Evaluation of strength and uniformity concrete in construction and ready-mixed concrete plants should be carried out in accordance with GOST 18105-72*.
1. Characteristics of concrete.
Concrete receive as a result of hardening of a properly selected mixture: binder, water, fine and coarse aggregates and, if necessary, special additives.
A mixture of cement and water is called cement paste, and its mixture with sand and rubble (or gravel) - concrete mix. Cement paste serves as a binding adhesive, which, when hardened, holds grains of sand or gravel together.
As a result of this, the concrete mixture laid in the form gradually turns into an artificial stone material - concrete.
The cement in concrete is the binder or binder.
Sand and crushed stone (gravel) do not participate in the formation of concrete, therefore they are called placeholders or inert materials.
There are small and large aggregates. Fine sand and other aggregates (for example, slag and expanded clay) with grains up to 5 mm; to large aggregates - crushed stone (stone, expanded clay, slag, etc.) and gravel with grain size 5…150 mm. Sometimes larger pieces of stone - "raisins" - are placed in concrete.
If the mixture contains, in addition to cement and water, only fine aggregates, then it is called cement mortar.
Most often used in industrial construction heavy concrete, consisting of a mixture of cement and water with sand and gravel or crushed stone.
The density of heavy concrete in the hardened state is 2200…2500 kg/m³.
In housing and civil construction, they are more often used fine-grained concretes(without coarse aggregate - gravel or crushed stone) with an average density of over 1800 kg/m³. Concrete density 1800 kg/m³ and less called light or warm, because they have low thermal conductivity.
The main difference between lightweight concrete and heavy is that they are cooked on light aggregates having a porous structure (expanded clay, perlite, granular slag, etc.).
The main requirement for concrete- acquisition by him at a certain time (usually in 28 days) given strength for compression.
Depending on the compressive strength concrete divided into classes:
- heavy concretes with large filler -
- B3.5; AT 5; Q7.5; AT 12; B15; IN 20; B25; B30; B40; B45; B50; B55; B60;
- from B3.5…B30- with fine sand, up to B40- with coarse sand;
- B20…B40- at concrete density 2000 kg/m³
- B5…B35- at density 1800…1900 kg/m³.
ATTENTION! Concrete class assigned in the project for the construction of the facility. For example, if the drawing says "concrete class B20", then this means that concrete strength compression (through 28 days) is 20 MPa.
heavy concrete classes up to V7.5 inclusive used only for non-reinforced structures.
Structures with prestressed reinforcement made of heavy concrete of a class not lower than IN 20 or from lightweight concrete of a class not lower than B15.
Concretes are subdivided:
- by volumetric weight -
- extra light - 500 kg/m³; lungs - 500…1800 kg/m³; heavy - 1900…2400 kg/m³; especially heavy - 2500 kg/m³.
- strength(in compression) on stamps -
- M-35, M-50, M-75, M-100, M-150, M-200, M-250, M-300, M-400, M-500 and M-600;
- frost resistance(in cycles of alternate freezing and thawing) on stamps -
- Mrz-10, Mrz-15, Mrz-25, Mrz-35, Mrz-50, Mrz-100, Mrz-150, Mrz-200, Mrz-300;
- in terms of water resistance on the brand, in which there is no seepage of water through a sample of 28 days of age -
- B-2, B-4, B-6, B-8- withstand water pressure, respectively, not less than 2, 4, 6, 8 kgf/cm²;
- in terms of hardness and mobility (see Table 1).
2. The main properties of the concrete mixture.
The main technological property concrete mix is workability. Concrete mix workability evaluated according to mobility and stiffness indicators in accordance with the test methods given in GOST 10181-76.
Concretes are subdivided by indicators rigidity and mobility according to table.1.
Table 1: Characteristics of concretes.
That's why when choosing the composition of concrete take into account not only required concrete strength, but also given by the conditions of work mobility of the concrete mix.
NOTE: Selection of the composition of the concrete mixture is that, to at the lowest consumption of cement to obtain a mixture of the required mobility, convenient for transportation and laying, which, after hardening will give concrete the required strength.
Mobility of the concrete mix measure"cone draft", for which a sheet steel mold is used, made in the form of a cone with a cut top: the upper diameter of the cone is 100 mm, lower - 200 mm, height - 300 mm.
The inner surface of the mold, which should be completely smooth, is slightly moistened with water and placed on a horizontal platform, also pre-moistened. The form is filled with concrete mixture in three layers of 10 cm and each layer is pierced (without impact) 25 times with a rod with a diameter of 15 mm. After filling the form, the excess concrete mixture is cut off flush with the edges and the form is slowly removed upward by the ring, while maintaining its strictly vertical position.
The molded concrete cone, after removal of the form, gives a draft, which is easy to measure by placing a ruler on the placed next to the concrete form. Measurement method rainfall cone molding concrete mix For determination of its mobility shown in Fig.1.
The more draft cone, topics more degree mobility of the concrete mix:
- concrete mix with a little water, not giving sediment or having a draft within 1…2 cm- is called rigid ( hard concrete);
- mixture with a high water content, which precipitates 3…16 cm- plastic ( plastic concrete);
- and above 16 cm- cast ( cast concrete).
Specified values mobility of the concrete mix, except for the latter, belong to hard and plastic mixtures, installed with vibrators.
Using cast concrete mixes, containing additive superplasticizers, mobility value can be installed at the construction site, depending on the nature of the structure and the method of transporting and laying mixtures within 16…24 cm.
Mobility of the concrete mix choose according with construction view according to table 2.
Table 2: Mobility of concrete mixtures placed in various structures /
| p/p | Types of work and construction | Cone draft, cm | Rigidity index, s |
| 1 | 2 | 3 | 4 |
| 1 | Preparation for foundations and floors, road foundations | 0…1 | 50…60 |
| 2 | Road surfaces, floors, massive non-reinforced structures (retaining walls, blocks of arrays, foundations) | 1…3 | 25…35 |
| 3 | Massive reinforced structures (slabs, beams, columns, large and medium sections) | 2…4 | 15…25 |
| 4 | Walls of industrial and residential buildings | 2…4 | 15…25 |
| 5 | Reinforced concrete structures heavily saturated with reinforcement (thin walls, columns, bunkers, silos, beams and slabs of small section), concreted in place, with reinforcement content up to 1%: | 4…6 | 10…15 |
| 6 | Structures that are especially saturated with reinforcement (strong and girder bridges, etc.), with a reinforcement content of more than 1%: | 6…8 | 10…15 |
| 7 | Structures concreted in sliding formwork: | ||
| 8 | - when compacted with vibrators | 6…8 | 10…15 |
| 9 | - with manual compaction | 8…10 | 5…10 |
Regardless of mobility concrete mix must be workable. This, in particular, means that when filling the mold and compacting, the mixture must remain homogeneous and not delaminate.
Workable concrete mix:
- easy to fit when laying in a cone shape;
- from under the form when it is filled water does not flow;
- after removing the cone, the concrete mix settles without falling apart and without crumbling.
