Selection of Superplasticizers for High-Performance Concrete

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Superplasticizers are used to fluidify the concrete in the field. Most of the superplasticizers helped to maintain high slump values for a time of 30 to 45 minutes. But some of the Portland cement do no behave well with the use of superplasticizers. This problem is understood only by the detailed study on the mode of action of superplasticizers.

Cement and Superplasticizer Reactivity and Compatibility

It is clearly impossible to explain the minute details about the working of superplasticizers. But among many problems, the most faced issue is the slump-loss problem which is observed in the making of high-performance concrete. When the intention is to create a large slump value with a lower water/ cement ratio the mentioned problem is met. Generally, 120 to 135 L/m³ of mixing water is used in the manufacture of high-performance concrete compared to a quantity of 160 to 180L/m³ of water in ordinary concrete. This value is dependent on the entrained air content, the maximum size of the coarse aggregate, nature and other related factors. On one hand, the rheology of the high-performance concrete is mostly influenced by:

• the rate at which different Portland cement mineral phases react with the water molecules
• the rate at which molecules of superplasticizers are trapped by compounds that are formed newly by the contact of water and the Portland cement

Calcium hemihydrates (Plaster of Paris) which are formed during the dehydration of gypsum and the ettringite (that is formed in the interstitial phase) are the two phases that consume a substantial amount of water and show a rapid hydration. Most studies have observed that the by the addition of superplasticizers on the Portland cement, their molecules are adsorbed over the di and tricalcium silicates. This is to control their hydration efficiently or to retard substantially at required situations. The cement's rheological reactivity can be called as the rate at which the water molecules are consumed by the cement during the first moment of mixing. It is seen that that during hydration process certain number of superplasticizers are also consumed. The first moment consumption of Superplasticizer molecules after the mixing is qualified as the cement/Superplasticizer compatibility. It is very difficult to make high-performance concrete with a very low water-cement ratio, that it will still have a slump value of 50mm 30 minutes with an initial slump value of 200mm. It is observed that the concrete surfaces become shiny as the time progress. And it is not found appropriate to add more superplasticizer with an intention to increase the slump value, as the mix lacks water. Such situations can ask for a small amount of water to recover the slump instantaneously, but the gained slump value will be lost once the reaction has been carried out within it. So, the optimization of high-performance concrete for a location can be carried out by conducting the following investigations:

• Whether the cement is having lower rheological activity
• Whether the superplasticizers used will not compete with the crystals of ettringite

There is no theoretical method to determine the above mentioned behavior. So, it is necessary to proceed with the trial and error method.

Rheopump to Study the Behavior of Superplasticizers and Cement

An empirical test was developed at the University of Sherbrook. The method is proved to be efficient and extremely reliable. A great variety of cement that varies in the content of tri calcium aluminate, tetra calcium alumino ferrite, limestone filler, di and tri calcium silicates and cement with different fineness works compatible with the test. The whole arrangement is called the rheopump. The test comprises an arrangement of small modified pump that will recirculate a cement grout having a water cement ratio of 0.35. The grout consists of a reference superplasticizer of naphthalene. The dosage is based on the type of cement that is used in the preparation of the grout. This is dosed for a time of four minutes. As shown in the figure-1, the flow time taken for 1L of the prepared grout through the Marsh Cone is measured. This marsh cone is used to check the fluidity of bentonite in the petroleum industry. Once the measurement of initial time is completed, the grout is placed in another plastic container and subjected to continuous agitation until the next measurement. The next set of measurement will be taken 40 minutes later. Hence the initial reactivity and the compatibility of the chosen cement with the specified superplasticizer is understood within an hour by employing a single bag of cement. The rheopump have never failed even used with concrete for slump loss measurement.

Fig.1: Schematic Diagram Representing Rheopump

The rheopump can also be used to find the retarder dosage whenever it is necessary to slow down the slump loss or to find the compatibility of a different variety of superplasticizer with a given type of cement.

Problems in Choosing Appropriate Superplasticizer of High-Performance Concrete

It is very essential to carefully choose the different constituents to make a high-performance concrete. The concrete producers lack a complete knowledge of this particular area and the following problems are faced. 1. Problems with the physical Nature: Because of a limited number of bins and service to the regular customer, the facilities allow a little latitude in this area. 2. Problems due to an Economical Nature: Based on the geographic location, resources that are locally and economically available are demanded first then selecting others. 3. Problems with the Social Nature: Limit the Superplasticizer selection due to the tie up with known cement or quarry companies. The concrete producers may have a personal dealing with any of the cement companies that limit the variety of selection.

