Concrete Works

Pulverized fly ash as cement replacement – how it works?

Pulverized fly ash is a type of pozzolans. It is a siliceous or aluminous material which possesses no binding ability by itself. When it is in finely divided form, they can react with calcium hydroxide in the presence of moisture to form compounds with cementing properties. During cement hydration with water, calcium hydroxide is formed which is non-cementitious in nature. However, when pulverized fly ash is added to calcium hydroxide, they react to produce calcium silicate hydrates which is highly cementitious. This results in improved

concrete strength. This explains how pulverized fly ash can act as cement replacement.

PFA vs GGBS

(i) Similarities: 

Both GGBS and PFA are by-products of industry and the use of them is environmentally friendly. Most importantly, with GBS and PFA adopted as partial replacement of cement, the demand for cement will be drastically reduced. As the manufacture of one tonne of cement generates about 1 tonne of carbon dioxide, the environment could be conserved by using less cement through partial replacement of PFA and GGBS. 

On the other hand, the use of GGBS and PFA as partial replacement of cement enhances the long-term durability of concrete in terms of resistance to chloride attack, sulphate attack and alkali-silica reaction. It follows that the structure would remain to be serviceable for longer period, leading to substantial cost saving. Apart from the consideration of long-term durability, the use of PFA and GGBS results in the reduction of heat of hydration so that the problem of thermal cracking is greatly reduced. The enhanced control of thermal movement also contributes to better and long-term performance of concrete.

In terms of the development of strength, PFA and GGBS shared the common observation of lower initial strength development and higher final concrete strength. Hence, designers have to take into account the potential demerit of lower strength development and may make use of the merit of higher final concrete strength in design.

(ii) Differences Between GGBS and PFA

The use of GGBS as replacement of cement enhances smaller reliance on PFA. In particular, GGBS is considered to be more compatible with renewable energy source objectives. The replacement level of GGBS can be as high as 70% of cement, which is about twice as much of PFA (typically replacement level is 40%). Hence, partial replacement of GGBS can enable higher reduction of cement content. As the manufacture of one tonne of cement generates about 1 tonne of carbon dioxide and it is considered more environmentally friendly to adopt GGBS owing to its potential higher level of cement replacement.

The performance of bleeding for GGBS and PFA varies. With PFA, bleeding is found to decrease owing to increased volume of fines. However, the amount of bleeding of GGBS is found to increase when compared with OPC concrete in the long term. On the other hand, drying shrinkage is higher for GGBS concrete while it is lower for PFA concrete. In terms of cost consideration, the current market price of GGBS is similar to that of PFA. As the potential replacement of GGBS is much higher than PFA, substantial cost savings can be made by using GGBS.

Purpose of setting minimum amount of longitudinal steel areas for columns

In some design codes it specifies that the area of longitudinal steel reinforcement should be not less than a certain percentage of the sectional area of column. Firstly, the limitation of steel ratio for columns helps to guard against potential failure in tension. Tension may be induced in columns during the design life of the concrete structures. For instance, tension is induced in columns in case there is uneven settlement of the building foundation, or upper floors above the column are totally unloaded while the floors below the column are severely loaded. Secondly, owing to the effect of creep and shrinkage, there will be a redistribution of loads between concrete and steel reinforcement. Consequently, the steel reinforcement may yield easily in case a lower limit of steel area is not established. 

In addition, test results showed that columns with too low a steel ratio would render the equation below inapplicable which is used for the design of columns:

                                      N=0.67fcuAc+fyAs

Concrete Works

Minimum area of reinforcement vs maximum area of reinforcement

Beams may be designed to be larger than required for strength consideration owing to aesthetics or other reasons. As such, the corresponding steel ratio is very low and the moment capacity of pure concrete section based on the modulus of rupture is higher than its ultimate moment of resistance. As a result, reinforcement yields first and extremely wide cracks will be formed. A minimum area of reinforcement is specified to avoid the formation of wide cracks. On the other hand, a maximum area of reinforcement is specified to enable the placing and compaction of fresh concrete to take place easily.

Mild steel vs high yield steel in water-retaining structures

In designing water-retaining structures, movement joints can be installed in parallel with steel reinforcement. To control the movement of concrete due to seasonal variation of temperature, hydration temperature drop and shrinkage etc. two principal methods in design are used: to design closely spaced steel reinforcement to shorten the spacing of cracks, thereby reducing the crack width of cracks; or to introduce movement joints to allow a portion of movement to occur in the joints.

For the choice of steel reinforcement in water-retaining structures, mild steel and high yield steel can both be adopted as reinforcement. With the limitation of crack width, the stresses in reinforcement in service condition are normally below that of normal reinforced concrete structures and hence the use of mild steel reinforcement in water-retaining structure will suffice. Moreover, the use of mild steel restricts the development of maximum steel stresses so as to reduce tensile strains and cracks in concrete.

However, the critical steel ratio of high yield steel is much smaller than that of mild steel because the critical steel ratio is inversely proportional to the yield strength of steel. Therefore, the use of high yield steel has the potential advantage of using smaller amount of steel reinforcement. 

On the other hand, though the cost of high yield steel is slightly higher than that of mild steel, the little cost difference is offset by the better bond performance and higher strength associated with high yield steel.

Mechanism of plastic settlement in fresh concrete

Within a few hours after the placing of fresh concrete, plastic concrete may experience cracking owing to the occurrence of plastic shrinkage and plastic settlement. The cause of plastic settlement is related to bleeding of fresh concrete. Bleeding refers to the migration of water to the top of concrete and the movement of solid particles to the bottom of fresh concrete. The expulsion of water during bleeding results in the reduction of the volume of fresh concrete. This induces a downward movement of wet concrete. If such movement is hindered by the presence of obstacles like steel reinforcement, cracks will be formed.

No fines concrete

In some occasions no fines concrete is used in houses because of its good thermal insulation properties. Basically no fines concrete consists of coarse aggregates and cement without any fine aggregates. It is essential that no fines concrete should be designed with a certain amount of voids to enhance thermal insulation. The size of these voids should be large enough to avoid the movement of moisture in the concrete section by capillary action. It is common for no fines concrete to be used as external walls in houses because rains falling on the surface of external walls can only penetrate a short horizontal distance and then falls to the bottom of the walls. The use of no fines concrete guarantees good thermal insulation of the house.

Over vibration of fresh concrete

For proper compaction of concrete by immersion vibrators, the vibrating part of the vibrators should be completely inserted into the concrete. The action of compaction is enhanced by providing a sufficient head of concrete above the vibrating part of the vibrators. This serves to push down and subject the fresh concrete to confinement within the zone of vibrating action. Over-vibration should normally be avoided during the compaction of concrete. If the concrete mix is designed with low workability, over-vibration simply consumes extra power of the vibration, resulting in the wastage of energy. For most of concrete mixes, over-vibration creates the problem of segregation in which the denser aggregates settle to the bottom while the lighter cement paste tends to move

upwards . If the concrete structure is cast by successive lifts of concrete pour, the upper weaker layer (or laitance) caused by segregation forms the potential plane of weakness leading to possible failure of the concrete structure during operation. If concrete is placed in a single lift for road works, the resistance to abrasion is poor for the laitance surface of the carriageway. This becomes a critical problem to concrete carriageway where its surface is constantly subject to tearing and traction forces exerted by vehicular traffic.

Concrete Works

Longer tension lap lengths at the corners and at the top of concrete structures

In BS8110 for reinforced concrete design, it states that longer tension lap lengths have to be provided at the top of concrete members. The reason behind this is that the amount of compaction of the top of concrete members during concrete placing is more likely to be less than the remaining concrete sections. Moreover, owing to the possible effect of segregation and bleeding, the upper layer of concrete section tends to be of lower strength when compared with other locations.

When the lap lengths are located at the corners of concrete members, the degree of confinement to the bars is considered to be less than that in other locations of concrete members. As such, by taking into account the smaller confinement which lead to lower bond strength, a factor of 1.4 (i.e. 40% longer) is applied to the calculated lap length.

