Cement-based concrete

10. Cement-based concrete

Concrete is an artificial composite material, obtained at hardening of rationally proportioned mixture of binding material, aggregates, water and other substances required for hardening. Mixture of these materials before their hardening is called concrete mixture.

Concrete is produced of cement, lime, gypsum and special binders. Concrete based on polymeric, magnesia, phosphate binders, liquid glass and some other belongs to the group of concrete on special binders.

Concrete on the cement binders is the most widely used in all areas of construction industry. Dominant position of concrete in construction is explained by availability and large supplies of non-metallic materials - sand, gravel, and crushed stone and by possibility to give concrete different properties and casting the structures different by shape; high longevity and reliability at correspondence of its composition and structure to the performance conditions.

Concrete is classified by the followings features: basic assignment, binder type and the type of aggregates, structure and average density.

Concrete is divided into structural and special (hydraulic, heat-resistant, radiation protective, chemically proof, heat-insulating etc.) depending on the basic assignment.

Concrete for load-bearing and inclosing structures belong to the structural concrete. They are made in accordance the requirements, which characterize their mechanical, and in some cases other properties. Concrete which meets particular requirements according to the performance conditions of structures and elements belongs to the special concrete.

Dense, porous and special aggregates are used for obtaining the concrete. The last (ore containing rocks, scrapped iron, chamotte and other) give the special properties to the concrete.

Concrete structure can be dense, porous, macroporous and cellular. In the concrete with dense structure (dense) all the space between grains of the aggregates is filled with hardening binder, including the pores of the entrapped air. The peculiarity of concrete with porous and macroporous structures consists in that at first case hardened concrete is poroused by foam- or gas-forming additives, and at second case cement paste or cement-sand mortar does not fully fills the space between grains of coarse aggregate. The concrete of cellular structure consists of hardening mixture of binder, silica component and uniformly distributed pores as cells, formed by gas or foaming makers.

Concrete with density 2000...2500 kg/m³ is called heavy-weight, 500...2000 kg/m³ – light-weight, over 2500 kg/m³ - extra heavy-weight and less than 500 kg/m³ – extra light-weight.

The increasing efficiency of concrete application in construction goes by the decline of materials content in the structures and labor intensiveness of their production.

10.1. Requirements to the initial components of concrete

Constructional-technical properties of concrete depend on the quality of initial components and their proportion in concrete mixture.

Cements. At the selection of cement there should be taken into consideration required concrete strength, intensity of its growing, aggressive influence of the environment, structural features of elements and conditions of concrete works conduction. The recommended ratio of compressive strength of cement and concrete changes from 3 for concrete with strength 10...15 MPa to 1.2...2 for concrete with strength 20...50 MPa for a heavy-weight concrete. At diminishing of these ratios the cement contents increase, shrinkage deformations develop and crack resistance of concrete reduces, and at increasing – due to insufficient cement content (if there are no additives of the filling agents and plasticizers) there is possible segregation of concrete mixture, decreasing of concrete density.

Requirements to chemical-mineral and material composition of cement are caused by conditions of hardening and performance. It is effective to apply high-early-strength Portland cement and blastfurnace cement for the production of precast reinforced-concrete structures which harden at thermal and moisture treatment. In concrete functioning at the systematic alternate freezing and thawing, moistening and drying, the application of cements with diminished content of mineral additives and tricalcium aluminate(С₃А < 8 %) is desired. The low-heat cements are required for hydraulic concrete, which is placed in dams.

Mixing water. An ordinary drinking-water is used for concrete mixing. It is possible to use the technical recirculated and natural mineralized waters which contain the allowable amount of admixtures (Table 10.1).