Selection of Superplasticizer for High-Performance Concrete

Certain general rules that each concrete producer must strictly follow to bring the desired result are mentioned below:

Type of Superplasticizer

By only focusing on the solid content it is recommended to use naphthalene superplasticizers when compared with the melamine. The naphthalene superplasticizers are marketed with a solid content from 40 to 42 percent. The melamine superplasticizers are marketed for a solid content of 22 to 30 %. New melamine superplasticizers are said to have solid content of 40 percent. The experience show that the efficiency of the superplasticizers depends on the following parameters:

• Quantity of solids present
• Quality of the solids
• The length of the molecular chains
• The amount of impurities that are present
• The amount of residual sulphates present

Hence it can be concluded that the choice of superplasticizer is not dependent on the solid content alone, but also on the economic efficiency, that is by thinking about the dollars that must be spent to reach the desired workability. Other factors that influence the choice of superplasticizer is the technical and the commercial problems. This include factors like quality service that is offered by the admixture company, the consistency of quality of superplasticizer, the regularity of superplasticizer delivery and the confidence laid on the admixture company. Superplasticizers may be chosen only by considering the technical factors. For example, the melamine superplasticizers where chosen by some of the precasters, because they provide surfaces free from bubbles. In North America, due to commercial reasons, melamine was chosen commonly for the concrete manufacture. This is mainly because of the reason that the market status of melamine superplasticizer was very well done. But now as the use of high performance concrete moved worldwide, the use of naphthalene superplasticizer also increased.

Brand of Superplasticizer

The number of superplasticizer manufacturers whether it be naphthalene or melamine, is very limited. This will lead to rise of several commercial superplasticizers that have the same reactor and showing similar properties. The origin of certain commercial superplasticizers can be determined by analyzing their infrared spectrum and subjecting their respective solids to thermal analysis.

Type of Superplasticizer from a Brand

There was a time where a single superplasticizer was only available where the contractors have no other choice of selection. But now it is not the case. A single brand provided different variety of superplasticizers. It is a time taking process to determine which products from the same product must be chosen. The Portland cement can be classified into different types. But each type shows different characteristics which is influenced by various factors. Superplasticizers with variation in types do show variability it character, but it is less compared with Portland cement. This is because the superplasticizers are manufactured in a straight forward process incorporating limited number of raw materials that are fairly pure.

Type of Formulation - Liquid or Solid

The superplasticizers are offered either in the solid or in the liquid form. When considering the ease of use and the limited time for mixing, liquid superplasticizers are mainly used. It must be kept in mind that the superplasticizers are very sensitive to ambient temperature changes. The naphthalene superplasticizers can freeze at a temperature of -4 degree Celsius. When the ambient temperature is below 5-degree Celsius their viscosity will decrease. This change in property will affect the efficiency that is guaranteed by the superplasticizer. In case of frozen superplasticizers, they must be kept at a maximum temperature of 35-degree Celsius for a time period of 24 hours. It is observed that certain superplasticizers will perform undesirable at high temperatures. They will develop bad odors, fungi, and bacteria. So, to take the full advantage of superplasticizers, it must be stored in temperatures between 10 and 30 degree Celsius.

Dosage of Superplasticizer

The dosage of superplasticizers is known in concrete plants as in liters of commercial solution per cubic meter of concrete. This representation is in contrast with the scientific practice of representing the superplasticizer dosage in terms of solid content. It is very important to know how to consider the amount of water that is contributed by the superplasticizer as it is in turn necessary to control the water cement ratio of high-performance concrete.

Utilization with a Water Reducer

Some concrete producers to save money ass lignosulphonate based water reducers to the superplasticizers. These will be dosed at the usual rate of 0.5 to 1.5L/m³. This is a common practice and a rule in Norway and producers in North America. This method is considered to save superplasticizers.

Utilization with a Retarder

Experience and observation have shown that the incorporation of a certain amount of retarding agent at the time of mixing the concrete along with the superplasticizer can help in solving the slump loss problem that is faced by the the producer. This retarder use is mainly done when rheologically active types of cement are used. The retarding agent can be added in an amount equal to 5 to 10 percent of the weight of superplasticizer that is added to the whole mix. The problem of slump loss is solved without retarding the setting of the concrete. Retarding agent based on sodium gluconate will work best when compared with lignosulphonate retarding agents. This is because the sodium gluconate helps in fewer entrapping of air bubbles when compared with a lignosulphonate retarding agent. Sodium gluconate is found to be more effective practically and is more controlled than lignosulphonate when it is used as a retarding agent. Conditions, where the superplasticizers and the retarding agents have to be used simultaneously, leads the producer to determine the optimum dosage in terms of cost as well as the compressive strength for short term. Any delay in the compressive strength that is to be gained within a single day will delay the work. The use of superplasticizer and the retarding agent is a tricky method. The climatic factors also must be considered while optimizing their amount, because the cement reactivity is greatly affected by the temperature of the concrete.