Placing and curing of concrete

Placing of fresh concrete

Placing of concrete includes the following operations:

• spreading concrete or filling concrete form;
• compaction;
• finishing.

Concrete is placed into previously prepared molds – concrete form which can be made of wood, metal, plastic, rubber, concrete or a combination of some of the above mentioned materials. The formwork should be easily assembled and disassembled, provide quality placing, i.e. it should be hermetically sealed, and it should provide concrete work without any mass loss. It gives shape to the construction and it must be removed without any damaging.

In order to place concrete, as homogeneous as possible, into the formwork or molds, with minimum cavities, air and water bubbles and thereby achieve a better strength and good looks, it is necessary to activate it, i.e. to excite mass so that each particular size take the correct position. An important factor in achieving high quality concrete is correctly placing of concrete.

The following methods for placing of fresh concrete are available and they can be used individually or combined at the same time:

• vibrating,
• centrifuging
• vacuuming and
• pressing.

Today, vibrating is certainly most widely spread method of placing of fresh concrete. During vibrating there comes to "packing" of particles and air extrusion. After completion of the procedure it usually stays 1-3% of residual air, and the volume mass increases to the projected value.

The intensity of vibration is characterized by amplitude and frequency of oscillations. Concrete with coarse aggregate - (larger aggregate size, D = 50-70 mm) requires a lower frequency and high amplitude of oscillations, so for concrete made with this type of aggregates a low frequency vibrators are used. For concrete with medium aggregate size (D = 10-50 mm) vibrators with mid-frequency and mid-amplitude are used, and for the small particle size of concrete, vibrators with high frequency and low amplitude.

Centrifuging is a method in which a mold with concrete mixture is rotated around the longitudinal axis. Due to the speed of rotation, under the action of centrifugal force, concrete mixture is spread to the outer sides of the mold and by pressing it distributes in a layer of uniform thickness. Concrete with low water-cement ratio and aggregate with a smaller particles size, rarely exceeding 16 mm, should be used for centrifuging. This technology of placing concrete is commonly used for production of electric poles and concrete pipes for higher pressures, as well as for hollow shaft carriers.

Vacuuming is usually used in combination with vibrating. This method aims to remove the excess of water from concrete already casted into a mold or formwork, namely that amount of water which is not needed for chemical activation of the cement process, and which had to be added to the concrete mixture in order to provide a better placing. This method is used more often during the concrete works of drainage channels and sewage collectors, then during concrete works of prefabricated wall blocks and during the construction of concrete roads and floors.

Pressing is also a method which doesn't apply separately, but in combination with vibrating and vacuuming. Concrete with low water cement ratio exposes simultaneously to vibration and pressing. In this way, it increases the compactness, or density, of concrete, and therefore the strength of concrete.

Pressing concrete blocks under high pressure gives a seemingly hard block, which can be transported while concrete still hasn't setting.

Curing of concrete

Curing of concrete represents a set of different procedures and measures to be taken in order to achieve the designed mechanical characteristics of hardened concrete. Curing prevents evaporation of water from the fresh concrete required for the hydration of cement. Also, this method provides protection from extreme temperature that could have negative affect to hydration of cement.

In order to ensure optimal conditions for the hydration of cement, concrete must be protected from the harmful effects of wind, sun and changeable weather.

There is a wide range of techniques for curing of concrete, from those that are designed to prevent water loss, to those which provide enough moisture to concrete during the early stages of this process.

Air temperature, humidity, concrete temperature and wind speed have an influence on water evaporation from freshly placed concrete.

In the average conditions (moisture content = 70 %, temperature = 21 °C and the wind speed of 9 m/s) concrete loses 0,5 l/m2 of water.

More about the reasons for curing and techniques you can found here.