Location of lifting anchors in precast concrete units

It is desirable that the position of anchors be located symmetrical to the centre of gravity of the precast concrete units. Otherwise, some anchors would be subject to higher tensile forces when compared the other anchors depending on their distance from the centre of gravity of the precast concrete units. As such, special checks have to be made to verify if the anchor bolts are capable of resisting the increased tensile forces.

Location of construction joints

Construction joints are normally required in construction works because there is limited supply of fresh concrete in concrete batching plants in a single day and the size of concrete pour may be too large to be concreted in one go.

The number of construction joints in concrete structures should be minimized. If construction joints are necessary to facilitate construction, it is normally aligned perpendicular to the direction of the member. For beams and slabs, construction joints are preferably located at about one-third of the span length. The choice of this location is based on the consideration of low bending moment

anticipated with relatively low shear force. However, location of one-third span is not applicable to simply supported beams and slabs because this location is expected to have considerable shear forces and bending moment when subjected to design loads. Sometimes, engineers may tend to select the end supports as locations for construction joints just to simplify construction.

Measurement of cement and aggregates – by weight vs by volume

Measurement of constituents for concrete is normally carried out by weight because of the following reasons:

(i) Air is trapped inside cement while water may be present in aggregates. As such measurement by volume requires the consideration of the bulking effect by air and water.

(ii) The accuracy of measurement of cement and aggregates by weight is higher when compared with measurement by volume when the weighing machine is properly calibrated and maintained. This reduces the potential of deviation in material quantity with higher accuracy in measurement for the design mix and leads to more economical design without the wastage of excess materials.

Movement accommodation factor for joint sealant

Movement accommodation factor is commonly specified by manufacturers of joint sealants for designers to design the dimension of joints. It is defined as the total movement that a joint sealant can tolerate and is usually expressed as a percentage of the minimum design joint width. Failure to comply with this requirement results in overstressing the joint sealants.

For instance, if the expected movement to be accommodated by a certain movement joint is 4mm, the minimum design joint width can be calculated as 4÷30% = 13.3mm when the movement accommodation factor is 30%. If the calculated joint width is too large, designers can either select another brand of joint sealants with higher movement accommodation factor or to redesign the arrangement and locations of joints.

Minimum distance between bars and maximum distance between bars

In some codes, a minimum distance between bars is specified to allow for sufficient space to accommodate internal vibrators during compaction.

On the other hand, the restriction of maximum bar spacing is mainly for controlling crack width. For a given area of tension steel areas, the distribution of steel reinforcement affects the pattern of crack formation. It is preferable to have smaller bars at closer spacing rather than larger bars at larger spacing to be effective in controlling cracks. Hence, the limitation of bar spacing beyond a certain value (i.e. maximum distance between bars) aims at better control of crack widths.

Concrete Works

Indirect tensile strength in water-retaining structures

The crack width formation is dependent on the early tensile strength of concrete. The principle of critical steel ratio also applies in this situation. The amount of reinforcement required to control early thermal and shrinkage movement is determined by the capability of reinforcement to induce cracks on concrete structures. If an upper limit is set on the early tensile strength of immature concrete, then a range of tiny cracks would be formed by failing in concrete tension.

However, if the strength of reinforcement is lower than immature concrete, then the subsequent yielding of reinforcement will produce isolated and wide cracks which are undesirable for water-retaining structures. Therefore, in order to control the formation of such wide crack widths, the concrete mix is specified to have a tensile strength (normally measured by Brazilian test) at 7 days not exceeding a certain value (e.g. 2.8N/mm2 for potable water).

Joint filler in concrete expansion joints – a must??

The presence of joint filler is essential to the proper functioning of concrete joints though some may doubt its value. For a concrete expansion joint without any joint filler, there is a high risk of rubbish and dirt intrusion into the joint in the event that the first line of defense i.e. joint sealant fails to reject the entry of these materials. In fact, the occurrence of this is not uncommon because joint sealant from time to time is found to be torn off because of poor workmanship or other reasons. The presence of rubbish or dirt inside the joint is undesirable to the concrete structures as this introduces additional restraint not catered for during design and this might result in inducing excessive stresses to the concrete structure which may fail the structures in the worst scenario. Therefore, joint filler serves the purpose of space occupation so that there is no void space left for their accommodation. To perform its function during the design life, the joint filler should be non-biodegradable and stable during the design life of the structure to enhance its functioning. Moreover, it should be made of materials of high compressibility to avoid the hindrance to the expansion of concrete.

Lifting hoops in precast concrete – mild steel vs high yield steel

The strength of high yield steel is undoubtedly higher than mild steel and hence high yield steel is commonly used as main steel reinforcement in concrete structures. However, mild yield steel is commonly used in links or stirrups because they can be subjected to bending of a lower radius of curvature.

For lifting hoops in precast concrete, it is essential that the hoops can be bent easily and hence mild steel is commonly adopted for lifting hoops because high yield bars may undergo tension cracking when it is bent through a small radius.

Lap length > anchorage length

In some structural codes, the lap length of reinforcement is simplified to be a certain percentage (e.g. 25%) higher than the anchorage length. This requirement is to cater for stress concentrations at the end of lap bars. A smaller load when compared with the load to pull out an anchored bar in concrete triggers the splitting of concrete along the bar because of the effect of stress concentration. A higher value of lap length is adopted in design code to provide for this phenomenon. 

Longitudinal steel – an enhancement of shear strength

In addition to shear resistance provided by shear reinforcement, shear forces in a concrete section is also resisted by concrete compression force (compressive forces enhances higher shear strength), dowel actions and aggregate interlocking. The presence of longitudinal steel contributes to the enhancement of shear strength of concrete section in the following ways:

(i) The dowelling action performed by longitudinal reinforcement directly contributes significantly to the shear capacity.

(ii) The provision of longitudinal reinforcement also indirectly controls the crack widths of concrete section which consequent affects the degree of interlock between aggregates.

Concrete Works

Disadvantages of excessive concrete covers

In reinforced concrete structures cover is normally provided to protect steel reinforcement from corrosion and to provide fire resistance. However, the use of cover more than required is undesirable in the following ways:

(i) The size of crack is controlled by the distance of longitudinal bars to the point of section under consideration. The closer a bar is to this point, the smaller is the crack width. Therefore, closely spaced bars with smaller cover will give narrower cracks than widely spaced bars with larger cover. Consequently, with an increase in concrete cover the crack width will increase.

(ii) The weight of the concrete structure is increased by an increase in concrete cover. This effect is a critical factor in the design of floating ships and platforms where self-weight is an important design criterion.

(iii)For the same depth of concrete section, the increase of concrete cover results in the reduction of the lever arm of internal resisting force.

Effect of concrete placing temperature on early thermal movement

The rate of hydration of cement paste is related to the placing temperature of concrete. The rate of heat production is given by the empirical Rastrup function:

An 12 degree C increase in placing temperature doubles the rate of reaction of hydration. Hence, concrete placed at a higher temperature experiences a higher rise in temperature. For instance, concrete placed at 32 degree C produces heat of hydration twice as fast when compared with concrete placing at 20 degree C. Hence, high concrete placing temperature has significant effect to the problem of early thermal movement.

Effect of rusting on steel reinforcement

The corrosion of steel reinforcement inside a concrete structure is undesirable in the following ways:

(i) The presence of rust impairs the bond strength of deformed reinforcement because corrosion occurs at the raised ribs and fills the gap between ribs, thus evening out the original deformed shape. In essence, the bond between concrete and deformed bars originates from the mechanical lock between the raised ribs and concrete. The reduction of mechanical locks by corrosion results in the decline in bond strength with concrete.

(ii) The presence of corrosion reduces the effective cross sectional area of the steel reinforcement. Hence, the available tensile capacity of steel reinforcement is reduced by a considerable reduction in the cross sectional area.

