Ensuring durability & Structural Integrity of Concrete

Concrete is a composite material composed of Cement, Aggregates (Coarse and Fine), Admixtures (Used to alter / modify the properties of concrete) and water. Concrete is the most popular construction material in the world due to following reasons.

• Its flexibility; can be virtually used in different types of constructions / different shapes.
• It is cheap; in comparison with other construction materials like steel etc due to economy of scale / production.
• It is widely available; due to the raw materials required for the production of cement i.e. the basic ingredient required for concrete.
• It is durable and long lasting.

The durability and structural integrity of concrete can be affected by the components of cement if they are not balanced and manufactured well as per the (Inter) National Standards. Cement soundness generally can be considered to be a resistance to, or the lack of, any swelling,cracking, or disintegration resulting from an expansive chemical reaction such as the hydration of free lime (CaO) or crystalline free magnesia (MgO) or any other factor in cement paste, mortar, or concrete.

Following are the Properties of Cement which are the major contributor to concrete disintegration.

1. Low Compressive and Flexural Strength of Cement / Concrete Mix
2. High Free Lime
3. High Free Crystalline MgO (Periclase)
4. High Alkali Content
5. High C3A Content
6. High Sulpahte content
7. Partial Hydration of Clinker / Cement---Less Expansion
8. Less Cement Fineness
9. High Heat of Hydration for Mass Concreting

Low Compressive and Flexural Strength of Cement / Concrete Mix:

One of the most easily understandable property of concrete which can cause structural failure is less compressive and flexural strength of cement / concrete; since if the concrete is not able to bear the load for which it has been designed for; than definitely building / concrete collapse will happen. However, this particular aspect is not within the scope of the article.

High Free Lime:

Free lime is the lime (CaO) which remained uncombined in the pyroprocessing process during the cement manufacturing. On cement hydration; this free lime (fCaO) slowly reacts with water to form Ca(OH)2 the volume of which is much more than the initial volume of the individual reactants hence causing cracks / stress in the building over period of time leading to structural failure.

The test most commonly used to determine expansion caused by free lime is Le-Chatlier Test and the limit in most of the standards is max 10 mm for a cement to be considered sound; hence to be used as a good cement for concrete.However, the Le-Chatlier test is a mild test and it can detect the expansion / unsoundness only due to free lime and any other factor which cause the cement to be unsound will not be detected by expansion test.

Free Lime in cement can be determined using wet method of analysis however that is prone to analysis bias and inaccuracy if the cement is partially hydrated; the accurate method is to use XRD or Microscopy.

Maximum safe limit of free lime in clinker depends upon the clinker reactivity, type of cement produced i.e. Type & % age of additives added during the cement milling stage as well as storage of cement. However general industry practice is to control clinker free lime < 1.50 % in order to avoid Le-Chatlier expansion; however, this limit needs to be reduced when testing for Autoclave expansion test since that is a rigid test and would capture all the factors which lead to cement expansion.

High Free Crystalline MgO (Periclase):

Another component of cement which causes the cement / concrete to be unsound / expansive is the hydration of Free Crystalline MgO (Periclase). The hydration reaction is very slow and once the cement is set and gained strength the slow reaction of MgO with Water will produce Mg (OH)2; the volume of which is much more than the individual reactants and hence leads to slow structural disintegration of concrete.

That is why in most of the cement standards the total MgO in cement is usually limited to 4---6 % Max; however, it must be understood that it is not the total MgO which cause the expansion; it is only the hard burnt free crystalline MgO which is produced during the pyroprocesing which causes the cement expansion.

MgO + H20-------------------Mg(OH)2

Periclase + Water-----------Brucite

Expansion due to MgO cannot be detected by Le-Chatlier test and can be detected by Autoclave test which is a rigid and harsh test in comparison with Le-Chatlier Test. It is possible that a cement many be sound / conforming to Le-Chatlier test limit but still it is unsound in Autoclave test; as per ASTM standard C 150 the limit of Max Autoclave Expansion is 0.8 %.

Clinker produced with slow cooling of clinker versus clinker produced with rapid cooling of clinker will be more prone to this expansion type; due to growth of periclase crystals when clinker is slowly cooled whereas in case of rapidly cooled clinker size of periclase will be much small and hence will be more reactive and will hydrate during the required hydration period instead of expanding later on and some quantity will be even entrapped in glass form rather than free crystalline MgO.

Total MgO can be tested by Chemical analysis and XRF; but crystalline MgO can be detected using XRD.

High Alkalis Content: (Na2Oeq)

If the cement contains high Alkalis (Often expressed as Sodium Equivalent Na2Oeq = (Na2O % + 0.658 * K2O %); than it can react with certain aggregates containing reactive Silica (Can be determined using Petrographic Examination) and form an Alkali Silica Gel (Alkali Silica Reaction ---ASR); that gel expand over a period of time by absorbing moisture and cause stress / exerting pressure that may be sufficient to expand and crack the concrete.

