High Performance Concrete Admixtures for Improving the Properties of Concrete

Pramod Pathak, Director, Multichem Group, Mumbai.

Admixtures are the ingredients in concrete which are other than the hydraulic cementitious material, water, aggregates or fiber reinforcement that are used as ingredients of a cementitious mixture to modify its freshly mixed, setting or hardened properties and that are added to the batch before or during mixing. Admixtures are usually further defined as a non–pozzolanic (does not require calcium hydroxide to react) admixture in the form of a liquid, suspension or water-soluble solid. 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. Not all admixtures are economical to employ on a particular project.
High Performance Concrete Admixtures for Improving the Properties of Concrete
Also, some characteristics of concrete, such as low absorption, can be achieved simply by consistently adhering to high quality concreting practices.

Water-reducing admixtures improve concrete’s plastic (wet) and hardened properties, while set-controlling admixtures are used in concrete being placed and finished in other than optimum temperatures. Both, when used appropriately, contribute to good concreting practices. Also, both admixtures should meet the requirements of ASTM C 494, (Table 1).

Water-Reducing Admixtures

High Performance Concrete Admixtures for Improving the Properties of Concrete
Water reducers decrease the amount of mixing water required to obtain a given slump. This can result in a reduction of the watercementitious ratio (w/c ratio), which leads to increased strengths and more durable concrete.

Reducing the w/c ratio of concrete has been identified as the most important factor to make durable, high-quality concrete. On the other hand, sometimes the cement content may be lowered while maintaining the original w/c ratio to reduce costs or the heat of hydration for mass concrete pours.

Water-reducing admixtures also reduce segregation and improve the flow ability of the concrete. Therefore, they are commonly used for concrete pumping applications as well.

Water-reducing admixtures typically fall into three groups: low-, medium- and high-range. These groups are based on the range of water reduction for the admixture. The percent of water reduction is relative to the original mix water required to obtain a given slump (Table 2). While all water reducers have similarities, each has an appropriate application for which it is best suited. Table 3 presents a summary of the three types of water-reducing admixtures, their ranges of water reduction and their primary uses. Their effect on air entrainment will vary depending on the chemistry.

How They Work?

When cement comes in contact with water, dissimilar electrical charges at the surface of the cement particles attract one another, which results in flocculation or grouping of the particles. A good portion of the water is absorbed in this process, thereby leading to a cohesive mix and reduced slump.

Water-reducing admixtures essentially neutralize surface charges on solid particles and cause all surfaces to carry like charges. Since particles with like charges repel each other, they reduce locculation of the cement particles and allow for better dispersion. They also reduce the viscosity of the paste, resulting in a greater slump.

High Performance Concrete Admixtures for Improving the Properties of Concrete
Table 4 presents some of the most common basic materials used for each range of water reducer. Other components are also added depending on the requirement of additional properties of concrete. Some water-reducing admixtures have secondary effects or are combined with retarders or accelerators. This will be discussed later.

Effects on Concrete

Water-reducing admixtures are primarily used to reduce the water-cementitious content of concrete, thus increasing strength. In some cases, they can be used to increase the workability or slump of the concrete providing for easier placement. Mid-range water-reducing admixtures were developed to increase the slump beyond the range available with regular water reducers without the excessive retardation that had been known to occur. High-range water reducers, commonly called superplasticizers, were developed for high-strength and high performance concrete applications.

Superplasticizers, e.g., Multiplast Super can take a 3- inch slump concrete to a 9-inch slump without risk of segregation and without compromising its strength. Many precasters can benefit from the use of a superplasticizer, especially because of its improved high early strength development.

All water-reducing admixtures increase strength development as a result of better dispersion of the cement. This increases the exposed surface area of the cement particles, allowing for more complete hydration of the cement.

Set-Controlling Admixtures

Set-controlling admixtures alter the rate of the cement’s hydration and, therefore, the rate of setting (stiffening) of the paste. Coincidentally, they also may affect the hardening or strength gain after the paste has set. Setcontrolling admixtures include retarding and accelerating admixtures.

Retarding Admixtures

These admixtures, Multiplast R slow down the hydration process. They may also reduce the setting time of cement. Retarding admixtures fall into two categories: regular and extended-set. Regular, most commonly referred to as just “retarders,” are used to place concrete in hot climates when long travel times are expected or, in case of emergency, when placement is delayed. They are also commonly used for mass concrete pours to prevent cold joints.

Extended-set control admixtures are those used to delay hydration for many hours or even days. These are usually a twocomponent admixture system. The first component is a retarder (stabilizer) which delays the setting of concrete. The second component is an accelerator (activator) which overcomes the retarder. The concrete typically reaches initial set in a few hours after the activator is applied.

How they work Retarders essentially slow early hydration by reducing the rate at which tricalcium silicate (C3S) reacts with water. Furthermore, retarders slow the growth of calcium hydroxide crystals. Both reactions develop the early setting and strength gain characteristics of paste. The effect remains until the admixture is incorporated into the hydrated material, thereby removing it from the solution and allowing for initial set to occur. The duration of retardation is based on the dose and chemistry of the retarder, cement composition, temperature and the time it was added to the mix.

Accelerators

These admixtures increase the cement’s rate of hydration. Multiplast ACC are designed to increase the rate of hydration of C3S, thereby increasing early strength. There are two types of accelerators: rapid and normal.

Rapid accelerators can set concrete in minutes and are used in shotcreting applications, to make repairs against hydrostatic pressure or when very rapid setting is required. These are typically not used in precast concrete applications.

Standard or normal accelerators are used to speed up construction in cold-weather concreting conditions; however, it is important to note that they are not antifreezing admixtures.

Effect on concrete: Both retarders and accelerators seem to have negligible effects on air entrainment. However, when water-reducing agents are included, such as lignosulfonates, some air may be entrained.

