The modern-day material chemistry has revolutionized all industries, and concrete is also not an exception. Adding few ml of chemicals to concrete modifies the property of concrete to a larger extent, and hence its application is increasing day by day.
Concrete is the most used material in the construction, made of cement, sand, aggregate, water, and concrete admixtures. The concrete admixtures or concrete additives are the important materials which modify one or more than one properties of concrete in the plastic state and the hardened state, such as an increase or decrease in setting time, increased hardening, workability, strength, unit weight etc.
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There are many kinds of admixtures available which can be used in concrete to modify the properties of concrete such as concrete accelerator, concrete retarder, plasticizers, Superplasticizer, air entraining admixtures etc. These admixtures are added before mixing or during mixing of concrete itself.
Here, we are going to discuss the concrete retarders in detail, so that you can get an idea about its working mechanisms, uses, advantages and disadvantages
According to ‘Indian Standard, IS : ’ (Concrete Admixtures- Specification (First Revision)), “An admixture which delays the setting of cement paste, and hence of mixtures, such as mortar or concrete containing cement is called retarders”. Concrete retarders are also Known as retarding admixtures or retarders.
Concrete retarders are added in the concrete to delay the setting time of the concrete. As you know, the procedure of setting or stiffening is a result of a chemical reaction between water and cement in the concrete. This reaction is called hydration. When retarders are used in the concrete, it temporarily slows down the chemical reaction (hydration). Hence the concrete remains in plastic stage and remains workable for a longer time.
Sometimes we need more time to place concrete because of difficult placing condition or delay may occur in transportation. Take an example, in ready mix concrete, concrete is manufactured in the central batching plant and transported over a long distance and often to the work sites which may take significant time depending on the mode of transportation. In that case, if you want the concrete in a plastic state for perfect final placing and compaction, then the setting time of concrete will have to be increased or simply delayed by retarding its hydration process.
Concrete retarders are also used to overcome the accelerating effect of high temperatures on setting properties of concrete in hot weather concreting.
‘The European Federation of Concrete Admixture Associations (EFCA)’ stated that, “Retarding admixtures are used to slow down the speed of the reaction between cement and water by affecting the growth of the hydration products and/or reducing the rate of water penetration to the cement particles”.
The mechanism of the concrete retarders can be understood in a simple way: the retarding admixtures form a film around the cement compounds (e.g., by absorption). This film prevents or slows the reaction with water. How much the rate of hydration will be retarded is governed by thickness of this film. When this film breaks down, then normal hydration starts. If in some cases when the dosage of concrete retarders exceeds a certain critical point, hydration of cement compounds will never proceed beyond a certain stage, and the cement paste will never set. Hence, it is important to avoid overdosing and take proper care while using a retarding admixture.
The factors influencing the degree of retardation are water-cement ratio, cement content, C3A and alkali contents present in the cement, type and dosage of the concrete retarder, and the stage at which the retarder is added to the concrete mix itself. The efficiency of retarder is increased if its addition to the fresh concrete is delayed for a few minutes.
Considered a boon for the construction process in the long run, concrete retarders assist with multiple advantages and some major advantages are being discussed below for your reference,
Everything has a slice of precautions or you may say disadvantages. Same stands true in case of concrete retarders. Laid below are some common disadvantages that one should be aware of before using concrete retarders for the construction process.
Concrete retarders are abundantly used or applied in various construction settings; however, some most popular uses or applications are given over here,
According to ‘M.S. Shetty’ (Author of the Concrete Technology Theory and Practice 16th edition), oil wells are sometimes taken up to a depth of about -meter-deep where the temperature may be about 200°C. The annular spacing between the steel tube and the wall of the well will have to be sealed with cement grout. Sometimes porous rock strata may also require grouting to prevent the entry of gas or oil into some other strata. For all these works, cement grout is required to be in workable condition for around 3 to 4 hours and not get set at that high temperature. For meeting these challenging requirements retarding admixtures are commonly applied and mostly yield preferable results.
The most commonly known concrete retarder is calcium sulphate – gypsum. Sugar is also one of the most common and effective retarding admixture which is used for delaying the setting time of concrete without detrimental effect on the ultimate strength of concrete itself.
There are number of other organic and inorganic materials used as concrete retarders, these are:
Lignosulphonic acids and their salts and Hydroxylated carboxylic acids and their salts which in addition to the retarding effect also reduce the quantity of water required for the given workability.
Nowadays, admixtures are manufactured to combine a set of retarding and water reducing properties. They are usually made by the mixing of conventional water reducing admixtures and sugars or hydroxylated carboxylic acids or their salts.
In the summary, the knowledge about the concrete retarders is essential for their proper application. It’s beneficial for maintaining workability in hot weather concreting, in difficult placing conditions, for long transportation etc. But it is highly recommended that before retarder is used, trial mixes with the actual cement to be used in construction, should be tested. Any mistake may Cause delay in construction, wastage of time and money too.
Table of Contents
Retarders are specialized admixtures used in concrete to delay the setting time and maintain its plasticity for extended periods. This characteristic makes them indispensable in scenarios where concrete placement and compaction require additional time due to complex structural designs, challenging weather conditions, or lengthy transportation.
