Chapter 1 Introduction (Part 2)
WET CONCRETE PROPERTIES
(a) Bulking of Aggregate
- The free moisture content in fine aggregate results in bulking of volume. Free moisture content forms a film around each particle. This film of moisture exerts what is known as surface tension which keeps the neighbouring particle away from it.
- It is interesting to note that bulking increase with increase in moisture content upto a certain limit and beyond that the further increase in the moisture content results in the decrease in the volume. and at a moisture content representing saturation point, the fine aggregates show no bulking. Maximum increase in volume is upto 4% and maximum bulking occurs at a moisture content near to 5%.
- Due to bulking, fine aggregate shows unrealistic change in volume. Therefore it is necessary that consideration must be given to the effect of bulking in proportion concrete by volume.
(b) Soundness of Aggregate
- Soundness refers to the ability of aggregate to resist excessive changes in volume as a result of changes in physical conditions. These physical conditions that affect the soundness of aggregate are freezing and thawing, temp. variation alternate wetting and drying.
- The soundness of cement is determined either by 'Le chatelier Method' or by means of an 'Autoclave Test'.
- The test to determine the soundness of aggregate is 'sulphate test'.
(c) Alkali Aggregate Reaction
- This reaction takes place between the alkalis in the cement and the active silica or carbonates of aggregates.
Factor promoting alkali aggregate reaction.
(a) Reactive type of aggregate
(b) High alkali content in cement-alkali in cement 0.6%
(c) Availability of moisture
(d) Optimum temperature conditions (Ideal temperat ure is 10ºC to 38º)
- Alkali silica gel formed is the major disruptive product of alkali silica reaction. This silica gel exerts osmotic pressure to cause pattern crakcing particular in thinner sections of concrete like pavements.
- The formation of pattern cracks due to the stress induced by the growth of silica gel results in subsequent loss in strength and elasticity. If further accelerates other processes of deterioration of concrete due to formation of cracks. Solutions of dissolved CO2 convert Ca (OH)2 to CaCO3 with subsequent increase in volume.
Control of Alkali Aggregate Reaction:
(a) Selection of non reactive aggregates
(b) By the use of low alkali cement
(c) By the used of corrective admixtures such as pozzolanas.
(d) By controlling the void space in concrete.
(e) By controlling moisture conditions and temperature.
(d) Specific Surface
- Specific surface increase with the reduction in size of aggregate particle so the fine aggregate attributes very much more to the surface area then does the coarse aggregate. Greater surface area requires more water for lubricating the mix to give workability but very very fine particles act differently. These particles themselves act like ball bearings to reduce the internal friction b/w coarse particle.
- Experience has shown that usually very coarse sand or very find sand is unsatisfactory for concrete making. The coarse sand results in bleeding and segregation, and the fine sand requires a comparatively greater amount of water to produce the necessary fluidity.
(e) Gap Grading
- Generally it is assumed that the voids present in the higher size of aggregate are filled up by the next lower size of aggregate, and similarly voids created by the lower size are filled by the one size lower than those particle and so on. This was called continous grading.
- But later it was realized that the voids created by a particular fraction are too small to accommodate the very next lower size. This would create "particle size interference", which prevents the large aggregate compacting to their maximum density.
Advantage of gap graded concrete:
1. Sand required will be less.
2. Requires less cement and lower w/c ratio.
(f) Workability of Concrete
- Hundred percent compaction of concrete is an important parameter for contributing to the maximum strength. Lack of compaction will result in voids whose damaging effect on strength and durability is equally or more predominant than the presence of capillary cavities.
- To enable the concrete to be fully compacted with given efforts, normally a higher w/c ratio than that calculated by theoretical consideration must be required that is to say the function of water is also to lubricate the concrete can be compacted with specified effort forth coming at the site of work and the too without segregation.
- The quality of concrete satisfying the above requirements is termed as workable concrete Every job requires a particular workability. A concrete which considered workable for concrete foundation is not considered workable for concrete to be used in roof construction. Therefore the word workability assumes full significance of the type of work, thickness of section, extent of reinforcement and mode of compaction. So workability is the property of concrete which determines the amount of useful internal work necessary to produce full compaction.
Factors affecting workability
(a) Water content
(b) Aggregate cement ratio or mix proportion
(c) Size of aggregate
(d) Shape of aggregate
(e) Surface texture
(f) Use of Admixture
Measurement of workability
(a) Slump test
(b) Compacting factor test
(c) Flow test
(d) Kelly ball test
(e) Vee Bee consistometer test
(g) Segregation of Concrete
- Segregation is defined as the separation of the constituent materials of concrete. A good concrete is one in which all the ingredients are properly distributed to make a homogeneous mixture.
- Segregation may be of three types Firstly the coarse aggregate settling down from the rest of the matrix, secondly the paste or matrix separating away from coarse aggregate and thirdly, water separating out from the rest of the material.
