1. Properties of Concrete
Lab Report
Submitted To:
Dr. Ayub Elahi
Submitted By:
TaseerRaza (17-CE-93)
Department of Civil Engineering,
University of Engineering &Technology, Taxila.
2. Durability:
Durability is the ability to last a long time without significant
deterioration. A durable material helps the environment by conserving
resources and reducing wastes and environment impacts of repair and
replacement.
Durability in terms of concrete:
Durability of concrete may be defined as the ability of concrete to
resist weathering action, chemical attack, and abrasion while
maintaining its desired engineering properties.
A durable concrete is one that performs satisfactorily under
anticipated exposure conditions during its life span. The materials and
mixed proportions used should be such as to maintain its integrity and,
if applicable, to protect embedder metal from corrosion. Even though
concrete is durable material requiring a little or no maintenance in
normal environment but when subjected to highly aggressive or hostile
environments it has been founded to deteriorate resulting in premature
failure of structures or reach a state requiring costly repairs.
Importance of durability:
• Durability is vital with regards to a structure's lifespan. It is the characteristic that dictates
how long a structure can live to perform its desired function.
• Durability has a direct impact on strength. It cannot be underestimated. More the
structure is durable more it has strength.
• The durability is about the ability of the structure to resist action from the weather over
time, fire, and any chemical attack, while maintaining the desired engineering properties.
So, it is vital
.
Factors contributing towards durability:
There are many factors that contribute in durability of concrete, amongst them few are following;
1. Cement Content
2. Aggregate Quality
3. Water Quality
4. Concrete Compaction
3. 5. Curing Period
6. Permeability
7. Moisture
8. Temperature
9. Abrasion
10. Carbonation
11. Wetting and Drying Cycles
12. Freezing and Thawing
13. Alkali-Aggregate Reaction
14. Sulfate Attack
15. Organic Acids
1.Cement content:
• Quantity of cement used in concrete mix will also be a factor affecting durability of
concrete.
• If cement content used is lower than the required, then water cement ratio becomes
reduced and workability also reduced.
• Adding more water to this mix results in formation capillary voids which will make
concrete as permeable material.
• If excess cement content is used, problems like drying shrinkage, alkali-silica reaction
may occur which finally effects the durability of concrete.
2.Aggregate quality:
• Use of good quality aggregates in concrete mix will surely increase the durability of
hardened concrete.
• The shape of aggregate particles should be smooth and round. Flaky and elongated
aggregates effects the workability of fresh concrete.
• For better bond development between ingredients rough textured angular aggregates
are recommended but they require more cement content.
• Aggregate should be well graded to achieve dense concrete mix.
• Aggregates should be tested for its moisture content before using. Excess moisture in
aggregate may lead to highly workable mix.
4. 3.Water quality:
• Quality of water used in concrete mixing also effects the durability of concrete. In
general, potable water is recommended for making concrete.
• pH of water used shall be in the range of 6 to 8.
• Water should be clean and free from oils, acids, alkalis, salts, sugar, organic materials etc.
• Presence of these impurities will lead to corrosion of steel or deterioration of concrete by
different chemical reactions
4. Concrete compaction:
• While placing concrete, it is important to compact the placed concrete without
segregation.
• Improperly compacted concrete contains number of air voids in it which reduces the
concrete strength and durability.
5.Curing period:
• Proper curing in initial stages of concrete hardening result in good durability of concrete.
• Improper curing leads to formation of cracks due to plastic shrinkage, drying shrinkage,
thermal effects etc. thereby durability decreases.
6.Permeability:
• Concrete durability gets affected when there is a chance of penetrability of water into it.
• Permeability of water into concrete expand its volume and lead to formation of cracks
and finally disintegration of concrete occurs.
• Generally concrete contains small gel pores and capillary cavities. However, gel pores do
not allow penetration of water through them since they are of very small size.
• But, capillary cavities in concrete are responsible for permeability, which are formed due
to high water cement ratio.
• To prevent permeability, lowest possible water cement ratio must be recommended.
• Use of pozzolanic materials also helps to reduce permeability by filling capillary cavities.
7. Moisture:
• Moisture present in the atmosphere will also affects durability of concrete structures.
5. • Efflorescence in concrete occurs due to moisture, which will convert salts into soluble
solutions and when it evaporates salts become crystallized on the concrete surface.
