SHAPE AND TEXTURE OF AGGREGATES

The shape of an aggregate is an important characteristic since it affects the workability of concrete. Not only the parent rock, but also the type of crusher used will influence the shape of aggregates. Basically the shape of aggregates may be rounded or angular. One of the methods of expressing the angularity qualitatively is by a figure called Angularity Number, as suggested by Shergold. This is based on the percentage of voids in the aggregate after compaction in a specified manner. If the void is 33% the angularity number of such aggregate is considered 0. If the void percentage is 44, the angularity number of such aggregate is considered as 11. An aggregate having angularity number between 0 and 11 is considered is considered suitable for making concrete. Angularity number 0 represents the most practicable rounded aggregates and the angularity 11 indicates the most angular aggregates that could be tolerated for making concrete.

Figure. Different Shape of Aggregate

Murdock suggested a different method for expressing the shape of aggregate by a parameter called Angularity Index ‘f_A’.

Where f_H = Angularity Number 

From the standpoint of economy in cement requirement for a given water/cement ratio rounded aggregates are preferable to angular aggregates. On the other hand, the additional cement required for angular aggregate is offset to some extent by higher strength and durability as a result of interlocking texture of the hardened concrete and higher bond characteristics between aggregate and cement paste. 

Super-imposing plus and minus points in favour and against these two kinds of aggregates it can be summed up as follows: 

For water/cement ratio below 0.4, the use of crushed aggregate has resulted in strength upto 38% higher than the rounded aggregate. With an increase in water/cement ratio the influence of roughness of the surface of the aggregate gets reduced and at a water/cement ratio of 0.65, no difference in strength of concrete made with angular aggregate or rounded aggregate. 

The surface texture is a measure of the smoothness or roughness of the aggregate. Surface texture depends on hardness, grain size, pore structure, structure of the rock and the degree to which forces acting on the particle surface have smoothed or roughened it. Hard, dense, fine grained materials will generally have smooth fracture surfaces. As surface smoothness increases, contact area decreases, hence a highly polished particle will have less bonding area with the matrix than a rough particle of the same volume.

BRICK LAYING

Brick masonry construction is a great art since laying must be systematically done with respect to bonding, jointing and finishing. Brick laying for wall construction is done in the following steps:

1. All the bricks to be used in construction are thoroughly soaked in water so that they do not absorb the water of the mortar.

2. Mortar is spread on the top of the foundations course, over an area to be covered by the edges of the wall. The depth of spread of mortar may be about 1.5 cm.

3. The corner of the wall is constructed first. For that, one brick is laid first at the corner and pressed with hand so that the thickness of the bed-joint remains only about 1 cm. the first closer is covered with mortar on its side and then pressed against the first corner brick, such that 1 cm thick vertical joint is obtained. The excess mortar from the sides will squeeze out, which is cleaned off with trowel.

Figure. Brick laying by Conventional Method

4. The level and the alignment is checked. If the brick or closer is not in level, they are pressed gently further. Similarly, the placement of the edges of the bricks is checked so that correct offset of concrete is available.

5. Few headers and stretchers are then laid in the first course, adopting the same method as described in step 3 for the closer brick. That is, mortar is applied on the side of the brick to be laid and it is pressed against the previous brick laid earlier, so that excess mortar squeezes out from the sides. The level and alignment of these are properly checked.

6. After having laid the first course at the corner, mortar is laid and spread over the first course, to a depth of about 1.5 cm and end stretcher is laid first, by pressing it into the mortar and then hammering it slightly so that the thickness of the bed-joint is 1 cm. mortar is then applied on the side of another stretcher and pressed to the side of the corner stretcher so that thickness of the vertical joint is about 1 cm. Excess mortar which oozes out is cleaned off. This way, stretchers and headers are laid for the second course.

7. Other courses (usually four to six) are then laid at the corner. Similarly, the corner at the other end of the wall is laid. Since the corner construction at each end works as a guide for filling in-between bricks of various courses, the corner construction should be done with great care. The plumb as well as alignment should be thoroughly checked. Plumbing up by means of plumb rule should be frequently restored to as new brick work has a tendency to overhang. Vertical face is obtained by tapping the handle of the trowel against the overhanging bricks. 

8. For building the in-between portion of the wall, a cord is stretched along the top of the first course laid at each other. A brick bat is attached at either end of the cord so that it remains tout. The course is then built. The line or cord is then shifted up, corresponding to the top level of the second course, and the second course is also constructed. The procedure is repeated till the in-between wall is constructed to the height of corner masonry.

9. The corners of the wall are then raised further, and steps 7 and 8 are repeated. All the walls should be uniformly constructed so that the load on the foundations is uniform. It should be ensured that the difference in height between two adjoining walls is not more than 1m.