(iii)The corrosion products occupy about 3 times the original volume of steel from which it is formed. Such drastic increase in volume generates significant bursting forces in the vicinity of steel reinforcement. Consequently, cracks are formed along the steel reinforcement when the tensile strength of concrete is exceeded.

Formation of pedestrian level winds around buildings

When a building blocks the wind blowing across it, part of the wind will escape over the top of the building. Some will pass around the edges of the building while a majority of the wind will get down to the ground. The channeling effect of wind for an escaping path, together with the high wind speeds associated with higher elevations, generates high wind speeds in the region at the base of the building. At the base level of the building, there are three locations of strong pedestrian level winds:

(i) Arcade passages – wind flow is generated by the pressure difference between the front and the back of the building.

(ii) At the front of the building – high wind is produced by standing vortex.

(iii)At the corners of the building – high wind is induced by corner flow.

Concrete Works

Critical steel ratio – only consider 250mm of concrete from outer face

The purpose of critical steel ratio is to control the cracking pattern by having concrete failing in tension first. If steel reinforcement yields first before the limit of concrete tensile strength is reached, then wide and few cracks would be formed. In the calculation of critical steel ratio, the thickness of the whole concrete section is adopted for analysis. However, if the concrete section exceeds 500mm in thickness, only the outer 250mm concrete has to be considered in calculating minimum reinforcement to control thermal and shrinkage cracks. It is because experimental works showed that for concrete section greater than 500mm, the outer 250mm on each face could be regarded as surface zone while the remaining could be regarded as core. The minimum reinforcement to control cracking should therefore be calculated based on a total maximum thickness of 500mm. 

Corrosion protection of lifting anchors in precast concrete units

The corrosion of lifting anchors in precast concrete units has to be prevented because the corroded lifting units cause an increase in steel volume leading to the spalling of nearby surface concrete. Consequently, steel reinforcement of the precast concrete units may be exposed and this in turns results in the corrosion of steel reinforcement and the reduction in the load carrying capacity of the precast units. To combat the potential corrosion problem, the lifting anchors could be covered with a layer of mortar to hide them from the possible external corrosion agents. Alternatively, galvanized or stainless steel lifting anchors can be considered in aggressive environment.

Concrete cover to enhance fire resistance

In the event of exposing the concrete structures to a fire, a temperature gradient is established across the cross section of concrete structures. For shallow covers, the steel reinforcement inside the structures rises in temperature. Generally speaking, steel loses about half of its strength when temperature rises to about 550 degree C. Gradually, the steel loses strength and this leads to considerable deflections and even structural failure in the worst scenario. Hence, adequate cover should be provided for reinforced concrete structure as a means to delay the rise in temperature in steel reinforcement. 

Differences between epoxy grout, cement grout and cement mortar

Epoxy grout consists of epoxy resin, epoxy hardener and sand/aggregates. In fact, there are various types of resin used in construction industry like epoxy, polyester, polyurethane etc. Though epoxy grout appears to imply the presence of cement material by its name, it does not contain any cement at all. On the other hand, epoxy hardener serves to initiate the hardening process of epoxy grout. It is commonly used for repairing hairline cracks and cavities in concrete

structures and can be adopted as primer or bonding agent.

Cement grout is formed by mixing cement powder with water in which the ratio of cement of water is more or less similar to that of concrete . Owing to the relatively high water content, the mixing of cement with water produces a fluid suspension which can be poured under base plates or into holes. Setting and hardening are the important processes which affect the performance of cement grout. Moreover, the presence of excessive voids would also affect the strength, stiffness and permeability of grout. It is versatile in application of filling voids and gaps in structures.

Cement mortar is normally a mixture of cement, water and sand (typical proportion by weight is 1:0.4:3). It is intended that cement mortar is constructed by placing and packing rather than by pouring. They are used as bedding for concrete kerbs in roadwork. They are sometimes placed under base plates where a substantial proportion of load is designed to be transferred by the bedding to other members.

Concrete Works

Bond breaker for joint sealant

Joint sealant should be designed and constructed to allow free extension and compression during the opening and closure of joints. In case joint sealants are attached to the joint filler so that movement is prohibited, they can hardly perform their intended functions to seal the joints against water and debris entry. Polyethylene tape is commonly used as bond breaker tape.

To facilitate free movement, it can be achieved by adding bond breaker tape in-between the joint sealant and joint filler. Primers may be applied to the sides of joints to provide a good bond between them.

Bonding performance to concrete: Epoxy-coated bars vs galvanized bars

The bonding of galvanized bars to concrete is lower in early age owing to hydrogen release when zinc reacts with calcium hydroxide in concrete and the presence of hydrogen tend to reduce the bond strength between galvanized bars and concrete. However, bonding will increase with time until the full bond strength of ungalvanized bars is attained. 

For epoxy-coated bars, there is a 20% decrease in bond strength for bars placed at the bottom of concrete sections while for bars placed on the top there is no major difference in bond compared with uncoated bars.

Coating on concrete – complete impermeability to moisture?

In designing protective coating on concrete structures, stoppage of water ingress through the coating is normally required. Since chloride ions often diffuse into concrete in solution and cause deterioration of concrete structures, the prevention of water transmission into the coating certainly helps to protect the concrete structure. However, if water gets behind the coating from some means and becomes trapped, its effect may not be desirable. Firstly, vapour pressure would be developed behind the surface treatment and this leads to the loss of adhesion and the eventual peeling off of the coating. Moreover, the water creates a suitable environment for mould growth on concrete surface.

In fact, the surface treatment should be so selected that it is impermeable to liquid water but it is permeable to water vapour. This “breathing” function enhances the concrete to lose moisture through evaporation and reject the uptake of water during wet periods.

Crack width limitation (Less than 0.5mm) equal to control reinforcement corrosion?

In many standards and code of practice of many countries, the allowable size of crack width is normally limited to less than 0.5mm for reinforced concrete structure to enhance the durability of concrete. The limitation of crack width can serve the aesthetic reason on one hand and to achieve durability requirement by avoiding possible corrosion of steel reinforcement on the other hand. Regarding the latter objective, site surveys and experimental evidence do not seem to be in favor of the proposition. There was no correlation between surface crack width and durability of r/f concrete structure. In practice, most corrosion problems to the r/f instead of surface cracks perpendicular to the reinforcement.

Chapter 4: Experimental Analysis of Recycled CDW Materials

4.1. INTRODUCTION

The experimental investigations have been done to check the qualities of the recycled materials from the construction and demolition waste. The Chapter contains the experimental results of the recycled materials (Recycled Coarse Aggregates & Recycled Fine Aggregates) and of some products made by these recycled materials (Different Concrete blocks). Also the comparison of the characteristics of the recycled materials and that of standard materials have been included for the suitability of the Recycled materials in Real Environment/Construction Industries.

The following laboratory tests have been done on the different materials containing recycled aggregates: -

a) Compressive Strength test of the eco-concrete

b) Compressive Strength test of the cement mortar cubes

c) Compressive Strength test of the Concrete Bricks

d) Laboratory Tests on the Recycled Coarse Aggregates

    i. Specific Gravity Test and Water Absorption Test

    ii. Aggregate Impact Test

    iii. Bulk Density, Fineness Modulus Test (FM)

On the basis of the results obtained from laboratory test. It has been decided that where it can be used in construction work. Use of the all the recycled materials have been explained in the next chapter.

4.2. Compressive Strength Testing of Eco-Concrete

Out of many test applied to the concrete, this is the utmost important which gives an idea about all the characteristics of concrete. By this single test one judge that whether Concreting has been done properly or not. The concrete cube size used under this project was 15cm x 15cm x 15cm as we have used recycled coarse aggregates of the size 10mm-20mm. These specimens are tested by compression testing machine after 7 days curing or 28 days curing. Load should be applied gradually at the rate of 140 kg/cm2 per minute till the Specimens fails. Load at the failure divided by area of specimen gives the compressive strength of concrete. (As per IS: 516)

APPARATUS: - Compression testing machine, cube mould of size 15cmx15cm x15cm, temping rod, vibrator, others.