- The conditions favourable for segregation are the badly proportioned mix where sufficient matrix is not there to bind and contain in the aggregation. Mixed concrete with excess water content shows a higher tendency for segregation. Dropping of concrete from heights also results in segregation. if too wet mix is excessively vibrated, it is likely that the concrete gets segregated.
- The tendency of segregation can be remedied by correctly proportionating the mix, by proper handling, transporting, placing, compacting and finishing.
- It is sometimes referred as water gain. It is a particular kind of segregation in which some of the water from the concrete coems out to the surface of concrete Bleeding is predominantly observed in a highly wet mix, badly proportioned and insufficient mixed concrete. In thin numbers like roof slab or road slabs and when concrete is placed in sunny weather. Show excessive bleeding. Sometimes certain quantity of cement also comes out to the surface with water. This formation of cement paste at the surface with water. This formation of cement paste at the surface is known as "Laitance". It results in higher shrinkage cracks and bad wearing quality. This could be avoided by removing the Laitance fully before the next lift is poured. Bleeding can be reduced by proper proportionating and uniform and complete mixing.
- Use of finely divided pozzolanic materials reduces bleeding by creating a longer path for the water to traverse use of air entraining agents is very effective in reducing the bleeding. Revibration may also be adopted to overcome the bad effect of bleeding.
Important Fraction of Wet Concrete other than Aggregate
Maximum permissible limits for solids as per IS:456–2000 in water:
1. Organic–200 mg/l
2. Sulphates (SO3) – 400 mg/l
3. Chlorides (CI–) – 2000 mg/l for concrete work not containing embedded steel. = 500 mg/l for RCC
4. Suspended – 2000 mg/l
5. Inorganic – 3000 mg/l
- The pH value of water shall be not less than 6.
- The initial setting time of test block made with the appropriate cement and the water proposed to be used shall not be less than 30 min and shall not differ by ± 30 min for the initial setting time of control test block prepared with the same cement and distilled water.
- Average 28 days compressive strength of at least three 150 mm concrete cubes prepared with water proposed to be used shall not be less than 90% of the average of strength of three similar concrete cube prepared with distilled water.
- IS 456–2000 prohibits the use of sea water for mixing and curing of reinforced concrete and presetressed concrete work.
- Admixture is defined as a material, other than cement, water and aggregates, that is used as an ingredient of concrete and is added to the batch immediately before or during mixing. Additive is a material which is added at the time of grinding cement clinker at the cement factory.
- Following Admixtures are used in Concrete: Plasticizers, super plasticizers, retarders and retarding platicizers, accelerators and accelerating plasticizers, air entraining admixtures, pozzolanic or mineral admixture, workability admixture.
Other Minor Admixture: Damp proofing and water proofing, gas forming admixtures, air entraining, grouting admixtures, bonding admixtures, fungicidal, germicidal and colouring admixtures.
(c) Plasticizers (Water Reducers)
- A high degree of workability is required in situation like deep beams, thin walls of water retaining structures with high percentage of steel reinforcement, tremie concreting, pumping concreting.
- The use of plasticizers can help in difficult conditions for obtaining higher workability without using excess of water.
- "The organic substances or combination of organic and inorganic substances, which allow a reduction in water content for the given workability, or give a higher workability at the same water cotent, are termed as plasticizing admixtures. Calcium, Sodium and Ammonium Ligno sulphonates are mostly used. They are used in the amount of 0.1% to 0.4% by wt. of the order of 5 to 15%. This naturally increases the strength.
- The increase in workability that can be expected at the same w/c, may be anything from 30 mm to 150 mm slump, depending on the dosage, initial slump of concrete, cement content and type.
(d) Super Plasticizers
- They are high range water reducers. Their use permit the reduction of water to the extent of 30% without reducing workability.
- Thus the super plasticizers produce a homogeneous, cohesive concrete and bleeding. Some super platicizers are:
1. Sulphonated melamine formaldehyde condensate (SMF)
2. Sulphonated napthalene formaldehyde condensates (SNF)
3. Modified Ligno Sulphonates.
- It slows down the chemical processes of hydration so that concrete remains in plastic and workable for a longer time than concrete without the retarder. Generally good for hot weather concretig. Retarders are used in grouting oil wells.
- The most common retarder is Calcium Sulphate. (Gypsum). Other materials used for this purpose are starches, cellulose products. sugars, acids or salt of acids, Ligno Sulphonic Acids and their salts, hydroxylated carboxylic acids and their salts. The last two also reduce the water content.
- All the plastizers and superplastizers by themselves show certain extent of retardation.
