Concrete Technology: A Comprehensive Guide
Concrete Technology: A Comprehensive Guide
Phase and Chemical Composition of Portland Cement Clinker
THE COMPONENT PHASES OF PORTLAND CEMENT
- •3CaO.SiO2 (C3S alite) 55-65 %
- •2CaO.SiO2 (C2S belite) 15-24 %
- •3CaO.Al2O3 (C3A tricalcium aluminate) 8-12 %
- •4CaO.Al2O3.Fe2O3 (C4AF brownmillerite) 8-12 %
CHEMICAL COMPOSITION OF CEMENTS
CaO 60 ÷ 67 % SiO2 18 ÷25 % Al2O3 2.5 ÷ 8 % Fe2O3 0.5 ÷ 6 % MgO 0,1 ÷ 4 % SO3 1,7 ÷ 4,6 % Na2O+K2O 0,2 ÷ 2 %
Physical and Mechanical Properties of Cement
• Cement is a gray fine powder
• The cement colour depends mainly on raw material
• Slag cements are more fair in colour
• Fly-ash cements are dark-grey
Phase Composition of Hardened Cement Paste
• Reactivity of cement with water is a function of its fineness.
•Generally, the finer a cement (higher specific surface), the more rapidly it will react.
It results in:
- decrease of initial setting time,
- increase of hydration heat,
- increase of strength growth dynamics,
- increase of tendency to shrinkage deformation.
Classification of Cements (Types, Classes)
• ordinary cements – without pointing special
Applications. (CEM I PORTLAND, CEM II PORTLAND COMPOSITE, CEM III BLAST FURNACE, CEM IV POZZOLANIC, CEM V COMPOSITE)
• special cements – pointing special performance
features for special application (Sulfate-Resisting Cements, Low heat cements, Very low heat special cements, Low alkali cement)
What Factors Have to Be Taken into Consideration When Cement is Selected for a Given Application?
•Sub Class N indicates normal early strength
•Sub Class R indicates rapid early strength
•Standard strength class
•Indicates the second main constituent, in this case pulverized fly ash
•Indicates the proportion of cement clinker (A) higher, (B) medium and (C) lower.
•Main cement type.
Definition and Classification of Concrete Additions
•Materials of an inorganic nature that are characterized by pozzolanic or
hydraulic properties; Finally, they can be added to the concrete and improve
its properties or special characteristics.
- •Granulated blastfurnace slag S
- •Siliceous fly ash V
- •Calcareous fly ash W
- •Natural pozzolana P
- •Industrial pozzolana Q
- •Burnt shale T
- •Limestone L, LL*
- •Silica fume D
* L – organic carbon content TOC should not exceed 0,5% by mass
LL – organic carbon content TOC should not exceed 0,2% by mass
Pozzolanic Addition – Definition, Examples and Properties?
•POZZOLANAS are natural or industrial materials containing silica, aluminate-silica or mixture, which are composed mainly from reactive silicon dioxide and aluminium oxide, and also iron and other matals oxides.
The content of reactive calcium oxide is not important.
vThey don’t harden after mixing with water.
vWhen they are finely grinded they react (in room temperature in the presence of water) with solved calcium hydroxide (which is a product of hydration of main clinker phases).
vMixture of hydrated calcium silikates and aluminates with growing strength is formed.
vThese compounds resemble the compounds formed during cement hydration.
8.Composition and properties of fly ash. What are the benefits of using fly ash inconcrete?
Type II addition, must comply with EN 450 „Fly ash for concrete”.
Fly ash is obtained by electrostatic or mechanical precipitation of dust-like particles from the flue gases from furnaces fired with pulverised coal.
Fly ash may be siliceous or calcareous in nature. The former has pozzolanic properties; the latter may have, in addition, hydraulic properties.
Benefits
Possible beneficial influence on workability of cencrete mix,
Possible water reduction,
Concrete mix is suitable for pump transportation,
Extended setting times,
Reduction of amount and rate of hydration heat emission,
Reduction of cement milk emission
9.Composition and properties of silica fume. What are the benefits of using silica fume inconcrete?
Silica fume is an extremely fine, spherical powder that is used as an additive for improving concrete performance. It is a very fine pozzolanic material, composed mostly of amorphous silica produced by electric arc furnaces.
Silica fume improves the strength and durability of concrete by creating a denser cement matrix when compared to conventional concrete.
Composition
Sume of calcium oxide (CaO), magnesium oxide (MgO) and silicon dioxide (SiO2) ≥ 66,7%
The rest contains (Al2O3) with small amount of other compounds.
Mass ratio (CaO + MgO)/SiO2 should be at least 1,0.
Benefits
·increase of strength
·resistance to acids
·reduction of alkali–silica reaction
·resistance to sodium sulfate attack
·reduction of eflorescence
·resistance to chloride ingress
·increase of freeze/thaw resistance
10.Definition and classification of concreteadmixtures.
Material added for concrete mix in order to modify properties of concrete mix or/and hardened concrete, amount does not exceede 5% of cement mass.
classification
·Water reducing admixtures
·water retaining admixtures
·air entraining admixtures
·set accelerating admixtures
·hardening accelerating admixtures
·set retarding admixtures
·waterresisting admixtures
·complex admistures
11.Mechanisms and effects of application of water reducingadmixtures
Water-reducing admixtures are added to obtain:
• better workability at a constant water-cement ratio (w/c).