Table 10.1

Allowable content of admixtures in mixing water, mg/l

Type of the concrete	Soluble salts	Ions SО42-	Ions СІ-	Weighted particles
For prestressed reinforced-concrete structures	2000	600	350	200
For structures with ordinary reinforcement, including water spillway structures, zones of alternate water level of massive structures	5000	2700	1200	200
For the non-reinfirced structures to which requirements on limitation of walls salinity are not specified	10000	2700	3500	300

Note. The pH-value of water should not be less than 4 and more than 12.5.

Dense aggregates. Aggregates with the density of grains more than 2 g/cm³ belong to dense aggregates. Required concrete properties at minimal possible cement content and water-cement ratio are provided by the selection of fine and coarse aggregate.

Sand – is a natural or artificial mineral mixture of grains from 5 to 0.16 mm by size, serving as a fine aggregate. Quartz sands are the most widespread dense fine aggregates. Due to the grains high strength they can be utilized practically for the concrete of all classes. Sands, which consist of grainsof loose igneous rocks, limestones, dolomites and others are utilized after laboratory tests and technical and economic substantiation.

The sands made of industrial wastes can be used in many causes after testing. Manufactured sands depending on strength of initial rock are divided into grades. Igneous and metamorphic rocks, used for the production of the manufactured sands should have the ultimate compressive strength not less than 60 MPa.

Basic properties of concrete depend on the adhesion of cement stone with aggregates, which is influenced by the shape and character of their surface, clay, dust, other harmful admixtures presence and aggregates’ chemical- mineralogical composition. Adhesion increases at an acute-angled shape and rough surface of grains, representative, for example, for mining (ravine) sand, or due to chemical interaction at carbonate rocks application.

Clay and dust-like particles due to highly developed surface substantially increase the water content of concrete mixtures, envelop grains of aggregates and diminish their adhesion to cement stone. Besides that, the finest dust-like particles (< 0.08 mm) reduce frost resistance of concrete.

Permissible content of dust-like and clay particles is specified depending on the aggregate type and concrete assignment. For ordinary natural sand it should not exceed 3 %, ground up - 4 %, for fractional natural and manufactured sands - 2 % and 3% correspondingly. The special limitations for dust-like and clay admixtures contents are specified for sand, used in the production concrete for hydraulic structures, culvert aqueducts, transport structures. For example, content of dust-like and clay admixtures in sand should not exceed 2 % for the concrete of variable water level zone of hydraulic structures, above-water concrete - 3 %, underwater concrete and concrete of internal zone - 5 %.

The mica inclusions, sulfurous and sulfate compounds, iron oxides and hydroxides, minerals which contain the amorphous types of silica, organic admixtures and others like that belong to other harmful aggregate admixtures. They impair concrete structure; influence negatively on the hardening process of cement stone, cause its corrosion. Limitations of harmful admixtures content are determined by the special researches taking into account sands function.

The sand with sulfurous and sulfate admixtures content according to the mass not more than 1% should be used for the concrete of hydraulic constructions; the mica content should not exceed 1 % for the concrete of variable water level zone, 2% for the concrete of above-water zone and 3 % for the concrete of underwater and internal zone.

Cement paste is spent for the filling of voids in the mixture of aggregates and on formation of envelop, which greases and cements separate grains in a strong conglomerate. That is why, the lower voids volume and grains surface, the more saving cement paste application in concrete is.

Sand fineness is characterized with the fineness modulus M_fin, which means the sum of complete residues (A_i, %), at screening of fine aggregate on standard sieves, divided by 100:

(10.1)

A complete residue is a sum of partial residues on current sieve and on larger sieves, which are included in screening set. A partial residue is a ratio of mass on this sieve to the mass of the screened sample.

Description of the sands according to fineness is represented in Table 10.2.