ATTENTION! Machinability test concrete mix can serve, for example, shovel test:
- hitting with a shovel for concrete mix, look, what trace does it leave shovel;
- If the solution did not fill the voids in the rubble- which means that it is not enough and the mixture is not workable;
- If on impact, the shovel sinks into concrete mass, leaving a hollow- it indicates for excess solution- such concrete may be overly porous.
Get the mobility you need concrete mix at a given concrete strength can be done in the following way:
- if to concrete mix add at the same time cement and water - concrete mobility will increase;
- without changing the water-cement ratio(relation mass of water to mass of cement), That the strength of concrete does not change.
The composition of concrete is set:
- as a ratio by weight of the amount of cement, sand and crushed stone (gravel), moreover the amount of cement is taken as a unit;
- amount of water indicate separately as a water-cement ratio W/C (for example, composition 1:2.5:4.5 by weight; W/C=0.6);
- as quantity of materials per 1 m³ of concrete[For example, 260 kg cement, 170 l(kg) water, 700 kg sand, 1280 kg rubble].
3. The strength of the concrete mixture.
Activity and content of cement in concrete directly affect the strength of concrete. However, the increase content cement in concrete P positive effect up to certain limits, after which the strength changes little, and other properties of concrete may deteriorate.
So, with increased cement consumption shrinkage and creep of concrete increase, as well as its exotherm in the initial period of hardening.
Under certain conditions it may cause the appearance of shrinkage and temperature cracks.
ATTENTION! IT IS FORBIDDEN to prescribe concrete compositions or water-cement ratio ONLY ACCORDING TO TABLES AND CHARTS or by thermal calculation (WITHOUT EXPERIENCED VERIFICATION) in a construction laboratory, because concrete grade depends:
- on water quality,
- varieties of cements and concrete mix aggregates,
- as well as dosing accuracy building materials in responsible building structures(columns, floor slabs, crossbars, beams and blocks of arrays).
Working composition and water-cement ratio of concrete appointed by selection, according to the test results of samples made from test batches.
Table 2-B: Correlation between grades and classes of concrete in terms of compressive strength.
| p/p | Compressive strength class of concrete (according to SNiP 2.03.01-..) | Concrete grade for compressive strength | Concrete class for compressive strength (according to SNiP 2.03.01-…) | |
| 1 | 2 | 3 | 4 | 5 |
| 1 | M-15 | IN 1 | M-300 | B-22.5 |
| 2 | M-25 | AT 2 | M-350 | B-27.5 |
| 3 | M-35 | B-2.5 | M-400 | B-30 |
| 4 | M-50 | B-3.5 | M-450 | B-35 |
| 5 | M-75 | AT 5 | M-500 | B-40 |
| 6 | M-100 | B-7.5 | M-600 | B-45 |
| 7 | M-150 | B-12.5 | M-700 | B-55 |
| 8 | M-200 | B-15 | M-800 | B-60 |
| 9 | M-250 | IN 20 | - | - |
Basic conditions from which strength of concrete depends:
cement quality.
The higher the strength (activity) of cement, the higher will be the strength of concrete. How cement hardens faster, the faster it will increase the strength of concrete.amount of cement.
The amount of cement used for 1 m³ concrete. The best indicator of strength is concrete with such a consumption of cement, in which a thick cement paste fills all the voids in the sand and envelops the sand particles with a thin layer. A cement-sand the solution fills all the voids in a large aggregate.The amount of water.
With the same amount of cement, the strength of concrete will be less, the more water it contains. This is explained by for hardening concrete about 20% mass (weight or volume) of cement. For example, when cement consumption 220…250 kg per 1 m³ concrete will be required 45…50 l water, but with this amount of water, the concrete mixture will turn out to be too dry, it cannot be mixed evenly enough and laid tightly in the form. That's why almost have to add water 3...4 times more- near 160…180 l on 1 m³ concrete. Excess water evaporates as it hardens, leaving pores (voids). The more water was added to the concrete mixture during its preparation, the more pores are formed in hardened concrete and the less because of this will be its strength.The quality of the fillers.
The quality of placeholders is their purity, shape and grain composition(the number of grains of different sizes and the maximum grain size). Irregular grain shape and rough surface contribute better adhesion of cement paste with aggregates And creates more strength. Contamination of aggregates, which worsens their adhesion to the cement paste, also reduces the strength of concrete.The order of laying the concrete mixture in the structure.
Interruptions in concrete placement great importance It has concrete joint surface treatment method laid after the break with laid before the break. Failure to comply with the rules for surface treatment (cleaning, notching, wetting) greatly reduces the strength of the joint.Compaction of the concrete mix.
Concrete, compacted in the form of a mixture by vibrators, has on 10…30% greater strength than hand-compacted concrete.The age of the concrete.
The strength of concrete is growing with his age and especially quickly - at an early age(before 28 days). Strength continues to increase more slowly over a number of years.hardening conditions.
The greatest strength of concrete obtained by hardening in a humid environment. Vice versa, hardening in dry and hot air Maybe lead to poor quality concrete. Freezing stops the hardening process of concrete, but when defrosted, the process continues. Concrete loses strength if he frozen until it reaches "critical strength". Even more harmful than premature freezing are alternate freezing and thawing fresh concrete, resulting in concrete in some cases may even lose the ability to harden.
4. Characteristics of heavy and light concretes.
Heavy concrete.
Extra heavy concrete apply for special protective structures, foundations for machines and for hydraulic structures, and almost never used in residential construction, so in detail we will not consider.
heavy concrete apply during construction monolithic concrete and iron concrete structures , manufacturing concrete products. Heavy concrete is made using sand, gravel, crushed stone from heavy rocks.
In projects of concrete and reinforced concrete structures include the use of the following brands heavy concrete - M-50, M-75, M-100, M-150, M-200, M-300, M-400, M-500, M-600.
ATTENTION! For reinforced concrete structures It is FORBIDDEN to use heavy concrete grade below M-100. For concrete structures DO NOT use concrete grade above M-300.
heavy concrete subdivided:
- on plastic- laid in forms (formwork) with moderate compaction;
- And hard- installation of which requires reinforced mechanical sealing.
Lightweight concrete.
Lightweight concrete apply for the manufacture of enclosing structures, products and construction parts to reduce their weight.
Varieties lightweight concrete on porous artificial and natural aggregates determined by the type applied coarse aggregate: expanded clay concrete, slag concrete, aggloporite concrete, tuff concrete etc.
Most common in practice lightweight concrete on porous aggregates having bulk mass 900…1400 kg/m³.
porous inorganic aggregates for lightweight concrete called bulk materials with a bulk density not higher 1200 kg/m³; with grain size up to 5 mm(sand) and no more 1000 kg/m³ with grain size 5…40 mm(rubble, gravel).
Origin aggregates are natural and artificial. The most widely used artificial fillers - expanded clay, agloporite, perlite, slag pumice, granulated slag best suited to their requirements.
Expanded clay gravel.
Expanded clay gravel obtained by firing fusible clays to swelling occurring in the temperature range between their softening and sintering.
Depending on destination lightweight concrete divided into three types:structural, structural and heat-insulating and heat-insulating.
Expanded clay density:
- for structural concrete - 400…800 kg/m³,
- for thermal insulation - 300…500 kg/m³.
Aggloporite crushed stone.
