10.2. Fundamentals of concrete mixtures technology

Carefully mixed mixture of binder, water, aggregates and additives, introduced in required proportion is meant under concrete mixture. Concrete mixtures are made on the concrete mixing units, plants or workshops of reinforced concrete elements plants.

Concrete mixture can be made in truck mixers in the process of transporting at the small distances from a factory to the building sites or directly at the object from the dry cement mix with aggregates.

The process of the concrete mixture production consists of the followings technological operations: materials preparations, dosage, mixing (Fig. 10.3).

T he materials preparation for concrete mixture includes aggregates cleaning from the harmful admixtures and segregation on factions which is executed on the grinding-sorting factories. The aggregates and water heating is made at the winter concreting. The aggregate and water cooling, and also replacement of water part with the ice can be executed at concreting to decline the heat generation of massive hydraulic constructions.

The quality of concrete mixture depends on the accuracy dosage of the components. The dosage of bulk materials is executed according to the mass. The porous aggregates are dosaged according to the volume with a mass correction. The error of cement, water and admixtures dosage should not exceed ± 2%, aggregates ± 2.,5%. The portioning and continuous batchers with a manual, semi-automatic or automatic control are used on the concrete mixing units. Batchers with a manual control are used only at the units with the small capacity.

The basic stage at the concrete mixture production is the mixing, on the care of which the strength and homogeneity of concrete depend. There are concrete mixers of two basic types depending on the method of mixing: free falling and forced mixing.

The concrete mixers of the free falling (gravity) have drums as a cylinder or two cones, combined with the bases. Mixing takes a place due to fascination of mixture by the blades at a drum rotation with its following free falling.

The mixers of the forced mixing are the most effective for rigid concrete mixtures mixing, and also porous aggregates based mixtures. The concrete homogeneity rises due to the blade rotation in the stationary drum, or in a drum rotating in the opposite direction.

The mixing duration of heavy-weight concrete mixtures in gravity batch mixers lasts, as a rule, 1...2 minutes, in the mixers of the forced mixing - 2...3 minutes. The mixing duration of the fine-grained concrete mixtures and concrete mixtures on porous aggregates is increased up to 3.5 minutes. The mixing duration is also increased in the winter.

Depending on the workability the concrete mixtures are divided into classes accordingly to EN 206-1:2000 (Table 10.6).

Таble 10.6

The workability classes of the concrete mixtures

Class	Vebe, secs	Class	Slump, mm
V0	> 31	S1	10...40
V1	30...21	S2	50…90
V2	20...11	S3	100 ...150
V3	10...6	S4	160 ...210
V4	5...3	S5	>220

Also, concrete mixtures can be divided by degree of compaction and spread (flow test) of Abrams cone (Table 10.7).

Table 10.7

Compaction and spread of Abrams cone classes

Class	Degree of compaction		Class	Spread of cone, mm
С0	>1,46		F1	>340
С1	1,45 ...1,26		F2	350 ...410
С2	1,25 ...1,11		F3	420 ...480
С3	1,10... 1,04		F4	490...550
			F5	560. ..620
			F6	>620

The next classification (Fig.10.4) of concrete mixturese depending on slump can be also used.

C oncrete mixtures with the classe S3 and S4 should be made with the obligatory using of plastifying agents.

Homogeneity of the concrete mixture, intensity of hardening, strength and other properties depend on duration and intensity of mixing.

The concrete mixing plants and factories are batch (batch-type) and continuous action. The plants of batch action are the most widespread; they include operations which repeat periodically: loading of charging feed, mixing and unloading of the prepared mixture. The processes of dosage, mixing and delivery of mixture flow without breaks at the concrete plants of continuous action.

For batch gravity mixer ready batch volume depending on the drum capacity hesitates from 65 to 3000 litres. Mixers with a capacity of the ready batch not more than 165 litres are movable, with greater capacity - stationary.

Workability, rigidity and segregation are the basic technological properties of concrete mixtures.

Under the workability of concrete is understood its ability to flow back under the action of self- weight.

The mixtures which slump close to the zero belong to the rigid. The rigidity of concrete (V) is characterized with the concrete mix cone vibration time required (at the special device using) for its full compaction.

The water content (W) is one of the major factors that determine the indexes of workability of concrete mix (Fig. 10.5). The water content of the concrete mixture, that means water content, required for achievement of required workability and rigidity, depends on to the type and amount of cement, specific surface and the aggregates voidage, shape and character of their surface, correlation of fine and coarse aggregate, conditioning duration of the mixture to its placing, kind and content of plasticisizing admixtures and other.

I ncreasing in cement normal consistency at 1 % causes approximately the increasing in concrete mixture water content at 1.5...3 %. Thus, concrete on cements with raised normal consistency at constant water content has lower slump and higher rigidity.

Change in the cement content at constant water content to C/W =1/1.68K_n.c (K_n.c is normal consistency of cement paste in the decimal fraction) does not influence significantly on the workability. This statement is called the rule of water content constancy, as it foreknows that with the increasing of the cement content to some limit at unchanging correlation of aggregates, the water content of mixture does not almost change (Fig. 10.6). The greater amount of cement paste is required with the increasing of the sand content, surface of aggregates, their voidage, and also the content of dust and clays in them. If it will remain permanent, the workability of the concrete mixture will go down. The crushed stone using instead of gravel also conduces to the increasing of concrete mixture rigidity.

As a result of partial cement hydration, some water evaporation and its absorption by the aggregates, workability of concrete mixtures goes down at seasoning them before casting and at transporting. The workability loss of concrete mixtures at transporting should not exceed 50 % at slump no more than 8 cm and 30 % - over 8 cm. The rational technological method of workability improvement of concrete mixtures is the plasticizing surface-active substances (SAS) using. They are divided into four groups depending on the effect: superplasticizers, high, medium and low plasticizing. Superplasticizers allow to obtain from low workable mixtures (Slump = 2...3 cm) cast concrete mixtures with the slump 20 cm and more at constant water content without strength reduction. Low plasticizing admixtures increase slump from 2...3 to 8 cm and less.