Тable 10.2

Sands description according to the fineness

Sand groups	Fineness modulus, %	A complete residue is on sieve No. 0.63 (by mass), %
Raised fineness	Over 3 to 3.5	Over 65 to 75
Coarse	Over 2.5 to 3	Over 45 to 65
Middle	Over 2 to 2.5	Over 30 to 50
Fine	Over 1.5 to 2	Over 10 to 30
Ultrafine	Over 1 to 1.5	To 10

Coarse, middle and fine sands are used as the fine aggregate for concrete. Fine sands cause the cement overrun, especially for the high classes' concrete. Using the sands with M_fin = 1.5...2 in concrete with ultimate compressive strength 20 MPa and higher is accepted only at the proper technical and economic substantiation.

Along with sand fineness its voidage or intergrain space volume (P) has an important value:

, (10.2)

where - bulk density, - absolute density of sand.

Voidage depends on the ratio of different grains according to the fineness that is on grain distribution of sand. It can increase in the sands of unsatisfactory grain distribution up to 40...47 %. The sand suitability for use according to the grain distribution for concrete is determined by graphing of sieving curve, which has to be placed in a certain area (Fig. 10.1).

It is possible to use sands, obtained by the previous mixing of separate fractions to provide the required grain distribution. Additives of large fractions of natural or manufactured sand are expedient when fine sands are used.

Approximately 50 % from all of concrete mass accounts for the coarse aggregate (gravel or crushed stone), which forms the stiff framework of concrete.

G ravel is fragile sedimental rock, formed as a result of weathering of dense rocks. Coarseness of the gravel grains ranges from 5 to 70 mm. Rounded shape of grains and in most cases raised content of dust-like particles and grains of weak rocks is typical signs of the gravel.

Crushed stone is a product of rock crushing. The crushed stone is obtained also from gravel, boulders and blast-furnace, steel-smelting and other slags.

Quality of the coarse aggregate, as well as sand, is determined by coarseness and grain distribution (Fig. 10.2), shape, grains surface and content of admixtures. Petrographic peculiarities, strength of initial rock, water- and frost-resistance, have a substantial value.

T he maximal size of the coarse aggregate should not exceed 1/4 of the minimum cross area of the structure. The coarseness of the aggregate is accepted less than 2/3 of distances between the reinforcing bars in the reinforced concrete structures.

The crushed stone or gravel is divided into separate fractions; mixed up in the recommended correlations to provide an optimum grain distribution (Table 10.3). Fractions 5...10, 10...20, 20...40, 40...70 mm are used as a rule.

It is allowed to use the crushed stone and gravel with coarseness up to 150 mm and more at technical and economic assessment in the concrete of the hydraulic and other massive structures.

One of the important requirements is grains strength of the coarse aggregate. The crushed stone can be made of the igneous rocks with ultimate strength more than 80 MPa, metamorphic rocks – more than 60 MPa and sedimentary with more than 30 MPa for heavy-weight concrete. Crushed stone strength made of natural stone should be higher concrete strength in 1.5...2 times.

Table 10.3

Recommended content of fractions in coarse aggregate, %

Maximum coarseness of grains, mm	Aggregate fraction, mm
	5...10	10...20	20...40	40...70	70... 120
20	25...40	60...75	—	—	—
40	15...25	20...55	40...65	—	—
70	10...20	15...25	20...35	35...55	—
120	5...10	10...20	15...25	20...30	30...40

The crushability (Cr) can serve as a parameter of gravel and crushed stone strength:

, (10.3)

where m - mass of a specimen, m_i- mass of material which passed through a sieve with the mesh size 4 times less than the smallest size of fraction after testing . The higher is the index of crushability of crushed stone or gravel, the lower is the expected concrete strength.

It is possible to use the aggregate with crushability grade no more than Cr 16, for the concrete with compression strength 25 MPa and lower; 30...35 MPa - Cr 12; 40 MPa and higher - no more than Cr 8.

The special requirements to strength of coarse aggregate are set at its application in hydraulic concrete, concrete for bridge structures, culvert aqueducts, reinforced concrete ties, coverages and bases for highways and aerodromes.

