SURFACE RETARDER FOR CONCRETE

SURFACE RETARDER FOR CONCRETE

INTRODUCTION

Concrete surface retarders are mainly applied for surface treatments, in case of concreting in unusual circumstances like hot weather or when unavoidable delays occur at last to prevent cold joints. The surface retarders delay the setting time of the concrete by expanding the dormant period of the hydration of the cement paste. In addition to retarding the concrete setting time, concrete surface retarders can also have the following side effects on the properties of the concrete: a decrease of early strength within the first 24 hours, better workability, an increase of slump, significantly more bleeding water, better freeze-thaw behaviour because of an increase of air entrainment and a slight increase of the plastic shrinkage. Creep, drying shrinkage and durability are not significantly affected by the inclusion of retarding admixtures.

HYDRATION OF CEMENT – HYDRATION MECHANISM ^[1]

When OPC cement is mixed with water its chemical compound constituents undergo a series of chemical reactions that cause it to harden (or set).  These chemical reactions all involve the addition of water to the basic chemical compounds listed in Table.  This chemical reaction with water is called "hydration".  Each one of these reactions occurs at a different time and rate.  Together, the results of these reactions determine how Portland cement hardens and gains strength. 

Tricalcium silicate (C₃S).  Hydrates and hardens rapidly and is largely responsible for initial set and early strength.  Portland cements with higher percentages of C₃S will exhibit higher early strength.

Dicalcium silicate (C₂S).  Hydrates and hardens slowly and is largely responsible for strength increases beyond one week.

Tricalcium aluminate (C₃A).  Hydrates and hardens the quickest.  Liberates a large amount of heat almost immediately and contributes somewhat to early strength.  Gypsum is added to Portland cement to retard C₃A hydration.  Without gypsum, C₃A hydration would cause Portland cement to set almost immediately after adding water.

Fig:-1 Heat dissipation curve

Tetracalcium aluminoferrite (C₄AF).  Hydrates rapidly but contributes very little to strength.  Its use allows lower kiln temperatures in Portland cement manufacturing.  Most Portland cement color effects are due to C₄AF.

Three principal reactions occur:- Almost immediately on adding water some of the clinker sulphates and gypsum dissolve producing an alkaline, sulphate-rich, solution. Soon after mixing, the (C₃A) phase (the most reactive of the four main clinker minerals) reacts with the water to form an aluminate-rich gel (Stage I on the heat evolution curve above). The gel reacts with sulphate in solution to form small rod-like crystals of ettringite. (C₃A) reaction is with water is strongly exothermic but does not last long, typically only a few minutes, and is followed by a period of a few hours of relatively low heat evolution. This is called the dormant, or induction period (Stage II). The first part of the dormant period, up to perhaps half-way through, corresponds to when concrete can be placed. As the dormant period progresses, the paste becomes too stiff to be workable. At the end of the dormant period, the alite and belite in the cement start to react, with the formation of calcium silicate hydrate and calcium hydroxide. This corresponds to the main period of hydration (Stage III), during which time concrete strengths increase. The individual grains react from the surface inwards, and the anhydrous particles become smaller. (C₃A) hydration also continues, as fresh crystals become accessible to water. The period of maximum heat evolution occurs typically between about 10 and 20 hours after mixing and then gradually tails off.

The equation for the hydration of tricalcium silicate (C₃S) is given by:

·         Tricalcium silicate + Water--->Calcium silicate hydrate+Calcium hydroxide + heat

2 C₃S + 6H ---> C₃S₂H₃ + 3 Ca(OH)₂

2 (CaO SiO₂ )+ 6 H₂O ---> 3 CaO^.2SiO₂^.3H₂O + 3 Ca(OH)₂ + 173.6kJ

·         The equation for the hydration of dicalcium silicate(C₂S) is given by:

Dicalcium silicate + Water--->Calcium silicate hydrate + Calcium hydroxide +heat

2 C₂S + 4H ---> C₃S₂H₃ + Ca(OH)₂

2 Ca₂SiO₄ + 4 H₂O---> 3 CaO^.2SiO₂^.4H₂O + Ca(OH)₂ + 58.6 kJ

·         The equation for the hydration of tricalcium aluminate(C₃A) is given by:

Tricalcium aluminate +Gypsum+ Water ---->  Calcium aluminate compound

C₃A + 6H ------> C₃ A H₆

C₃ A H₆+ Ca SO₄-------> monosulphoaluminate

·         The equation for the hydration of C₄AF is given by:

Tetra calcium aluminoferrate ------> Calcium ferrite hydrates

C₄AF + H -----> C₃FH₆

MECHANISM OF RETARDATION ^[2]

Adding a retarder, dissolved in the mixing water or sprayed on the surface of the concrete, temporarily interrupts the hydration reactions, especially at the lowest point of the graph, after reaction of C₃A( Though C₃A reacts first, Reaction occurs in stage 1), which creates a longer dormant period. There are four different mechanisms of actions between retarders and cement to interrupt those reactions. The mechanisms that appear depend on the combination of the type of retarder and the type of cement. Most retarders normally act by several actions. It’s also important to realize that the mechanisms of retardation are temporary. After a predictable period, the effects of the mechanisms disappear and the hydration continues. Below, all four mechanisms are described.

Adsorption Mechanism

On the surface of the cement particles, a retarding admixture is adsorbed. This layer of retarding admixture creates a protective skin (diffusion barrier) around the cement particles. Due to this diffusion barrier the water molecules are hindered to reach the surface of the hydrated cement particles and the hydration is slowed down. The result is that there is no considerable amount of hydration products to give rigidity to the cement paste so the paste remains plastic for a longer period. The retarding admixture is removed from the solution, among other by reacting with the C3A from the cement, and is incorporated into the hydrated material.

Nucleation Mechanism

When water is added to the cement, calcium ions and hydroxyl ions are released from the surface of the cement particles. When a critical value of the concentration of those ions is reached, the hydration products C₂S and C₃S start to crystallize. A retarding admixture, which is incorporated into the cement, is adsorbed by the calcium hydroxide molecule, which prevents the growth of the calcium hydroxide nucleus until some level of super saturation. So the induction period has been extended because of the increase of the level of calcium hydroxide super saturation before crystallization starts.

Complexation Mechanism

During the first few minutes, some kind of complexes with calcium ions, released by the cement grains, are formed. The formation of those complexes causes an increased solubility of the cement. During the hydration, in the presence of a retarding admixture, an increased concentration of Ca²+, OH-, Si, Al and Fe will occur in the aqueous phase of the cement paste. The accumulation of the calcium and hydroxyl ions in the solution prevent the precipitation of those ions to form calcium hydroxide. In that way, hydration is retarded.

Precipitation Mechanism

Precipitation is nearly similar to adsorption but in the case of precipitation some kind of insoluble derivatives of retarder are formed by a reaction with the highly alkaline solution. Because of that, the pH of the solution rises over 12 after the first few minutes of the contact between water and cement. The precipitation of protective coatings of these insoluble derivatives around the cement particles suppresses the cement hydration. The protective coating acts as a diffusion barrier so the water molecules can’t make a good contact with the cement particles.

  

TYPES OF RETARDERS ^[5]

There are two categories of retarders. Both categories work according to the four mechanisms as mentioned earlier.

Among the inorganic retarders, only the phosphonates are commercially utilized. Some retarders also have other effects. In many cases, retarders can act as super plasticizers / water-reducers. Ligno-sulphonates are normally used as water reducer but they also have secondary retarding effects. On the opposite, hydroxyl-carboxylic acids normally are retarders but they have secondary water reducing effects. It works in both directions; this is because water reducers and retarders have some similar chemical components. Sometimes this can create problems. To reduce the retarding effect of super plasticizers they can be combined with accelerators. The dosage is an important parameter in this respect, and is different for each retarder.^.[8]

