Concrete Work

operations carried out in the erection of monolithic concrete and reinforced concrete structures and installations made of cement concrete. Concrete work includes the following basic processes: preparation of the concrete mix; delivery of the mix to the construction site; feeding, distribution, and compaction of the mix in the formwork (molds); the curing of the concrete while it is hardening; and quality control of the concrete work. By 1975 the volume of concrete work carried out in the USSR in the construction of industrial buildings, public buildings, and other installations made of monolithic concrete and reinforced concrete will reach approximately 150–160 million cu m per year. In heavy hydroengineering projects, the volume of concrete poured amounts to 5–7 million cu m.

Preparation. Preparation of the concrete mix is usually carried out at concrete plants or in movable concrete-mixing units. Also used for this purpose are stock (sectional-assembly) plants, whose equipment, transportable on railroad flatcars or trailer trucks, may be set up in large units. The productivity of concrete plants and units operating in the USSR ranges from 5 to 240 cu m per hour. Concrete plants include equipment for receiving concrete components from the means of transport, storage areas for cement and aggregates, the apparatus for conveying the materials from the storage areas to the delivery hoppers, the delivery hoppers themselves, and the batching and mixing units. According to the nature of the technical process employed, concrete plants are divided into cyclical-action plants, in which the preparation and output of concrete mix is conducted in batches that correspond to the capacities of the concrete mixers, and continuously operating plants, in which the basic technical operations are conducted simultaneously and the prepared mix is delivered in an uninterrupted flow. The basic technical processes at concrete plants are batching, mixing of the components of the concrete mix, and transport and storage operations. Automation has been developed for these processes. Materials from automated storage areas of cement and aggregates are conveyed upon signals from sensory monitors which indicate the level of the material in the corresponding delivery hopper. The automatic batching units weigh out the necessary portions of each component according to set formulas (at cyclical-action plants) or provide a flow of materials corresponding to a set degree of productivity (at continuously operating plants). The components are remixed in concrete mixers. All technical processes are controlled by an operator who works from a centralized remote-control panel. There are also automated plants which prepare the concrete mix after the truck driver has dumped the load of ingredients into a programmed and computerized unit; this unit is equipped with a punched card or counter which contains the code designating the required formula and quantity of the mix.

Delivery. Delivery of concrete mix to the construction site is usually carried out by truck transport. In addition to dump trucks, concrete trucks are used which are specially equipped for hauling concrete mix; for long distances, concrete-mixer trucks are utilized. They are loaded at the concrete plant with the dry components of the mix, which are mixed with water either en route or upon arrival at the construction site. Ready-made concrete mix can also be transported in such concrete-mixer trucks. If unloading the concrete mix directly from the body of the truck into the formwork is impossible, the mix is unloaded into buckets, which are then conveyed by cranes (truck-mounted, track-laying, tower-type, and other types) to the location where the concrete is to be placed.

Feeding. The feeding of concrete mix is carried out by belt conveyors, concrete pumps, concrete hoists, pneumatic blowers, and vibration chutes. Feeding and distributing concrete mix for the laying of concrete footings under structural components and under equipment to be used in industrial buildings are also carried out by mobile concrete placement units, equipped with directable belt conveyors. In highway construction the distribution of concrete mix along the breadth of the strip to be concreted is accomplished primarily by concrete placement units which move along in forms mounted on rails. There are prospects for railless concrete placement units with sliding forms and automatic braking at markers indicating the strip where concrete is to be placed.

Compaction. Compaction of concrete mix is an extremely important process in concrete work. It ensures that the mix will compactly fill all the interstices between the reinforcement rods as well as between the reinforcement rods and the form-work in order to achieve the required strength, impermeability to water, and frost resistance of the concrete. The basic method of compaction—vibrating the concrete mix— is the forced action on the mix of high-frequency oscillating impulses, during which the concrete mix becomes mobile (fluid) and is compacted by the action of its own weight. Vibration allows the use of so-called stiff and slightly workable concrete mixes, to economize on cement and to obtain high-strength concretes. Depending upon the type of structural element to be concreted, various kinds of vibrators are employed: internal (immersed in the concrete mix), surface (compacting the mix from above), and sometimes external (clamped to the form-work). At large hydraulic engineering construction sites, sets of heavy-duty vibrators are utilized that are transportable by mechanized means. When necessary the surface of compacted concrete may be smoothed over by concrete-finishing machines.