• This will damage the concrete structure and durability will be reduced.
8.Temperature effect:
• Concrete is heterogeneous material, when fresh concrete is subjected to high temperature
rate of hydration gets affected and strength and durability becomes reduced.
• Concrete ingredients have different thermal coefficients, so at higher temperatures,
spalling and deterioration of concrete happens.
9.Abrasion:
• Deterioration of concrete also occurs due to severe abrasion.
• When concrete is subjected to rapidly moving water, steel tires, floating ice continuously
wearing of surface occurs and durability gets affected.
• Higher the compressive strength higher will be the abrasion resistance.
10.Carbonation:
• When moist concrete is exposed to atmosphere, carbon dioxide present in atmosphere
reacts with concrete and reduces pH of concrete.
• When pH of concrete reaches below 10, reinforcement present in the concrete starts
corroding.
• Corrosion of reinforcement causes cracks in concrete and deterioration takes place.
11.Weeting and drying cycle:
• When concrete is exposed to alternate wetting and drying conditions such as tidal waves
from sea etc. secondary stresses are developed in concrete.
• Due to these stresses cracks will form and reinforcement is exposed to atmosphere.
• When chlorides or sulfates from sea water meets reinforcement, corrosion occurs, and
durability of concrete is reduced.
6. • Use of low-permeable concrete, proper cover for reinforcement can prevent this type of
problems.
12.Freezing and thawing:
• When fully saturated concrete is exposed to repeat cycles of freezing of thawing, it is
deteriorated by the action of freezing and softening of water in it.
• It causes cracking on concrete surface in the form of maps which is called map cracking
and effects durability of concrete.
• The coarse aggregate presents the concrete also effected by freeze and thaw cycles,
spalling of concrete may occurs.
• In this case, durability of concrete can be achieved by adding air-entraining admixtures to
the mix and also reduce the maximum size of coarse aggregate.
13.Alkali aggregate reaction:
• Alkali-aggregate reaction or alkali-silica reaction, takes place between alkali content of
cement and silica content of aggregates is also a major factor effecting durability of
concrete.
• Due to this reaction, Concrete expansion occurs which finally lead to severe cracking and
concrete gets deteriorated.
• Use of cement with less alkali content, Non-reactive aggregates, pozzolanic materials like
fly ash or slag cement, Lithium-based admixture in concrete will help to overcome this
problem.
14.Sulfate attack:
• When concrete structures are attacked by sulfates like sodium sulfate, magnesium sulfate
etc. concrete disintegration happens.
• This reaction is due to the chemical reaction between hydrated cement products and
sulfate solutions.
• Sulfate attack generally happens when water used for concrete mix contains high sulfate
content, Due to unwashed aggregates, when soil around the concrete structure contains
sulfates in it etc.
7. • This can be prevented by using sulfate resisting cement, by adding slag cement, by
decreasing permeability etc.
15.Organic acid:
• When concrete surface is subjected to organic acids like acetic acid, lactic acid, butyric
acid etc., concrete durability gets affected severely.
• Formic acid on concrete surfaces can lead to corrosion of concrete.
Alkali silica reaction (ASR) in concrete:
The alkali silica reaction (ASR), more commonly known as "concrete cancer", is a
swelling reaction that occurs over time in concrete between the highly
alkaline cement paste and the reactive non-crystalline (amorphous) silica found in many
common aggregates, given sufficient moisture.
Mode of action:
This reaction causes the expansion of the altered aggregate by the formation of a soluble
and viscous gel of sodium silicate. This hygroscopic gel swells and increases in volume
when absorbing water. It exerts an expansive pressure inside the siliceous aggregate,
causing spalling and loss of strength of the concrete, finally leading to its failure.
Effect on concrete:
ASR can lead to serious cracking in concrete, resulting in critical structural problems that
can even force the demolition of a structure. There are many hydraulic as well as other
structures that are demolished or damaged due to ASR. These structures include bridges,
highways and dam walls.
Leading cause:
ASR is mainly due to use of reactive aggregates.
Causes of ASR in concrete
Causes of alkali silica reaction are many but they are broadly classified into two parts,
External causes:
8. • Alkalinity in the water to which the concrete is exposed may cause problems if reactive
aggregate was used in the concrete.
