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建筑学论文代写 The Special Properties Of Concrete

6.3 Slump loss

Retarding admixtures are useful for helping to reduce slump loss, particularly at elevated temperature but it is still important to have a high initial workability.

Retarding water reducing admixtures are very effective at reducing slump loss when used to increase the initial workability of the mix, but less so when used as a water reducer. Indeed, if water reduction is taken at the expense of high initial workability, initial slump loss may be slightly faster and will slow when about half the initial slump is reached.

6.4 Setting time

The prime function of a retarder is to extend the setting (stiffening) time of concrete, usually in order to prevent the formation of cold joints between deliveries of concrete. Even if workability has fallen to almost zero slump, fresh concrete can be vibrated into, and will bond with, a preceding, older pour.

In hot weather, even a small delay in deliveries or a short breakdown of the pump can result in the first concrete pours setting before subsequent pours can be placed and vibrated to form a monolithic joint. In deep pours, if concrete placed early starts to set, the heat generated can cause faster setting of concrete above it and again lead to cold joints. In this situation, retarder dosage can be progressively reduced as the pour proceeds.

6.5 Air entrainment

Retarding admixtures do not normally entrain air, and some types, especially those based on hydroxycarboxylic acid, may actually reduce air content. This may cause these retarded mixes to feel harsher and have more tendency to bleed.

Most types of retarder can be used effectively in combination with an air entraining agent.

6.6 Bleeding

The total volume of bleed water arising from concrete is often related to its setting time because once setting starts, bleeding stops. Thus retarded concretes are always more prone to bleed. Any reduction in air tends to aggravate this potential problem.

The plasticising component of a retarding water reducing admixture may help to offset this effect and some types are formulated to slightly air entrain in order to reduce bleed.

6.7 Heat of hydration

Retarding admixtures do not reduce the heat output of concrete but do serve to delay the time of peak temperature rise by exactly the same time interval by which it was retarded. In small sections this may allow slightly more heat dissipation and so peak temperature may be a little lower.

In thick sections there will be no reduction in peak temperature and there is evidence that the peak temperature may even be increased slightly.

6.8 Volume deformation

Creep and drying shrinkage are not significantly affected by the inclusion of retarding admixtures.

If the concrete is water reduced by the use of a retarding water reducing admixture, then drying shrinkage will be reduced.

6.9 Durability

Provided that the concrete is correctly cured, then retarded concrete should be stronger and just as durable as equivalent plain concrete. However, because of the extended plastic stage, more attention needs to be paid to protecting the concrete before it sets. Retarded water reduced concrete will have a lower water content than the equivalent plain concrete, and will be correspondingly more durable.

MECHENISM OF RETARDING ADMIXTURES

Retarding admixture is an admixture that retards the setting of concrete. A retarding admixture causes cement set retardation by one or more offollowing mechanisms:

(1) Adsorption of the retarding compound on the surface of cement particles, forming a protective skin which slows down hydration;

(2) Adsorption of the retarding compound on to nuclei of calcium hydroxide, poisoning their growth, which is essential for continued hydration of cement after the end of induction period;

(3) Formation of complexes with calcium ions in solution, increasing their solubility and discouraging the formation of the nuclei of calciumhydroxide .

(4) Precipitation around cement particles of insoluble derivatives of the retarding compounds formed by reaction with the highly alkaline aqueous solution, forming a protective skin .

Detailed Explanation

According to the first mechanism, a retarding admixture is adsorbed on the surface of cement particles. This layer of retarding admixture around the cement particles acts as a diffusion barrier. Due to this diffusion barrier, it becomes difficult for the water molecules to reach the surface of the unhydrated cement grains and hence the hydration slows down, and the dormant period (period of relatively inactivity) is lengthened. Due to the slow hydration, no considerable amount of the hydration products giving rigidity to the cement paste will be formed and thus the paste remains plastic for a longer time. Later, when the admixture is removed from solution by reaction with C3A from cement or by some other way it is removed and incorporated into the hydrated material, further hydration is eliminated. On first contact of water with cement grains (C3S and C2S) calcium ions and hydroxyl ions are rapidly released from the surface of the cement grains. When concentration of these ions reaches a critical value (at which the solution becomes saturated), the hydration products calcium hydroxide and calcium silicate hydrate start to crystallize from the solution and then hydration proceeds rapidly.

According to the second mechanism, a retarding admixture incorporated into cement paste is adsorbed on the calcium hydroxide nuclei and prevents its growth until some level of super saturation is reached during the induction period of hydration. Thus, retarder lengthens the induction period by causing an increase in the level of calcium hydroxide super saturation before crystallization begins. This is analogous to the poisoning of crystal growth of calcium hydroxide by the retarding admixture as both calcium and hydroxyl ions are present in the solution but unable to precipitate as a result of poisoning of the calcium hydroxide nuclei.

According to the third mechanism, a retarding admixture incorporated into cement paste forms some kind of complexes with calcium ions released by the cement grains during the first few minutes. Formation of the complexes increase the solubility of cement, i.e., increased concentration of Ca2+, OH, Si, Al and Fe in the aqueous phase of the cement pastes will occur when hydrated in the presence of the retarding admixture. Thus the calcium ions and hydroxyl ions will accumulate in solution and will be unable to precipitate to form calcium hydroxide. For example, when ordinary Portland cement is hydrated in sucrose solution, lime is solubilised and a sucrose calcium complex (R – -O —Ca+ – -OH) is formed in which Ca+ – -OH group is attached to the five membered ring (R) of the sucrose molecule. Such sucrose—calcium complex will be able to become absorbed on the growing calcium hydroxide nucleus. The adsorption of the complex on the calcium hydroxide nucleus will inhibit its growth as the calcium and hydroxyl ions will not be able to precipitate. In this way, hydration is retarded.

The fourth mechanism is similar to the first but here some kind of insoluble derivatives of retarder are formed by reaction with the highly alkaline solution as pH of the solution rises to over 12 within few minutes after first contact of water with cement. For example, inorganic salt admixtures (borates, phosphates, zinc and lead salts etc.) give insoluble hydroxides in alkaline solution. The cement hydration is suppressed through the precipitation of protective coatings of these insoluble derivatives around the cement grains.

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