APPLICATIONS

Surface treatment: exposed aggregate finish

Since the 80’s, in Belgium and nearby areas, exposed aggregate finish is applied on 95% of the surfaces of the cement concrete pavements^[6].The surface Retarder is applied after the placement and finishing of the concrete, is to give the concrete surface a great skid resistance while a limited tire/pavement noise is achieved, even at high speeds and on wet road surfaces. An exposed aggregate finish is primarily used for motorways and roads with intense and high speed traffic. It is also used for pavements in public spaces in order to emphasize the aesthetic characteristics of specific aggregates. In other cases like high traffic area, Industrial roads, parking areas the surface treatment usually consists of brooming in the transverse direction.The first step in the execution of exposed aggregate finish is spraying a retarding agent on the surface of the fresh concrete about 30 minutes, or earlier in hot weather, after the concrete has been finished. The setting retarder has to prevent the concrete mortar skin from hydrating during a certain period that depends on the quality of the concrete and on the weather conditions.

Special attention has to be paid to the following items during the application of Surface retarder:-

• the surface has to be very flat before the treatment
• the surface treatment may not disrupt the flatness nor may it obstruct surface drainage
• the composition of the concrete mix at the surface must be homogeneous.
• although the presence of water will not affect or prevent the operation of the setting retarder, the concrete surface should be free from standing water. Because more water in the concrete will allow the surface retarder to penetrate inside the concrete deeper, hence it may affect the setting time of the concrete also. 

DOSAGE

For large areas surface retarders are mostly applied by the use of paving trains but small surfaces can also be sprayed by hand. Before the work, the contractor regulates the height, the flow rate of the spray and the movement speed as a function of the required amount of retarding agent that has to be sprayed on the surface. The spray has to be shielded from the wind. When the paving train has stopped, it shall be avoided that too much retarding agent is sprayed at the same location. To achieve this, a gutter can be placed under the sprayer whenever the paving equipment stands still. The surface retarder has a bright colour, in FOSROC Range it is available in Blue colour because a pigment is usually added to it. This way, a visual inspection of the homogeneous distribution is possible. On the one hand it must be sufficiently viscous in order not to run off after being sprayed, regardless of the slope. On the other hand it must also remain possible to spray it with a suitable apparatus (pump, spraying nozzle). Although most products are harmless to the environment, it is appropriate to absorb the cement paste so it does not get into the sewerage.

The amount of surface retarder to be applied is determined in accordance with the supplier's instructions and is in function of the intended result. However, there are differences between organic retarders and chemical retarders. Organic retarders can be sufficiently dosed (doses > 250g/m²). Chemical retarders are applied in economical consumption with reduced doses of 100 to 200 g/m², depending on the product. This results in a higher risk of inadequate dosing and inadequate washout depth, especially when used in strong winds.

Mostly, any overdose will result in an increase in setting time, which can be significant for some retarder types. Accidentally retarded concrete will normally set and recover strength within two or three days if the overdose is no more than double than which was intended and the concrete is well cured to prevent it from dehydration. Re-vibration is advisable if the concrete remains fluid for an extended period so any settlement cracks can be closed before the concrete. The concrete may not reach its strength in a reasonable time if large overdoses occur or where large overdoses of water reducing retarders have been used without a correspondingly large water reduction. As a general rule, if concrete has not set hard in 5 days after an overdose of a retarding admixture, then it may not gain useful mechanical strength^.[4]

PROTECTION

The Surface Retarder can protect the concrete against drying out due to weather or wind. However, the effectiveness of this protection is limited. Usually, immediately after spraying, the surface should be protected against drying out either by means of a watertight plastic sheet, which is kept in place until the skin of concrete mortar is removed, or by applying a special curing compound. This curing compound has to be compatible with the retarding agent. In case of organic retarders, the covering of the concrete pavement with a plastic film must always be done immediately after applying the retarder agent in order to protect the retarder from drying out. Chemical retarders on the other hand have the advantage that they also protect the concrete from drying out due to the membrane formation, making a plastic film unnecessary.

