Introduction. General Aspects.Concrete. Reinforced Concrete. Prestressed Concrete.

Z.Kiss, T.Onet - Proiectarea Structurilor de Beton dupa SR-EN-1992-1

T.Onet, R.Olar - Reinforced Concrete Handbook - Part 1.

SR EN 1992-1-1 - Proiectarea structurilor de beton. Partea 1-1: Reguli generale si reguli pentru clădiri.

Eurocode 2 (EC2) - Design of concrete structures.

Concrete

material with a mixed structure inhomogeneous anisotropic

[SR EN 1992] ; [EC2]

1. Analysis

2. Design

3. Detailing

Concrete Mixture. Characteristic and Design Strengths.Loads. Combinations of Loads.

100m10-1m10-2m10-3m10-4m10-5m10-6m10-7m10-8m10-9m

macromicronano

Sand dimension < 4 mm = 4 million nm

Multi-walled nanotubes ext.diameter : 2-25 nm

Steel bars reinforcementd = 6…28mm

Reinforced concreteelements

Ex. The effect of silica fume in concrete

The smaller particles have greater surface area per unit mass compared with larger particles

d=30 nm => 5% of atoms/surface

d=10 nm =>20% of atoms/surface

d= 3 nm =>50% of atoms/surface

Decrease the size => Increase the reactivity.

Decrease the size => Increase thedensity and compressive strength.

The mechanical behaviour of concrete is directly affected by the processes that occur at micro- and nano-scale.

Concrete Mixture

I. PortlandCement

MineralAdmixtures

waterIII. Chemical admixtures

II. Aggregates

•Cement is a material that hasadhesive and cohesive propertiesenabling it to bond mineralfragments into a solid mass.•It can set and harden in thepresence of water (the so‐calledhydraulic cement).•consists primarily of silicates andaluminates of lime made fromlimestone and clay (or shale)which is ground, blended, fused ina kiln, and crushed to a powder.•the usual hydraulic cement usedfor r.c. is known as PortlandCement.

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• Concrete made with Portland cement ordinarily requires about 14 days to attain adequate strength so that forms can be removed and construction and dead loads carried. • The design strength of such concrete is reached at  28 days.

Aggregates are inert granular materials such as:• sand, • gravel,• crushed stone. 

For a good concrete mix, aggregates need to be:• clean,• hard,• strong particles,• free of absorbed chemicals or coatings of clay and other fine materials that could cause the deterioration of concrete.

Aggregates, which account for 60 to 75 percent of the total volume of concrete, are divided into two distinctcategories:• fine• coarse.

0‐4 mm

4‐8 mm

8‐16 mm

16‐31 mm

crushed stone 0‐4,4‐8,8‐16,

16‐25,25‐63 mm

Grading ‐ refers to the determination of 

the particle‐size distribution for aggregates. 

Grading limits and maximum aggregate size are specified because grading and size affect the amount of aggregate used as well as cement and water requirements, workability, pumpability, and durability of concrete. 

Particle shape and surface texture influence the properties of freshly mixed concrete more than the properties of hardened concrete. Rough‐textured, angular, and elongated particles require more water to produce workable concrete than smooth, rounded compact aggregate. 

Consequently, the cement content must also be increased to maintain the water‐cement ratio. Generally, flat and elongated particles are avoided or are limited to about 15 percent by weight of the total aggregate.

The amount of water in the concrete mixture must be adjusted to include the moisture conditions of the aggregate. 

Abrasion and skid resistance of an aggregate are essential when the aggregate is to be used in concrete constantly subject to abrasion as in heavy‐duty floors or pavements. Different minerals in the aggregate wear and polish at different rates. Harder aggregate can be selected in highly abrasive conditions to minimize wear.

Admixtures are used to modify the properties of the concrete to make it better serve its intended use or for better economy.

The main types of admixtures are classed according to their function:• air‐entraining, • water‐reducing, • retarding, • accelerating,• plasticizers (superplasticizers). 

Other functions of special admixtures :• corrosion inhibition, • shrinkage reduction, • workability enhancement, • bonding, • coloring. 

Air‐entraining admixtures 

• reduce the required water content for a concrete mixture by about 5 ‐ 10 %,• concrete needs less water to reach a required slump than untreated concrete,• the treated concrete can have a lower water‐cement ratio => a higher strength concrete can be produced without increasing the amount of cement.

