No_6_1959 — Page 52

ADMIXTURES FOR CONCRETE-A SURVEY

N modern concrete technology, the

IN

at

use of admixtures to promote special properties in concrete for the work hand is finding great favour among en- gineers and architects.

Some of the more important proper- ties obtained by the use of admixtures are High early strength; retardation of initial stiffening: improved work- ability; reduced

increased bleeding; durability; denser concrete;

improved

surface texture; increased bond to steel reinforcement; reduced capillary flow and permeability to liquids; in grouting operations, improved penetration and reduced segregation; in grout for seating machinery, column bases, etc., prevent settlement by slight expansion of the grout.

The most common admixtures can be grouped into the following classes: Ac- celerators, retarders, air entraining

agents, gas forming agents, pozzolans, damp-proofing and permeability reduc- ing agents, workability agents.

the

rate

of per- forms,

Accelerators are used to shorten time of set and to increase the of hardening for the purposes mitting the early removal of shortening the curing period, advanc- ing the time when a structure can be put into service, offsetting the retarding effects of low temperatures, and gency repairs.

CARE NEEDED IN CHOICE

OF ACCELERATORS

emer-

Care must be exercised in the choice of accelerators available. Some of them, even when used in small quanti- ties, can cause a flash set in some ce- ments. Such accelerators normally re- duce the ultimate strength and should only be used for emergency patching and stopping of leaks.

Undue delay between mixing and the placing of the concrete should be avoid- ed.

Accelerators can be used with advantage in the manufacture cast concrete products.

great

of pre-

to off.

Retarders are normally used set the accelerating effect of hot wea- ther on the setting of concrete, to eli- minate the tendency of some cements to exhibit false set, and to delay the stiffening of grout and concrete for dif- ficult conditions of placing or when the mixer is sited a considerable dis- tance from the working area.

Examples of the use of retarders are the cementing of oil wells at tempera- tures up to 400 deg. F, the pumping of cement grout slurries for consider- able distances along narrow grout holes, and the transportation of concrete from ready-mix concrete plants.

Another application of retarders is their use in the control of false sets in some cements.

Over the last 20 years ог So, the superior durability of air-entrained con- crete has been proved under severe natural weathering conditions.

50

By R. Leong, B. Sc.

of

Entrained air, in the form microscopic bubbles evenly distributed throughout the mix, materially changes the properties of both the plastic and

Air-entrained hardened concrete.

con-

crete is much more workable than

ordinary concrete and can be placed with less segregation and there is less tendency for bleeding and water gain. The lower water cement ratio reduces the number of capillaries in the concrete.

All these factors help in promoting durability by increasing uniformity and eliminating planes of weakness at the top of vertical lifts. In the hardened state, the air bubbles may act as pansion chambers to relieve the pressure created in the concrete due to volume changes caused by variations in tem- perature and moisture.

ex-

and

While improving workability durability, air-entrainment may have an adverse effect upon strength. How- ever, by using less sand and water to obtain a constant workability, the ten- be to decrease dency

strength can partially offset and, with not more than 7 percent of entrained air, the reduction in compressive strength rarely exceeds 15 percent.

Air-entraining agents are increasisgly used in precast concrete products factories to reduce breakages and wear on machinery, obtain denser and cleaner looking blocks with vastly improved surface texture.

In the field of lightweight concrete, air-entraining agents are also used to entrain greater percentages of air with adequate structural strength and superior thermal insulation.

These agents are similar to air-en- training agents but differ in that they produce a gas upon reaction with the hydroxides present in hydrating cement. This action, when properly controlled, causes a slight expansion in the plastic mortar or concrete and thus reduces or eliminates settlement or bleeding and conscquently may increase bond horizontal reinforcing bars and improve the effectiveness of grout under machine bases.

EFFECTS ON STRENGTH VARY

WITH MIX

to

Gas forming agents are also used in the manufacture of lightweight con- crete.

Pozzolanic materials are used in con- junction with portland cement, often as a replacement of the cement. In the presence of water, the pozzolanic

some-

material reacts with the lime from the hydrating cement to form cementitious compounds. These materials are times used in large hydraulic structures where high temperatures are to be avoid- ed or in structures exposed to seawater or sulphate-bearing waters.

When used as an addition to concrete, pozzolans improve workability imper- meability, and resistance to chemical attack.

The effects of pozzolans on strength vary with the mix and the material. Generally, the strength of lean mixes is increased while the strength of rich mixes is often decreased.

Water passes through concrete by two different processes.

In one, water under pressure is forced through channels con- necting the inner and outer surfaces. In the other,

draws the capillary action water into and through the concrete without any appreciable pressure present.

There are admixtures which used in carefully designed and well compacted concrete will minimise considerably the passage of water by the above two processes.

Admixtures of this type should be used with care since most of them reduce the strength of the concrete.

They are normally used in floors in contact with the ground, basements, lift wells, swimming pools, etc.

Good workability is essential in all concrete and should be the maximum obtainable. Where the concrete is harsh due to aggregate characteristics or must be placed in sections with con- gested reinforcement, extra workability is necessary.

EFFECTS OF REDUCED

WATER CONTENT

Where a mix is deficient in "fines" the addition of mineral admixtures im- proves the workability, reduces bleeding and increases the strength.

The use of other workability agents permits a lower water content in a mix without loss in workability. The effects of this reduced water content in the hardened concrete are improved im- permeability and compressive strength.

Air-entraining agents are also used as workability agents because they improve cohesiveness in the mix and accordingly reduce segregation and bleeding at a reduced water content with no loss in workability.

Apart from the above briefly discussed admixtures, there are other admixtures such as

and integral floor hardeners alkali

aggregate expansion inhibitors which may be useful in particular cases. In conclusion, it will not be out of place to note that the American Con- crete Institute Committee 212 endorses the statement of P. H. Bates that "the use of admixtures should not be frown-

ed upon or scored. On the contrary, their development and study should be highly commended and fostered".

In its introduction, the Concrete Manual issued by the Bureau of Re- clamation, United States Department of the Interior, also says: Thus, where concrete was once considered to be a simple mixture of coarse aggregate, sand, cement, and water, mixed and placed in a haphazard manner, the modern concept is a carefully con- trolled mixture combining admixtures as needed to obtain the optimum in quality and economy for any use."

THE HONG KONG & FAR EAST BUILDER

VOLUME 14, NUMBER 6