PREPARATION OF CUBE SPECIMENS: - The proportion and material for making these test specimens have been done as per code IS: 516.

SPECIMEN: - 6 cubes of 15 cm size Mix. M25 

HAND MIXING 

a) Mix the cement and fine aggregate on a water tight none-absorbent platform until the mixture is thoroughly blended and is of uniform color 

b) Add the coarse aggregate and mix with cement and fine aggregate until the coarse aggregate is uniformly distributed throughout the batch 

c) Add water and mix it until the concrete appears to be homogeneous and of the desired consistency. 

SAMPLING 

a) Clean the mounds and apply oil 

b) Fill the concrete in the molds in layers approximately 5cm thick 

c) Compact each layer with not less than 35strokes per layer using a tamping rod (steel bar 16mm diameter and 60cm long, bullet pointed at lower end.

d) Level the top surface and smoothen it with a trowel. CURING The test specimens are stored in moist air for 24hours and after this period the specimens are marked and removed from the molds and kept submerged in clear fresh water until taken out prior to test. 

PROCEDURE 

a) Remove the specimen from water after specified curing time and wipe out excess water from the surface. 

b) Take the dimension of the specimen to the nearest 0.2m 

c) Clean the bearing surface of the testing machine 

d) Place the specimen in the machine in such a manner that the load shall be applied to the opposite sides of the cube cast. 

e) Align the specimen centrally on the base plate of the machine.

f) Rotate the movable portion gently by hand so that it touches the top surface of the specimen. 

g) Apply the load gradually without shock and continuously at the rate of 140kg/cm2/minute till the specimen fails 

h) Record the maximum load and note any unusual features in the type of failure.

CALCULATIONS 

Size of the cube =15cm x15cm x 15cm 

Weight of the Specimen (Kg) = W Kg 

Area of the specimen = 225cm2 

Compressive strength (fck) at 7 days or 28 days = Load/Cross Section Area of cube (N/mm2) 

RESULT 

Average compressive strength of the concrete cube =13.775 N/ mm2 (at 7 days) 

Average compressive strength of the concrete cube = 22.665 N/mm2 (at 28 days)

Percentage strength of concrete at various ages:

The strength of concrete increases with age. Table shows the strength of concrete at different ages in comparison with the strength at 28 days after casting.

Compressive strength of different grades of concrete at 7 and 28 days:

CONCLUSION OF THE TEST

As per the result obtained, we can conclude that the recycled coarse aggregates can be used widely in the construction industry for the construction works including both Plain cement concrete work and reinforced cement concrete works as well. The another benefit is that the Recycled fine aggregates (less than 1.18mm size) can be used in place of the natural sand in concrete for the plain cement concrete along with recycled coarse aggregates.

(This chapter is continue...)

Fineness of Cement by Dry Sieving (Very Simple Procedure)

To find fineness of cement by dry sieving method. In this method the cement sample is sieved dry on IS sieve no. 9. The residue left is expressed as percent of weight of sample, and in an indirect measure of fineness of cement.

Apparatus:

1. 90 micron IS sieve

2. Weighing balance (2kg) accurate to 0.1g, with weights

3. Bristle brush 25mm size.

Material: Cement sample weighing about 100g.

Procedure:

1. Weigh cement sample accurately and record weight W1. Place the sample on IS sieve no.9 (It is preferable to take W1= 100g for simplifying calculations).

2. Break air set humps in the sample with fingers.

3. Hold the sieve in both hands and sieve with a gentle wrist motion without spilling the cement and keeping cement well spread on the screen. Carry out continuous circular motion of the sieve for a period of 15 minutes.

4. Collect the residue left on the sieve, using brush if necessary, and weigh and residue. Let the weight of residue be W2.

Observations and Calculations:

1. Weight of the cement sample = W1 gms.

2. Weight of residue = W2 gms.

    Hence,  Percentage residue = P = (W2/W1) x 100 %

Results: Percent of residue of cement sample by dry sieving = P %. It is less than/ more than 10%

Requirement: As per IS 269-1976, the residue by weight on 90 micron IS sieve by dry sieving should not exceed 10% by weight in case of Ordinary Portland Cement. Coarser material more than 10% signifies less fine cement and vice versa.

Conclusion:

The given sample of cement contains less than/ more than 10% by weight of materials coarser than 90 micron sieve. Therefore it satisfies/ does not satisfy the criteria as specified by IS.

Road Works

Skid resistance of wearing course

The skid resistance of wearing course in a bituminous pavement is contributed by the macrotexture (i.e. the general surface roughness) and the microtexture (i.e. the protruding from chippings) of the wearing course. These two factors affect the skid resistance of flexible carriage in different situations. For instance, when the carriageway is designed as a high-speed road, the tiny channels among the macrotexture help to drain rainwater to the side of the road and avoid the occurrence of aquaplaning. In low speed roads the microtexture has particular significance in providing skid resistance by gripping the car tyres to the road surface.

Tack coat – emulsified asphalts vs cutback asphalts

Emulsified asphalt is a suspension of asphalt in water by using an emulsifying agent which imposes an electric charge on asphalt particles so that they will join and cement together. Cutback asphalt is simply asphalt dissolved in petroleum. The purpose of adding emulsifying agent in water or petroleum is to reduce viscosity of asphalt in low temperatures.

The colour of emulsion for tack coat is brown initially during the time of application. Later, the colour is changed to black when the asphalt starts to stick to the surrounding and it is described as “break”. For emulsified asphalts, when water has all evaporated, the emulsion is said to have “set”. Cutback emulsion is described to have been “cured” when the solvent has evaporated. There are several problems associated with cutback asphalts:

(i) Emulsified asphalt can be diluted with water so that a low application rate could be achieved.

(ii) The evaporation of petroleum into atmosphere for cutback asphalt poses environmental problem.

(iii)The cost of production of petroleum is higher than that of emulsifying agent and water.

Unsealed contraction joints in concrete pavement

For unreinforced concrete pavement, the contraction joint is an approximately 3mm wide groove with a depth of about one-third to one-fourth of slab thickness and a regular spacing of normally 5m. The grooves are designed such that they are too narrow for stones to fall into when the cracks are open due to the contraction of concrete. The groove location is l a plane of weakness and the groove acts as a potential crack-inducing device where any potential cracks due to shrinkage and thermal contraction may form will be confined to the base of the groove. It will not cause any unpleasant visual appearance on the exposed surface of unreinforced concrete pavement. The above-mentioned contraction joints can be designed as unsealed.

These grooves are very narrow so that stones can hardly get into these grooves even when the joint undergoes contraction. The fine particles or grit entering into the groove are likely to be sucked out by the passing vehicles. The joints can be self-cleansing and it may not be necessary to seal the joints for fear of attracting the accumulation of rubbish and dirt.

Road Works

“Pumping” at joints in concrete carriageway

Pumping at joints in concrete carriageway occurs in the presence of the following factors:

(i) Fine-grained subgrade;

(ii) Seepage of water into subgrade due to improper or inadequate drainage design;

(iii)The presence of heavy vehicular loads.

It involves the pumping out of water-borne particles of the subgradeowing to the deflections at the end of concrete slab. The first mechanism of pumping involves the softening of subgrade by water and the reduction in bearing capacity. It causes a larger instantaneous deflection at the slab ends under heavy traffic loads. During deflection, water containing fine soil particles is pumped out at the joints. Consequently, voids are formed in subgrade region and the void size grows by repeating the above sequence.