- Accelerating admixture are added to concrete to increased the rate of early strength development in concrete to:
1. Permit earlier removal of formwork.
2. Reduce the required period of curing.
3. Advance the time that the structure can be placed in service.
4. In the emergency repair work.
5. In cold weather to compensate for retardation.
- Some accelertors ar e soluble carbonates, silicates, some of the organic compounds such as trietheriolamine are used.
(g) Air Entraining Admixture
- Air entrained concrete is made by mixing a small quantity of air entraining agent or by using air entraining cement. These air entraining agents incorporate millions of non-collapsing air bubbles, which will act as flexible ball bearings and will modify the properties of plastic concrete regarding workability, segregation, bleeding and finishing quality of concrete.
- It also modifies the properties of hardened concrete regarding its resistance to frost action and permeability.
- Air entraining agents and Natural wood resins, animal and vegetable fats and oils and their fatty acids and such as stearic and oleic Acids, wetting agents such as alkali salts or sulphated or sulphated or sulphonated organic compounds.
Setting time of concrete
- Setting time of concrete depends upon the w/c ratio, temperature conditions, type of cement, use of minerals admixture, use of plasticizers.
- The setting time of concrete is found by penetrometer test. Plot a graph of penetration resistance as ordinate and elapsed time as abscissa.
- From penetration resistance equal to 3.5 MPa draw a horizontal line. The point of intersection of this with a smooth curve is read on the X-axis which gives the initial setting time. Similarly a horizontal line is drawn from the penetration resistance of 27.6 MPa and the point it cuts the smooth curve is read on the X-axis which gives the final set time.
Various Stages of Manufacture of Concrete (a) Batching (b) Mixing (c) Transporting (d) Placing (e) Compacting (f) Curing (g) Finishing
- It is of two types, volume batching and weight batching. Volume batching is not considered good.
- when volume batching is done the bulking of sand is consider
- Hand mixing and machine mixing are the commonly adopted mixing methods. Hand mixing is adopted for small scale and unimportant work while machine. mixing is carried out for reinforced concrete work for medium or large scale concrete work. Machine mixing is not only efficient but also economical (when the quantity of concrete to be provided is large)
- Concrete mixers are generally designed to slum at a speed of 15 to 20 rpm. For proper mixing it is seen that about 25 to 30 revolutions are required in a well designed mixer.
(c) Transporting of Concrete
The methods adopted for transportation are:
(a) Mortar pan
(b) Wheel barrow
(c) Crane, bucket and rope way
(d) Truck Mixers and dumpers
(e) Belt conveyor
(g) Skip and Hoist
(h) Transit Mixer
(i) Pump and pipe line
(d) Placing of Concrete
- Concrete must be placed in systematic manner to yield optimum results.
- Concrete is often required to be placed under water or in a trench filled with bentonite slurry. In such cases use of bottom dump bucket or tremie is made use of.
(e) Compaction of Concrete
- It is process adopted for expelling the entrapped air from the concrete. The lower the workability, higher is the amount of air entrapped. In other words, stiff concrte mix has high percentage of entrapped air and therefore would need higher compacting efforts than high workable mixes. It must be borne in mind that 100% compaction is important not only from the point of view of strength but also from point of view of durability.
Methods of Compaction
(a) Hand Compaction
1. Rodding 2. Ramming 3. Jumping
(b) Compaction by Vibration
1. Internal Vibrator (Needle Vibrator)
2. Form Work Vibrator (external Vibrator)
3. Table Vibrator
4. Platform Vibrator
5. Surface Vibrator 6. Vibratory Roller
(c) Compaction by Pressure and Jolting
(d) Compaction by Spinning
(f) Curing of Concrete
- Curing can be considered as creation of a favourable environment during the early period of uninterupted hydration.
- More elaborately it can be described as the process of maintaining a satisfactory moisture content and a favorable temperature in concrete during the period immediately following placement so that hydration of cement may continue until the derived properties are developedto a sufficient degree to meet the requirement of service.
Curing Methods may be broadly divided into 4 categories.
(a) Water Curing (b) Membrane Curing (c) Application of heat (d) Miscellaneous
- Water curing is considered the best method of curing. Even if membrane method is adopted, it is desirable that a certain extent of water curing is done before the concrete is covered with membranes. Immersion, ponding, spraying or fogging, wet cover are commonly adopted for water curing. Membrane curing is adopted where there is acute shortage of water. Membrane curing is the application of membrane or scaling compound. The materials used for this purpose are bituminous compounds, polyester film, water proof paper or rubber compound etc. Bituminous compound, polyster film, water proof paper or rubber compound etc. Bituminous compound are generally not used now as they have a great heat absorbing property.
- In the application of heat, generally steam curing is adopted as it fulfills both teh purposes of maintaining temperature and that too without loss of m/c. Steam curing is done normally at ordinary pressure but in certain circumstances under high pressure curing is done by infrared radiation Electrical curing are other curing methods.
- Curing is done at the temperature of 27 ± 3ºC and humidity 90%.