Materiali da costruzione svanzati - Parte 2

Concrete structure

Binder (cement) + water + aggregates + admixtures. Sometimes we have normal cement but binders, this maintain connect the aggregates.

Concrete is composite material composed by the particles inside the cement paste. In traditional concrete aggregate have high mechanical properties and cement paste has low mechanical properties.

In new concrete admixtures are commonly used.

Characteristics

• Good compressive strength (Rc) and resistance to the environment, concrete is a material that can give an high resistance to the exposure to environment, this is some that we can see in the ancient structures (like Pont du Gard, Nimes, built in 17 before Christ) the production of cement start around 1800, because it need a temperature around 1450 celsius degree.
• Can be reinforced with carbon steel bars (high Rt), resistance to tension is poor because concrete is a porous material and has a brittle behavior, but we can use inside carbon steel reinforcement.

In past they can’t use reinforcement in steel because they didn’t have the cement and so, there wasn’t the formation of passive film on the steel rebars and so there was corrosion. Now with the concrete, that has high alkalinity, there is the formation of the passive film on the steel rebars and this give protection and prevent the corrosion.

• Easy to produce and to place.
• Low cost, raw materials easy to achieve everywhere, because we can use lime and clay that have low cost.

Presence of the rebar without corrosion, concrete that has aggregate with different dimensions, and the cement paste that maintain in position the aggregates and give the adhesion to aggregates and to the carbon steel elements.

We can see some bubbles that is the entrapped air, this air is embedded during the mixing and after is impossible to remove.

Concrete is a composite material that contain one matrix that is cement paste that is the cement paste and two reinforcement, one are the aggregates and the other is the rebar.

Constituents

• Binder (cement).
• Water.
• Aggregates.
• Admixtures.

In Europe we can use EN 197 to have the information of cement, when the designer can consider to use the CEM I or CEM III?

Designers have to select the right cement for every specific application, the problem is that some cements are produced only in some part of the Italy, like CEM III is not easy to find in Milano, because it is produced only in Taranto.

CEM I is the pure Portland Cement that contain 90% of clinker (element that we can obtain after the cooking of the different constituent of the cement).

Gypsum is fundamental for the cement production because

CEM III is used in precasted element, in normal construction we normally use CEM II.

• Fresh concrete.
• Hardened concrete.

Example of designation

CEM II / B - L 32.5 R.

Strength classes (EN 197) (standard mortar specimens: w/c = 0,5; sand/c = 3).

Contain an amount of limestone that is between 21 and 25%, this do not give advantage.

32.5 is the resistivity after a period of 28 days of curing, this number give the mechanical properties, this cement has a resistance to compression > 32.5 MPa with a relative humidity > 95%, fundamental for the hydration of the cement.

The water cement ratio (w/c) of 0.5 is the result of test of mortar sample, we have 1 part of cement and 3 part of sand (sand/c).

The letter R mean rapid, cement that can give us rapidly the strength.

The letter N mean normal.

The letter L mean Low hydration, sometimes we need a low hydration because during hydration we have the production of an high amount of heat because there is an isotherm reaction and this sometimes create disaster.

Because if we have a variation in temperature between the internal part of concrete and the external part, that is higher that 20 celsius degree, there is an expansion that give crack in concrete. For example in the dam we have to use only cement with low hydration.

This different characteristics generally are obtained only with the different size of grains inside the cement. Like the 32.5 has bigger dimensions and in 52.5 has grain very fine (that give a easy reaction) and the grains dimension is lower than 1000mm.

Cement hydration

Inside of the cement due to production there is a very high amount of C3S. In old cement there is the C2S. The difference is that the C3S give a rapid strength and we can obtain high mechanical performance after few days of curing, the cement that is produced nowadays has very rich of this constituent.

In the powder of cement there are also aluminate like C3A and C4AF and 5% of gypsum (CaSO4).

When we put this constituent in contact with water initially we have the fast reaction of aluminate with water and there is the formation of hydration products that are calcium aluminate constituent, this, if we do not stop the reaction, give the formation of crystal (crystal stop the movement inside of the cement and so we can not use this material, for this reaction is enough very few time) but the gypsum is added and thanks the reaction with the aluminate and the creation of constituent, that is Ettringite, this stop the hydration of aluminate and give time to maintain in fresh state the cement for 1/2 hours.

After 1/2 hours we can start to have the hydration of calcium silicate that produce a gel (C-S-H) this give the structure of our material, it is like a glue. There is also the formation of an alkaline constituent that is calcium hydroxide (Ca(OH)2) called hydrated lime, this give a maximum pH of 12,6.

Hydration —> exothermic reaction (heat production). If I maintain the concrete in contact with water the needle grow. In our concrete there are particles of cement that isn’t hydrated, and this is normal. The hydration is fundamental for many characteristic of concrete. During the hydration have an increment of temperature because the hydration is an exothermic reaction.

