UFC 4-150-07
19 June 2001
aggregate is the only available material, it must be washed thoroughly with
freshwater to remove the salt.
In some applications, special aggregates may be used to make
lightweight concrete for lightweight structures. Most lightweight concretes have a
density between 1,280 and 1,760 kg/m3 (79.9 and 109.87 pounds/cubic foot)
compared to 2,400 kg/m3 (149.83 pounds/cubic foot) for normal weight concrete.
Lightweight aggregate concrete is generally more durable in the marine
environment than concrete made with normal weight aggregate.
3-3.2.3 Water. Water quantity and quality will affect the durability, strength,
and workability of concrete. In practice, use water equal in quality to drinking
water. The ratio of water to cement has a direct effect on the strength of the
concrete and its permeability. A maximum water-to-cement ratio of 0.40 by
weight is crucial for concrete used in a marine environment. Seawater should
never be used for making concrete because the salts will dramatically increase
the corrosion rate of reinforcing steel.
3-3.2.4 Admixtures. Various chemical admixtures are used to give specific
properties to the concrete to improve durability, finishability, or workability. If
admixtures are required, they should meet the appropriate ASTM or ACI
specifications.
3-3.2.4.1 Water Reducing. Water-reducing admixtures are available to allow a
reduction in the water-to-cement ratio while maintaining a workable slump.
Normal range water reducers and superplasticizers are admixtures that permit
the reduction of the water-to-cement ratio.
3-3.2.4.2 Air-Entraining. Air-entraining admixtures are available to improve the
concrete's ability to resist freeze-thaw conditions and enhance workability. Air-
entraining agents should be used to incorporate from 5 to 7 percent of entrained
air into the concrete. ASTM C 233, Air-Entraining Admixtures for Concrete,
covers the air-entraining agents. Air-entraining agents also improve the
workability of the concrete.
3-3.2.4.3 Accelerator. Accelerator admixtures are available for rapid setting
products. These increase the early strength of concrete but have little effect on
the final strength. Accelerators containing excessive chloride should not be used
since they increase corrosion of reinforcing steel.
3-3.2.4.4 Pozzolan Minerals. Pozzolan minerals (fly ash Class F) are highly
recommended as a replacement of 25% of the Portland cement. The ash
reduces permeability, eliminates alkali-silicon reactions, and improves
workability. Silica fume can reduce permeability but may cause finishing
problems and surface cracks. Avoid silica fume dosages about 5% by weight to
concrete.
3-14
Previous Page
