a- Briefly explain how various environmental factors affect the durability of concrete? b- What is the mechanism of sulphate attack on concrete; how it can be controlled?

 

Q#  a-      Briefly explain how various environmental factors affect the durability of concrete?                    

b-      What is the mechanism of sulphate attack on concrete; how it can be controlled?            

 

Answer A

Durability

       A DURABLE CONCRETE IS ONE THAT PERFORM satisfactorily in workig enviromet during its anticipated exposure conditions service. 

Durability is the ability of a physical product to remain functional, without requiring excessive maintenance or repair, when faced with the challenges of normal operation over its design lifetime. There are several measures of durability in use, including years of life, hours of use, and number of operational cycles. In economics, goods with a long usable life are referred to as durable goods.

A DURABLE CONCRETE IS ONE THAT PERFORM satisfactorily in workig enviromet during its anticipated exposure conditions service.

dURABILITY Of concrete is its ability to resist weathering action, chemical attack, abrasion or any other process deterioration 

Environmental factors affect the durability of concrete;

Physical Durability

Physical durability is against the following actions

1.   Freezing and thawing action

2.   Percolation / Permeability of water

3.   Temperature stresses i.e. high heat of hydration

Chemical Durability

Chemical durability is against the following actions

1.   Alkali Aggregate Reaction

2.   Sulphate Attack

3.   Chloride Ingress

4.   Delay Ettringite Formation

5.   Corrosion of reinforcement

 

TEMPERATURE

Durability of concrete may be defined as the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. ... When the temperature increases the volume of the concrete increases and when the temperature falls the concrete contracts

MOISTURE

Higher water-to-cement ratios result in greater spacing between the aggregates in cement, which affects compaction. Similarly, increased moisture levels reduce the concrete’s compressive strength and durability. As concrete’s surface area increases, particularly with the addition of fine aggregates, so does the demand for water. The increased water leads to a higher water-to-cement ratio.

When excess water creates greater spaces between aggregate materials, the voids fill with air after the moisture evaporates. The resulting inadequate compaction reduces the concrete’s strength. Concrete with trapped air levels as little as 10 percent experience reductions in strength of up to 40 percent.

Quality of water

if the water has pollutant i it, it will result in decrease in strength of concrete and the workability is goig to decrease

Freezing and Thawing:

Deterioration of concrete from freeze thaw actions may occur when the concrete is critically saturated, which is when approximately 91% of its pores are filled with water. When water freezes to ice it occupies 9% more volume than that of water. As the seasons pass, concrete goes through the process of freezing and thawing resulting in repeated loss of concrete surface With the addition of an air entrainment admixture, concrete is highly resistant to freezing and thawing.

Alkali aggregate reaction:

 Use of non-reactive aggregate from alternate sources . use of low alkali ordinary Portland cement having total alkali content not more than 0.6 percent (as Na20 equivalent).Measures to reduce the degree of saturation of the concrete during service such as use of impermeable membranes.

Answer B

sulfate attack

    sulfate attack of concrete is a complex process, which includes physical salt attack due to salt crystallization and chemical sulfate attack by sulfates from soil, groundwater, or seawater. Sulfate attack can lead to expansion, cracking, strength loss, and disintegration of the concrete

Sulfate attack can be 'external' or “internal”. “External”         

EXTERNAL:

Due to penetration of sulphates into the concrete from outside For example : high-sulphate soils and ground waters, or atmospheric or industrial water pollution.        

INTERNAL:

Due to a soluble source being incorporated into the concrete at the time of mixing, gypsum in the aggregate.       

MECHANISM:

        Sulfate attack has occurred at various locations throughout the world. Some common sulfate environments are soils, groundwater, transport fluids, contained soils or fluids, and seawater. Many concrete structures are exposed to these environments and accumulation of sulfates at an exposed face increases the potential for deterioration.

The mechanism of distress for sulfate attack is an expansive pressure caused by the transformation of monosulfoaluminate to ettringite. This mechanism can be described as a sequence of processes. First, the external sulfate reacts with calcium hydroxide to saturate the pore solution and precipitate gypsum (CaSO4 • 2H2O). The increased concentration of SO4 promotes the transformation of monosulfoaluminate to ettringite (3CaO• Al2O3• 3CaSO4 • 32H2O). This transformation causes an increase in solid volume, which results in the deterioration of concrete by inducing cracking, softening, and spalling. The expansion mechanism is caused by pressure from ettringite crystal growth or swelling due to absorption of water. Another measurable concrete property identified with the sulfate attack mechanism is a decline in compressive strength.

Different sulfates can effect the concrete differently. Magnesium sulfate is the most severe because of the presence of magnesium ions. These ions can cause additional corrosive reactions through the formation of Mg(OH)2 and ettringite. This also decomposes the C-S-H.

 

PROTECTION:         

The methods given below can be adopted to protect concrete from sulphate attack.

USE OF SULPHATE RESISTING CEMENT:

1.QUALITY CONCRETE:-

A well designed, placed and compacted concrete which is dense and impermeable exhibits a higher resistance to sulphate attack. Similarly, a concrete with low water/cement ratio also demonstrates a higher resistance to sulphate attack.

