Superplasticizers are linear polymers containing sulfonic acid groups attached to the polymer backbone at regular intervals (Verbeck 1968). Most of the commercial formulations belong to one of four families:
Sulfonated melamine-formaldehyde condensates (SMF)
Sulfonated naphthalene-formaldehyde condensates (SNF)
Modified lignosulfonates (MLS)
The sulfonic acid groups are responsible for neutralizing the surface charges on the cement particles and causing dispersion, thus releasing the water tied up in the cement particle agglomerations and thereafter reducing the viscosity of the paste and concrete.
Effect of Superplasticizers on Concrete Properties. The main purpose of using superplasticizers is to produce flowing concrete with very high slump in the range of 7-9 inches (175-225 mm) to be used in heavily reinforced structures and in placements where adequate consolidation by vibration cannot be readily achieved. The other major application is the production of high-strength concrete at w/c's ranging from 0.3 to 0.4.
The ability of superplasticizers to increase the slump of concrete depends on such factors as the type, dosage, and time of addition of superplasticizer; w/c; and the nature or amount of cement. It has been found that for most types of cement, superplasticizer improves the workability of concrete.
The capability of superplasticizers to reduce water requirements 12-25% without affecting the workability leads to production of high-strength concrete and lower permeability. Compressive strengths greater than 14,000 psi (96.5 MPa) at 28 days have been attained (Admixtures and ground slag 1990). Use of superplasticizers in air-entrained concrete can produce coarser-than-normal air-void systems. The maximum recommended spacing factor for air-entrained concrete to resist freezing and thawing is 0.008 inch (0.2 mm). In superplasticized concrete, spacing factors in many cases exceed this limit. Even though the spacing factor is relatively high, the durability factors are above 90 after 300 freeze-thaw cycles for the same case. A study conducted by Siebel indicated that high workability concrete containing superplasticizer can be made with a high freeze-thaw resistance, but air content must be increased relative to concrete without superplasticizer. This study also showed that the type of superplasticizer has nearly no influence on the air-void system.
One problem associated with using a high range water reducer in concrete is slump loss. In a study of the behavior of fresh concrete containing conventional water reducers and high range water reducer, Whiting and Dziedzic found that slump loss with time is very rapid in spite of the fact that second-generation high range water reducer are claimed not to suffer as much from the slump loss phenomenon as the first-generation conventional water reducers do. However, slump loss of flowing concrete was found to be less severe, especially for newly dev