T he effect of plasticizing substanses can be used with the purpose of simultaneous decline of water and cement content at constant mixture workability and concrete strength, or for diminishing of water-cement ratio and increasing of concrete strength. Traditional plasticizing agents as technical lignosulphonates due to diminishing of the water content allow to decrease cement content at 8...12 %, the cement economy at the superplasticizers using can achieve 20 %.

The workability and rigidity of concrete mixtures is assigned depending on the peculiarities of structures, degree of reinforcement, method of transporting and compaction. The greater workability of mixtures is required for the thin-walled elements saturated with reinforcement, than for the massive low-reinforced structures. At the using of the modern methods of compaction, especially in industrial conditions, the possibility to increase rigidity of concrete mixtures appears, which conduces to the cement economy and improves the row of constructive properties of concrete.

Along with required workability of concrete mixtures it is important to provide their homogeneity which is measured by the segregation degree of mixtures at storage and transporting. Segregation of concrete mixtures develops as a result of the sedimentary processes caused by the different density of the concrete components. Application of segregating mixture conduces to the uneven distributing of aggregates in the concrete and cement stone, and deterioration of the concrete properties. Diminishing of segregation is observed at the water content declining, introduction of fine ground up mineral admixtures, surface-active substances.

Concrete mixtures are transported at construction sites by high-discharge mixers or truck mixers. Transportation of concrete mixtures with dump truck is permitted. The concrete mixture can be given with cargo cranes, on runners, band conveyers, by the pipeline transportation to the place of work implementation.

Placement of concrete is carried out in forms or mold with the preliminary placed reinforcement and embedded items.

Quality of placement is determined by efficiency of material compaction of all the volume, preventing the segregation. Concrete density reducing at 1 % causes decreasing of its strength approximately at 5 %. Compaction ratio K_c should be not less than 0.97...0.98, and for the concrete of raised density not less than 0.99:

, (10.4)

де - actual average density of compacted concrete mixture; - calculated average density.

The compaction of concrete mixtures can be carried out by vibration, degassing, pneumatic concreting (shortcreting), and also by rolling, pressing, centrifugation etc.

The vibration is the most widespread and effective method of compaction. The vibration frequency 10 - 50 Hz is used for its compaction. The two types of vibration are used for cast-in-place structures - internal and surface vibration. An internal vibration is based on the immersion of the executive device of vibrator into the mixture; it is utilized for the compaction of concrete mixtures with the slump over 1 cm. Surface vibration is recommended for the compaction of concrete coverings, roads, slabs etc. The layer with the depth 20...30 cm is well consolidated at a surface vibration.

At the plants of the precast reinforced concrete elements are formed, as a rule, at the special vibro-areas. The surface, internal and external vibrators are used at elements cast in the stationary forms. The last vibrators are attached to the form.

Vibrostamping is applied at production of elements of the complicated configuration; it consists in the compaction by the vibrator of certain shape. The progressive method of forming is the powered vibrorolling - combination of vibration of mobile layer of concrete on a stripe with pressure at the concrete by vibromills of rolling mill.

Efficiency of vibration of rigid concrete mixtures rises at the simultaneous action of small pressure (vibrations with cantledge) - to (1...3) 10³ Pa at the opened surface of the element.

The operating conditions of vibration, intensity and duration of fluctuations, also method and time of their application influence on compaction quality. With the intensity increasing until the optimal magnitude compaction coefficient and concrete strength grows, but at exceeding of optimal magnitude the segregation of mixture is possible.

The repeated vibration after 1.5...3 hours after a placing (when the setting of concrete mixture did not take place) allows to increase strength of concrete at 10...15 % or to reduce cement content at 20 %, to increase water impermeability in 1.5...2 times, and also to improve a frost resistance and adhesion with reinforcement.

The effective method of compaction is vacuum compaction along with the repeated vibration for the thin-walled structures from mixtures with raised water content. At the vacuum compaction by the shields and bushes which have vacuum chamber, it is sucked off 10...20 % of the general amount of water from the concrete, which improves it quality.

The centrifugation is utilized for pipe forming, bearing supports, piles and other structures of tubular profile, at which distributing and compaction of concrete mixture takes place under the action of centrifugal forces, developing in the revolving forms.

The effective method of compaction is pressing with the help of which it is possible to get the concrete of high strength and longevity at the using of fine-grained rigid concrete mixtures The different methods of pressing (vibropressing, radial and axial pressing) are used, for example, at making of concrete pipes.

The superplasticizers using allows to produce at moderate water content cast mixtures, using of which enables to reject the vibrocompaction and carry out the gravity filling of molds. The cast mixes are required for production composite and heavily reinforced concrete structures, when high-quality compaction of concrete mixture by vibration is difficult to realize.

The pneumatic methods of concreting - shortcreting and jetcreting are applied in the hydraulic engineering and other industries of construction for concreting of the channel slopes, dikes, screens and tunnel facing; they are based on repulsion of concrete mixture under the pressure and application of it with the dynamic forces at the surface.

The shortcrete does not contain coarse aggregate; its rational compositions range from 1:2 to 1:4. The jetcrete contains a coarse aggregate and allows to create the layer of higher thickness (20...30 cm). Shortcrete is also used also for repair, strengthening and reconstruction in various structures, grouting of the precast reinforced concrete, for concreting of the thin-walled bearing structures of different shapes. Air cement guns, pneumatic delivers and pneumatic rackers are used for application on the surface of shortcrete and jetcrete.