Primarily, retarders slow the chemical process of hydration, ensuring that concrete remains workable even in high-temperature environments where rapid hardening is a concern. They are commonly employed in ready-mix concrete applications, large-scale pours, and specialized tasks such as oil well grouting or achieving unique surface finishes like exposed aggregates.
By mitigating the risk of cold joints, improving workability, and providing flexibility in placement schedules, retarders play a crucial role in enhancing the quality and durability of concrete structures. Their versatility and adaptability make them an essential component in modern construction practices.
The effectiveness of retarders in concrete lies in their ability to temporarily delay the hydration process, thereby extending the setting time. This delay occurs due to various chemical and physical interactions between the retarder and the cement particles, categorized into four main mechanisms:
Retarders form a protective layer on the surface of cement particles by adhering to them. This layer acts as a diffusion barrier, preventing water molecules from effectively reaching the unhydrated cement particles. As a result, the hydration process slows, keeping the concrete in a workable state for a longer duration.
In normal conditions, the ions released during the initial interaction of water and cement crystallize to form hydration products like calcium hydroxide. Retarders interrupt this process by adsorbing onto the calcium hydroxide nuclei, delaying their growth and subsequent hydration.
Retarders react with calcium ions released by the cement grains to form complexes, which increase the solubility of cement components in the aqueous phase. This accumulation of calcium and hydroxyl ions inhibits their precipitation, effectively delaying the formation of hydration products.
Some retarders react with the alkaline solution in the cement paste to form insoluble derivatives. These derivatives create a protective coating around the cement particles, further impeding the water-cement interaction and slowing hydration.
Temporary Nature of Retardation
It is important to note that the retardation effects are temporary. After a predictable period, the hydration process resumes, ensuring that the concrete eventually achieves its intended strength and durability.
Retarders used in concrete can be broadly categorized into organic retarders and inorganic retarders based on their composition. Each type has unique properties that make it suitable for specific applications.
Organic retarders are carbon-based compounds that effectively delay the setting time of concrete. Common examples include:
These are mineral-based compounds that interact chemically with cement to slow down hydration. Examples include:
Modern construction often requires admixtures that combine retardation with other benefits, such as water reduction. These hybrid admixtures, like retarding plasticizers or superplasticizers, offer enhanced performance and convenience in concrete mix designs.
The use of retarders in concrete significantly influences its fresh and hardened properties. While their primary purpose is to delay the setting time, their effects extend to other aspects of workability, strength, and durability.
Retarders extend both the initial and final setting times, allowing for better placement, compaction, and finishing. This is especially beneficial in hot weather, where accelerated hydration can lead to premature setting.
Retarders improve the workability of concrete by increasing its initial slump, which is especially useful for complex placements or long-distance transportation. They also reduce slump loss over time, ensuring the mix remains workable until placement.
While most retarders do not significantly affect air content, certain types, such as those based on hydroxycarboxylic acids, can reduce air content, potentially impacting freeze-thaw resistance.
Retarded concrete is more prone to bleeding because of the extended plastic state. Proper mix design and curing practices can minimize this issue.
Although retarders do not reduce the total heat output, they delay the rise of the peak temperature, making them valuable for mass concrete pours where thermal cracking is a concern.
Properly cured retarded concrete exhibits durability comparable to conventional concrete. It is resistant to environmental stressors, provided the mix design and curing practices are appropriate.
Plastic shrinkage may slightly increase in retarded concrete, but creep and drying shrinkage are generally unaffected.
The incorporation of retarders in concrete offers several benefits that enhance its performance and usability, particularly in challenging conditions. Below are the key advantages:
Retarders counteract the rapid setting caused by high temperatures, allowing sufficient time for mixing, transportation, and placement.
In large or sequential pours, retarders ensure that successive layers bond seamlessly, preventing weak points or cold joints in the structure.
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Retarders are instrumental in creating exposed aggregate finishes. By delaying surface hardening, they enable the easy removal of the cement paste, revealing aggregates for aesthetic appeal.
For intricate structures or areas with difficult access, retarders extend the working time, facilitating proper placement and compaction without compromising quality.
Retarders are essential in ready-mix concrete to maintain workability during long transportation times, ensuring the mix remains in a plastic state upon arrival at the site.
By delaying the heat of hydration, retarders reduce the likelihood of thermal cracking in large concrete pours, promoting structural integrity.
Retarders are used in grouting deep oil wells, where high temperatures demand extended workability of the cement slurry.
Modern retarders often combine with plasticizers or superplasticizers, providing additional benefits like water reduction, enhanced slump retention, and improved strength development.
These advantages make retarders a crucial component in achieving high-quality concrete in a wide range of construction scenarios.
Retarders are used in various construction scenarios where controlling the setting time and maintaining workability are essential. Below are some of the key applications:
In regions with high ambient temperatures, retarders slow down the hydration process, preventing rapid setting and allowing sufficient time for placement and finishing.
Retarders are crucial for maintaining workability during the transportation of ready-mix concrete over long distances, ensuring the mix remains plastic until it reaches the site.
