Principally, air entrainers are used to result in concrete which is highly resistant to severe frost action and cycles of wetting and drying or freezing and thawing. Air entraining agents also provide a number of other important benefits to concrete including higher degrees of workability and durability. Furthermore, they reduce bleeding and improve the cohesion of mix components which may normally be susceptible to segregation, which provides the concrete with a better quality finish.
Since these admixtures are particularly effective in combating frosty conditions they are mainly used in places where there is potential for high freeze and thaw cycles. For example, applications of its use in industry include:
- Driveways and paths
- Concrete Roads
- Yards and aprons
- Ground-slabs
- Most other external paved areas
2.1.2 Water Reducers.
The purpose of water reducing agents is to reduce the water content within the concrete mixture. There are three different varieties of water reducers, low range, mid range and high range. In general, these admixtures improve the quality of the concrete by providing the required slump with less water in the mix and so a lower water: cement ratio is produced. Thus, higher strength concrete is achieved without increasing the amount of cement, which has cost saving benefits. They also improve the properties of concrete containing marginal or low-quality aggregates and help in placing concrete under difficult conditions whilst minimising segregation. In addition, they aid the production of a dense, closed textured surface improving the concrete’s durability.
2.1.2.1 Low Range Water Reducers.
Low range water reducers (LRWR’s) reduce water content by a minimum of 5% and reduce slump by 1.5 to 2 inches without the addition of water.
2.1.2.2 Mid Range Water Reducers.
Mid range water reducers (MRWR’s) reduce water content by at least 8% and up to 15% and work most effectively in mixes designed to have a slump in the range of 4 to 5 inches. Compared to low and high range reducers these tend to be more stable over a wider range of temperatures and tend to give more consistent setting times. They also produce less permeable, more durable concrete with significantly early and ultimate compressive strengths. It is for these reasons that MRWR’s are sometimes used as finishing enhancers for concrete. This is apparent in commercial and residential flatwork and formed concrete applications.
2.1.2.3 High Range Water Reducers.
High range water reducers (HRWR’s) reduce water content from 12% up to 40% and are typically used in concretes designed to have slumps of 8 to 11 inches. They can also be used to either increase slump by 4 to 8 inches or lower the water content of hot weather concrete mixes which produces flowing concrete. HRWR’s as a result can be used to enhance the flowability of concrete giving a highly fluid but workable mixture, which can then be placed with very little, if any, vibration or compaction, saving time and money.
These water reducers are particularly useful in applications such as:
- Difficult wall placements
- Narrow forms
- Sections with blockouts, penetrations, or embedded items
- Pumping high vertical distances
- Fast placement of concrete
- Increased lift heights and free fall distances
In general, water reducers have been used primarily in bridge decks, low-slump concrete overlays, and patching concrete. Other applications include:
- Direct finish floor-slabs for commercial uses
- High quality formwork finishes
- Pumped concrete
Typically, water reducers are used wherever there is the need for a large amount of concrete as they considerably reduce the cost of construction since higher strengths can be attained from lower water: cement ratios. As a result, huge cost savings are made were less cement is required.
2.1.3 Retarders.
Retarding admixtures are used to slow the setting rate of concrete. This is sometimes necessary as high temperatures often cause an increased rate of hardening in concrete which makes placing, compacting and finishing difficult. Retarders work by counteracting the accelerating effect of hot weather on concrete setting. This delays the initial set time of the concrete and keeps it workable for longer periods.
Benefits of the concrete staying workable are that a better quality finish can be achieved as more time is given for placement, compaction and finishing. The setback in hardening times is particularly useful in situations where there is a delay of some kind, a delivery problem for instance. Set retarders also permit the application of higher temperature curing of pre-stressed concrete without negatively affecting ultimate strength. Another advantage of retarding admixtures is that they can also function as water reducers and entrain some air in concrete and so are sometimes used for multipurpose reasons, reducing the cost of construction considerably.
