 |
1.0 Introduction
Admixtures are materials other than cement, aggregate and water that are added to concrete either before or during its mixing to alter its properties, such as improved retention of workability; higher early strengths; higher ultimate strengths; reduced shrinkage; improved durability of reinforced structural concrete; and enhanced quality of surface finishes. Some admixtures have been in use for a very long time, such as calcium chloride to provide a cold-weather setting concrete. Others are more recent and represent an area of expanding possibilities for increased performance.
The chemistry of concrete admixtures is a complex topic requiring in-depth knowledge and experience. A general understanding of the options available for concrete admixtures is necessary for acquiring the right product for the job, based on climatic conditions and job requirements. Based on their functions, admixtures can be classified into the following five major categories:
* Air-entraining admixtures
* Retarding admixtures
* Accelerating admixtures
* Water reducing/plasticizing admixtures
* Superplasticizers
Among other important admixtures that do not fit into these categories are admixtures whose functions include bonding, shrinkage reduction and damp proofing. The following paragraphs provide details on the above-mentioned categories of concrete admixtures.
2.0 Air-entraining admixtures
Air-entraining agents entrain small air bubbles in the concrete. The major benefit of this is enhanced durability in freeze-thaw cycles, especially relevant in cold climates. While some strength loss typically accompanies increased air in concrete, it generally can be overcome by reducing the water/cement ratio via improved workability (due to the air-entraining agent itself) or through the use of other appropriate admixtures
Air entrainment is the process whereby many small air bubbles are incorporated into concrete and become part of the matrix that binds the aggregate together in the hardened concrete. These air bubbles are dispersed throughout the hardened cement paste but are not, by definition, part of the paste. In addition to increased freeze-thaw resistance, air-entrained concrete is more workable than non-entrained concrete. The use of air-entraining agents also reduces bleeding and segregation of fresh concrete
2.1 Factors affecting air-entrainment
Constituent materials also influence the air-void system created by using air-entraining agents in concrete. Concrete materials such as cement, sand, aggregates, and other admixtures play an important role in maintaining the air-void system in concrete. For example:
* It has also been found that air content will increase as cement alkali levels increase and decrease as cement fineness increases significantly.
* Mineral admixtures such as fly ash also affect the formation of void systems in concrete. Whereas, the effects of fly ash on air-void stability of concrete containing fly ash produced relatively stable air-void systems. However, fly ash types affect the volume of air retained. In mixtures containing fly ashes, the amount of air-entraining agent required to produce a given percentage of entrained air is higher, and sometimes much higher, than it is in comparable mixtures without fly ash.
* Silica Temperature can also have a significant effect on air entrainment. Air entrainment varies inversely with temperature. The same mix will entrain more air at 10°C than at 35°C.
2.2 Air content control
Measurement of air content is an important checking "sensor" for the concrete user to know whether concrete will resist freeze-thaw damage. Because average void spacing decreases as air content increases, an "optimum" air content at which void spacing will prevent the development of excessive pressure due to freezing and thawing will exist.
It is important to check the air content of fresh concrete regularly for control purposes. Air content should be tested not only at the mixer but also at the point of discharge into the forms, because of losses of air content due to handling and transportation.
3.0 Retarding admixtures
Retarding admixtures (retarders) slow down the hydration of cement, lengthening set time. Retarders are beneficially used in concrete to offset the effect of high temperatures, which decrease setting times, or to avoid complications when unavoidable delays between mixing and placing occur. Because most retarders also act as water reducers, they are frequently called water-reducing retarders.
3.1 Composition and Mechanism
Many water reducers have a retarding tendency. Therefore, some of the ingredients in water reducers, such as lignosulfate acids and hydroxycarboxylic acids, are also a basis for set-retarding admixtures. Other important materials used in producing set retarders are sugars and their derivatives.
The role of retarding admixtures can be explained in a simple way: the admixtures form a film around the cement compounds (e.g., by absorption), thereby preventing or slowing the reaction with water. The thickness of this film will dictate how much the rate of hydration is retarded. After a while, this film breaks down, and normal hydration proceeds. Other factors influencing the degree of retardation include the water/cement ratio, cement content, C3A and alkali contents in cement, the type and dosage of the admixture, and the stage at which the retarder is added to the mix. The effectiveness of retarder is increased if its addition to the fresh concrete is delayed for a few minutes.
3.2 Effect on Concrete Properties
In addition to their role in controlling setting time, retarders - like any other admixtures - influence the properties of fresh and hardened concrete. Air entrainment of concrete is affected and fewer air-entraining agents need to be used because some retarders entrain air (see water reducers). Workability (slump) loss might increase even when abnormal setting behaviour does not occur.
Because of retarding action, the 1-day strength of the concrete is reduced. However, ultimate strength is reported to be improved by using set-controlling admixtures. Rates of drying shrinkage and creep could increase by using retarders, but the ultimate values cannot increase.
4.0 Accelerating admixtures
Accelerators shorten the set time of concrete, allowing a cold-weather pour, early removal of forms, early surface finishing, and in some cases, early load application. Proper care must be taken while choosing the type and proportion of accelerators, as under most conditions, commonly used accelerators cause an increase in the drying shrinkage of concrete.
Calcium chloride is a common accelerator, used to accelerate the time of set and the rate of strength gain. However, excessive amounts of calcium chloride in concrete mix may result in rapid stiffening, increase in drying shrinkage and corrosion of reinforcement. Chloride free accelerators based on triethanolamine, calcium formate and sodium thiocyanate are widely available.