Water‐reducing admixtures 

• the amount of entrained air is usually between 5 ‐ 8 % of the volume of the concrete, but may be varied as required by special conditions. • the concrete is highly resistant to severe frost action and cycles of wetting ‐ drying or freezing ‐thawing • the concrete has a high degree of workability and durability. 

• slow the setting rate of concrete,• used to counteract the accelerating effect of hot weather on concrete setting. • retarders keep concrete workable during placement and delay the initial set of concrete.

Accelerating admixtures 

• increase the rate of early strength development, • reduce the time required for proper curing and protection, • speed up the start of finishing operations. • useful for modifying the properties of concrete in cold weather.

Retarding admixtures 

Superplasticizers: 

• are also known as plasticizers or high‐range water reducers (HRWR),• reduce water content by 12 ‐ 30 % ,• can be added to concrete with a low‐to‐normal slump and water‐cement ratio to make high‐slump flowing concrete,• flowing concrete is a highly fluid but workable concrete that can be placed with little or no vibration or compaction,• the effect of superplasticizers lasts only 30 to 60 minutes, depending on the brand and dosage rate, and is followed by a rapid loss in workability,• as a result of the slump loss, superplasticizers are usually added to concrete at the jobsite. 

• contribute to the properties of hardened concrete through hydraulic or pozzolanic activity (a pozzolan is a material which, combined with calcium hydroxide Ca(OH)2, exhibits cementitious properties),

Typical examples are:• natural pozzolans, • fly ash,• ground granulated blast‐furnace slag,• silica fume.

These materials react chemically with calcium hydroxide released from the hydration of portland cement to form cement compounds. 

These materials are often added to concrete to make concrete mixtures more economical, reduce permeability, increase strength, or influence other concrete properties. 

Fly ash

• the most commonly used pozzolan in concrete,• a finely divided residue that results from the combustion of pulverized coal, • carried from the combustion chamber of the furnace by exhaust gases. • commercially available fly ash is a by‐product of thermal power generating stations. 

Blast‐furnace slag

• known also as iron blast‐furnace slag,• nonmetallic product consisting essentially of silicates, aluminosilicates of calcium, and other compounds• is developed in a molten condition simultaneously with the iron in the blast‐furnace. 

Silica fume

• also called condensed silica fume or microsilica, • finely divided residue resulting from the production of elemental silicon or ferro‐silicon alloys that is carried from the furnace by the exhaust gases. • silica fume, with or without fly ash or slag, is often used to make high‐strength concrete. • consists primarily of amorphous (non‐crystalline) silicon dioxide (SiO2),• the individual particles are extremely small, approximately 1/100th the size of an average cement particle, •because of its fine particles, large surface area, and the high SiO2 content, silica fume is a very reactive pozzolan when used in concrete.• silica‐fume concrete with a low water content is highly resistant to penetration by chloride ions. • used in concrete for construction of new bridges or rehabilitation of existing structures.

Characteristic Material Strengths

The characteristic strengths of materials, corresponding to the 5% fractile, arethose strengths values of the tests results below which less than 5% will fall.

the normal (or Gaussian) distribution

Characteristic Loads

represent the limits within which at least 95% of values will lie in practice. it’s expected that not more than 5% of cases will exceed the upper limit.

* usually, the maximum value of the actions on a structural member is critical, so the positive sign in the expression is used,** minimum value is used when for stability cases or for the behaviour of the continous members.

Partial Factors of Safety

Design situations - sets of physical conditions representing the realconditions occurring during a certain time interval for which the design willdemonstrate that relevant limit states are not exceeded.

persistent design situations, (the conditions of normal use), transient design situations, (temporary conditions), accidental design situations, (exceptional conditions), seismic design situations.

Limit States

are defined as the states beyound which the structure no longer satisfies the design performance

Ultimate Limit State (ULS)associated with collapse, loss of equilibrium of the whole structure, failure by excessive deformation, rupture or loss of stability

Serviceability Limit States (SLS)correspond to conditions beyond which specified service requirements for a structure or structural member are no longer met:• deformations,• deflections, • vibrations,• crackings of the concrete that affect durability.

Loads types. Loads transmission to the ground. Ground reactions.

Actions

• permanent [G]: self-weight of the structure, fixed equipment…

• variable [Q]: live loads, wind, snow, temperature…

• accidental [A]: fire, explosions, impact, seism.

seismic load

snow load wind load

Combinations of Actions for ULS

Range of values for variable actions

Recommended values of ř factors for buildings.