Purpose of using capping layers in pavement construction

When the California Bearing Ratio of subgrade is checked to be below a certain percentage (e.g. 5%), a capping layer is normally provided to reduce the effect of weak subgrade on the structural performance of the road. It also provides a working platform for sub-base to be constructed on top in wet weather condition because the compaction of wet subgrade is difficult on site. The effect of interruption by wet weather can be reduced significantly and the progress of construction works would not be hindered. Most importantly, the cost of capping layers is low because the material can be readily obtained locally.

Purpose of tar in bituminous materials

Tar is commonly incorporated in bituminous materials because of the following reasons:

(i) Blending of tar with bitumen possesses better binding performance with roadstone than bitumen.

(ii) Resistance to fuel oil erosion is high. Tar is used in roads where there is frequent spillage of fuel from vehicles.

Roadbase vs basecourse in flexible carriageway

Roadbase is the most important structural layer in bituminous pavement. It is designed to take up the function of distributing the traffic loads so as not to exceed the bearing capacity of subgrade. In addition, it helps to provide sufficient resistance to fatigue under cyclic loads and to offer a higher stiffness for the pavement structure. However, the basecourse is normally provided to give a well-prepared and even surface for the laying on wearing course. Regarding the load distribution function, it also helps to spread traffic loads to roadbase but this is not the major function of basecourse.

Sand layer vs cement sand used as bedding of precast concrete paving units

Cement sand is a mixture of cement and sand and it acts as a cohesive mass once mixed. Normally, a 20mm to 30mm sand layer is laid underneath precast paving block units. However, in locations of steep gradients where it stands a high possibility that rain runoff will wash out infilling sand and sand layers, cement sand should be sued instead. Similarly, when high pressure jetting is anticipated to be employed frequently in routine maintenance, sand layers beneath precast paving block units is not preferable owing to the reason of potential washing out of sand.

Sub-base for concrete carriageway – non-strength provider

Basically, sub-base for a concrete carriageway is provided for the following reasons:

(i) It provides a smooth and even surface between the subgrade and concrete slab. This avoids the problem of uneven frictional stresses arising from the uneven interface under thermal and shrinkage movement. It also improves the uniformity of support provided to concrete slab to enhance even distribution of wheel load to the subgrade.

(ii) For heavily trafficked carriageways with frequent occurrence of a high water table, it serves to prevent the occurrence of mud pumping on clayey and silty subgrade. The loss of these clayey soils through carriageway joints such as contraction and expansion joints will cause structural failure of concrete slab under heavy traffic load.

The stiffness of concrete slab accounts for the strength of rigid road structure. It is normally uneconomical to employ sub-base as part of the strength provider because a much thicker layer of sub-base has to be adopted to reduce the thickness of concrete slab by a small amount.

Hence, it is more cost-effective to increase the depth of concrete slab rather than to enhance foundation strength in order to achieve a higher load-carrying capacity of the concrete pavement.

Road Works

“Mortar mechanism” vs “stone contact mechanism” in bituminous materials

“Stone contact mechanism” applies to well graded aggregates coated with bitumen (e.g. dense bitumen macadam) where the traffic loads on bituminous roads are resisted by stone-to-stone contact and by interlocking and frictional forces between the aggregates. It is essential to adopt aggregates with a high crushing strength. The bitumen coatings on the surface of aggregates merely serve to cement the aggregates together.

“Mortar mechanism” involves the distribution of loads within the mortar for gap-graded aggregates (e.g. hot rolled asphalt). The mortar has to possess high stiffness to prevent excessive deformation under severe traffic loads. It is common practice to introduce some filler to stiffen the bitumen.

Noise absorptive materials – how it works

The basic mechanism of noise absorptive material is to change the acoustic energy into heat energy. The amount of heat generated is normally very small due to the limited energy in sound waves (e.g. less than 0.01watts). The two common ways for energy transformation are: 

(i) Viscous flow loss

The absorptive material contains interconnected voids and pores into which the sound energy will propagate. As sound waves pass through the material, the wave energy causes relative motion between the air particles and the absorbing material and consequently energy losses are incurred.

(ii) Internal fiction

The absorptive materials have some elastic fibrous or porous structures which would be extended and compressed during sound wave propagation. Other than energy loss due to viscous flow loss, dissipation of energy also results from the internal friction during its flex and squeezing movement.

Necessity of air voids in bituminous pavement

If the presence of air voids is too high, it leads to an increase of permeability of bituminous pavement. This allows the frequent circulation of air and water within the pavement structure and results in premature hardening and weathering of asphalt. Therefore, too high an air void content poses detrimental effect to the durability of the bituminous pavement.

If the presence of air voids is too low, flushing, bleeding and loss of stability may result under the effect of prolonged traffic loads because of the rearrangement of particles by compaction. Aggregates may become degraded by traffic loads leading to instability and flushing for such a low air void content. The air void space can be increased by adding more course or fine aggregates to the asphalt mix. Alternatively, if asphalt content is above normal level, it can be reduced to increase air voids.

Oil interceptors

Grease and oils are commonly found in stormwater runoff from catchments. They come from the leakage and spillage of lubricants, fuels, vehicle coolants etc. Since oils and grease are hydrocarbons which are lighter than water, they form films and emulsions on water and generate odorous smell. In particular, these hydrocarbons tend to stick to the particulates in water and settle with them. Hence, they should be trapped prior to discharging into stormwater system. Oil interceptors are installed to trap these oil loads coming from stormwater. In commercial areas, car parks and areas where construction works are likely. It is recommended to establish oil-trapping systems in these locations. Typical oil interceptors usually contain three compartments:

(i) The first inlet compartment serves mainly for the settlement of grits and for the trapping of floatable debris and rubbish. 

(ii) The second middle compartment is used for separating oils from runoff.

Optimum binder content in bituminous pavement

The amount of binder to be added to a bituminous mixture cannot be too excessive or too little. The principle of designing the optimum amount of binder content is to include sufficient amount of binder so that the aggregates are fully coated with bitumen and the voids within the bituminous material are sealed up. As such, the durability of the bituminous pavement can be enhanced by the impermeability achieved. Moreover, a minimum amount of binder is essential to prevent the aggregates from being pulled out by the abrasive actions of moving

vehicles on the carriageway.

However, the binder content cannot be too high because it would result in the instability of the bituminous pavement. In essence, the resistance to deformation of bituminous pavement under traffic load is reduced by the inclusion of excessive binder content.

Purpose of reinforcement in concrete roads

The main purposes of reinforcement in concrete roads are:

(i) to control the development and pattern of cracks in concrete pavement.

(ii) to reduce the spacing of joints. In general, joints and reinforcement in concrete structures are common design measures to cater for thermal and shrinkage movement. 

Hence, the inclusion of reinforcement allows the formation of tiny cracks in concrete pavement and this allows wider spacing of joints.

In fact, the amount of reinforcement in concrete slab is not substantial and its contribution to the structural strength of roads is not significant.

Road Works

High temperature in laying bituminous pavement

In general, bituminous materials are also broadly classified into two types, namely bitumen macadams and hot-rolled asphalts. During compaction, the increase of temperature causes the reduction of viscosity of binder. The binder acts as a lubricant among aggregates particles because it is mobile in a fluid state under high temperatures. The internal resistance between the bituminous materials is drastically reduced resulting in the formation of a mixture with better aggregate interlock. Bitumen macadams mainly contain continuously graded aggregates. Compaction of this type of bituminous material is eased with an increase of mix temperature as the lubricating effect of reduced viscosity of binder helps in the rearrangement of aggregates. The aggregate of hot-rolled asphalt are not well graded. With a rise in mixing temperature, the binder will stay unset and the mixture has little resistance to compaction.

