5.1 Admixture Selection
Where predictable set retardation is required, a retarding admixture is the best choice. Where set needs to be continually amended, a graph of dosage rate versus setting time can be determined for a given mix, at a given temperature. For phosphate based products, the retardation time graph may often be linear, and this is a major advantage for this type of retarder.
Sucrose and other polysaccharides are more efficient, but a dosage versus setting time graph may often be exponential, making accurate prediction more difficult. These types are often blended with lignosulphonate to produce cost effective, retarding/water reducing admixtures.
Hydroxy carboxylic acid salts will often reduce cohesion in the mix potentially enhancing bleed and segregation. Polysaccharides, especially if blended with a lignosulphonate, tend to stabilise some air and may enhance cohesion.The choice between the different types of retarding water reducing admixtures is often determined by other properties such as mix cohesion. Selection may therefore be based on the particular mix characteristics of the concrete.
Retarding admixtures based on phosphates and phosphonates are designed to have a linear effect of dosage upon setting time. The dosage rate used may be quite high, as they do not have any effect other than retardation. Depending on the molecule chosen, typical dosages are 0.1% to 3.0% by weight of cement to yield a delay of set of 1 hour to 35 hours.
Retarding admixtures based on sucrose and other similar polysaccharides are very powerful, and their retarding effects are rarely linear so that small increases on the intended dosage can lead to large increases in retardation. They are very cost effective, but are more difficult to control than the higher dosage phosphate based types. Typical dosages are 0.1% to 1.5% by weight of cement to yield a delay of set of 3 hours to 50 hours. Retarding water reducing admixtures are very commonly used at a low dose, typically 0.2% by weight of cement, to reduce the water content of the concrete by 7 to 10% with only a small delay in setting. They can be used at higher dosages; typically 0.3 to 0.6% to reduce water content, but simultaneously to delay the setting time. Greater water reductions may be achieved by using retarding high range water reducers, generally at 0.5 to 1.5% dose, depending on the type.
Retarders are quite sensitive to temperature. At low temperatures retardation will be further extended. At very high temperatures, the converse is true and it may be difficult to achieve the required workability retention and extension of stiffening time.
5.3 Cement type
Retarding admixtures and water reducing retarding admixtures can be used with all types of Portland cement, including all those covered by EN197-1. However, it is very important to note that their effectiveness in terms of retardation of set is very dependent upon the type of cement.
Cements such as CEM1 require the highest level of retarder to achieve a given level of retardation. However, as the proportion of slag or pulverised fuel ash, as used in CEM 2 and CEM3 types increases, then the level of retarder will decrease in order to achieve a given level of retardation. The chemistry of the cement is also important in determining the effect of retarders. Cements low in tricalcium aluminate (C3A) require significantly less retarder for a given degree of retardation than normal cements.
Retarding admixtures do not have any significant effect upon the yield of concrete. Retarding water reducing admixtures, when used to reduce the water content of concrete, will reduce the yield in direct proportion to the water reduction made. This needs to be taken into account when modifying the mix design.
The level of retardation achieved is related to the dosage used. Any overdose will result in an increase in setting time. Large overdoses of retarders can produce very long setting times and even small overdoses can have this effect if the initial dose is high. Provided the overdose is no more than double that which was intended, and the concrete is well cured to prevent it from desiccation, accidentally retarded concrete will normally set and recover strength within two to three days. Where a dosage range is given, the normal dose should be taken as the bottom of the range. Where very large, accidental overdoses occur or where large overdoses of a water reducing retarders have been used without a correspondingly large water reduction, the concrete may not recover its strength in a reasonable time.
As a general rule, if concrete contains an overdose of a retarding admixture and has not set hard in 5 days, then it may not gain useful mechanical strength within a reasonable time.
Effects upon properties of concrete
As with water reducing admixtures, ultimate strength gain is increased with increasing water reduction Retardation of set allows the slower formation of a more ordered, smaller, denser cementitous matrix. This has the effect of increasing ultimate strength relative to an unretarded mix with the same water cement ratio.
Acceleration of strength by heat produces the opposite effect, with the rapid formation of a coarse matrix. This explains why steam cured precast concrete rarely produces the same ultimate strength as concrete cured at normal temperatures and produced from the same concrete.
Retarding admixtures do not have a significant effect upon initial workability. However, they generally have a beneficial effect upon workability retention, particularly at elevated temperature.
Retarding water reducing admixtures, have a pronounced effect upon workability. Typically, an increase in slump of 60-100mm results from the addition of a dosage of 0.25% by weight cement. Set retarding high range water reducing/plasticizing admixtures may be used to enable workability to be increased to a greater extent, at a typical dosage level of 0.3 to 1.0%.