Introduction
Many important characteristics of concrete are influenced by the ratio (by weight) of water to cementitious materials (w/cm) used in the mixture. By reducing the amount of water, the cement paste will have higher density, which results in higher paste quality. An increase in paste quality will yield higher compressive and flexural strength, lower permeability, increase resistance to weathering, improve the bond of concrete and reinforcement, reduce the volume change from drying and wetting, and reduce shrinkage cracking tendencies (PCA, 1988).
Reducing the water content in a concrete mixture should be done in such a way so that complete cement hydration process may take place and sufficient workability of concrete is maintained for placement and consolidation during construction. The w/cm needed for cement to complete its hydration process ranges from 0.22 to 0.25. The existence of additional water in the mixture is needed for ease of concrete placing and finishing (workability of concrete). Reducing the water content in a mixture may result in a stiffer mixture, which reduces the workability and increases potential placement problems.
Water reducers, retarders, and superplasticizers are admixtures for concrete, which are added in order to reduce the water content in a mixture or to slow the setting rate of the concrete while retaining the flowing properties of a concrete mixture. Admixtures are used to modify the properties of concrete or mortar to make them more suitable to work by hand or for other purposes such as saving mechanical energy.
Water reducing admixtures (WRA)
The use of WRA is defined as Type A in ASTM C 494. WRA affects mainly the fresh properties of concrete by reducing the amount of water used by 5% to 12% while maintaining a certain level of consistency, measured by the slump as prescribed in ASTM C 143-90. The use of WRA may accelerate or retard the initial setting time of concrete. The WRA that retards the initial setting time more than three hours later is classified as WRA with retarding effect (Type D). Commonly used WRA is lignosulfonates and hydrocarboxylic (HC) acids. The use of HC acids as WRA requires higher water content compared to the lignosulfonates. Rapid bleeding is a problem for concrete treated with HC acids.
Increase of slump is different according to its type and dosage. Typical dosage rate is based upon the cementitious material content (milliliters per hundred of kilograms). The figure below illustrates the influence of dosage of Lignosulfonates and HC acid on slump. It is shown in the figure that HC acids give a higher slump compared to lignosulfonates with the same dosage.
Figure 1 Influence of Dosage of Retarders on Slump (Neville, 1995).
WRA has been used primarily in hot weather concrete placing, pumping, and tremie. Careful concrete placement is required, as the initial setting time of concrete will take place an hour earlier. It is also shown that the use of WRA will give a higher initial concrete compressive strength (up to 28 days) by 10% compared to the control mixture. Other benefit of using WRA is that higher concrete density is achieved which makes the concrete less permeable and have a higher durability.
Retarding admixtures
The use of this admixture is defined in ASTM C494. There are two kinds of retarders, defined as Type B (Retarding Admixtures) and Type D (Water Reducing and Retarding Admixtures). The main difference between these two is the water-reducing characteristic in Type D that gives higher compressive strengths by lowering w/cm ratio.
Retarding admixtures are used to slow the rate of setting of concrete. By slowing the initial setting time, the concrete mixture can stay in its fresh mix state longer before it gets to its hardened form. Use of retarders is beneficial for:
- Complex concrete placement or grouting
- Special architectural surface finish
- Compensating the accelerating effect of high temperature towards the initial set
- Preventing cold joint formation in successive lifts.
Retarder can be formed by organic and inorganic material. The organic material consists of unrefined Ca, Na, NH4, salts of lignosulfonic acids, hydroxycarboxylic acids, and carbohydrates. The inorganic material consists of oxides of Pb and Zn, phosphates, magnesium salts, fluorates, and borates. Commonly used retarders are lignosulfonates acids and hydroxylated carboxylic (HC) acids, which act as Type D (Water Reducing and Retarding Admixtures). The use of lignosulfonates acids and hydroxylated carboxylic acids retard the initial setting time for at least an hour and no more than three hours when used at 65 to 100 oF.