Aggloporite crushed stone obtained by crushing porous pieces, formed in the process sintering on agglomeration grids clay mixtures(or loam) with waste fuel containing over 10% unburned coal. Volumetric density of agglomeration rubble - 400…800 kg/m³. Expanded clay and aggloporite sand- result crushing crushed stone (gravel) with grinding to fraction less 5 mm.
Perlite.
Perlite (crushed stone and sand) density 250…400 kg/m³ is the result heaving process of volcanic rocks with their increase in volume 6 ... 12 times when heated before 1100°C.
Slag pumice.
slag pumice, processed into crushed stone and sand, receive as a result of porosity molten blast-furnace slags when cooled by steam, A granular slag- as a result of porization of the same slags when rapidly cooled with water. Density pumice slag - 500…1200 kg/m³, granulated slag - 800…1200 kg/m³.
natural porous materials are volcanic rocks pumice and tuffs, crushing which get crushed stone and sand. These materials are used where they are local.
The consistency of the concrete mix(her stiffness and mobility) appoint directly on site, taking into account the size and reinforcement of the structure(Table 3 and Table 4).
Table 3: Approximate consumption of cement in concrete for monolithic concrete and reinforced concrete structures.
| p/p | Design grade of concrete | Grade of cement | Consumption of cement in kg per 1 m³ of concrete for structures |
|
| everyone except thin-walled | thin-walled | |||
| 1 | 2 | 3 | 4 | 5 |
| 1 | 100 | 300 | 225 | - |
| 2 | 150 | 300-400 | 250 | - |
| 3 | 200 | 400-500 | 270 | 300 |
| 4 | 300 | 500-600 | 320 | 350 |
| 5 | 400 | 600 | 440 | 440 |
| 6 | 500 | 600 | 500 | 550 |
| 7 | 600 | 700 | 560 | 600 |
NOTE: feature hardening lightweight concrete is that porous aggregates in the initial period absorb and retain moisture in the capillaries and then return it cement stone. This creates conditions for more long-term and complete hydration of cement in concrete that increases its strength.
Table 4: Specifications for lightweight concrete.
| p/p | Name of concrete | The maximum density in the dried state, kg/m³ | Grade of lightweight concrete | Coefficient of thermal conductivity, kcal/(m*h*t°C) |
|
| strength | frost resistance | ||||
| 1 | 2 | 3 | 4 | 5 | 6 |
| 1 | Constructive | 1800 | M-100, M-150, M-200, M-250, M-300, M-400 | Mrz-25, Mrz-35, Mrz-50, Mrz-100 | Not standardized |
| 2 | 1400 | M-25, M-35, M-50, M-75, M-100 | Mrz-10, Mrz-15, Mrz-25, Mrz-35 | 0,5 | |
| 3 | heat insulating | 800 | Not less than M-10 | Not standardized | 0,25 |
Given the relatively small strength and porosity of aggregates, it is recommended to limit their highest allowable fineness.
For example, the fineness of the porous gravel (expanded clay, slag, tuff) should be no more than 40 mm, A rubble- before 20 mm.
For mobile concrete mixes apply gravel size up to 20 mm, A rubble- before 10 mm.
For sedentary mixtures indicative cereals aggregate compositions For lightweight concrete on expanded clay gravel Can accept according to table 5.
| p/p | Name concrete | The largest grain size, mm | Grain composition of aggregates in %% | |||||
| Aggregate grain size | ||||||||
| up to 1.25 mm | 1.25…2.5mm | 2.5…5mm | 5…10mm | 10…20mm | 20…40mm | |||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 1 | Constructive | 10 | 25% | 20% | 10% | 45% | - | - |
| 2 | 20 | 20% | 15% | 15% | 20% | 30% | - | |
| 3 | Structural and heat-insulating | 10 | 25% | 15% | 10% | 50% | - | - |
| 4 | 20 | 20% | 15% | 10% | 25% | 30% | - | |
| 5 | 40 | 15% | 10% | 10% | 15% | 20% | 30% | |
| 6 | heat insulating | 20 | - | 10% | 15% | 35% | 40% | - |
| 7 | 40 | - | 10% | 10% | 20% | 25% | 35% | |
Average cement consumption depending from the brand of concrete, nature and size placeholder, as well as from the activity cement should be assigned according to table 6.
Table 6: Consumption of cement for the preparation of lightweight concrete on crushed stone and gravel.
| p/p | The largest fineness grains, mm | For concrete | Consumption of cement per 1 m³ of lightweight concrete, in kg |
||||
| M-35 | M-50 | M-75 | M-100 | M-150 | |||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| 1 | 10 mm | on rubble | 130…150 | 150…170 | 170…190 | 190…210 | 250…270 |
| 2 | on gravel | 100…115 | 115…130 | 130…145 | 145…160 | 175…190 | |
| 3 | 20 mm | on rubble | 140…160 | 160…180 | 180…220 | 220…240 | 290…320 |
| 4 | on gravel | 110…125 | 125…140 | 140…160 | 160…180 | 200…225 | |
| 5 | 40 mm | on rubble | 160…180 | 180…200 | 210…260 | 260…280 | 340…380 |
| 6 | on gravel | 120…135 | 135…200 | 160…180 | 180…200 | 240…280 | |
NOTE: For mobile concrete mixes cement consumption increases by 10…15% .
5. Preparation of concrete mix.
Cooking concrete mix at reinforced concrete factories and in construction conditions on mobile units (concrete mixers). Concrete mixing plants of cyclic and continuous action are used.
ATTENTION! Manually full-fledged concrete cook impossible.
Table 7: Approximate composition of the concrete mix and consumption of materials per 1m³ of concrete.
| p/p | Concrete brand | Consumption of materials, in kg | The mobility of the mixture cm | workability, With |
|||
| cement | rubble | sand | water | ||||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| For Portland cement brand M-300 | |||||||
| 1 | M-100 | 200 | 1200 | 800 | 155 | 1…2 | 35…25 |
| 2 | 210 | 1218 | 745 | 165 | 3…5 | 20…15 | |
| 3 | 220 | 1210 | 748 | 175 | 6…8 | 15…10 | |
| 4 | 235 | 1198 | 740 | 185 | 9…12 | 10…5 | |
| 5 | M-150 | 225 | 1211 | 726 | 155 | 1…2 | 35…25 |
| 6 | 270 | 1215 | 701 | 165 | 3…5 | 20…15 | |
| 7 | 290 | 1215 | 675 | 175 | 6…8 | 15…10 | |
| 8 | 305 | 1220 | 658 | 185 | 9…12 | 10…5 | |
| For Portland cement brand M-400 | |||||||
| 9 | M-100 | 215 | 1225 | 750 | 155 | 1…2 | 35…25 |
| 10 | 230 | 1215 | 747 | 165 | 3…5 | 20…15 | |
| 11 | 245 | 1200 | 724 | 175 | 6…8 | 15…10 | |
| 12 | 260 | 1209 | 676 | 185 | 9…12 | 10…5 | |
| 13 | M-150 | 155 | 1188 | 750 | 155 | 1…2 | 35…25 |
| 14 | 265 | 1215 | 704 | 165 | 3…5 | 20…15 | |
| 15 | 280 | 1202 | 685 | 175 | 6…8 | 15…10 | |
| 16 | 300 | 1200 | 660 | 186 | 9…12 | 10…5 | |
| 17 | M-300 | 335 | 1220 | 635 | 155 | 1…2 | 35…25 |
| 18 | 360 | 1215 | 630 | 165 | 3…5 | 20…15 | |
| 19 | 380 | 1202 | 588 | 175 | 6…8 | 15…10 | |
| 20 | 400 | 1200 | 560 | 185 | 9…12 | 10…5 | |
I. When compiling the table material density accepted:
- cement - 1300 kg/m³,
- sand - 1800 kg/m³,
- rubble - 1400 kg/m³.