In projects involving large volumes of concrete, such as dams or foundations, retarders help prevent cold joints by extending the workable time between successive pours.
Retarders are sprayed on formwork surfaces to delay setting at the concrete-formwork interface. This enables easy removal of the paste to expose aggregates, creating visually appealing finishes.
In continuous construction methods like slip forming, retarders help regulate the setting time to ensure a consistent and smooth process.
In deep oil wells with high temperatures (up to 200°C), retarders are used in cement slurries to maintain mobility for extended periods, ensuring proper sealing and preventing gas or oil leakage.
Retarders are employed to manage the heat of hydration in mass concrete pours, minimizing thermal cracking and ensuring structural stability.
For architectural projects requiring intricate designs or textures, retarders provide the flexibility to achieve desired effects without compromising the concrete’s integrity.
These applications highlight the versatility of retarders in addressing diverse construction challenges.
When choosing and using retarders in concrete, several factors must be considered to ensure optimal performance. The selection process involves understanding the specific requirements of the project, the type of cement, and the environmental conditions. Here’s a guide to effectively selecting and handling retarders:
The effectiveness of retarders can vary depending on the type of cement used. Ordinary Portland cement (OPC) typically responds well to retarders, while specialized cements like high-alumina or sulfate-resistant cements may require different retardation agents. Always match the retarder to the specific type of cement for best results.
Hot weather is the primary condition where retarders are used, but extreme cold can also impact the effectiveness of retarders. In both cases, it is crucial to adjust the dosage of the retarder according to the temperature. Additionally, in environments with high humidity, retarders may work more effectively, while dry conditions may necessitate higher doses to achieve the same results.
The amount of retarder to be added depends on the required retardation time and the temperature of the working environment. Overuse can lead to excessive delays in setting time, affecting the concrete’s strength development and workability. Always refer to manufacturer guidelines for dosage and conduct small-scale tests to determine the appropriate quantity for the job.
It’s essential to have proper inspection and control when using retarders, especially if compounds like gypsum or sugar are involved. Excessive amounts can cause undesirable expansion, delayed setting, or inconsistencies in concrete performance. Regular monitoring of the setting time and hydration process ensures that the retarder is working as intended.
Retarders should be thoroughly mixed into the concrete to ensure even distribution. Commercial retarders often come pre-mixed with other admixtures, such as plasticizers, to maintain stability. Ensure that the retarder is stored in a cool, dry place, as exposure to extreme temperatures can degrade its effectiveness.
While retarders are beneficial, improper use can lead to issues like excess bleeding, increased risk of cracking, or reduced early strength. Therefore, careful calculation and application are vital to ensure that the retarder enhances the concrete’s performance without causing adverse effects.
Retarders play a crucial role in modern concrete construction by controlling the setting time and maintaining workability under challenging conditions. From hot weather concreting to large-scale pours, these admixtures offer significant advantages by allowing more time for mixing, transporting, and placing concrete. Additionally, retarders are essential in applications like ready-mix concrete, oil well grouting, and exposed aggregate finishes, where precise control over setting times is required.
While retarders provide numerous benefits, selecting the right type and dosage is essential to avoid potential drawbacks such as reduced early strength or excessive delay in setting. Proper handling and monitoring during mixing, transportation, and placement ensure that the concrete meets the desired performance criteria.
Ultimately, the use of retarders enhances the flexibility and efficiency of concrete, enabling it to meet the demands of diverse construction projects. Whether it’s combating the effects of high temperature or achieving specialized surface finishes, retarders are a vital tool in the concrete industry, supporting the delivery of high-quality, durable structures.
A retarder is an admixture added to concrete to delay its setting time, ensuring the mix remains workable for an extended period. This is particularly useful in hot weather, complex placements, or long transportation scenarios.
Retarders are specialized chemicals used to slow down the hydration process of cement in concrete, maintaining its plasticity and preventing premature setting.
The quantity of retarder depends on factors like the desired setting time, ambient temperature, and the type of cement. It’s crucial to follow the manufacturer’s guidelines and conduct small-scale tests to determine the appropriate dosage.
Retarders delay the setting time of concrete by slowing the hydration process, improving workability, reducing the risk of cold joints, and enabling better placement and finishing.
Retarders function by temporarily delaying the hydration of cement. They form protective layers around cement particles, interrupt the growth of hydration products, or create insoluble compounds that slow down the reaction between cement and water.
The effect of retarders is temporary and designed to extend the setting time for a specific period. After this period, the hydration process resumes, allowing the concrete to set and harden as intended.
Retarders are used to counteract rapid setting in hot weather, prevent cold joints in large pours, and enable better control in complex structural designs or decorative finishes.
Retarders are categorized into organic (e.g., lignosulphonates, sugars) and inorganic types (e.g., borates, phosphonates). Some modern admixtures combine retardation with other benefits, like water reduction, for enhanced performance.
Yes, retarders can affect the strength of concrete. While early strength may be slightly reduced, properly cured retarded concrete can achieve similar or even higher long-term strength compared to untreated concrete.
Improper use of retarders can lead to issues like excessive bleeding, delayed strength development, or uneven setting. Proper dosage and monitoring are essential to prevent these problems.
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