• the reduction in water needed to obtain the desired slump – it causes increase of the strength. These admixtures can reduce the amount of water %required by 20 to 30%. The compressive strength may increase as much as 25% and the flexural
strength may increase as much as 10%.,
• certain strength (constant w/c) at given workability at lower cement content.
Mechanisms of water reducing admixtures action
Water-reducing chemicals belong to a group of chemicals known as ‘dispersants’.
The action of the dispersant is to prevent the flocculation of fine particles of cement.
12.Mechanisms and effects of application of air entraining admixtures.
It helps protect against damage caused by repeated freezing and thawing cycles.
It also imparts better workability, improved homogeneity, decreased segregation and decreased bleeding.
When you increase the air content of concrete by about 1%, you decrease the compressive strength by about 5%.
13.What is the difference between „designed concrete” and „prescribed concrete”.
•Designed concrete – Concrete for which the required properties and additional characteristics are specified to the producer who is responsible for providing a concrete conforming to the required properties and additional characteristics.
•Prescribed concrete – Concrete for which the composition of the concrete and the constituent materials to be used are specified to the producer who is responsible for providing a concrete with the specified composition.
14.Definition and classification of exposure classes (environmental actions).
•Those chemical and physical actions to which the concrete is exposed and which result in effects on the concrete or reinforcement or embedded metal that are not considered as loads in structural design.
•The exposure classes to be selected depend on the provisions valid in the place of use of the concrete. This exposure classification does not exclude
consideration of special conditions existing in the place of use of the concrete or the application of protective measures such as the use of stainless steel or other corrosion resistant metal and the use of protective coatings for the concrete or the reinforcement (X0, XC1, XC2, XC3, XC4, XD1, XD2, XD3, XS1, XS2, XS3, XF1….XA1)
15.Definition and classification of concrete strengthclasses.
•The characteristic compressive strength of either 150 mm diameter by 300 mm cylinders or of 150 mm cubes may be used for classification.
defined as compressive strength at 28 days in N/mm2 , below which not more than 5% of specimens give the result.
•Compressive strength classes for normalweight andheavyweight concrete:
C 8/10, C 12/15, C 16/20, C20/25, C 25/30, C 30/37…..
16.Classification of fresh concrete (concrete mix)consistencies.
Compaction classes: C0, C1,C2, C3
Flow classes: F1, F2, F3, F4, F5, F6
Slump classes: S1, S2, S3, S4, S5
17.Describe one method of testing fresh concreteconsistence.
Among the tests that exist to determine the consistency, the most used is the Cone de Abrams.
It consists of filling a 30 cm high conical mold with fresh concrete.
The decrease in height that occurs when the mixture is demold is the measure that defines the consistency
18.What factors influence concrete compressivestrength?
Water / Cement Ratio (A / C).
Concrete Cement Content.
Aggregates and their influence on concrete.
19.What factors influence concretedurability?
For high durability, concrete must not only be “strong” but also impermeable, especially in the areas near the surface. The lower the porosity and the denser the hardened cement paste, the higher the resistance to external influences, stresses and attack.
20.Stages of concreteproduction.
•dosing and mixing the raw materials
•transporting (delivery)
•placing
•compacting
•finishing
•curing
21.Methods of curing fresh concrete and protective measures in hot weather.
Curing methods
•Applying liquid curing agents
•Leaving in the forms
•Covering with sheets
•Laying water-retaining covers
•Spraying or “misting” continuously with water, keeping it
effectively submerged and
•A combination of all of these methods
Hot Weather Concrete
The protective measures are dependent on:
•the outside temperature,
•air humidity,
•wind conditions,
•fresh concrete temperature,
•heat development
•dissipation and the dimensions of the pour.
22.Methods of curing fresh concrete and protective measures in cold weather.
•Curing methods
In heavy frosts or long periods of freezing temperatures, the air surrounding the fresh concrete must be heated and the concrete surfaces must stay damp. Good sealing is important (e.g. by closing window and door openings and using enclosed working tents).
•It is important to prevent wide temperature differences (>15 K) between the core and the surface in fresh and new concrete and exposure to abrupt temperature changes in partially hardened concrete.
23.Which factors influence duration ofcuring?
The principle of the European draft is incorporated in E DIN 1045-3. Its basis is that curing must continue until 50% of the characteristic strength fck is obtained in the concrete component. To define the necessary curing period, the concrete producer is required to give information on the strength development of the concrete. The information is based on the ratio of the 2 to 28 day average compressive strength at 20°C and leads to classification in the rapid, average, slow or very slow strength development range.
The minimum curing period prescribed according to DIN 1045-3 is based on these strength development ranges.
24.What are the protective measures against premature drying of fresh concrete?
Premature drying can be prevented by keeping the surface continuously damp by continuously wetting the concrete surfaces.
25.Give example of specification ofconcrete.
Specification of Concrete
Example: Pumped concrete for ground slab in ground water area
Specification conforming to EN 206-1 (designed concrete):
•Concrete conforming to EN 206-1
•C 30/37
•XC 4
•Cl 0.20
•Dmax 32 (max. particle size)
•C3 (degree of compactability)
•Pumpable
26.Explain the
If type II additions are used (fly ash and Silica fume), the k-value permits them to be taken into account in the calculation of the water in the fresh concrete.
(The k-value concept may differ from country to country).
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