II. In all compositions as coarse aggregate provided rubble size up to 40 mm.
NOTE: When cooking concrete mix in forced concrete mixers mixing time must be at least 2 min., (of them 30…60 sec. - mixing dry ingredients).
6. Preparation for concreting.
Before starting concreting, necessary:
- thoroughly inspect the formwork and supporting her forests,
- check the reliability of the installation racks of scaffolding and wedges under them formwork fasteners, no gaps.
wooden formwork must carefully clear of chips and debris and plentifully pour water. At the same time, everything small cracks in the formwork swell, and the tree absorbs water and does not subsequently suck moisture out of the concrete mixture laid in the formwork.
For large volumes of work formwork cleaning performed with compressed air supplied by the sleeve from the compressor.
Metal formwork must be coated with grease. Monitoring the condition of formwork and scaffolding should conduct also during concreting and all emerging troubleshoot immediately.
You can start concreting only after how will they be checked laid down and fixed fittings and embedded parts. There, where the reinforcement lies directly on the formwork(for example, a grid in slabs), it is necessary lay lining for education protective concrete layer not less than 10 mm.
7. Protection of concrete during the hardening period.
In conclusion. Important have and ways of care behind hardening concrete, because especially large shrinkage of concrete during the initial hardening period, reaching in the first day 60…70% monthly shrinkage.
ATTENTION! Strength growth curing concrete in time is possible only under certain temperature and humidity conditions, preventing premature evaporation of water from concrete(Tables 8 and 9).
Table 8: The increase in the strength of concrete on Portland cement -
ATTENTION! How previously protected concrete from drying out, the higher the final strength.
ATTENTION! It should be remembered that shortcomings of concrete care in the early days almost impossible to compensate for aftercare.
Air curing concrete dries up and shrinks, moreover drying out is faster than inside. That's why if the moisture content of the concrete during hardening was insufficient, on its surface small shrinkage cracks appear.
Table 9: Effect of curing conditions on concrete strength R30 -
| p/p | Place and conditions of concrete hardening | The amount of mixing water for 30 days, V %% | R30 | |
| MPa | %% | |||
| 1 | 2 | 3 | 4 | 5 |
| 1 | Outdoors (in the sun), covering the first 14 days with parchment | 37 | 24,8 | 100 |
| 2 | The same, with a solution of bitumen in gasoline | 13 | 24,4 | 98,5 |
| 3 | The same, with a layer of sand 3 cm for 14 days | 47 | 22,4 | 90 |
| 4 | indoors at temperature 20…23°С and relative humidity 50% without cover and gravy | 70 | 14,8 | 59,5 |
| 5 | Outdoors (in the shade under a canopy) with average temperature 20…23°С without cover and gravy | 77 | 13,2 | 53 |
ATTENTION! For curing laid concrete it is necessary to create a temperature and humidity regime. To this end, it is necessary shelter with moisture-absorbing materials and watering of concrete to begin no later than after 10…12 hours after the completion of concreting, A in hot weather - after 2…3 hours after mixing.
In dry weather concrete watered during:
- on Portland cement - at least 7 days,
- on aluminous cement - at least 3 days,
- concrete on other cements and concrete with plasticizers - at least 14 days.
At a temperature 15°C and higher concrete is poured during first 3 days:
- afternoon - every 3 hours;
- at night - once;
- at a later time - 3 times a day.
ATTENTION! When covering concrete moisture resistant materials sawdust, sand etc.) duration of breaks between waterings May be increased.
At air temperature below 5°С - watering - not needed.
ATTENTION! Covering and pouring concrete require significant labor, That's why for large areas of concrete appropriate replace watering with concrete coating protective films or water-bitumen emulsion.
Adding milk of lime to the emulsion gives it a lighter color, That's why increases the reflectivity of films And reduces the intensity of heating of the surface by the sun's rays.
For ordinary people, the main quality of concrete is its strength, which is determined by the brand of the mixture. But experts always add concrete mobility to strength. This term is based on such a property of the solution, in which concrete, under the influence of its own mass or with a small impact (vibration, tamping), fills the form intended for it. That is, the mobility index, which is indicated in a special table, determines the ease of use of the solution. For large volumes construction works it is important.
How to determine the mobility of a solution?
There is no need to use laboratory equipment for this. The definition process is quite simple. You will need a special cone made of sheet steel with a thickness of 1.5 mm.
Cone dimensions:
- height - 30 cm;
- large diameter- 30 cm;
- small diameter - 10 cm.
This standard size. But there are additions that are determined by the fraction used in the crushed stone solution.
If the crushed stone fraction does not exceed 70 mm, then the dimensions of the cone will be as follows: 30 × 20x10 cm (height - large diameter - small diameter). If the fraction exceeds 70 mm, then the dimensions will be as follows: 45 × 30x15 cm.
Two handles are soldered on the sides of the figure for the convenience of the testing process.
Trial
The prepared concrete solution is laid in a cone in three layers from the wide side of the figure. The inner surface of the cone must be moistened. Each layer is compacted with a piece of reinforcement. The total number of bayonet movements should be 25 times, that is, 8-9 times per layer. If an enlarged cone is used, then you will have to bayonet 56 times.
Excess mixture that will stick out must be cut off with a spatula. After that, the cone is turned over and removed from the concrete, which has taken a conical shape.
In this state, the solution must stand for a while so that its natural shrinkage occurs. After that, the height of the concrete cone is measured and compared with the height of the metal figure (30 cm).
To accurately determine the height difference between the two cones, it is recommended to do two test runs. The average number is the required indicator.
Types of mobility
If the height difference is zero, then the concrete mortar belongs to the category of rigid concrete (they are designated in the marking with the letter “Zh”). They are used very rarely. In private housing construction is not used at all. It is very difficult to work with such mixtures, their rigidity is high.
If the height difference is 1-5 cm, this is a sedentary solution. If 6-14 cm - this is plastic concrete. There is a fourth type, in which the difference between the cones is more than 15 cm. Specialists call such solutions "cast mass". This mobility of concrete allows the use of the material only in certain conditions for special structures.
Practice shows that the density determines the strength of the structure being poured. Therefore, when choosing one or another concrete solution in terms of mobility, it is necessary to know exactly under what conditions the solution will be poured, and for what purposes it is intended Basic structure Houses. That is, for each individual filling option, you will have to select the composition both in terms of mobility and stiffness.
pivot table
A table of various indicators simplifies the search for the desired parameters or characteristics. The same is true with concrete solutions. There are combined tables that include all the characteristics of mixtures, and there are separate tables for different composition parameters. The table below shows only the mobility of the material.