10. Prepends must be kept vertical. This should be checked, as the work proceeds, with the help of straight edge and the square. The straight edge is placed flat on the course and slightly projecting beyond the face. The stock of the square is then set against the underside of the straight edge with the blade coinciding with the last-formed vertical joint.

11. Bricks with one frog should be laid with its frog on its top face to ensure that they will be completely filled with mortar.

12. In the case of thick walls, mortar is first spread over the entire bed and the outer bricks are laid as described above. The inner bricks are then pressed and rubbed into position to cause some of the mortar to rise between the vertical joints, which are finally filled flush with liquid mortar so that no hollow spaces are left.

13. All loose materials, dirt and set lumps of mortar which may be lying over the surface on which the brick work is to be freshly started, should be removed with wire brush and wetted slightly.

14. After having constructed the wall, jointing and pointing is done. The procedure for jointing and pointing has been described separately. However, all the joints should be cleaned and finished after every day’s work.

BULKING OF SAND

The increase in volume of a given mass of fine aggregate caused by the presence of water is known as bulking.

Free moisture forms a film of water around each particle. The bulking of fine aggregate is caused by these films of water which pushes the particles apart. Therefore, no point of contact is possible between the particles. This causes increase in volume of the mass of fine aggregate. The extent of bulking depends upon the percentage of moisture present in the sand and the fineness of sand particles.

Figure 1 shows the effect of moisture content on bulking. It is seen that bulking increases with the increase in moisture content up to a certain limit and beyond that the further increase in the moisture content results in the decrease in the volume. This is due to the fact that after addition of certain amount of water in the fine aggregates, the further addition of water breaks the film around the particles and hence, volume gradually decreases.

Figure 1. Effect of moisture content on the bulking of sand

For ordinary sands the bulking usually varies between 15 to 30 percent. Fine sands bulk more and the maximum bulking is obtained at a higher water content that the coarse sand. In extremely fine sand, the bulking may be as much as about 40 percent at a moisture content of 10 percent but such sand is unsuitable for concrete. In case of coarse sand, the increase in volume is negligible due to the presence of free water as the thickness of the moisture film is very small compared with the size of the particle.

The percentage of bulking is to be determined as per IS: 2386-1963 (Part III). Put sufficient quantity of the sand loosely into a container until it is about two-thirds full. Level off the top of the sand and pushing a steel rule vertically down through the sand at the middle to the bottom, measure the height. Suppose this is h cm.

Empty the sand out of the container into another container where none of it will be lost. Half fill the first container with water. Put back about half the sand and rod it with a steel rod, about 6 mm in diameter, so that its volume is reduced to a minimum. Then add the remainder of the sand and rod it in the same way. Smooth and level the top surface of the inundated sand and measure its depth at the middle with the steel rule. Suppose this is h₁ cm.

The percentage of bulking of the sand due to moisture shall be calculated from the formula: 

REGULATORY REQUIREMENTS OF WASTE MANAGEMENT

Solid waste policy in the US is aimed at developing and implementing proper mechanisms to effectively manage solid waste. In the US, the Environmental Protection Act (EPA) regulates household, industrial, manufacturing and commercial solid and hazardous wastes under the 1976 Resource Conservation and Recovery Act (RCRA). The RCRA is the principal federal law in the US governing the disposal of solid waste and hazardous waste. The US Congress enacted RCRA to address the increasing problems the nation faced from its growing volume of municipal and industrial waste. RCRA amended the Solid Waste Disposal Act of 1965. It sets national goals for:

• Protecting human health and the natural environment from the potential hazards of waste disposal. 
• Energy conservation and natural resources. 
• Reducing the amount of waste generated, through source reduction and recycling. 
• Ensuring the management of waste in an environmentally sound manner.

The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), also known as “Superfund” was enacted in 1980 to address the problem of remediating abandoned hazardous water sites, by establishing legal liability, as well as trust fund for clean up activities.

In general, CERCLA applies to contaminated sites, while RCRA’s focus is on controlling the ongoing generation and management of particular waste streams. In 1984, the US Congress expanded the scope of RCRA with the enactment of Hazardous and Solid Water Amendments (HSWA). The amendments strengthened the law by covering small quantity generators of hazardous waste and establishing requirements for hazardous waste incinerators and the closing of substandard landfills. In 1986, SARA (Superfund Amendments and Reauthorization Act of 1986) addressed clean up of leaked underground storage tanks and other leaking waste storage facilities. The amendments established a trust fund to pay for the clean up of leaking underground storage tank sites where responsible parties cannot be identified.

In India, waste management is governed by Ministry of Environment, Forest and Climate Change (MoEF) who work together with State Pollution Control Board set up in various states.