Cement paste - structure

The pores that is present inside the concrete is 3 types:

• Gel pores (~ 1 nm; 10^-9 m), is like a lamella of few nanometres, this pores for us is negligible.
• Capillary pores, is the pore between the needles. This pores is connected together. I can have micropores and macropores. Micropores is very small then for whatever I have outside can’t enter in these pores. Capillary pores give permeability.
• Entrapped air voids; compaction (1 - 10 mm), I make compaction in order to remove this air but generally I can reach the total extraction of the air. These voids are in contact with the capillary pores, but that is not important for the durability.

We can control the porosity of the material with the water cement ratio and with the curing of the concrete. Water cement ratio give an idea about the distance between the cement grain and the initial stage. If we increase the water cement ratio we can not reach a very closed structure.

With the curing we increase the percentage of the grain that is hydrate.

The perfect structure is a structure in which we have big pores (macro pores) that are present in different positions but they are also connected with micropores (dimension below 500 nanometers) this is called pores segmentation. In this way is not easy for the substance from outside to penetrate inside of the material and to reach all the part.

In order to promote this segmentation I have to manage degree of hydration and the curing time related to the w/c.

For the segmentation of the pores we need curing.

Blended cements and mineral additions

• Pozzolanic materials:
  • Natural pozzolans.
  • Pulverised coal fly ash (FA), obtained during the burning of coal.
  • Silica fume (SF), very fine powder, is something that we can use inside of the paste.
• Ground granulated blastfurnace slag (GGBS) it is obtained during the production of cast iron, the problem is that we can obtain a slow cement.

It is fundamental the curing of the cement that contain mineral addition in order to obtain the properties that we need.

Characteristic

• Slow hydration rate and slow development of strength.
• A longer curing periods is necessary.
• Slow rate of heat evolution.
• Refinement of the capillary pore microstructure (lower permeability).
• Higher resistance to penetration of aggressive species (Cl-, SO4^2-).
• Lower alkali content (NaOH, KOH).

Aggregates

Are another constituent very important.

In the concrete I have a high amount of aggregate, more or less 80%. The aggregates give me the mechanical properties. They affect the properties of fresh concrete and the shrinkage. We can use what we want but this depend from the properties that we need.

Natural or artificial materials with particles of size (fine and coarse) and shape (gravel, crushed ...) suitable for the production of concrete.

• They fill the volume of concrete, reducing the amount of cement paste (reduce heat, cost ...).
• They affect the properties of fresh concrete (workability, without segregation ...).
• They influence the properties of hardened concrete (strength, shrinkage ...).

—> Suitable grading (grading curves).

We need a structure like this, with aggregates with big dimension and among this aggregates with low dimensions and other very smaller aggregates in order to seal completely the volume that we want to fill, we need very small amount of cement paste to glue the different elements. If we reduce the cement paste we have advantages.

For the analysis of the dimensions of the grain we have to do a special analysis related to the curve of the distribution of the particles present inside of the cement.

Further requirements (EN 12620)

Related to the mechanical performance:

• Compressive strength.
• Resistance to fragmentation of coarse aggregate.
• Wear and abrasion resistance of coarse aggregate.
• Resistance to freeze-thaw cycles.
• Volume stability (drying shrinkage).
• Alkali-silica reactivity.
• Content of chlorides and sulphates.
• Content of organic impurities.

Mixing water

Mixing water, it is important to analyze the composition of water in order to not create problems.

• Drinkable water OK→.
• Water recovered from concrete manufacturing plants.
• Underground water.
• Natural surface water or industrial water.
• Seawater.
• Sewage water.

Admixtures

Now it is possible to use some admixtures or additives in order to obtain specific properties, this help the change of the characteristics in the fresh state and in the hardened state. When we have problems related to the winter period, low temperature and due to this there is a slow hydration of cement and the possibility to freeze the cement (this create cracks) to avoid this problem is use an admixture like accelerator that contain salts. In summer we have a fast hydration of cement and in order to reduce this we can use organic compounds that contain sugar.

• Water reducers (Plasticisers).
• Superplasticisers, to increase the workability.
• Water retaining agents.
• Air entraining.
• Setting accelerators.
• Hardening accelerators.
• Retardants.

Properties of concrete in fresh state

For normal building we need concrete with good workability and good mechanical performance, for a dam we need concrete with zero workability and high mechanical performance. Fresh state and hardened state are different and sometimes they are completely different.

In fresh state is fundamental to have material prepared in the correct way, we need all the characteristics below without the segregation.

Fresh concrete

Concrete, before setting, should be:

• Mixed.
• Transported.
• Placed.
• Compacted.
• Finished.

Easily, avoiding segregation of the mixture, the correct mix of the aggregates avoid the problem of segregation.

Workability

The workability is calculated with a slump test. This test give also results easiness of flowing (fluidity) + cohesion + easiness of compaction about segregation of cement. We have 5 classes, from S1 to S5.

We can use concrete with different workability for different applications, like for dam we use concrete that has workability lower than S1.