It is required to use crushed stone for the hydraulic structures, which strength is not less than 2.5...3 times higher than concrete strength for the zone of variable level and in 2...2.5 times for underwater, internal and upper zones.

Crushed stone with the grains density not lower than 2.5 g/cm³, water absorption not more than 0.5 % is applied for the concrete of variable water level zone, and for other zones - not lower 2.3 g/cm³ and 0.8% accordingly. Water absorption of crushed stone made of sedimentary rocks can be a little higher, but not more than 1...2 %.

For abrasive- and cavitation-resistant concrete there are additionally established grades of coarse aggregate by abrasive resistance by testing in the special drum.

Crushed stone according to the grains shape is divided into three groups: cube-shaped with content of plate-like (flaky) and needle grains not more than 15 %, improved - 25 %, and ordinary - 35 %. The content of plate-like and needle-shaped grains can exceed 35 %, but on conditions that assigned workability of concrete mixture and density of concrete without the cement overrun are provided.

The allowed content of dust-like and clay particles in coarse aggregate from igneous and metamorphic rock should not exceed 1 %, from sedimentary of rocks with strength 20…40MPa - 3%, from sedimentary rocks of higher strength - 2 %. The amount of dust-like and clay particles in crushed stone and gravel depending on the type of rock for the concrete of variable water level and upper zones should not exceed 1 %, and for underwater and internal zones - 2 % for hydraulic structures.

The aggregate frost resistance should provide the required frost resistance of concrete. It depends on the average monthly temperature of the coldest month in the performance region for the concrete of hydraulic structures. If this temperature hesitates from 0 to minus 10°C, frost resistance of crushed stone and gravel should be not less than 100 cycles, but from minus 10 to minus 20°C is not less than 300 cycles.

Concrete aggregates can be made on the basis of metallurgical and fuel slags, and other industrial waster. The required condition obtaining the aggregates from the metallurgical slags is their resistance to the different types of disintegration. The sulphur, containing in the metallurgical slags can cause corrosion of reinforcing steel. Its content should not exceed 2,5 % from the mass in the crushed stone made of blast-furnace slag. The sulphur presence should be taken into account at the production of prestressed reinforced concrete structures, where possibility of the slag crushed stone using should be supported with the special researches.

Porous aggregates. Aggregates with the grains density up to 2 g/cm³ belong to porous. They are utilized for obtaining light-weight concrete and mortars, and also thermal- and sound-proof compositions. The porous aggregates are divided into natural obtained in the form of sand and crushed stone from porous rocks, and artificial ones. Last aggregates are taken to the artificial inorganic ones as: claydite and its varieties (haydite gravel, clay-ash claydite), obtained by burning with expansions of granules from clay rocks, haydite slates, tripoli powder, ash-slag mixture or fly ash from the thermal power-stations; aggloporite is a product of calcination of sand-clay rocks, ashes, wastes of coal-cleaning industry; slag pumice is a product of porization during melting of metallurgical and chemical industries slags; granulated slag; perlite and vermiculite, obtained be expansion at the burning-out of effusive rock and hydrated mica grains.

Organic porous aggregates are obtained by the processing of wood and other vegetable raw materials and also polymers.

The type of porous aggregates is represented in the name of light-weight concrete (claydite concrete, aggloporite concrete, perlite concrete and other). Cement wood is made on the basis of organic aggregates. Claydite concrete is the most widespread among the light-weight concrete.

The porous gravel and crushed stone according to the bulk density are divided into grades 250...1200 kg/m³, porous sand - on grades 100...1400 kg/m³. Maximally allowed grade of all of the of porous gravel types and perlite crushed stone - 600 kg/m³, aggloporite and slag pumice - 900 kg/m³, crushed stone from porous mining rocks 1200 kg/m³.

The strength grades are also set for the coarse porous aggregates, which are determined by squashing in a cylinder. A frost resistance of the coarse aggregate should be not less than F15, the maximum possible loss of mass after testing allowed does not exceed 8 %.