The reaction between the alkaline pore solution and silica minerals such as opal, chalcedony, micro and cryptocrystalline quartz, cristobalite and tridymite and volcanic glasses. ASR has also been observed with aggregates such as quartzite, greywacke, argillite, hornfelsed shale, phyllite, granite, and gneiss.

ASR requires three factors to proceed:

(a) Sufficient moisture in the pore structure of the concrete. ASR damage is unlikely to occur when the equilibrium internal relative humidity in the concrete is less than 75%.

(b) Sufficient alkali in the pore solution. Alkali can be supplied by cement and other binder constituents, chemical admixtures and/or the aggregate.

(c) Reactive mineral(s) in the aggregate. Some reactive aggregates will only cause significant expansion if they are present in a critical amount known as a pessimum proportion. This is the proportion of reactive aggregate at which greatest expansion occurs.

 If any one of these three factors is absent, then ASR will not proceed. Once all available water or alkali is used up the reaction will stop, but it may recommence if the conditions (a) and/or (b) are again satisfied.

Once ASR has started in a structure, there is no way of stopping it if sufficient moisture remains in the concrete. It will continue until the alkalinity of the pore solution falls below the required concentration for reaction, or until the reactive component of the aggregate is exhausted.

How to Avoid Alkali Silica Reaction:

If the aggregates contain reactive silica; than one way to avoid the Alkali Silica Reaction is to use Low Alkali Cement (Na2Oeq < 0.60 %) the others are by using SCM like Pozzolonas, Fly Ash, Blast Furnace Slag, Silica Fume, instead of Ordinary Portland Cement or by blending with OPC.

High Tri Calcium Aluminate (C3A):

C3A has the highest reactivity among all of the clinker minerals and in order to slow down the immediate reactivity Gypsum is added in cement. However, in areas where sulphate is readily available from the soil /water in the form of Sulphate Salts (Particularly Magnesium Supahte and Sodium Sulphate) than that sulphate react with excess C3A to form Tri Calcium Sulpho Aluminate Hydrate which is of higher volume (227 %) than the initial reactants and cause internal stress within the concrete structure which has already hardened and lead to its structural failure.

In case of having high sulphate salts in the area / water where concrete work is being done; than in those areas Sulphtae Resistant Cement (SRC) is used; SRC is a cement having low C3A content. As per ASTM High Sulphate Resistant Cement is classified as one having C3A < 5 %; whereas as per certain other standards the limit may be 3.5 % C3A or even 0 % C3A European standard EN 197.

SRC is not recommended to be used for Soil / water rich in chloride content; the reason for this is that C3A binds Chloride in Calcium Chloroaluminate; in the absence high chloride content can cause steel corrosion in the concrete.

High Sulphate Content:

Gypsum (Sulphate Source) is added to control the rapid reaction of C3A; however, when there is excess Gypsum added over and above required for C3A; than that gypsum can also contribute to structural failure of concrete by causing slow expansion once the cement is set / gained strength. That is the reason international standards limit the SO3 content in cement to 3.0 or 3.5 %. ASTM C 150 / EN 191.

Partial Hydration of Clinker / Cement:

If the clinker / cement is partially hydrated than the cement expansion will be low; the rationale is that the free lime will react with water to form Ca(OH)2 before the actual concreting work and will not contribute to cement / concrete expansion at the later stage. However, this partial hydration leads to many other problems like less compressive strength, high setting times, more chances of false set etc.

This is one route to ensure conformance to expansion testing; however, needs a proper balance between high free lime (Partially Hydrated) and low free lime clinker (Fresh).

Low Fineness of Cement:

Cement having low fineness will contribute to more expansion as tested in the autoclave test; it has been observed by researchers that when the fineness of cement was increased the autoclave expansion got decreased in spite of all other parameters remaining equal.

This is one of the routes to ensure conformance to autoclave expansion testing.

High Heat of Hydration for Mass Concreting:

The hydration of cement compounds is exothermic. Because the thermal conductivity of concrete is comparatively low, it acts as an insulator, and in the interior of a large concrete mass like Dams (Not Ordinary Concrete Structures), hydration can result in a large rise in temperature. At the same time, the exterior of the concrete mass loses some heat so that a steep temperature gradient may be established and, during subsequent cooling of the interior, serious cracking may result.

For this reason, it is necessary to limit the rate of heat evolution of the cement used in this type of structure: a greater proportion of the heat can then be dissipated and a lower rise in temperature results. Cement having such a low rate of heat development are known as low heat Portland cement or Low Heat of Hydration Cement.

Conclusion:

Durability of concrete can be ensured by carefully optimizing the cement properties during the cement manufacturing process as highlighted in the above article. Please feel free to share your knowledge and understanding of the factors which lead to structural failure of concrete.