2.USE OF AIR-ENTRAINED ADMIXTURE:

Use of air-entrainment to the extent of about 6% has beneficial effect on the sulphate resisting qualities of concrete. The beneficial effect is possibly due to reduction of segregation, improvement in workability, reduction in bleeding and in general better impermeability of concrete.

3.USE OF POZZOLANA:

Incorporation of or replacing a part of cement by a pozzolanic material reduces the sulphate attack. Admixing of Pozzolana converts the leachable calcium hydroxide into non-leachable cementitious product. This pozzolanic action is responsible for impermeability of concrete. Secondly the removal of calcium hydroxide reduces the susceptibility of concrete to attack by magnesium sulphate.

4.HIGH PRESSURE STEAM CURING:

High pressure steam curing improves the resistance of concrete to sulphate attack. This improvement is due to the change of C3AH6 into a less reactive phase and also to the removal or reduction of calcium hydroxide by the reaction of silica which is invariably mixed when high pressure steam curing method is adopted.

 

a- What do you mean by the workability of concrete; why do we need workable concrete? b- How various properties of aggregates affect workability of concrete?

 

Q#  a-      What do you mean by the workability of concrete; why do we need workable concrete? 

b-      How various properties of aggregates affect workability of concrete?                                         

 

Solution#4 (to be typed here by the student)

Workability

Workability is a property of raw or fresh concrete mixture. In simple words, workability means the ease of placement and workable concrete means the concrete which can be placed and can be compacted easily without any segregation.

The property of concrete which determines the amount of useful internal work ,necessary to produce full compaction i.e workability is the amount of energy to overcome Friction while compacting. Also defined as the relative ease with which concrete can be mixed ,transported, moulded and compacted..

Compaction and workability are very close to each other. Workability can also be defined as the amount of useful internal work necessary to produce full compaction.

Workability is the property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity

 

why do we need workable concrete?

    In every construction work we use different quantitative or qualitative terms to express workability. Before specifying workability for any work a concrete technologist must keep the following things in mind.

            1-Type of construction work

            2-Method of mixing

            3-Thickness of section

            4-Extent of reinforcement

            5-Mode of compaction

            6-Distance of transporting

            7-Method of placing

            8-Environmental condition

      Concrete that can be placed readily without segregation or separation in a mass dam could be entirely unworkable in a thin structural member.

     Workable concrete compacted by means of high frequency vibrators would be unworkable if vibrators could not be used and hand tamping and spading were required.

Concrete having suitable workability for a pavement might be unsuitable for use in a heavily reinforced section.

Answer B

water contents

Water/Cement Ratio or Water Content of Concrete

Higher the water/cement ratio, higher will be the water content per volume of concrete and concrete will be more workable. Higher water/cement ratio is generally used for manual concrete mixing to make the mixing process easier.

Mix Proportions

Rich concrete mix (cement content is high) is more workable because due to sufficient cement aggregates will have proper lubrication for easy movement which means more workability.

Size of Aggregates

Finer particles require more water for a larger surface, hence aggregate with finer particles need more water to make it workable. On the other hand, bigger particles have less surface area, demand less water for wetting surface and require less amount of paste for lubricating.

So bigger particles give higher workability for fixed water content. But maximum size of aggregate depends on some practical considerations like handling-mixing and placing equipment, concrete section, and spacing of reinforcement.

Grading of Aggregates

Well-graded aggregates tend to fill up voids and easily get workability. Less amount of water can make it workable. If grading is better, there will be fewer voids and excess paste will be available to give better lubricating effect. Due to excess paste, the mixture gets cohesive and prevent segregation. It also makes it get compacted easily i.e. increases the workability.

Use of Admixtures

There are some admixtures which can improve workability. Some admixtures are mixed intentionally to increase workability and some admixtures increase workability as a side effect of its main purpose.

The Surface Texture of Aggregates

The surface texture of aggregate can be either smooth or rough. A smooth surface can improve workability, yet a rougher surface generates a stronger bond between the paste and the aggregate creating a higher strength

 

a- What do you mean by concrete mix design; what are the major factors that must be considered while designing concrete mixes? [5] b- Name various methods used for concrete mix design worldwide? Which method is commonly used Pakistan?

 

Q#      a-      What do you mean by concrete mix design; what are the major factors that must be considered while designing concrete mixes?                                                                                          

b-      Name various methods used for concrete mix design worldwide? Which method is commonly used Pakistan?                                                                                                                                

Aswer A

concrete mix design

The art of selectig suitable ingrediets of concrete and determining their relative proportions with object of producing concrete with certain minimum strength

Concrete mix design involves a process of preparation in which a mix of ingredients creates the required strength and durability for the concrete structure. Because every ingredient in the mix consists of different properties, it’s not an easy task to create a great concrete mix. It is imperative that all ingredients be tested to determine their physical properties and the bearing capacity of the project location.

The following must be considerd  while designing concrete mixes

Ø  Cost

Ø  WORKABILITY

Ø  COST

Ø  STRENGTH

Ø  DURABILITY

 

1-WORKABILITY

Basic Considerations Workability • The following points related to workability shall be kept in mind while designing concrete mixes. The consistency of concrete should no more than that necessary for placing, compacting and finishing.