Such testing is carried out for mixtures that use crushed stone with sizes of 5-40 mm. For this, a special measuring tool is used - a viscometer.
Tools
For the accuracy of the experiment, you will need a vibrating plate and a cone (as in the first case). A conical form of concrete is being prepared, which is installed on a vibrating plate.
Then a tripod is stuck into the concrete, on which a disk is put on, which acts as a press. The tripod has marks along the length of the instrument.
Measurement process and result accounting
The stopwatch is turned on simultaneously with the vibrating plate. In this case, the disk, under the action of vibration and its mass, begins to compact the concrete form. As soon as he reaches a certain risk, the stove and stopwatch turn off, the passage time is recorded.
The time indicator is multiplied by a factor equal to 0.45. This is the standard value. The result obtained is the stiffness or mobility of concrete. On large construction sites, the result of each check is recorded in a special log.
To do this, you need to prepare a cubic form of sheet iron. For solutions where crushed stone up to 70 mm in size was used, a cube of 20 × 20x20 cm is prepared. Where crushed stone up to 20 mm in size was used, a cube with a side of 10 cm is prepared.
The cube is installed on a vibrating plate. Then, a conical-shaped concrete prepared according to the recipe described above is placed in it. After that, the vibrating plate and the stopwatch are turned on.
It is necessary to measure the time during which the concrete cone falls apart, fills all the corners of the cube and its surface becomes horizontal. This time indicator is multiplied by 0.7. This is mass mobility.
Concrete designation
The mobility indicator is marked with the letter “P” with the addition of a digital value from 1 to 5. That is, P1, P2 ... And the higher the numerical indicator, the higher the mobility of the solution. Therefore, there is a certain division of concrete in terms of mobility:
- P1, P2, P3 - sedentary;
- P4, P5 - with high mobility.
Sedentary
The first group has a large number of sand in relation to cement, so the consistency of such concretes is thick. They are usually used for the construction of monolithic structures. When filling them, vibrators must be used.
Please note that in addition, to increase their fluidity, it is impossible. The brand immediately decreases, and hence the strength of the entire structure as a whole. IN this case Fluidity can be increased only by adding special plasticizers.
highly mobile
Concretes from the second group are used for pouring into formworks where a frequent reinforcement cage is installed, or into formworks in which it is difficult to compact. For example, it can be columns or narrow but high foundations.
By the way, experts believe that P4 concrete is optimal. It does not need to be rammed or vibrated.
Mobility and composition of the mixture
Determining the mobility of the concrete mix affects the quality of the final result, so such testing must be carried out. And if the quality of the solution (or rather, its mobility) does not suit you, then you can change the recipe of the mixture or change the parameters and brands of the constituent components. That is, add cement of a different brand to the solution, a finer or coarser fraction of sand or gravel, change the volume of water.
Cement
With an increase in the water-cement ratio towards the liquid, the mobility of the concrete mixture increases. At the same time, the strength and rigidity of the composition immediately decreases. Plasticizers and modifiers added to cement reduce mobility.
If, according to the recipe, the volume of cement introduced is increased, then the fluidity of the mass also increases. However, the strength of the solution does not change. The thing is that with such a content of cement, the volume of cement paste increases. It fills the entire space between the fillers and prevents them from touching each other. And this reduces the friction force, hence the high mobility of the mass.
Sand and gravel
The size, surface quality and shape of large aggregates also affect the flowability of the concrete mix. For example, the smooth surface of gravel (crushed stone) makes it possible to reduce friction between its elements. This, in turn, increases the mobility of the mass, but as a result, the rigidity and strength of the entire structure decreases. Therefore, river gravel for concrete solutions is not used.
As for sand, it practically does not affect the mobility index. Of course, you should not use fine sand, which will increase fluidity, but greatly reduce the strength of the composition.
Fill conditions
The pouring conditions will also affect the mobility of the concrete mixture. These mainly include the frequency of the reinforcing frame and the shape of the structure being poured.
The more often the reinforcement is installed in the frame, the more fluid the solution will have to be made. This is done for the convenience of work. After all, it will be difficult to work with the same vibrator in such conditions. And if a hard solution is poured into this structure, then there is a high probability that its density after vibration will not correspond to the norm. Shells and pores will appear, and this is a decrease in quality.
The dimensions of the structure to be poured also affect the choice of plasticity of the concrete mass. And in this case, the main reason is the convenience of work. The larger and more complex the structure, the more plastic the concrete will have to be prepared.
Concrete is a very complex system in which many chemical processes take place inside during the entire service life.
The mobility of the concrete mix - how to determine?
Nowadays, there are many types of concrete, with different properties for specific structures and operating conditions. When organizing work on concreting, it is important to know such a property of concrete as workability.
Concrete mix workability- this is the ability of concrete during concreting to fill a form, formwork under the influence of its own weight or an applied external force (vibration, compaction).
The workability of the concrete mixture is determined by the mobility of the concrete mixture (P) or the draft of the cone (OK, S). The mobility of the concrete mixture is determined according to the method of DSTU B V.2.7-114-2002, where the draft of the cone OK (S) is determined, see. Standard cones are used to test the concrete mixture ( photo 2) depending on the coarse aggregate fraction:
- with a crushed stone fraction of not more than 70 mm - 300 × 200 × 100 mm (H × D × d);
- with a crushed stone fraction of more than 70 mm - 450 × 300 × 150 mm (H × D × d),
Where H is the height of the cone; D is the lower diameter of the cone; d is the top diameter of the cone.
The essence of determining the draft of the cone is that the prepared concrete mixture is poured into a truncated standard cone in three stages with sealing with a bayonet (usually a piece of smooth rod reinforcement). The upper surface of the cone is leveled, removing the remains of the concrete mixture, and then the form is raised vertically and placed near the formed cone. The height difference between the shape and the mixture is the value of the cone slump.
Based on DSTU B V.2.7-176:2008, all concrete mixtures, depending on the consistency, are divided into the following grades ( tab. 1)
Table 1. Grade of concrete mix by consistency
| Grade of concrete mix by hardness | |
| brand | Cone draft, mm |
| S1 | 10…40 |
| S2 | 50…90 |
| S3 | 100…150 |
| S4 | 160…210 |
| S5 | 220 |
| Brand of concrete mix by hardness (methodVebe) | |
| brand | Time, s |
| V0 | 31 |
| V1 | 30…21 |
| V2 | 20…11 |
| V3 | 10…6 |
| V4 | 5…3 |
Also, the consistency of the concrete mix can be defined by the following terms:
- rigid concrete mix: OK from 0…1 cm;
- sedentary concrete mix: OK from 1…5 cm;
- mobile concrete mix: OK from 6…14 cm;
- poured concrete mix: OK over 15 cm.
By table 1 it can be seen that the thickest concrete mix has the following characteristics: S1, V0. The most liquid concrete mix has the following grades: S4 or S5, V4. Rigid mixtures S2, S3 are used for concreting building objects using vibration and compaction.
If seals and vibrators are not used, then voids are formed in rigid mixtures, violating the integrity and solidity of the structure and thereby reducing strength, photo 4.