Factors which influence workability

• Water content (high fluidity; high segregation).
• Aggregates (gravel high workability; high Dmax high workability, but high segregation).
• Plasticisers / superplasticisers or air entraining.
• Temperature (high T, low workability).
• Time (workability low in time, “slump loss”).

Workability and compaction

We need to make a compaction in order to remove the air inside the concrete. This machine can be with multiple elements or single element that with vibration remove the air. Vibration is fundamental because we can obtain concrete that is perfect for our applications, because the air reduce a lot the mechanical performance.

Problem of cold joints, if there is not the perfect adhesion between the elements and there is a not normal load, like earthquake the effect is that is represented in the picture.

When we have new fresh concrete that need to connect with the previous concrete we have to prepare the surface of old concrete, like remove all the powder and we have to clean the surface, and than we have to saturate the old concrete.

Curing

After the casting we need to start curing. Curing give us the mechanical performance and the hydration of the cement grain. The hydration is fundamental to create structure with high mechanical performance and high durability.

We have to maintain humid the surface of the concrete like with the use of plastic film on the surface or wet with water the surface.

Low water cement ratio after few days of curing we have mechanical performance and segmentation of product.

At the end of curing we have the hardened concrete that has the characteristics that we want.

Hardened concrete

Properties

• Mechanical strength (compressive, tensile, flexural, ...).
• Resistance to deformation (loads, changes in moisture and T).
• Resistance to degradation (Durability).

Main factors

• W/c.
• Curing.
• Compaction.
• Cement type (blended, 32.5, 42.5).
• Aggregates.
• Admixtures (air entraining).

Generally the test that is used for the hardened state is the compressive test, Why we have two numbers? For example we can have a concrete with this class of resistance C 30/37.

The first one is for cylinder and the second one is for cube.

Two numbers because one is related to anglosassone standard and the other for the general standard.

This number give also information about water cement ratio.

Deformation - loads

The properties are not only related to the compressive strength but we have to evaluate the effect related to deformation because concrete is viscoelastic material.

In a specific time we apply a load and maintain this load in a period.

In elastic material the strength is related to the stress and the load is maintain the same but when we remove the load the material return in the initial situation.

The strength has an increase in time because this is a porous material and when we apply a load we reduce the dimensions of pore. When we remove the stress we have a partially recover. The recover depend from the conditions that we have outside.

If we are in a situation with submerged structure in water we have lower effect because we have the advantage that the structure is completely saturated, we have a lower effect related to the grip and we can obtain a recover that is more or less completely.

For the dry conditions we have the water that goes out from the small pores and reach the macro-pores, for this reason it is not so easy to water to goes outside from the macro pores, this phenomena is called creep.

Creep is the effect related to the strength, the phenomena linked to the stress is called relaxation, with a constant strength in the material we have a reduction of the stress in the time (for viscoelastic material).

Deformation - changes in moisture

Water is present inside of the pores (in micro pores) water molecule is connected with the surface of the pore. The water inside the pore maintain fix the pore dimension. If we remove the water we have a decrease in the diameter of the pore and we have the shrinkage effect. This is a problem of the concrete and the cement paste. The reduction of the pores can create cracks.

We have two type of deformation.

Plastic shrinkage

Plastic shrinkage (during setting; water evaporation, formwork absorption).

Related to the not correct placement, we cast concrete and we don’t consider curing, this occur for example for the flooring (pavimento). It is related to the plastic period of the concrete, when the concrete is in the fresh state, there is the fast evaporation of water and this create cracks. Fundamental to cure after casting.

Drying shrinkage

Drying shrinkage (after curing; water evaporation, hydration only if w/c<0.3). We have normally, when we reach the end the curing period we remove the system that we use for curing and the structure goes in the equilibrium with the humidity that there is in the environment.

Generally we have a reduction in the humidity and this create shrinkage. If we wet the structure we can have a partially recover of shrinkage (reversible shrinkage).

In the flooring we have quadratic elements with some joints, but this are not joints.

After the realization of the flooring we have to cut the concrete element because of the shrinkage. We create artificial cut to obtain cracks below this cut.

Plastic shrinkage related to repaired mortar.

Advanced concrete and special cementitious materials

The problem of this concrete is related to the low knowledge in the behavior of this materials.

This concrete is used for special needs:

• Mechanical.
• Workability.
• Durability.
• Sustainability.
• Special application.

Type of special concrete

High performance concrete (HPC).

Higher mechanical performance Development of HPC:

This concrete has resistance that is closed to the resistance of steel.

• Powers’ theory (’50): low w/c —> high of capillary porosity and high Rc (with nil porosity, Rc —> 250 MPa).

If we consider reduce the porosity to zero we can obtain a compressive strength of 250 MPa. This only in theory because in 50’ the technology and the knowledge were limited.

The problem related to the theory is related to the workability, if we use low water cement ratio we need to use an high amount of water, but in this way the workability is very very low.

• Superplasticizers (’70): w/c can be low without compromising worka.