• The water may acquire its alkalinity from soil through which it passes.
• Deicing solutions may provide alkalis that could cause ASR if reactive aggregate was
used in the concrete
• Sea water and water from industrial processes may also introduce alkali to concrete
Internal causes:
• Alkali in the cement and reactive aggregate in the concrete in sufficient concentrations
will cause ASR.
• Use of reactive aggregates that has high content of alkali.
• Internal humidity may cause ASR if the concrete is not permitted to dry properly.
• Using mix water with high alkalinity will increase the concrete’s alkali content
Defects produced due to ASR
The cracking caused by ASR can have several negative impacts on concrete, including:
• Expansion:
The swelling nature of ASR gel increases the chance of expansion in concrete
elements.
• Compressive Strength:
The effect of ASR on compressive strength can be minor for low expansion levels,
to relatively higher degrees at larger expansions compressive strength is not very accurate
parameter to study the severity of ASR; however, the test is done because of its simplicity.
• Fatigue:
ASR reduces the load bearing capacity and the fatigue life of concrete.
ASR Mitigation techniques
1. Use a low alkali cement (<0.60% equivalent Na2O). This may not remain a viable
solution because changing technology used by the industry to conform to environmental
regulations inherently produces cement with a higher alkali content. The alternative may
be to use Type IP or IS cements.
9. Implementing this recommendation does not imply that alkali-silica reaction will
not occur. If a highly reactive aggregate has been used in the concrete, only a
small amount of alkali is required to initiate the reaction.
ASR may also occur in a low-alkali cement concrete if the structure experiences
wetting and drying cycles. Even though only a small concentration of alkali is
present, the alkali will become concentrated in the drying zone of the concrete and
may be sufficient to initiate reaction.
2. Use fly ash, natural pozzolans, granulated blast-furnace slag, or silica fume as a cement
replacement. They make the concrete mix less permeable therefore making it more
difficult for water to reach the aggregate. However, before specifying them for a project
mixture, test their effectiveness.
If Class C fly ash is used, replace at least 25% of the cement by mass with it.
Class F fly ash is better than Class C fly ash and is certain to solve the problem.
Although any amount of class F fly ash will reduce alkali-silica reaction, 25%
replacement of cement by mass is also recommended.
If silica fume is used, it should replace 10% of the cement by mass.
If blast-furnace slag is used, it should replace between 40% and 70% of the
cement by mass.
3. Control access to moisture and alkali from external sources. Water imbibed into the gel
causes expansion. Not allowing water access to the material and maintaining the
concrete’s internal relative humidity below 80% will stop further gel growth and alkali-
silica reaction. Accomplish this by applying a sealer such as paint or a moisture barrier to
the concrete surface. Sealers must be reapplied periodically to remain effective. For
example, a solvent-based silicon coating will provide protection for about 2.5 years.
4. Use proven non-reactive aggregates or "sweeten" (replace 30% of the aggregate with
crushed limestone) the mixture with non-reactive aggregates. This may be impossible or
very expensive. River gravel is often the most suspect because it contains only small
amounts of reactive rock. The performance history of a given aggregate source is
important in evaluating its usability about ASR.
5. Alter the alkali-silica gel.
6. Use concrete mixes with a low w/c ratio. This will make the hardened concrete less
permeable thus allowing less of the water necessary for expansion of the gel to reach
affected areas and limiting mobility of water and alkali around the concrete mass.
7. The use of lithium and barium salts as admixtures is known to reduce ASR.
8. Air-entrainment can reduce the effect of ASR expansion.
9. Use low cement concrete. Less cement provides less alkali to the system.
10. Use a coarse aggregate with a relatively porous microstructure.
11. If reactive aggregate must be used, either use only a small amount or a large amount. The
worst situation is to incorporate an amount of reactive aggregate between large and small
amounts.
10. 12. Periodic cleaning of the structure may help prevent ASR by washing away salts before
they dissolve and penetrate the concrete.
13. Use beneficiation of the aggregate to remove the undesirable portions
References:
• Properties of Concrete by Adam Neville
• Concrete Technology by Adam Neville and J.J Brooks
• Concrete Technology by MS Shetty
• Portland Cement paste and Concrete by Itzhak Soroka