The mobility of a concrete mix depends on many factors:
- type of cement;
- amount of water;
- water-cement ratio (W/C);
- the absence or presence of additives;
- type of additives used;
- quality and form of aggregates;
- fineness of aggregates (fine and coarse).
How to choose the right mobility of the concrete mix?
The most important factor responsible for the properties of concrete is the water/cement ratio (W/C). Therefore, it is strictly unacceptable to dilute the concrete mixture with water to give it increased mobility. The strength of concrete directly depends on the water-cement ratio W/C. If W / C is violated by adding water to the concrete mixture, the main characteristics of concrete are violated. In this case, the strength of concrete may decrease by several classes, for example, C30 can be obtained from strength class C40.
There is an opinion that concrete with high mobility has better strength. Concrete grades S4, S5 will be more expensive in consistency than concrete grade S1, but this does not mean that it is stronger. The strength class of concrete with a draft of the cone S1, S2, S3, S4, S5 will be the same, but the consumption of cement will be different, which determines the price of concrete. For more mobile concrete mixtures, it is necessary to consume more cement than for less mobile ones in order to ensure the same concrete strength. Thus, it is not necessary to order concrete with S5 mobility for concreting an open area or slab, where it is possible to compact the concrete mixture with the help of vibrators - this is an extra unreasonable cost of money.
If it suddenly happened that a concrete mixture was brought to the construction site below the required mobility, it can be increased with the help of plasticizer additives. The addition of plasticizers will not significantly reduce the strength of concrete. When concreting in winter at negative temperatures, it is necessary to use antifreeze additives that can provide the necessary mobility for up to 6 hours.
IN tab. 2 the rational area of application of concrete mixture of different mobility for different construction needs is given.
Table 2. Scope of concrete mixture depending on mobility
| Grade of concrete mixture according to the draft of the cone | Cone draft, mm | Application area |
| S1 | 10…40 | For monolithic structures, concreting walls, non-reinforced or rarely reinforced structures, massive foundations (OK - 30 ... 60 mm) |
| S2 | 50…90 | For standard monolithic construction, for slabs, crossbars, columns, densely reinforced structures, concrete stuffed piles (OK - 40 ... 50 mm) |
| S3 | 100…150 | |
| S4 | 160…210 | It is used for concreting structures with a small cross section, densely reinforced elements, hard-to-reach places, columns, when concreting with a concrete pump, you can not use a vibrator |
| S5 | 220 |
When calculating the composition of concrete to determine the required amount of water for a given mobility, you can use the following graphs, rice. 1.
Rice. Fig. 1. Graph of water demand for (a) plastic and (b) rigid concrete mixture made using Portland cement, medium-sized sand (water demand 7%) and gravel of the largest size: 1 - 70 mm; 2 - 40 mm; 3 - 20 mm; 4 - 10 mm
Most of all in construction, the draft of the cone is used to describe the consistency of the concrete mixture. But in some cases, they use such a characteristic as hardness of the concrete mix.
Rigidity of the concrete mixture (W) defined as the vibration time in seconds required to measure and compact a preformed concrete cone using a stiffness tester (Vebe type tester) – rice. 2. This characteristic more accurately reflects the property of rigid or slow-moving mixtures and is found in construction.
Rice. 2. Determination of the stiffness of the concrete mixture: І - device of the Vebe type; ІІ - concrete mixture on the device before vibration; ІІ - concrete mixture on the device after vibration; 1 - cylindrical ring, 2 - truncated cone, 3 - funnel, 4 - tripod, 5 - disk with 6 holes, 6 - rod, 7 - vibrating table
Konev Alexander Anatolievich
The popularity of concrete is due to its functionality and excellent characteristics, the possibility of using it in the construction of a variety of objects. One of the main characteristics this material is the ability to fill out any form. Filling is carried out under the influence of vibrations, and this characteristic is denoted by the concept of mobility or shrinkage of the cone.
Using the Abrams cone, the mobility and ductility of concrete, as well as its shrinkage, are measured.
The mobility of concrete can only be measured in a very short period of time, since over time the components of the solution set, and its fluidity decreases to zero when the material turns into a monolith. This process takes 28 days.
The mobility or plasticity of a concrete mixture is the ability of a freshly made mortar to spread under its own weight in a prepared form. To measure the plasticity of concrete, laboratory equipment called the Abrams cone is used. Using this device, the diameter of the spreading of the concrete cone and the spreading time of the mixture up to 500 mm in diameter are calculated.
Measurement of shrinkage of concrete in the Abrams cone
This standard instrument is a stainless steel or galvanized iron cone. Outside of the device, 2 metal strip supports and 2 handles are welded for easy measurement. Device complete set:
- funnel made of the same material as the main body;
- a metal base plate with a circle drawn on it with a diameter of 500 mm, a thickness of 3 mm, with dimensions from 700 × 700 to 1000 × 1000 mm;
- ruler 500 mm.
Overall dimensions of the device in the assembly:
- width - 36 cm;
- height - 38.4 cm;
- top hole diameter - 10.2 cm;
- bottom hole diameter - 20 cm;
- assembly weight up to 3 kg.
To measure such a parameter as shrinkage of a cone, the following tools are needed:
- Abrams cone;
- trowel or trowel;
- bayonet.
The sequence of measuring the consistency of the concrete solution:
- The device is moistened from the inside with water and installed on the base plate, also previously moistened with a small amount of water.
- The device is filled with concrete mixture in 3 passes, concrete is laid in each pass with a layer of 10 cm.
- Each layer for sealing is pierced with a bayonet in a non-impact way about 25 times.
- The filled product stands for 90 seconds, after which it rises vertically upwards with the help of handles.
- concrete mix begins to spread, the time of spreading along a circle with a diameter of 500 mm and the time of completion of the deformation process are measured with a stopwatch.
A high-quality solution should reach the boundary of the outlined diameter in 3-6 seconds, the cone should completely spread in 45 seconds or more.
The mobility of concrete depends on the type of cement, the amount of water, sand, the size and structure of the aggregate, the presence of plasticizing additives.
The plasticity of the mixture is indicated by the letter P and numbers from 1-5. For ordinary concrete work, a solution with mobility P-1 according to P-3 is used. The pouring of reinforced structures, columns, hard-to-reach places is carried out with a concrete mixture with P-4 mobility.
I p \u003d A s (h o - y) 2 + n P d4/64
where A s is the sectional area of the reinforcement, h o is the distance from the top of the element to the center of gravity of the reinforcement bars, y is the height of the concrete compression zone, n is the number of reinforcement bars.
For example, for a structure with h o = 8 cm, reinforced with 1 rod (n = 1) with a diameter d = 1 cm and a height of the compressed zone y = 5 cm, the moment of inertia of the tension zone of the section will be:
I p \u003d 3.14 1 2 (8 - 5) 2 /4 + 1 3.14 1 4 /64 \u003d 7.068 + 0.049 \u003d 7.11 cm 4
And if the same reinforcement works as a separate structural element, then its moment of inertia will be:
I a p = n P d 4 /64 + 1 3.14 1 4 /64 = 0.049 cm 4
And thus, the efficiency of using reinforcement in this case decreases by 7.11 / 0.049 = 144 times and such reinforcement practically does not affect bearing capacity designs. In concrete, devoid of interaction with reinforcement, the height of the compressed zone of the reduced section is significantly reduced, which leads to a multiple decrease in the moment of inertia and the moment of resistance of the reduced section.