For porous aggregates, as well as for dense, grain distribution and content of harmful admixtures is specified. In addition, requirements to the coefficient of grain form, water absorption, humidity, disintegration resistance and other are set.

The selection of the porous aggregates is regulated depending on their assignment and requirements to the strength and density of concrete.

The slag pumice belongs to the most effective types of artificial porous aggregates.

The ash-and-slag mixtures can be used as aggregates for heavy-weight and light-weight concrete.

The porous aggregates based on the slags and ashes from thermal power stations belong also to the least power-consuming. The ashes with raised content of noncombustible particles which are impermissible for the production of a series of other materials (porous concrete, lime-sand brick, cements and others like that) are suitable for production of porous aggregates.

The porous aggregates, made of the ashes can be obtained by the burning-out of specially prepared mass and by unfired technology. Materials of the first group are obtained by the calcination of granular batch which consists of the ash and little amount of binding additives (clays, technical lignosulphonate and other), in rotatary kilns (ashy gravel and lumnite claydite) or on the grates of the sintering machines (ashy aggloporite, aggloporite gravel). The basic material of the second group is unfired ashy gravel, with using of mineral binders additives.

Admixtures to concrete. The inorganic and organic substances or their mixtures (complexes) due to introduction of which (Table 10.4) properties of the concrete mixtures and concrete are regulated, belong to the concrete admixtures. The admixtures according to the basic action effect are divided into the followings groups:

• those, which·regulate of rheological properties, setting and porosity of concrete mixtures, concrete hardening;

• those, which give the special properties to the concrete;

• those, which regulate simultaneously different properties of concrete mixtures and concrete (multifunctional action);

• mineral powders - cement substitutes.

Names of some widespread admixtures of the first three groups with pointing their introduction quantity are resulted in the Table 10.4.

Таble 10.4

Some types of concrete admixtures

Type of the admixtures	Name of the admixtures	Content, % by cement weight	Effect
Plasticizers and superplasticizers	Naphtalene-formaldehyde superplasticizer	0.5...1	Increasing of concrete mixture slump or water content decreasing
	Technical lignosulphonates	0.1...0.3
	Technical lignosulphonates, modified by alkalines	0.1...0.3
	Plasticizer formate- spirit	0.2...0.8
Air-entraining and plastifying	Naphthenate soap	0.01...0.2
	Alkaline float of the caprolactam manufacturing	0.5...0.5
Air-entraining	Neutralized air-entraining oleoresin	0.005...0.035	Increasing of concrete frost-resistance and corrosive resistance in 2 times and more
	Ligneous saponated resin	0.005...0.035
	Sulfanole	0.05...0.035
Hardening accelerators	Calcium nitrate	0.3...1	Acceleration of hardening and providing hardening on a frost
	Sodium sulfate	0.3...0.8
Antifreeze	Potash	3...12
	Calcium nitrate	3...12
	Carbamide	3...12
Setting time retarding	Sugar treacle	0.1...0.2	Increasing conservation time of concrete mixture
Steel corrosion inhibitors	Sodium nitrite Calcium nitrite-nitrate	0.5...3 0.5...3	Increasing of reinforced-concrete structures durability

Basic directions of the using of chemical admixtures are related to reduction of labor contents on the plants of the precast concrete and in the monolithic construction, considerable reduction of a part of difficult manual operations, improving concrete quality, its strength, frost resistance, water impermeability and corrosive resistance, saving cement. By admixtures adding it is possible to change and regulate terms and time of concrete hardening, giving it new properties, for example bactericidalness, hydrophobicity, ability to harden on the frost and others like that.

The plasticizers and superplasticizers, are the most widespread in the concrete technology. The superplasticizers using allows to reduce the labor contents at reinforced-concrete elements casting in 2...4 times and concreting of monolithic structures in 5...7 times, in a number of cases fully to eliminate vibration or replace it with the brief shaking, to cut down fuel and electric power expenses, save up to 20…25 % of the cement.