WAITING TIME

The waiting time is 6 to 24 hours after the concrete has been finished. The minimum waiting time is extended if it becomes apparent that on the inside the concrete has not hardened enough to start brooming without the risk to damage the concrete. Usually, the ambient temperature and the concrete composition is the decisive factor in the determination of the timing. If necessary, the waiting period is extended a couple of hours. In winter period, the waiting period can be extended to 48 hours after the concrete has been finished. The estimation of the waiting time is thus based on experience^.[6]

WASHING OR BROOMING

Afterwards, the non-hydrated layer of concrete mortar is washed out by means of a steel broom or, for small surfaces, by a high-pressure water jet at 5 - 10 N/mm2. The texture depth can depend between 1mm and 3 mm. The amount of cement mortar that is removed is little affected by the amount of applied retarder. It is rather determined by the type of Surface Retarder, the outside temperature, the time of application and the time of washing. While the brooming continues, the watertight plastic sheet is removed progressively to avoid that the retarder would dry up. In hot weather it can be useful to moisten again before threatening the surface. It is important that the concrete should be protected against drying out for at least 72 hours after washing or brooming. Therefore, after washing or Brooming of the concrete surface and before it is dried out, a new protection has to be applied to the surface. A possible way of doing this is for example by spraying a curing compound or by placing a watertight sheet.

USE IN DIFFICULT WEATHER CONDITIONS

Hot and dry weather may have two adverse effects: faster drying of the concrete, with shrinkage deformation as a result (cracking due to plastic shrinkage) and thermal deformations due to temperature variations in the concrete mass. High wind speeds can also cause or reinforce these effects. At air temperatures above 30⁰C or at a humidity below 50%, special precautions have to be taken to protect the fresh concrete against dehydration. The use of concrete setting retarders is one of the solutions that can be used in this situation.

CONCLUSION

This paper contains a general overview of different aspects of concrete Surface retarders and is far from being complete, for more information please consult the necessary specialized sources. The mentioned effects in this paper cannot be generalised for all types of concrete setting retarders. Depending on the type of retarder, the dosage, the used cement types and the combination with other admixtures, the effects can be more or less explicit. Special attention has to be paid on the dosage of the retarding agent. The manufacturer's instructions should be followed properly and the efficacy of the Surface retarder must be verified.

REFERENCES

[1] WHD Microanalysis Consultants Ltd, (2005). “Understanding Cement”. Retrieved from http://www.understanding-cement.com/hydration.html

[2] KHAN B., ULAH M., “Effect of a Retarding Admixture on the Setting Time of Cement Pastes in Hot Weater”, JKAU, Eng. Sci., Vol 15, No 1. P 63-79, 2004. Retrieved from http://www.kau.edu.sa/Files/135/Researches/54945_25263.pdf

[3] Federal Highway Administration, (2011). “Set-Retarding”. Retrieved from http://www.fhwa.dot.gov/infrastructure/materialsgrp/setretrd.htm

[4] Cement Admixtures Association, (2012). “Admixture Technical Sheet – ATS3 – Set Retarding”. Retrieved from http://www.admixtures.org.uk/downloads/ATS%203%20Retarding%20admixtures.pdf

 [5] MYRDAL R., (2007). “Retarding Admixtures for Concrete”. Retrieved from http://www.sintef.no/upload/Byggforsk/COIN/STAR%202%20in%201.2%20F%20Retarding%20admixtures%20for%20concrete.pdf

[6] Information provided by e-mail by the company Robuco, specialized in concrete treatments

[7] Febelcem, (2001) “Road Pavements Of Cement Concrete – Execution Of Monolithic Pavements”. Retrieved from http://www.eupave.eu/documents/graphics/inventory-of-documents/febelcem-publicaties/road-pavements-of-cement-concrete.pdf

[8] DE WEERDT K., REYNDERS D., (2006).“Combining Plasticizers/Retarders And Accelerators”. Retrieved from http://bwk.kuleuven.be/mat/publications/masterthesis/2006-de-weerdt_reynders-msc.pdf