To prevent this, the concrete mixture is compacted during the laying process.
However, "consolidation" is a rather conditional term, since compaction should be understood not as a change in the density of the concrete mixture, but as an increase in the volumetric weight of the structure due to the filling of all possible holes and cracks with the concrete mixture that have arisen after the concrete mixture has been laid. And in this sense, the compaction of the concrete mixture is more like the process of moving tenants into apartments, also called compaction. Recently, many methods have been invented for compacting concrete mixtures. The essence of most compaction methods is to increase the inert mass of the concrete mixture, since the gravitational mass for compaction is often not enough. The choice of compaction method depends on the workability of the concrete mix. And the workability of the concrete mix, in turn, is characterized by the mobility or rigidity of the concrete mix.
GOST 7473-94 "Concrete mixes. Specifications" defines 3 main groups of concrete mixes: mobile (P), rigid (ZH) and superrigid (SZh). Rigid and super-rigid mixtures are used in the manufacture of structures in the factory. To compact such mixtures, tamping, rolling, pressing, vibrating with a load are used. We will not consider methods for determining the stiffness and compaction of such mixtures in more detail.
On construction sites, mobile concrete mixes are commonly used. To characterize mobile mixtures by workability, the following grades are used:
Table 256.2. Workability grades(according to GOST 7473-94)
In general, the draft of the cone shows how many centimeters it will sink, and the spread of the cone - how much the molded concrete mixture will spread after removing the cone.
The draft of the cone (OK) is used to assess the workability of more rigid mixtures, the flow of the cone (RK) is used for the so-called cast mixtures. To determine the mobility of the concrete mixture, a special cone is used with the dimensions specified in GOST 10181-200 (the height of a normal cone is H = 30 cm, the upper diameter d = 10 cm, the lower diameter D = 20 cm), a ruler for measuring the draft of the cone, a funnel, a trowel , a stopwatch, a smooth steel or plastic sheet measuring 70x70 cm, as well as a metal rod with a diameter of 16 mm and a length of 60 cm with rounded ends (why we need all this, we will find out a little later). The normal cone is used to determine the flowability of a concrete mix with coarse aggregate grains ≤ 40 mm. For concrete mix with aggregate of larger sizes, an enlarged cone is used. A normal cone looks like this:
Figure 328.1. Standard cone for testing: 1 - handles, 2 - body made of sheet steel with a thickness of at least 1.5 mm, 3 - foot rests, 4 - welded seam.
The mobility of the concrete mixture is determined as follows:
A clean sheet of 70x70 cm is laid on a horizontal flat surface, the surface of the sheet is wetted. In order for the cone to adjoin the sheet tightly, they usually stand on the stops. Concrete mixture is poured into the cone through a funnel in three layers of approximately the same height. Each layer is sealed with a bayonet. For this, a metal rod (pin) is used. The essence of bayoneting is to apply strong blows to the concrete mixture. Moreover, the blows are applied not in one place, but over the entire area of the concrete mixture. Each layer should be compacted with 25 strokes. Concrete mixtures of grades P4-P5 are poured into a cone in one layer and compacted with 10 blows. If an enlarged cone is used, then the number of blows increases to 56. No more than 3 minutes are allotted for filling the cone with concrete mix and bayoneting with GOST. In the process of filling and bayoneting, the cone must be firmly pressed against the sheet.
After compaction of the last layer, the funnel is removed, the excess concrete mixture is removed with a trowel flush with the edge of the cone, the remaining concrete mixture is smoothed out. After that, the cone is taken by the handles and carefully removed, moving it vertically so as not to catch the molded concrete mixture, and the cone is placed next to the mixture on the sheet. It takes 5-7 seconds to remove the cone.
After that, the draft of the cone is determined. To do this, a pin is placed on top of the cone, and the distance from the bottom of the pin to the top of the concrete mixture is determined with a ruler with an accuracy of 0.5 cm. If the molded concrete mixture fell apart during the removal of the cone, then the measurement is not carried out, but the test is repeated on a new sample from the same concrete mixture.
To determine the draft of the cone, two measurements are taken. The draft of the cone is determined, rounded to the nearest centimeter, as the arithmetic mean of the results of 2 tests. In this case, the discrepancies in the results should not be more than
1 cm when OK< 9 см
2 cm at OK = 10-15 cm
3 cm with OK > 16 cm.
If the discrepancies in the results of 2 tests are more than indicated, then repeated tests are carried out. The total time for 2 tests should not exceed 10 minutes.
The spread of the cone of the concrete mixture is determined by the lower diameter of the cake
which was formed as a result of the flow of the concrete mixture. The diameter is determined by measuring the cake in two mutually perpendicular directions with an accuracy of 0.5 cm. The spread of the cone is determined, rounded to the nearest centimeter, as the arithmetic mean of the results of 2 tests. In this case, the discrepancies in the results should be no more than 3 cm. In case of large discrepancies, repeated tests are carried out.
Of course, when building a small house for themselves, their wives and children, few people acquire the above ingenious devices and tools. Nevertheless, it is possible to estimate approximately the mobility of the concrete mix at home. This will require an old iron bucket without a bottom. And if you suddenly don’t have such a bucket, then contact your mother-in-law; she should have several of these. Well, everything else necessary for testing, if you already started a construction site, you will definitely have it.
Let me remind you that all this fuss with determining the draft of the cone is needed in order to choose the optimal compaction method. However, on small construction sites of all possible ways seals are used or vibrating or bayoneting.
The essence of the bayonet is described above and is used only in extreme cases, if it is necessary to concrete a structure of a small volume, while a bayonet shovel can be used as a bayonet. But still, this method of compaction is very unreliable, and if you are going to compact the concrete mixture using this method, then the calculated strength of concrete should be reduced by multiplying the strength of concrete by the quality factor of work. In this case, the value of the work quality factor can be taken equal to γ k = 0.6-0.8.
vibratory seal
Concrete mixture can be compacted by volumetric, surface, immersion or contact method of vibration impact transfer. On small construction sites, surface or submersible vibrators are usually used. During vibration, the compaction of the concrete mixture is achieved by shaking the concrete mixture and also due to thixotropy - the transition of the concrete mixture into a liquid state due to a significant decrease in viscosity during vibration.
Surface vibrators are effective when the height of the concreted structure is not more than 25 cm with reinforcement only in the tension zone, 12 cm with reinforcement in both tension and compression zones. High-frequency vibrators with an oscillation frequency of 4500 oscillations / min and an amplitude of 0.15-0.2 mm are considered the most effective. When using vibrators with a normal oscillation frequency of 300 oscillations / min, the oscillation amplitude should be at least 0.3-0.35 mm; for hard mixtures, the oscillation amplitude is 0.5-0.7 mm.