Retarders tend to reduce one-day strengths and usually increase later-age strengths . Retarders may also increase slump loss and cause an early stiffening of the mixture, even though the strength gain has been delayed. Retarders tend to lose their effectiveness as concrete temperature increases. They also tend to increase the plastic shrinkage.

Accelerators typically increase early strengths. However, laterage strengths may be reduced relative to the same concrete without the accelerator. They also tend to increase early-age shrinkage and creep rates, but tests have shown that ultimate values seem to be unaffected.

Combinations

Some admixture chemistries provide for a combination of effects such as water reduction with retardation or acceleration. Advantages of this include reducing the number of admixtures that have to be stored and added to the concrete; less admixture incompatibility; and cost savings. Disadvantages include less flexibility and limited use when an accelerating or retarding effect is not desired. ASTM C 494 lists specifications for these combination admixtures.
Click Here
To Know More or to Contact the Manufacturer
Please let us know your name.
Invalid Input
Please let us know your Designation.
Please let us know your Contact Number.
Please let us know your email address.
Please brief your query.
Our other Value-Added Services:

To receive updates through e-mail on Products, New Technologies & Equipment, please select the Product Category(s) you are interested in and click 'Submit'. This will help you save time plus you will get the best price quotations from many manufacturers, which you can then evaluate and negotiate.

Invalid Input
Invalid Input
Invalid Input
Samir Surlaker, Director, Assess Build Chem Private Limited, emphasizes the importance of a clear cover for a concrete structure since concrete as a porous material needs protection of its reinforcement. Along with the thickness (quantity) of cover, the porosity of

Read more ...

Concrete technology has come a long way since the Romans discovered the material, with a number of ingredients, which include a host of mineral and chemical admixtures, besides of course, the Portland cement, aggregates (coarse and fine), and water. These ingredients

Read more ...

Anil Kumar Pillai, GM, Ramco Cements, discusses two major softwares (Life 365 and DuraCrete), used in the industry for protection of RCC structures. The common design approach is faulty because we consider only the loading aspect, whereas the environmental aspect is equally

Read more ...

Fibre Tuff, macro synthetic polypropylene fibres, are heavy-duty synthetic fibres that are specially engineered for use as secondary reinforcements, providing excellent resistance to the post cracking capacity of concrete. They are replacing steel fibres in a range

Read more ...

Reinforced concrete design and construction practice has historically focused on the use of bonded straight or bend rebar as a method for rebar anchorage. This relies on bond integrity between the rebar and the concrete so that sufficient anchorage

Read more ...

Innovation and entrepreneurship are essential ingredients in building a successful commercial venture. The ways in which these two concepts fuel enterprise are something entrepreneur's never stop exploring. There is no doubt that innovation were

Read more ...

Alite and belite are the predominant phases of Portland cement formulation. Alite is impure tricalcium silicate (C3S) and belite is impure dicalcium silicate (C2S). The impurities are an integral part as cement is manufactured

Read more ...

Concrete is a versatile construction material and day by day its consumption is increasing globally. It is second only to water in the global consumption. No civil engineering structure is feasible without using concrete

Read more ...

The use of Graphene with concrete has been talked about and researched ever since Graphene was invented in 2010 which grabbed its inventors a Nobel prize. Nanospan is the first company in the world to break technological and commercial

Read more ...

Fosroc is the foundation of the JMH Group. It employs over 1700 employees in 17 operating companies based in Europe, the Gulf & Middle East, India, South Asia, and China. Through FGT, its trading company, it services another 50 countries

Read more ...

Established in 1983 by French expatriate entrepreneurs, the Dextra Group has a long history of growth and development, driven by strong entrepreneurship and innovation. It has diversified into three main activities: manufacturing, trading and freight forwarding

Read more ...

Jyotirmoy Mishra, Ph.D. Scholar, Department of Civil Engineering, Veer Surendra Sai University of Technology, Burla, Odisha, presents his research on the feasibility and compressive strength performance of geopolymer concrete

Read more ...

As every one ton of Cement (OPC) produced, emits 0.96 tons of CO2, there is an urgent need to promote blending materials (ex. GGBS &PSC) and screened slag, to achieve lower CO2 emissions, reduce greenhouse gas effect, reduce exploitation

Read more ...

In most of the developing countries, demand for steel for use as a reinforcing material is increasing day by day. However, when steel is in short supply, one can consider bamboo as an alternative material for reinforcement

Read more ...

There is high demand for white cement in countries with hot climates, as more heat is reflected from white concrete surfaces as compared to standard grey concrete. As a value-added product, white cement is becoming

Read more ...

Garry Martin, Director - Major Projects, Low & Bonar Construction Fibres, presents a new examination of the benefits of micro fibres in both the plastic and hardened state of concrete and their contribution to increased sustainability.

Read more ...

An integrated material and structural design strategy of strength through durability is the need of the hour since structures are designed for ductility and structural integrity. Dr. S. B. Hegde, President – Manufacturing

Read more ...

The demand for structural strengthening of ageing structures is growing rapidly in buildings, industrial structures, infrastructure projects like bridges, dams, etc. Structural Strengthening also

Read more ...

Durability and strength are two most important criteria and requirements for the long-term performance of concrete structures against weathering action, chemical attack and abrasion. Any deficiency

Read more ...

Cement is a key binder component of concrete production in the building industry. It is a complex hydraulic binder, made up of four main clinker components; alite (Ca3SiO5), belite (Ca2SiO4)

Read more ...

×
Sign-up for Free Subscription
'India Construction Week'
Weekly e-Newsletter on Construction Industry
Get the latest news, product launches, projects announced / awarded, government policies, investments, and expert views.
Click here to subscribe.