Applications of retarding agents within the construction industry comprise of situations where greater set times and workable periods are needed such as:
- Long hauls
- Undermanned placement crew
- Slow pour rate
- Hot weather conditions
2.1.4 Accelerators.
Accelerating admixtures work in the opposite way to retarding admixtures by reducing the setting times of concrete. The presence of accelerating agents accelerates the cement hydration process which causes these shorter times of setting.
They are particularly useful in counteracting the effects of cold weather conditions where the rate of hardening of the concrete is slowed due to a low temperature. This increase in the rate of setting is advantageous, especially at cooler temperatures since it produces an increase in the rate of early strength development and protection against frost attack. Accelerators can also be used in conjunction with superplasticizers in order to facilitate better mould utilisation on pre-cast works. The downside to these admixtures is that the required time for proper curing and protection is reduced whilst finishing operations are sped up and have to begin earlier which puts pressure on the rest of the schedule and operations.
Applications of their use within industry include highway and bridge construction and placing concrete on roof systems. Accelerators are particularly useful in these areas of construction as the faster set times provide the concrete with greater dimensional stability, preventing emergencies during concrete operations from occurring such as falling debris. They are also used in a number of other applications:
- Cold weather conditions
- Pre-cast concrete
- Concrete pipes
2.1.5 Speciality Admixtures.
2.1.5.1 Corrosion Inhibitors.
Corrosion-inhibiting admixtures are used to slow the rate of corrosion in concrete. They work in two ways, firstly, the time before corrosion begins is increased and secondly, the amount of corrosion that takes place is reduced.
Corrosion inhibitors can either be anodic or cathodic or a combination of the two. They can work by providing a protective barrier which works to stabilise the layer of corrosion surrounding the concrete or work by providing a thin protective layer which prevents chlorides from reacting with the concrete.
As the name suggests, corrosion inhibitors are used to fight against corrosion by acting as a defensive barrier. Hence, they are used in environments which are highly susceptible to chloride induced corrosion and so they have many applications within the construction industry. For example, they are particularly effective in parking structures, highway bridges and marine environments as these will be exposed to high concentrations of chloride and diecing salts.
2.1.5.2 Shrinkage Reducers.
Shrinkage reducing admixtures (SRA’s) are used to control and reduce drying shrinkage and minimize cracking within concrete.
As concrete dries out, its hydrated cement loses moisture from its very small capillaries causing it to shrink with time. This loss of moisture means that the surface tension of the remaining water pulls the small capillaries together resulting in a reduction in volume, which can often lead to the concrete cracking. SRA’s work by reducing these drying shrinkage effects by decreasing the amount of surface tension within the capillaries.
The major benefit derived from the use of SRA’s is a drastic reduction in the occurrence of cracks which are caused by drying shrinkage. A side benefit is that the durability of the concrete is improved.
As a result, applications for SRA’s exist where concrete is put under large stresses and there is a high chance of cracking, such as:
- Marine environments
- Pre-stressed concrete
- Tunnel linings
- Bridge girders
- Watertight and water retaining structures
- Concrete pipes
2.1.5.3 Alkali Silica Reactivity Inhibitors (ASR’s).
mean that the concrete is vulnerable to corrosion, see Fig 1, sulphate attack and freeze thaw damage resulting in even further deterioration. ASR is a major problem within the construction industry and hence, its prevention is very important. This can be achieved, or at least reduced, through the use of ASR inhibitors.
ASR inhibitors work by controlling durability problems associated with alkali silica reactivity and reducing the level of deterioration within concrete in two ways. They extend the time in which ASR begins and reduce the amount that takes place.
These admixtures are particularly useful in concrete structures such as highways, runways, bridges, parking lots and garages although they are required in many other concrete applications where ASR is found.
2.1.5.4 Colouring.
Colouring admixtures can be used to alter the colour of concrete. The colour is determined by the pigments which make up the colouring agents that are added to the concrete mixture during mixing and these are generally divided into two categories, natural and synthetic.