Local vehicle parapet strong enough to contain vehicles

The majority of local parapets are 1.1m high and they are designed to resist impact from a 1.5ton car moving at a speed of 113km/hr. In some locations such as in the vicinity of railway lines, barriers with 1.5m high are provided to contain a vehicle with 24ton at a speed of 50km/hr. The impact situation for vehicles varies from event to event and they are dependent on the speed, size and angle of incidence of the impacting vehicle. Though full-scale crash test is the simplest way to prove their performance, computer simulation has been used extensively owing to its lower in cost. Based on the results of computer simulation and crash tests, it is established that the said parapets comply with international standard for safe usage. Mechanism of compaction by paver, steel-wheeled roller and pneumatic tire rollers Paver, steel-wheeled roller and pneumatic tire roller compact bituminous material by using the following principles:

(i) The static weight of the paving machines exerts loads on the bituminous material and compresses the material directly beneath the machine. The compacting effort increases with the period of contact and larger machine weight.

(ii) Compaction is brought about by the generation of shear stress between the compressed bituminous material under the machine and the adjacent uncompressed bitumen. 

Mechanism of taking up loads for concrete paving blocks 

The paving for concrete blocks consists of closely packed paving blocks in pre-determined patterns and the tiny joint spaces between individual blocks are filled with sand. The presence of sand avoids the displacement of a single block unit from the remaining blocks. Moreover, the horizontal interlocking provided by the arrangement of paving blocks in special patterns (e.g. herringbone pattern) prevents any single block from moving relative to one another. For instance, vertical loads acting directly on one concrete paving block are not only resisted by the block itself, but also by the blocks adjacent to it.

Fineness of Cement

Experiment: To determine the fineness of cement.

Purpose: The purpose of this test is to find out the quantity of coarse material present in cement. It is an indirect test of fineness of cement.

What is fineness?

Cement is in the form of powder, which is obtained by grinding the various raw materials after calcimining. The grinding produces finer particles of cement. The degree to which the cement is ground to smaller and  smaller particles is called fineness of cement

Effect of fineness on properties if cement: 

During use of cement in structure water is mixed with cement. A chemical reaction takes place between water and cement, and it is called hydration. The strength of cement concrete or mortar develops with hydration. More the rate of hydration faster is the development of strength. This is because finer cement offers greater surface area of particles for hydration. At the same time the rate of development of heat due to hydration also increases.

Advantage of using finer cement:

The cement develops strength earlier and so formwork can be removed earlier thus reducing the cost of construction.

Disadvantage of using finer cement:

The finely ground cement is likely to deteriorate earlier due to setting because of moisture in air. Also the drying shrinkage is higher in case of finer cements.  
Methods of finding fineness of cement:
Fineness of cement is found by two methods
A) By method of dry sieving
B) By specific surface method- finding fineness of cement using Blanine's air permeability apparatus.
Though method B is more accurate it is rarely used for specific purpose.Method A is quite good for field work.

DETERMINATION OF FINENESS BY DRY SIEVING (IS 4031 Part 1 : 1996)

SIEVING METHOD

Principle: The fineness of cement is measured by sieving it on standard sieve. The proportion of cement of which the grain sizes are larger than the specified mesh size is thus determined. A reference sample having a known proportion of

material coarser than the specified mesh size is used for checking the specified sieve.

Apparatus:

a) Test Sieve: It comprises a firm, durable, non-corrodible, cylindrical frame of 150 mm to 200 mm nominal diameter and 40 mm to 100 mm depth, fitted with 90 pm mesh sieve cloth of woven stainless steel, or other abrasion-resisting and non-corrodible metal wire. The sieve cloth shall comply with the requirements of IS 460 ( Part 1) : 1985 and IS 460 ( Part 3 ) : 1985

and shall be free of visible irregularities in mesh size when inspected optically by the methods of IS 460 ( Part 3 ) : 1985. A tray fitting beneath the sieve frame and a lid fitting above it shall be provided to avoid loss of material during sieving.

b) Balance: Capable of weighing up to 10 g to the nearest 10 mg.

c) Brush: A nylone or pure bristle brush, preferably with 25 to 40 mm bristle, for cleaning the sieve.

Material for Checking the Sieve:

A Standard reference material of known sieve residue shall be used for checking the sieve. The material shall be stored in sealed, airtight containers to avoid changes in its characteristics due to absorption or deposition from the atmosphere. The containers shall be marked with the sieve residue of the reference material.

Procedure: 

a) Determination of the Cement Residue

Agitate the sample of cement to be tested by shaking for 2 min in a stoppered jar to disperse agglomerates. Wait 2 min. Stir the resulting powder gently using a clean dry rod in order to distribute the fines throughout the cement.

Fit the tray under the sieve, weigh approximately 10 g of cement to the nearest 0.01 g and place it on the sieve, being careful to avoid loss. Disperse any agglomerates. Fit the lid over the sieve. Agitate the sieve by swirling, planetary and linear movement until no more fine material passes through it. Remove and weigh the residue. Express its mass as a percentage, R1, of the quantity first placed in the sieve to the nearest 0.1 percent. Gently brush all the fine material off the base of the sieve into the tray. 

Repeat the whole procedure using a fresh 10 g sample to obtain 5. Then calculate the residue of the cement R2 as the mean of R1 and R2 as a percentage, expressed to the nearest 0.1 percent. When the results differ by more than 1 percent absolute, carry out a third sieving and calculate the

mean of the three values. The sieving process is carried out manually by a skilled and experienced operator.

NOTE - Alternatively a sieving machine may be used provided that it can be shown to give the same results as the manual operation.

b) Checking the Sieve

Agitate the sample of cement to be tested by shaking for 2 min in a stoppered jar to disperse agglomerates. Wait 2 min. Stir the resulting powder gently using a clean dry rod in order to distribute the fines throughout the cement.

Fit the tray under the sieve. Weigh approximately 10g of the reference material to the nearest 0.01 g and place it in the sieve, being careful to avoid loss. Carry out the sieving procedure as above including the repeat determination of residue to yield two values p1 and P2 expressed to the nearest 0.1 percent. The two values of P1 and P2 for a satisfactory sieve should differ by not more than 0.3 percent. Their mean P characterizes the state of the sieve.

Given the known residue on the 90 m mesh of the reference material, R0 calculate R0/P as the sieve factor, F, expressed to the nearest 0.0 1. The residue, R determined as in 1 step shall be corrected by multiplying by F, which may have a value of 1.00+-0.20. 

EXPRESSION OF RESULTS

Report the value of R, to the nearest 0. I percent, as the residue on the 90 pm sieve for the cement tested. The standard deviation of the repeatability is about 0.2 percent and of the reproducibility is about 0.3 percent.

Chapter 3: Recycling Processes & Techniques (as per my project)

3.5. RECYCLING OF CONSTRUCTION WASTE

i. STEEL

The Steel pieces during the construction of buildings and other structure are left due to cutting of the steel bars and also by some other process. The salvage value of metals partially offsets the added labor costs for processing materials for recycling. Mixed salvaged steel can be sold at good price to salvage companies. Unprepared ferrous metal exceeding ¼ inch thickness has substantially more salvage value than light gauge scrap metal.

ii. WOOD

Today’s construction practices include the increased use of engineered wood products with high adhesive content, such as plywood, oriented strand board and laminated lumber, including glue-laminated beams and wood I-beams. Wood waste processors may have concerns with this high adhesive content, and should be consulted during preparation of a waste management plan. Engineered wood products may account for up to 50% of wood C&D wastes on a new construction project. This high adhesive content has been a major challenge in recycling, as it is not suitable for mulches and some other products. Engineered wood is often ground up and used as a daily landfill cover product, which is a waste. “Clean” wood wastes from new construction are usually relatively uncontaminated and can be more easily used as feedstock for engineered lumber, than from the demolition process. Other uses for wood waste are pallet production, landscape mulch, wood pellets, animal bedding & compost mediums.

iii. GLASS

During the construction process, the glass wastes are also generated. The recycling process is same as explained in the demolition waste part.

iv. PACKAGING

Packaging materials are plentiful during construction and protect materials, components and finishes during transportation and storage from dust and moisture. If segregation of packaging materials is possible, recyclers are usually easy to find that will accept these materials, often with reduced or even eliminated tipping fees. Care must be taken to avoid contamination of materials.

v. CARDBOARD

Corrugated cardboard has a well-developed end market in most communities. Boxes, packaging and protective covers can easily be flattened. Most recyclers will offer reduced tipping fees for clean cardboard. Some may even accept clean cardboard at no cost.

vi. PLASTIC

While some mixed plastics may not be easily recycled, plastic packaging is now recycled into various useful materials and products including plastic lumber, composite lumber (plastic mixed with wood), injection molded materials, construction materials and home-use items.