The compressive and the flexural strengths of concrete containing accelerating admixtures will develop more rapidly compared to plain concrete. Benefits to concrete construction and to the manufacture of concrete products because of the increase in early strengths are as follows:
* Earlier stripping and re-use of forms for walls and pre-cast work
* Earlier structural use of concrete, as in lift-slab construction, tilt-up, paving and floors.
* Earlier tensioning in post-tensioned concrete
* Earlier de-tensioning in pre-stressed concrete
* Potential energy savings in accelerated steam-cured applications (pre-stressed concrete)
* Potential energy savings in accelerated steam-cured applications (pre-stressed/precast) by reducing curing temperatures and/or the curing time necessary to reach the desired strengths.
5.0 Water reducing/plasticizing admixtures
Water-reducing admixtures are groups of products that are added to concrete to achieve certain consistencies (slump) at lower water/cement ratios than that of control concrete. Water-reducing admixtures are used to improve the quality of concrete and to obtain specified strength at lower cement content. They also improve the properties of concrete containing marginal- or low-quality aggregates and help in placing concrete under difficult conditions. However, water-reducing admixtures can entrain air into the concrete mix via its effect on water's surface tension, thereby also, obtaining some of the benefits of air-entrainment.
5.1 Effect of water-reducing admixtures
Water reducing admixtures are based on modified lignosulfonic acid derivatives, hydroxycarboxylic acids or hydroxylated polymers. When incorporated into concrete the use of water reducing admixtures reduces water demand by approximately 7 - 10%. Typical, water-reducing admixtures are used to:
* Increase the compressive strength by reducing the water content, whilst maintaining the cement content
* Reduce the water content but keep the strength constant by reducing the cement content (an environmental benefit, the less cement that is used the less raw materials and fuel for production that are consumed).
* Increase consistence (slump) whilst maintaining the cement content and compressive strength
A higher dosage of admixture can lead to more water reduction; however, excess retardation of the hydration mechanism and or excessive entrainment of air may be encountered.
6.0 Superplasticizers
The first generation of superplasticizers were commercially launched in the early 1960's and had an effective working life of less than one hour. Superplasticizers were first used in the United Kingdom in 1973. The original application of superplasticizers was to produce flowing concrete to be used in heavily reinforced structures and in placements where adequate consolidation by vibration cannot be readily achieved. However, they now have far wider applications including the production of high-strength concrete (compressive strengths greater than 100 MPa) at water/cement ratios ranging from 0.3 to 0.4.
Superplasticizers can be used in the same three ways as a conventional plasticizer:
* To impart extreme workability (beyond that obtainable with a conventional plasticizer)
* To permit a large water reduction to be made beyond the limits of normal plasticizing admixtures
* To achieve economic and environmental benefits (e.g. reduction of the cement content) whilst maintaining performance
The mode of action of superplasticizers is similar to conventional plasticizers; the admixture particles are adsorbed onto the cement particles, causing them to become mutually repulsive and thus having a dispersing effect.
The materials used to manufacture superplasticizers include:
* Melamine formaldehyde condensates (introduced in Germany in 1964)
* Naphthalene Formaldehyde (introduced in Japan 1963)
* Polycarboxylic ether (PCE)
As with most types of admixtures, superplasticizers can affect other concrete properties as well. One problem associated with using superplasticizers is slump loss whereas slump loss with time is very rapid in spite of the fact that second-generation high range water reducers are claimed not to suffer as much from the slump loss phenomenon as the first-generation conventional water reducers do. However, slump loss of flowing concrete was found to be less severe, especially for newly developed admixtures based on Polycarboxylic ether formulations.
7.0 New generation Polycarboxylic ether (PCE) based superplasticizers
The most important improvement of the new generation of polycarboxylate (PCE) based superplasticizers, compared to traditional superplasticizers, is their ability to maintain consistence (or reduce slump loss) over a prolonged period of time, even at low water/cement ratios. With appropriate mix design, loss of workability can be negligible during transportation of concrete. This can be of particular benefit in the production, transportation and placing of ready-mixed concrete. These new generation admixtures have been successful proved in ready mixed plants.
Problems with slump loss can be particularly troublesome with central batch plants, for example where summer temperatures are high and delivery radials long, especially if through congested traffic. Consequently, the benefits to the ready-mixed industry can include: increased delivery radials; faster truck loading; faster truck discharge and turn-round at site; less risk of rejected loads; economies gained by being able to rationalize the design high slump mixes at low water/cement ratios. Also product differentiation to customers, for example self-compacting concrete (SCC).
The new generation of polycarboxylate (PCE) based superplasticizers, act as powerful cement dispersants, consequently water reductions of 40% are achievable at comparatively low admixture dosage rates. Such exceptionally high water reductions can give significant benefits including:
* Higher early strengths
* High ultimate strengths
* Reduced drying shrinkage
Polycarboxylate ether (PCE) based superplasticizers, derived from new chemistries originally developed in Japan, have now established a strong track record of successful use all over the world.
These new technologies have been used to solve a variety of concrete problems in many major construction projects. Examples include high performance concrete (HPC) for high-rise residential structures, self-compacting concretes (SCC), and high quality precast concrete. Significant structural and engineering benefits have been achieved, and cost savings made in the construction processes.
|
|