Natural pigments are made from mined ores such as carbon (blacks), chromium (yellows, reds, browns, greens) and cobalt (blues). Synthetic pigments meanwhile are made from iron salts and comprise mainly of the colours, red, yellow and black.
Also, to gain a more permanent colour, the aggregates can also be exposed to the colouring admixtures. This has the advantage of minimising the effects of ultraviolet lights and efflorescence on the concrete. The disadvantage to colouring agents however is that the effects of efflorescence becomes more apparent with coloured concrete compared to traditional grey untreated concrete due to a greater differentiation in colour.
The main benefits however gained from the use of colouring admixtures are that of cost and aesthetics. Due to advances in technology they provide architects and the construction industry with the ability to achieve a variety of different finishes and colours. There use is so wide ranging that from being used in historical buildings to retain traditional colours they can be used to enable artistic images to be incorporated into the very fabric of a modern day building. Furthermore, the aesthetically pleasing can be achieved at a reduced cost of construction since adding colouring agents to the concrete mix is likely to yield better financial results than employing sub-contractors to carry out the necessary work to achieve the same desired effects, such as painting the concrete (a time consuming process).
2.2 Mineral
Mineral admixtures are powdered or pulverised which are generally natural or by-product materials. They affect the nature of the hardened or plastic concrete through hydraulic or pozzolanic1 activity. There similar to chemical additives in that they provide economic benefits and reduce the cost of construction. However, they also reduce permeability, increase strength and influence other concrete properties. Mineral admixtures fall into the following different functions:
- Fly ash
- Silica fume
2.2.1 Fly Ash.
Fly ashes are finely divided residue, which are derived from the combustion of coal. They work by helping to form the cementitious2 compounds of the concrete together and this has three main general effects, which are increased strength, greater durability and workability.
Fly ash has many benefits when used in concrete and there are two main classes of fly ash, class C and F, where each offers different advantages.
Class C ashes are used to provide the concrete with increased permeability and unique self-hardening characteristics. These are particularly good for use in applications where high early strength is required such as in pre-stressed concrete. Class F on the other hand, minimise bleeding and segregation of concrete in a plastic state. In a hardened state class F ashes reduce drying shrinkage, permeability, heat of hydration and creep, inhibit alkali-aggregate reaction and increase ultimate strength and sulphate resistance.
Fly ashes are generally applied in situations where a large quantity of concrete is required making the cost of construction more economical. For instance, they are frequently used in the construction of mass concrete pavement.
2.2.2 Silica Fume.
Silica fume, also known as microsilica3, is a by-product from producing silicon metal and ferrosilicon alloys. It works by reacting with the hydration products in Portland cement within concrete which forms a calcium silicate hydrate gel. The production of this gel enhances the strength of the concrete and provides it with greater durability.
Silica fume can improve concrete in two ways. The first is a basic pozzolanic reaction where the addition of silica fume strengthens bonding within the concrete and aids a reduction in permeability. The second is a result from silica fume being a microfiller, its fineness means that it can fill the microscopic air voids between the cement particles. This has the effect of reducing permeability and provides the concrete with a much better bond between aggregates compared to conventional concrete whilst increasing abrasion resistance, which leads to higher early age compressive and flexural strengths.
Applications for silica fume in the construction industry include high-strength structural columns, less permeable parking garage decks, and abrasion resistant hydraulic structures.
3. SUMMARY
In summary, admixtures are an essential component of concrete and key to the construction industry. They provide concrete with the vital characteristics and performances to enable the construction of a wide range of buildings. Without them, many of the buildings and structures that we see would not exist.
4. APPENDICES
1. Pozzolan
A cementitious material that occurs naturally and is produced as a by-product of coal combustion.
2. Cementitious
A construction material made with binder that has the properties of cement.
3. Microscilia
A type of pozzolan.
5. ACKNOWLEDGEMENTS
The structure of this report was based on the many recommendations given in a document by R H Mayo on the preparation and presentation of a technical report.
6. REFERENCES
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