When the mobility of the concrete mixture OK ≤ 1 cm, submersible vibrators should be used. The efficiency of submersible vibrators depends on the range of the vibrator. At high frequencies, the range of the vibrator is less than at low frequencies. As a rule, the radius of action of submersible vibrators is 8-10 diameters of the tip (mace). The greater the mobility of the concrete mixture, the greater the radius of the vibrator.
The efficiency of using vibrators also depends on the vibrating time. With a short vibration time, the concrete mixture does not have time to compact, with a long time, the concrete mixture begins to delaminate, which is also undesirable. In general, the greater the mobility of the concrete mix, the less time it takes to vibrate one area. As a rule, the optimal vibration time for mobile concrete mixes is 20-40 seconds. In addition, the sufficiency of vibration is determined visually: if the concrete mixture has ceased to settle and cement milk has appeared on the surface, then vibration can be stopped.
Note: The topic of concrete compaction is quite extensive. Here, only basic information about the principles and methods of compaction is presented.
However, compacting the concrete mix alone is not enough to ensure reliable anchoring of the reinforcement. If the distance between the reinforcement rods is less than the grain size of a large filler - crushed stone, then no matter how much the concrete mixture is compacted, the crushed stone will not pass between the rods.
Minimum spacing between rebars
SNiP 2.01.03-84 imposes the following requirements on the distance between reinforcement bars:
1. If an immersion vibrator is to be used to compact the concrete mixture, the clear distance between the bars (the distance between the centers of the section of the reinforcing bars minus the diameter of the bar) must allow free passage of the vibrator tip.
2. The clear distance between individual rods of longitudinal non-stressed reinforcement, as well as between the longitudinal rods of adjacent welded flat meshes (frames) must be at least largest diameter reinforcing bars, as well as:
- ≥ 25 mm - for horizontal or inclined bottom reinforcement bars;
- ≥ 30 mm - for horizontal or inclined top reinforcement bars;
- ≥ 50 mm vertically - for horizontal or inclined bottom reinforcement bars arranged in 2 rows in height;
- ≥ 50 mm and ≥ 1.5 coarse aggregate size - for vertical rods (column concreting);
The diameter of the reinforcement of a periodic profile is taken at the nominal value (excluding protrusions and ribs).
But compliance with the requirements for the minimum distance between the reinforcement bars is not enough. The concrete mixture must be in contact with the reinforcement from all sides, and not just from above and from the side. For this, the requirements for compliance with the protective layer of concrete follow.
Protective layer of concrete
The protective layer of concrete not only partially protects steel reinforcement from corrosion and temperature effects, but is also designed to ensure the joint operation of concrete and reinforcement.
1. For longitudinal reinforcement, accepted by calculation, the thickness of the protective layer must be ≥ d of the rod or rope and:
- ≥ 10 mm - in plates and walls ≤ 100 mm thick;
- ≥ 15 mm - in slabs and walls > 100 mm thick, in ribs and beams with a height< 250 мм;
- ≥ 20 mm - in ribs and beams ≥ 250 mm high, in columns;
- ≥ 30 mm - in foundation beams and prefabricated foundations;
- ≥ 35 mm - in monolithic foundations in the presence of concrete preparation;
- ≥ 70 mm - in monolithic foundations in the absence of concrete preparation;
2. For single-layer structures made of porous and lightweight concrete of classes B ≤ 7.5, the thickness of the protective layer is taken ≥ 20 mm, for external wall panels made without a textured layer - ≥ 25 mm. For single-layer cellular concrete structures, the protective layer thickness must be ≥ 25 mm in all cases.
3. For structural, transverse and distributive reinforcement, the protective layer of concrete is taken ≥ the diameter of the specified reinforcement and:
- ≥ 10 mm - at element section height< 250 мм;
- ≥ 10 mm - with element section height ≥ 250 mm;
For elements made of porous and lightweight concrete of classes B ≤ 7.5, regardless of the height of the section, as well as for elements made of cellular concrete, regardless of the class of concrete, the protective layer of transverse reinforcement is assumed to be ≥ 15 mm.
4. To ensure free laying in the form (formwork) of solid reinforcing bars, frames or meshes running along the entire length or width of the product, the dimensions of the bars, frames or meshes are taken smaller than the dimensions of the form: 20 mm (10 mm on each side = Δl to ) - with element length ≤ 9 m, by 30 mm (15 mm on each side) - with element length ≤ 12 m - by 15 mm, by 20 mm - with element length >12 m.
5. In hollow elements of a box-shaped or annular section, the distance from the inner surface of the concrete to the bars of the longitudinal reinforcement is taken in accordance with paragraph 1 and paragraph 3.
But that's not all. In reinforced concrete elements working in bending or tension, cracks form in the tensile zone of the section, and in order for the reinforcement to work together with concrete, its ends must be securely clamped in areas without cracks. However, the anchoring of the reinforcement can be provided in another way.
Rebar anchoring
Reinforcing bars of a periodic profile, and smooth bars of welded meshes and frames are made without hooks. The rods of knitted nets and frames, working in tension, must end with hooks, loops or paws.
1. The rods of the longitudinal tension and compression reinforcement must be wound behind cross section, in which they are taken into account with the total design resistance, for a distance of at least l an = λ an d and no less:
l an = (ω an R s /R b + Δλ an)d (328.1)
where R s - design resistance of reinforcement, R b - design resistance of concrete. The values ω an , Δλ an , λ an and the minimum allowable distance l an are determined from table 328.1:
Table 328.1(According to SNiP 2.01.03-84)
In this case, smooth reinforcement should end with hooks or have welded transverse reinforcement along the length of the embedment, which ensures anchoring of smooth reinforcement. The value of the design resistance of concrete R b can be multiplied by the coefficients of the concrete working conditions (except for γ b2).
2. For elements of fine-grained concrete of group B, the value of l an, determined by the formula (328.1), should be increased by 10 d - for tensioned concrete and by 5 d - for compressed.
3. If the cross-sectional area of the reinforcement is taken with a margin relative to the area required by the strength calculation, then the anchoring length l an calculated by formula (328.1) can be reduced by multiplying the anchoring length by the ratio of the required by the calculation and the actual cross-sectional areas of the reinforcement (l f an = l an A p s /A f s).
4. If cracks form along the tensioned reinforcement bars, then the bars must be embedded in the compressed zone of concrete for a distance l an determined by the formula (328.1).
5. If it is not possible to fulfill the above requirements, then special measures should be taken to anchor the longitudinal rods to ensure the operation of the rods with full design resistance in the considered section. To do this, indirect reinforcement is installed, the ends of the rods are welded to the embedded parts or rods of the anchor plates, or the anchor rods are bent. In this case, the value l an should be taken ≥10 d reinforcement.
Note: Features of the calculation of embedded parts are not considered here.
6. To ensure the anchoring of reinforcement in bending elements, all longitudinal rods that are inserted beyond the edge of the support (into the support section of the element, as a rule, these are the supports of single-span beams and slabs or the extreme supports of multi-span elements) must meet the following requirements:
a) with the strength of the element in the area under consideration, which allows the absence of transverse reinforcement, the tension rods are inserted beyond the inner face of the free support at a distance of ≥ 5 d;
b) if the calculation requires transverse reinforcement, then the tensioned rods are inserted beyond the inner face of the free support at a distance of ≥ 10 d;