will almost cease and thus not affect the steel.

Testing: Depth of Carbonation Methodology

To assist with the assessment Carbonation laboratories can determine the depth within concrete in accordance with BS EN 14630.  (Determination of carbonation depth in hardened concrete by the phenolphthalein method. BSI, London, 2006). This testing can be carried out through several methods.

One method is to drill several small holes in to the concrete and break a fresh piece of concrete away from the face of the holes to allow a clean face to be exposed, phenolphthalein indicator solution is then sprayed on to the exposed area and the depth measured to where the concrete changes colour, as shown in image below. The affected depth from the concrete surface can be readily shown. The solution becomes a pink/purple colour in the uncarbonated concrete (alkaline) and can be differentiated from the carbonated concrete (alkalinity is lost) and where the indicator will not turn pink.

(Image taken: POSSAN, E., THOMAZ, W.A., ALEANDRI, G.A., FELIX, E.F. and DOS SANTOS, A.C.P. CO2 uptake potential due to concrete carbonation: A case study. Case Studies in Construction Materials, Vol.6, June 2017, pp.147–161.)

A second method is to diamond drill a 50mm diameter core (aggregate size dependant) and split the core down the centre on its longitudinal axis using hand tools. The phenolphthalein indicator solution is then sprayed onto the freshly exposed surface and the depth of colour change recorded.

Interpreting the Depth of Carbonation Results

Understanding the action of chloride ions 

Exposure of reinforced concrete to chloride ions is the primary cause of premature corrosion of a steel reinforcement. Chloride ions, present in de-icing salts applied to road surfaces and seawater can cause steel corrosion if oxygen and moisture are also available to sustain the reaction.

Chloride Ion additions along with unwashed aggregates containing chloride ions were also commonplace in the 1960’s and 1970’s manufacturing of concrete. The additions were used to speed up the hydration process before the relationship between the chloride and degradation of the steel was related.

Chloride Ions Analysis

Concrete dust samples are usually taken incrementally from a location to obtain samples at specific depths within the concrete structure.

The chloride ions are then extracted from concrete dust by boiling in Nitric acid, in accordance with the principles of BS1881-124. The presence and subsequent concentration of chloride in the test sample is then determined via Aquakem Discrete Selective Photometric Analyser as chloride reacts with Mercury(II) thiocyanate to form a soluble non-ionic compound. The thiocyanate ions released react in an acid solution with Iron(III) nitrate to form a red/brown Iron(III) thiocyanate complex.

The resulting intensity of the stable colour produced is measured spectrophotometrically at a wavelength of 480nm and is related to the chloride concentration by means of a calibration curve. The chloride content is calculated and finally expressed as Cl as a percentage of the cement content.

Compressive Strength Testing

There are two ways to measure the compressive strength of concrete. The initial way is to sample fresh concrete and cast test cubes. If concrete has already been cast, test cores must be drilled to obtain samples.

Test Cubes

Compressive strength testing is used as a quality check for when fresh concrete is being laid. The concrete is cast into steel moulds which are stripped when the concrete has hardened. The concrete is then placed into temperature-controlled water for a specified period of time, before the test cubes are then crushed. The usual times for crushing test cubes are often 7 days, 28 days and 56 days. This gives an indication of the strength gain over curing time of the concrete. This form of testing is carried out in accordance with BS EN 12390-3:2019. The density of the concrete is also recorded at this stage to assess the durability of the concrete. This is carried out in accordance with BS EN 12390-7:2019.

Test Cores

Where a structure is already built, test core samples are diamond drilled in accordance with BS EN 12504-1: 2019 to allow the test samples to be obtained correctly. The cores are required to be a minimum of 3 x diameter of the aggregate size within the concrete to allow the concrete to be tested as a complete mix, rather than any high strength aggregate compromising the overall strength. The cores are also to be drilled perfectly perpendicular to the surface and without any abnormalities along the length of the core. No steel is allowed to be present within the test core section as this affects the strength and density of the concrete.

The core is prepared so it replicates a cube which is 1:1 height/diameter ratio, then tested in a cube crushing machine until the crushing strength is reached.