Fig. 3. Continuous concrete mixer

1 – mixing drum; 2 – blades; 3—water-supply funnel; 4—charge funnel for solids; 5 – electric motor with reducing gear

Vibro-mixers have been specially constructed for preparing stiff and extra-stiff concrete mixes, the ingredients being mixed by combined stirring and vibration, and. in some types of mixers, by vibration alone.

Transportation of concrete mixes. During transportation of the concrete mix to the site of placement, its homogeneity and mobility should be preserved. On the way, concrete mixes may thicken because of hydration of cement, absorption of water by the aggregate and evaporation; in no case, however, should a mix be placed in a constraction unless its mobility is in compliance with the design value.

Placing Concrete Mixes, Curing Concrete and Quality Control

Placing concrete mixes. The placing of a concrete mix and its compaction inside a form is one of the most labour- and power consuming operations. These operations are currently performed with the aid of concrete placers or more simple machines – concrete distributors. Concrete placers allow to mechanize to a large extent the distribution of a concrete mix inside forms. A concrete mix should be placed in a form with no voids; corners and narrow sections of the form should be filled with particular care. After a concrete mix is placed, it may be compacted by vibration, vibro-stamping, centrifuging, vacuumizing, rolling.

One of the most widely used techniques for compacting concrete mixes is vibration. The degree of compaction of a concrete mix by vibrators depends chiefly on the frequency and the amplitude of oscillations and the duration of vibration.

By the kind of prime mover, vibrators are subdivided into electromechanical, electromagnetic and pneumatic types, most of the vibrators being of the eleclromechanical kind.

The type of vibrator should suit the kind, shape and size of the construction being concreted. Large open concrete surfaces (floors, slabs, roads) should be compacted with the aid of external vibrators which transmit vibrations to the concrete mix via a metallic platform (float) which carries the vibrators. Oscillations penetrate as deep as 20-30 cm into the layer of concrete, the duration of vibration in one place being about 1 min, after which the vibrator is moved to an adjacent place.

Internal vibrators are used for compacting concrete mixes in massive constructions of great depth (thickness). Used as internal vibrators are:

• needle vibrators with an electric motor carrying eccentric weights which produce the vibrations;

• high-frequency (up to 7000 osc/min) vibrators with a flexible shaft and a tip housing an eccentric.

Hardening and curing of concrete. Curing of concrete involves means that provide normal hardening of concrete mixes and prevent damage to green concrete structures. Curing should be organized immediately after concrete is placed and compacted. First, it is imperative to protect the surface against drying.

An efficient means for curing freshly placed concrete in road construction is to coat its surface by film-forming substances, such as bitumen emulsions, latex, synthetic rubber, etc. Along with this, horizontal surfaces of products are covered with sand or sawdust after concrete has set, the materials being wetted at regular intervals. The time of moistening is a function of atmospheric conditions: in hot weather, it is up to two weeks; in cold weather, several days.