(Go to Chapter 4)

Chapter 2: Construction and Demolition Wastes

2.1. INTRODUCTION

     The Construction and demolition wastes are the wastes generated whenever any construction/demolition activity takes place, such as, building roads, bridges, fly over, subway, remodeling etc. It consists mostly of inert and non-biodegradable material such as concrete, plaster, metal, wood, plastics etc. A part of this waste comes to the municipal stream. These wastes are heavy, having high density, often bulky and occupy considerable storage space either on the road or communal waste bin/container. It is not uncommon to see huge piles of such waste, which is heavy as well, stacked on roads especially in large projects, resulting in traffic congestion and disruption. Waste from small generators like individual house construction or demolition, find its way into the nearby municipal bin/vat/waste storage depots, making the municipal waste heavy and degrading its quality for further treatment like composting or energy recovery. Often it finds its way into surface drains, choking them. It constitutes about 10-20 % of the municipal solid waste (excluding large construction projects). Construction and demolition wastes are one of the largest waste streams in the country, it must be recycled on the large scale so as to reduce the amount of the C&D waste.

2.2. WASTE CHARACTERIZATION

   There are two components to the characterization of construction and demolition waste: (1) composition and (2) quantity. The composition of the waste is defined by the type of included constituent components. The quantity of C&D waste is based either on the volume or weight of the debris depending on the requirement of the hauler/processor. One objective of this report is to add two additional components within this description: recyclability and salvageability. The recyclability of a material is defined as its potential for reuse after some form of substantial processing. Processing may take the form of reduction of the debris to its constituent materials.

The salvageability of materials is defined as the potential to reuse them in their current state.

                                                                

(This Chapter is continue.....)

Road Works

Good surface regularity for sub-base in concrete pavement

The surfaces of sub-base material for concrete carriageway should be constructed in a regular manner because of the following reasons :

(i) One of the main functions of sub-base in concrete pavement is to provide a smooth and even interface between concrete slab and subgrade so that a uniform support is established. A regular surface of sub-base assists in reducing the frictional and interlocking forces between concrete slab and sub-base and allowing easier temperature and shrinkage movement.

(ii) A uniform sub-base surface is essential in the construction of concrete slab of uniform thickness adopted in design. It saves the higher cost of concrete to make up the required level.

High-yield steel vs mild steel as road reinforcement

High yield steel is the preferred material for the reinforcement of concrete carriageway because of the following reasons :

(i) The principal function of steel reinforcement in concrete pavement is to control cracking. If mild steel is adopted for reinforcement, upon initiation of crack formation mild steel becomes overstressed and is prone to yielding. High yield steel offers resistance to crack growth. The above situation is commonly encountered where there is abnormal traffic loads on concrete carriageway exceeding the design limit.

(ii) High-yield steel is less prone to deformation and bending during routine handling operation.

(iii)In the current market, steel mesh reinforcement is normally of high-yield steel type and the use of mild steel as road reinforcement requires the placing of special orders to the suppliers.

Function of prime coat in bituminous pavement

The principal function of prime coat in bituminous pavement is to protect the subgrade from moisture and weathering. Since the presence of moisture affects the strength of subgrade, the prevention of water entry during construction is essential to avoid the failure of the pavement. In cold countries, by getting rid of moisture from subgrade, the danger of frost heave can be minimized.

Prime coat is an asphalt which, when applied evenly to the surface of sub-base or subgrade, serves to seal the surface to hinder the penetration of moisture into subgrade. Vehicular traffic should be avoided on the surface sprayed with prime coat because the traction and tearing action of vehicles would damage this asphalt layer. 

Road Works

Concrete crash barriers – its application

Concrete crash barriers are not considered as the best barrier design because of the following reasons when compared with flexible barriers:

(i) Concrete barriers possess rough surface which, when impacted by moving vehicles, tend to cause considerable damage to the vehicles.

(ii) Since concrete is a rigid material and the deceleration of collided vehicles is comparatively large when compared with flexible barriers.

However, concrete crash barriers have particular application in locations where the deflection of barriers is not allowed. For instance, in the central divider of a carriageway, if flexible barriers are adopted and vehicles crush into the barriers, the deformation resulting from the hitting of vehicles would result in an intrusion to the adjacent carriageway. This is undesirable because this may trigger further collisions in the adjacent carriageway and hence rigid barriers like concrete crash barriers should be adopted in this scenario.

Direction of placing the main weight of reinforcement in concrete pavement

The reinforcement of concrete pavement is usually in the form of long mesh type. A road usually has length is generally much longer than its width and therefore cracking in the transverse direction has to be catered for in design. Reinforcement is required in the longitudinal direction to limit transverse cracking while transverse steel acts to provide rigidity to support the mesh fabrics. For long mesh in concrete slab, the main weight of reinforcement should be placed in the critical direction (i.e. longitudinal direction) to control cracking. However, if the concrete road is quite wide, certain reinforcement has to be placed in the transverse direction in this case to control longitudinal cracking.

Function of waterproof (or separation) membrane for concrete carriageway

A layer of waterproof (or separation) membrane is normally placed between sub-base and concrete slab for the following reasons:

(i) It prevents the loss of water from cement paste which affects the strength of concrete slab.

(ii) It enhances the movement of concrete slab relative to sub-base layer and reduces the frictional forces developed at their interface.

(iii)It avoids the possibility of active aggressive agents from soil water being attached to the concrete slab.

(iv) It prevents the intermixing of freshly placed concrete with loose materials on the surface of sub-base.

Chapter 3: Recycling Processes & Techniques (as per my project)

3.1. INTRODUCTION

    The recycling of the construction and demolition wastes is of the main concern. The chapter includes the process and techniques which have been used in this project. First of all, we have planned and scheduled all the activities of the project. The suitable site of the construction and demolition waste generation source has been selected. The source selected was very much suitable as some parts (upper floors) of the building were under construction and some other parts of the building were being demolished because of some reasons. The source was perfect as it availed both the construction waste as well as demolition waste for our project. The working site i.e., Civil Labs are also in basement of that building. Hence the hauling/transportation of the waste materials was easy as distance between source and civil labs is short. The compositions of the construction and demolition waste have been identified. It contains non-biodegradable inert materials.

Recycling process includes the process of the separation, sorting, collection, storage, transportation and then re-use & conversion of these waste into the valuable fresh materials which can be used again. The conversion of the waste materials into valuable fresh materials can be termed as “Recycling Techniques”.

3.2. ONSITE SEPARATION/SORTING

     Onsite separation means the separation of the waste materials at the source of the generation of the construction and demolition wastes. The categorized or classified storage of the different components (concrete, steel, brick, etc.) of the construction and demolition wastes can be termed a sorting. In this project the on-site separation and sorting have been done manually as per requirement of the project work. For the large projects, onsite separation and sorting must be done carefully to clear the site properly. The bigger size particles/materials must be removed first separately. The heaps of the sorted materials be made for the purpose of the collection of the materials and for further processes.

3.3. COLLECTION, TRANSPORTAION & STORAGE

After the onsite separation/sorting of the waste materials, these wastes are collected separately in containers as per availability and amount of the waste materials at the site. The collected waste materials in the containers are then transfer from the source site of the construction and demolition wastes to the recycling plants. Then these are stored separately at storage points at the recycling plants. The stored construction and demolition wastes are then sent for the further process re-use and the waste materials which cannot be reused are sent for the recycling process. In this projects, the concrete wastes of the particles size ranging from 100mm to 10mm are collected in the plastic bags and then transported manually to the civil concrete testing laboratory for the further crushing and other recycling process. The steel bars, bricks, wood, plastics, metals and other waste materials are collected for further analysis.