A study performed on the influence of air temperature over the retardation of the initial set time (measured by penetration resistance as prescribed in ASTM C 403 – 92) shows that decreasing effect with higher air temperature (Neville1995). The table below describes the effect of air temperature on retardation of setting time:
Table 1 Air Temperature and Retardation of Initial Setting Time
| Admixture Type | Description | Retardation of initial setting time (h:min) at temperature of | ||
| 30oC | 40oC | 50oC | ||
| D | Hydroxylic acid | 4:57 | 1:15 | 1:10 |
| D | Lignin | 2:20 | 0:42 | 0:53 |
| D | Lignosulfonates | 3:37 | 1:07 | 1:25 |
| B | Phosphate-based | — | 3:20 | 2:30 |
The use of retarding admixture has the main drawback of the possibility of rapid stiffening, where rapid slump loss will result in difficulty of concrete placement, consolidation, and finishing. An extended-set admixture has been developed as another retarding admixture. The advantages of this admixture compared to the conventional one is the capability to react with major cement constituents and to control hydration and setting characteristics of concrete while the conventional one will only react with C3A.
Careful usage of retarder is required to avoid excessive retardation, rapid slump loss and excessive plastic shrinkage. Plastic shrinkage is the change in fresh concrete volume as surface water evaporates. The amount of water evaporation is influenced by temperature, ambient relative humidity, and wind velocity. Proper concrete curing and adequate water supply for surface evaporation will prevent plastic shrinkage cracking. The amount of water needed to prevent plastic shrinkage cracking is given by the chart below:
Figure 2 Rate of Surface Moisture Evaporation
The extended-set admixture is widely used as a stabilizing agent for wash water concrete and fresh concrete. Addition of extended-set admixture enables the reuse of wash water to the next batch without affecting concrete properties. This admixture can also be used for long haul concrete delivery and to maintain slump. Factors affecting the use of this admixture include the dosage rate and the ambient temperature of the concrete.
Superplasticizers (High Range Water reducer)
ASTM C494 Type F and Type G, High Range Water Reducer (HRWR) and retarding admixtures are used to reduce the amount of water by 12% to 30% while maintaining a certain level of consistency and workability (typically from 75 mm to 200 mm) and to increase workability for reduction in w/cm ratio. The use of superplasticizers may produce high strength concrete (compressive strength up to 22,000 psi). Superplasticizers can also be utilized in producing flowing concrete used in a heavy reinforced structure with inaccessible areas. Requirement for producing flowing concrete is defined in ASTM C 1017. The effect of superplasticizers in concrete flow is illustrated in the chart below:
Figure 3 Relation between Flow Table and Water Content of Concrete with and without Plasticizers (Neville, 1995).
Another benefit of superplasticizers is concrete early strength enhancement (50 to 75%). The initial setting time may be accelerated up to an hour earlier or retarded to be an hour later according to its chemical reaction. Retardation is sometimes associated with range of cement particle between 4 – 30 m m. The use of superplasticizers does not significantly affect surface tension of water and does not entrain a significant amount of air. The main disadvantage of superplasticizer usage is loss of workability as a result of rapid slump loss and incompatibility of cement and superplasticizers.
Superplasticizers are soluble macromolecules, which are hundreds of times larger than water molecule (Gani, 1997). Mechanism of the superplasticizers is known as adsorption by C3A, which breaks the agglomeration by repulsion of same charges and releases entrapped water. The adsorption mechanism of superplasticizers is partially different from the WRA. The difference relates to compatibility between Portland Cement and superplasticizers. It is necessary to ensure that the superplasticizers do not become fixed with C3A in cement particle, which will cause reduction in concrete workability.
Typical dosage of superplasticizers used for increasing the workability of concrete ranges from 1 to 3 liters per cubic meter of concrete where liquid superplasticizers contained about 40 % of active material. In reducing the water cement ratio, higher dosage is used, that is from 5 to 20 liters per cubic meter of concrete. Dosage needed for a concrete mixture is unique and determined by the Marsh Cone Test.
There are four types of superplasticizers: sulfonated melamine, sulfonated naphthalene, modified lignosulfonates and a combination of high dosages of water reducing and accelerating admixtures. Commonly used are melamine based and naphthalene based superplasticizers. The use of naphthalene based has the advantage of retardation and affecst slump retention. This is due to the modified hydration process by the sulfonates
Admixtures Dispensers
The basic function of a dispenser as defined in ACI Bulletin E4-95 is:
- To transport the admixture from storage to batch
- To measure the quantity of the admixtures required
- To provide verification of the volume dispensed
- To inject the admixture into the batch.
Admixtures have been dispensed in liquid form to ensure proper dispersion in the concrete mixture. WRA should be dispensed with the last water batch. Proper timing is very important, as any delay ranges between one to five minutes after the water addition will result in excessive retardation of setting time. The Superplasticizers should be dispensed on to the batch immediately before discharge for placement (Type F) or with the last portion of the water (Type G).