Concrete admixtures reduce the cost of construction, modify properties of hardened concrete, ensure quality of concrete during mixing, transporting, placing, and curing, and overcome certain emergencies during concrete operations.
Chemical admixtures are used to improve the quality of concrete during mixing, transporting, placement and curing. They fall into the following six categories:
- Water reducers
- Set accelerators
- Air entrainers
- Set retarders
- Specialty admixtures:
corrosion inhibitors, shrinkage control, alkali-silica reactivity inhibitors, and coloring
Water-Reducers are chemicals used to lower the water content and increase strength or to obtain a higher slump using the same water content for pumping the concrete or when you are dealing with hot weather.
A water reducer is recommended for use in:
- Conventionally placed concrete mixes containing a wide range of cements, granulated slags, Class C and F fly ashes, silica fume and aggregates.
- Reinforced, precast, prestressed, lightweight or normal weight concrete and pumped concrete
- Residential/commercial flatwork and formed surfaces
An accelerator are chemicals that reduce the initial set time of concrete and are recommended in cold weather.
An admixture is recommended for use in concrete applications where accelerated time of setting and increased early strengths are desired.
A accelerator provides the following advantages:
- Accelerated setting time across a wide range of temperatures
- Earlier finishing of flatwork
- Earlier stripping and reuse of forms
- Earlier finishing of slabs – reduced labor costs
- Increased early and ultimate compressive and flexural strength
The set-retarding characteristics of a Stabilizer aid in the production of concrete with the following special qualities:
- Improved workability
- Reduced segregation
- Superior finishing characteristics for flatwork and cast surfaces
- Moderate to extended retardation – depending on the dosage rate used
- Provides flexibility in the scheduling of placing and finishing operations
- Offsets the effects of slump loss during extended delays between mixing and placing
- Helps eliminate cold joints
- Allows for dead-load deflection to take place (before concrete sets) in extended pours for bridge decks, cantilevers, nonshored structural elements, etc.
- Effective as a singular admixture
- Reduces waste associated with concrete washwater and returned concrete
A Stabilizer is recommended for use in all types of concrete where moderate to extended retardation of set time and improved performance are desired.
This admixture improves pumped concrete, shotcrete (wet mix) and conventionally-placed concretes. It improves plain, reinforced, precast, prestressed, lightweight and normal weight concrete.
*** Primary Applications for use of a Stabilizer
- Stabilization of concrete washwater
- Stabilization of returned plastic concrete
- Stabilization of freshly batched concrete for long hauls
Stabilizers will not initiate or promote corrosion of reinforcing steel in concrete.
Concrete durability research has established that the best protection for concrete from the adverse effects of freeze/thaw cycles and deicing salts results from: proper air content in the hardened concrete; a suitable air-void system in terms of bubble size and spacing; and adequate concrete strength, assuming the use of sound aggregates and proper mixing, placing, handling and curing techniques.
When unusually low or high amounts of an air-entraining admixture are required to achieve normal ranges of air content or if the required amount of air-entraining admixture necessary to achieve required levels of air content is observed to change significantly under given conditions, the reason should be investigated. In such cases, it is especially important to determine: (a) that a proper amount of air is contained in the fresh concrete at the point of placement; and (b) that a suitable air-void system (spacing factor) is being obtained in the hardened concrete.