Great possibilities of water content reduction of concrete mix by superplasticizers and considerable decline of water-cement ratio provide the obtaining the high-strength concrete. Along with individual admixtures complex of them with more universal effect become more widespread. For example, application of complex admixtures containing plasticizers and hardening accelerators allows along with the improvement of flow characteristics of concrete mixtures to accelerate the concrete hardening at the thermal treatment. The complex admixtures give an opportunity to remove or weaken the negative and often to stress on the positive features each of components, to improve the general positive effect.

Application of some chemical admixtures requires taking into consideration of their possible negative action (setting acceleration of the concrete mixture, reinforcement corrosion, equipment and accessories; wall salt formation; the decline of structures stability under the action of ground current; development of such types of concrete corrosion as interaction of reactive silica of aggregates with alkalines and other). For example, admixtures containing chlorous salts can be used as hardening accelerators for only reinforced-concrete structures with nonprestressed reinforcement with the diameter over 5 mm. It is not allowed to use admixture of salt-electrolytes at making of reinforced-concrete structures, intended for the electrified transport and industrial enterprises which use direct electric current.

Dispersible mineral admixtures (filling agents) are divided into active and inert. Inert admixtures are obtained by fine grinding of sands, limestones, dolomites, loess and other rocks to which hydraulic activity is not inherent. To active mineral admixtures (diatomites, tripoli powders, silica clays, ashes, granulated blast-furnace slags and other) which are introduced to the concrete mixture, the same requirements are set, as at the obtaining the Portland cement, portland-pozzolan cement and blast-furnace cement. They should be close to the cement according to the grinding fineness.

Increasing in concrete mixture water content is undesirable at the mineral additives using.

Fly ash from the heat engine generating stations is one of effective admixtures which are introduced to the concrete to replace the part of the Portland cement part. Its part can achieve 30 % by binder mass.

The ash is divided into few kinds depending on the using area: I – for the reinforced-concrete structures and elements; II - for the concrete structures and elements; III- for the structures of hydraulic constructions. The requirements, which are requested according to the ash chemistry, are indicated in the Table 10.5.

Таble 10.5

Requirements to the fly ash from the heat engine generating stations

as to the concrete admixtures

Indexes	The index value of ash kind
	І	II	III
Content of SiО2 + Аl2О3 + Fe2О3, % to the mass, not less than, for ash:
Anthracite and mineral carbon	Not specified		70
Brown coal	70	The same	50
Content of sulfur and sulfuric-acid catenation in enumeration on SО3 % to the mass, not more than	3	3.5	3
Content of free calcium oxide СаО, % by the mass, not more than	3	5	2
Content of magnesium oxide MgO, % by the mass, not more than: for ash
Anthracite	15	20	5
Mineral carbon	7	10	5
Brown coal	5	5	3
Humidity % by the mass, not more than	3	3	3

The ash acts of not only as active mineral admixture in the concrete mix, which increases binder content but also is microfilling agent which improves the sand grain distribution and actively influences on the processes of structure-forming in the concrete.

The declination of minimum the cement content for the non-reinforced elements up to 150 kg/m³, and for reinforced concrete elements – up to 180 kg/m³ is assumed at the using of the fly ash. Total cement and ash content at that should be accordingly not less than 200 and 220 kg/m³. Introduction of fly ash is especially effective at simultaneous introduction of plasticizer additives to concrete mixtures.

Application of superplasticizers has allowed to increase considerably technical effect which have been achieved at the filler introduction. The highly refined active mineral fillers as microsilica in the plasticized cement systems with the reduced water content substantially change the terms of structure forming and synthesis of the concrete properties. The superdispersed silica substances using as the fillers for the plasticized concrete mixtures is one of the most effective ways of decisions cement economy problems, obtaining high-strength and durable reinforced-concrete structures