2- COST

Ø  Material cost

Ø  equipmentd cost

Ø  labor cost   

cements are several times costliar than than aggregates

cements content need to be minimized

 

3-STRENGTH

Out of all the physical characteristics of concrete compressive strength is often taken as an index. Therefore, the mix design is generally carried out for a particular compressive strength of concrete coupled with adequate workability so that the fresh concrete can be properly placed and compacted. In addition the mix proportions are also checked against the requirement of adequate durability for the type of exposure condition anticipated in service.

 

4-DURABILITY

durability requirements in terms of miimum cement content, maximum w/c ratio and minimum grade of concrete

Durability is the property due to which concrete withstand the enviromental condition to which it is exposed for long period of time

 

ANSWER B 

Name various methods used for concrete mix design worldwide

1- Maximum density method

2 -Road note 4 methods

3- finenes modulus method

4- surface area method

5-ACI method

6-Mix design for high strength concrete

7-Mix design for pumpable concrete

 

 

ACI method and superpave method is used in pakistan commonly

a- What are the main differences between plasticizers and superplasticizers? b- What is pozzolanic material; what benefits can be achieved by using Pozzolana with cement in concrete?

 Q#  a-      What are the main differences between plasticizers and superplasticizers?                                   

b-      What is pozzolanic material; what benefits can be achieved by using Pozzolana with cement in concrete?                                                                                                                                    

Solution#

 Answer A

 Plasticizer super plasticizer

          Both are different things Plasticizers are the chemicals used to increase plasticity or fluidity of a material they are added to. For example moulding clay.

Superplasticizers(water reducers) are the chemicals used in concrete to increase its usability or workability for example less water should be used to make stronger concrete but this affects its workability because it makes concrete difficult to mix now since strength is inversely proportional to the amount of water added,thereby for this purpose both plasticizers and superplastizers are used which makes the concrete much efficient.

 

Plasticizer

1.      A plasticizer is an additive used to improve the plasticity of a certain substance.

2. Increase the plasticity of polymer materials such as PVC & as a water reducer in concrete admixtures.

3. Also called water reducers.

4. Can reduce the need for water by 5-15%

5. Added 0.1 – 0.5% by weight of cement

6. Plasticizers are usually based on lignosulphonate, which is a natural polymer

Superplasticizer

 

1.  Superplasticizer is a water-reducing admixture capable of producing large water reduction or great flowability without causing undue set retardation or entrainment of air in mortar or concrete.

2. Further increase the water requirement for concrete mixtures, increasing the strength & durability of concrete.

3. Also called high range water reducers.

 

pozzolanic material

                 A pozzolan is a siliceous or siliceous and aluminous material that in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds having cementitious properties.

 

a- What are the broad types of admixtures? b- How would you improve the bond of fresh concrete to hardened concrete?

Q#   a-      What are the broad types of admixtures?                                                                            

b-      How would you improve the bond of fresh concrete to hardened concrete?                           

Solution

 for increasing the bond strength between fresh concrete and hardened concrete.

 

1. Providing reinforcement above than the hardened concrete to make bond strength

 

2. Using the some Epoxy resns and Epoxy Chemicals to increase the bond strength

 Classification:

·         Water reducers

·         Set-controlling chemicals

·         Air – entrains

·         Specialty admixtures

-Shrinkage reducing

-Viscosity modifiers

-Corrosion inhibitors

-Styrene – Butadiene latexes(SBR Latexes)

-Anti – freeze compounds

-Water proofing

-Alkali – Silica reaction mitigating admixtures

1.Water Reducers

For a given workability, the water demand is reduced, thus resulting in higher strength and durability.

·         For a given W/C and strength, workability can be increased.

·         For a given W/C, strength and workability, the quantity of cement can be reduced.

Normal water reducers (5-8) %

High range water reducers (15-25) %

 

2.Set-Controlling Chemicals

·         Set controllers are organic or inorganic chemicals that interfere with the hydration process of cement.

·         The rate of dissolution of cement compounds, that is necessary for hydration to occur, is either speeded up or slowed, depending on the chemical.

There are two types of set – controlling chemicals,

I. Accelerating chemicals

II. Retarding chemicals

 3.Air-Entrains

Air – entraining agents are also surface – active chemicals. Unlike the water – reducing surfactants, the hydrocarbon chains does not have any polar groups, and is entirely hydrophobic.

The hydrophobic polar groups (- coo- , - So3 - , - NO3 - ) are similar to water reducers.

 5.Viscosity Modifying Agents (VMAs)

To provide stability to extremely flow able concrete (which may be prone to segregation). To prevent the wash – out of concrete in under water application – in this case the VMA is also called “Anti – Washout Admixture”.

·         The dosage of VMA is generally 0.03 – 0.08 % by weight of cement.

·         Biopolymers and synthetics:

-Diutan gum

-Alginates

-Xanthan gum

-Hydroxy propyl methyl cellulose

-Hydroxy propyl starch