Curing. Curing of cement consists in the establishment of a temperature-humidity system required for the hardening of a compacted concrete mix and in the protection of the concrete from jarring vibrations, impacts, and so on. Effective methods of curing concrete consist of covering its surface with a protective polymer film or applying a water-asphalt emulsion, ethynol varnish [a many-component varnish with washing, naphthenic, and other fractions], and other compositions to prevent the evaporation of moisture. After placing the concrete, horizontal surfaces may also be covered with sand or sawdust, which is then wetted periodically.

In erecting thin-walled structures (for example, storage tanks, shells, and so on), concrete work is sometimes done by spraying the concrete mix, employing compressed air for this purpose. This method, called gunite work (in Russian, torkretirovanie), has also been adopted for repairing defects in concreting, as well as for strengthening and restoring concrete and reinforced concrete structural elements. In a number of cases an increase in the strength of concrete and a speed-up in its hardening process in the initial period have been achieved by vacuuming, that is, by suction of excess water and air from the concrete mix after it has been placed and compacted in the form-work. For this purpose the concreted surface is enclosed with boards which have vacuum channels and are covered with a filtering material. As the result of reverse pressure created in the vacuum channels by a vacuum pump, the boards adhere to the concrete surface and water is sucked from the concrete into the channels, while particles of cement are retained by the filter.

A special method of carrying out concrete work is the so-called separate method of concreting; it consists of forcing a cement-sand paste into crushed stone (gravel), previously placed in the form-work, by means of pipes or special injectors inserted into the aggregate. Such a method is efficient in concreting densely reinforced structural components in places which are difficult of access.

For constructing the underwater portion of docks, sluices, bridge piers, deep foundations, and other installations without draining off the water, underwater concreting is employed. Its basic methods are the vertically movable pipe method and the “rising mortar” method. In the vertically movable pipe (VPT) method, the concrete mix is fed underwater through a pipe (with a diameter of 200–300 mm); the lower end of the pipe, in order to avoid having the mix washed away by the water, is submerged in the mass of concrete which is being placed. The “rising mortar” method is a variety of the separate method of concreting.

Quality control. Quality control of the concrete work includes making concrete samples at the work site, maintaining them under conditions close to those of production, and testing the samples for strength. When special requirements for the concrete are called for, the samples are tested for impermeability to water, resistance to freezing, and other factors. In order to check the density and strength of the concrete, the sclerometric, ultrasonic, and radioisotopic methods of testing, all of which are nondestructive, are used. In addition to this testing, checks are regularly conducted on the conformity to engineering specifications of the quality of the materials used in the concrete, the exactness of the batching, the proper preparation of the structural components for concreting, the correct procedures in pouring the concrete, the observation of proper time periods for removal of forms, and so on.

In the USSR, in contrast to foreign countries, concrete work is carried on extensively not only during the summer months but also during the winter. The methods of winter concreting are subdivided into the so-called heatless methods (the “thermos” method and the “thermos with antifreeze admixtures” method), which are used primarily in the concreting of massive structures, and methods with artificial heating (electric heating, steam heating), utilized in building thin-walled structures. A combination of the above-mentioned methods is also possible. In the “thermos” method the hardening of concrete prepared from heated material occurs after the placing of the concrete mix in ordinary or heated forms because of the heat given off by the cement during the hardening process. The concrete reaches its required strength before it cools down to 0° C. In order to speed up the hardening and to increase the period of cooling down of the concrete, the concrete mix is frequently given a supplementary heating of up to 50°–70° C before placement by passing an electric current through it. Antifreeze additives (calcium chloride, sodium chloride, potassium carbonate, sodium nitrite, and others), by lowering the freezing point of the concrete, allow the mix to be placed under certain conditions and ensure that the concrete will harden without subsequent heating at an air temperature of below 0°C. In artificial heating up to temperatures of 40°–90° C there is a speed-up in the hardening of the concrete as well as in its attainment of the required strength. In steam heating of concrete, steam is fed into the space surrounding the concrete or into channels in the form-work. Electric heating may be carried out by passing an electric current through a body of hardening concrete, for which purpose special metallic electrodes are placed either on the surface of or inside the concrete. In addition, various electric heaters are utilized. Two types in particular are the heaters which are installed in the form-work and the induction heaters, which cause the steel forms and reinforcement framework to heat up.

In specific instances concrete work is performed in locally heated temporary winter shelters: adjustable (sectional) shelters, shelters on rollers (for horizontal work), or sliding shelters (for vertical work).