3.4. RECYCLING OF DEMOLITION WASTE

     The Recycling process and Techniques of the Demolition waste components are explained below: -

i. CONCRETE: - The demolished concrete waste contains the concrete as well as some brick pieces, hydrated cement mortar, plastering parts. During collection of the concrete waste, it cannot be possible to remove all the brick pieces and other dissimilar materials from the concrete waste. Also for the Reinforced Cement Concrete waste, the extraction of the steel bars must be done on-site for the collection of pure concrete waste. If concrete contains the steel bars in it, in that case the crushing must not be done as it cause losses of the machinery part. The drilling Machines are generally used for this purposes.

The demolished concrete wastes after collection at working site have been crushed and sieve analysis has been done for the grading of the aggregates with IS Sieves: 40mm, 20mm, 10mm, 4.75mm, 2.36mm, 1.18mm. The aggregates made by crushing and sieving were collected separately for the further testing and their uses. The experimental analysis has been explained and given in the next chapter. On the basis of the experimental analysis, the recycled aggregates can be used in plain cement concrete works, for making pre-cast units of the building, decorative concrete blocks, paver units, brick blocks, road base, etc.

In this project, the Crushing of the Concrete waste has been done manually with the help of heavy hammers. Also the on-site sieving and grading of the smaller pieces of the concrete waste have been done at the source. The aggregates obtained from the crushing and sieving of the concrete waste have been categorized in two parts: -

a) Recycled Coarse Aggregates (RCA): - The recycled coarse aggregates are considered the aggregates that retained on 4.75mm IS Sieve. The aggregates of the size between 4.75mm-10mm and 10mm-20mm have been used as coarse aggregates for the different test for its suitability. All the experimental analysis have been presented and their suitability has also been explained in chapter 4.

b) Recycled Fine Aggregates (RFA): - The recycled fine aggregates are considered the aggregates that passing the 4.75mm IS Sieve. The aggregates of the size less than 1.18mm, 1.18mm-2.36mm, 2.36mm-4.75mm all have been used as fine aggregates for the different test for its suitability. All the experimental analysis have been presented and their suitability has also been explained in chapter 4.

Cement mortar can be crushed along with the concrete waste and can be used as fine aggregates. These are easily crushed and after sieving, it can be graded in different range. Generally the crushed cement mortar falls under recycled fine aggregates of the size less than 1.18mm and in the range 1.18mm to 2.36mm.

ii. STEEL

  The Steel from the demolition of the buildings/structures is very recyclable due to its lack of contamination by dissimilar materials. Small pieces of steel from the construction and demolition wastes can be easily collected manually or by magnetic process of collection of steel; but the difficulties come when the extraction of the steel be done from the Reinforced Cement Concrete (R.C.C.) part of the demolition waste. The extraction of the steel from RCC units can be done by using heavy machinery like Drilling Machine, Hydraulic Pincers, Hydraulic Breaker, etc. More than 90% of the Steel from demolition can recycled by melting it and again use it for the purpose of the manufacturing of the different grades of the steel. The steel obtained by the demolition of the buildings can be used as raw material for the manufacturing of the steel bars. Good markets exist for ferrous metals such as iron and steel, as well as other non-ferrous metals such as copper, brass and aluminum. When separated, heavy gauge (1/4” or thicker) steel may have a much higher salvage value (2-3 times more) than light gauge, cold rolled or mixed steel.

iii. BRICK

   The brick waste can be directly used. It does not need any recycling technique. The brick has salvage value for CDW recyclers. The process of cleaning mortar from brick, however, can be labor intensive, removing much of the profit from this process. Brick remains, however, a very recyclable C&D material that recyclers will often accept at no cost. Non-salvageable brick can be crushed and used as aggregate base or backfill material or in foundation of flooring.

iv. WOOD

   Wood from the demolition process sometimes requires more labor-intensive disassembly of materials to remove fasteners and finishes. Structural wood (6x or larger) often has salvage value if deconstruction is practiced prior to demolition by salvage companies. Recycled wood can be ground into wood chips or wood flour and used to make composite or engineered lumber products, mulch, animal bedding, compost or many other products. Unseparated waste wood is sometimes burned to produce electricity.

v. GLASS

  Glazing can sometimes be salvaged, but typically is restricted by sizes and thermal properties. Tempered or laminated glass cannot easily be cut to size, and the thermal properties of new glazing today can be far superior to yesteryear’s glazing. Construction glass is usually separated from other glass such as drinking glass. Float glass is highest in quality, followed by rolled plate glass, container glass and fiberglass. The higher the quality of glass is, the higher the demand. Glass can be recycled back into flat glass or into other glass products including container glass, fiberglass, floor tile, or even as aggregate for concrete. Glass recycling can be a very complex industry, and is affected by contaminants, laminated or fire resistant glass, and tinted or colored glass.

vi. METAL

a) FERROUS METAL:  It is by far the most profitable and recyclable material. In India more than 80% scrap arising is recycled. Scrap steel is almost totally recycled and allowed repeated recycling. 100% steel can be recycled to avoid wastage at construction site.

b) NON FERROUS METAL: The main non-ferrous metal collected from construction and demolition sites q are aluminum, copper, lead and zinc. In India aluminum usage is up to 95000 tonnes q with about 80% recycled. Copper is recycled up to 119000 tonnes out of a national market of 262000 tonnes.

vii. PLASTIC

   The demolitions of the buildings also generate plastic, poly vinyl pipes, other plastic waste. These waste can be collected and sold to plastic recyclers for the recycling and we can get profit here too. While some mixed plastics may not be easily recycled, plastic packaging is now recycled into various useful materials and products including plastic lumber, composite lumber (plastic mixed with wood), injection molded materials, construction materials and home-use items.

viii. ROOFING

    ASPHALT COMPOSITION: - Many tonnes of asphalt shingle C&D waste is generated by the construction industry each year. Tear-offs from reroofing accounts for 91 percent, and manufacturing waste accounts for 9 percent. Oftentimes, builders install roofing over old roofing that is already 20 years old. Consequently, when builders tear off old roofs, it is not uncommon to find some roofing below that is ±40 years old. This material is composed of asphalt (19-36%), mineral filler (8-40%), mineral granules (20-38%) and felt backing (2-15%). Roofing debris is ground to a specific size for the product being made, and contaminants such as nails and wood are removed. Such products include aggregate base, asphalt pavement and pavement cold patch.

METAL ROOFING: - Most types of metal roofing are not only highly recyclable, but have salvage value as well like steel.

TILE ROOFING: - Clay roof tile is very salvageable and in high demand. Due to the brittle nature of aged clay tile, careful deconstruction is needed to remove tile from roof areas without damaging them. Likewise, concrete roof tile can be salvaged for reuse. Damaged tiles can be recycled and combined with other inert materials such as brick, asphalt and concrete.

ix. CARPET

   Some carpeting has salvage value and can be reused, restored and/or resold (typically tiles). Nylon carpet face fiber can be separated from backing and recycled into numerous products including carpet, carpet pad and carpet backing. Backing components can be recycled to make new carpet backing.

The entire carpet composite (face fiber and backing) can be re-processed and used as a raw material for other products including carpet backing, erosion control products, industrial flooring, parking stops, synthetic plywood and building materials, railroad ties and marine timbers. The carpet fiber can be used as an added filler or reinforcement for concrete or asphalt paving.

Several carpet manufacturers now offer to take back and recycle old carpet when consumers purchase new carpeting. Sometimes this “recycling”, however, consists of use as a fuel source in waste to energy (WTE) operations instead of using coal to produce energy. Carpet has low Sox emissions, and can also be used as a feed for cement kilns.

(This chapter is continue.....)