Certain laws are also present in the legal set up which helps in regulation of waste in India. The National Environment Policy, 2006 laid emphasis not only on disposal of waste but also recycling and treating waste. Some of the laws for the purpose of waste regulation are stated as under:

THE ENVIRONMENT PROTECTION ACT (EPA)

This act was enacted in 1986 by the Parliament of India and it aims to establish a sufficient protection system. This act confers powers to the Central Government to regulate all forms of waste. It is one of the primary legislatures to protect the environment and regulation of waste.

BIO-MEDICAL WASTE RULES, 1998

The aim of these rules is to ensure that bio-medical wastes are safely disposed of. Biomedical wastes can be defined as any waste or by-product generated during treatment, immunization and treatment of human beings or animals or in research activities. The BMW rules apply to various institutions like nursing homes, animal dispensaries, veterinary homes, blood banks, dispensaries, pathological laboratories, etc. The BMW rules prohibit mixing of biological wastes with any other types of wastes. The general rule provided is that biomedical wastes cannot be kept stored beyond a period of 48 hours without being treated. Further, all institutions covered under the rules are to mandatorily set up treatment facilities like microwave system, autoclave etc.

THE BATTERIES RULES, 2001

The Batteries Rules were notified to set up a mechanism in place which dealt with the disposal of lead acid batteries.  The Rules apply to every manufacturer, recycler, dealer, importer, assembler, bulk consumer and consumer. The Rule makes it compulsory for every consumer to deposit the used batteries back with the dealer, manufacturer, recycler or labelled collection centres. If a recycler wants to import used batteries in India, for the purpose of recycling, he must obtain custom clearance. Additionally, import of batteries will be allowed only upon producing valid registration with Reserve Bank of India and MoEF and providing an undertaking in prescribed format along with a copy of the latest half-yearly return.

THE HAZARDOUS WASTES RULES, 2008

Management of hazardous waste is a very complex issue. The Rule places an obligation on the occupier of hazardous waste to safe and sound handling of environmental waste. The occupier is that person under whose charge there is a plant or unit or factory which produces hazardous wastes as a result of their operation. The occupier must sell or send the hazardous waste to a re-processor or recycler, who is authorized by the government to dispose of the waste in a safe manner. Any person who is engaged in storage, package, collection, destruction, conversion, processing, etc., also has to take authorization from the State Pollution Board. Sale or transfer of hazardous waste can be done only after obtaining a valid registration from Central Pollution Board (CPCB).

THE PLASTIC WASTE RULES, 2011

The PWM Rules are set up to control the use, manufacture and recycling of plastic waste. The Rule has uniform applicability towards all distributors, users, retailers, and manufacturers of plastic products. The Rule makes it compulsory for every manufacturer of plastic products and recycler to obtain registration from State Pollution Control Board. The Rule also states that no retailer can provide plastic bags free of cost. This is done to ensure that people use plastic bags judicially.

THE E-WASTE RULES, 2011

The primary aim of the EWM is to put in place a system which manages e-waste in an environment-friendly way by regulating the issue of recycling and disposal of e-waste. E-waste is a problematic issue in India. With the growing economy and the technological advancement, India is becoming a hub for the IT sector. This creates a lot of e-wastes, disposal of which is necessary. The Rule empower the concerned state agencies to control, supervise and regulate relevant activities connected with e-wastes management such as collection, segregation, dismantling and recycling.

PLASTIC BENDING OF BEAMS

Let us consider a beam of homogenous material and symmetrical section subjected to a bending moment M. The distribution of bending stress flows a linear law with zero stress at the neutral axis and a maximum stress at the outermost fibres, when the deformations are within the elastic limit. In case the magnitude of M increases, the stress distribution also changes. These are shown in the following stages.

Figure 1. Stages of Stress Distribution

Stage 1. The deformation is within the elastic limit. The maximum bending stress is f. If the section modulus is Z, we have M = f.Z. (Figure 1(a)).

Stage 2. If the bending moment is gradually increased so that the extreme fibre reaches the yield stress f_y, the corresponding bending moment is given by, M_y = f_y.Z. (Figure 1(b)).

Stage 3. The bending moment, if further increased, will not increase the maximum fibre stress which remains at the yield stress value f_y, but the yield will spread into fibres for a depth e called the depth of penetration. (Figure 1(c)).

Stage 4. If the bending moment is further increased, a stage will be reached when the yield will spread into all the fibres resulting in a stress diagram shown in figure 1(d).

The beam section in this stage has reached its maximum resisting capacity. Any further increase in the bending moment cannot be resisted by the section and an instability is reached as would happen if a hinge was provided at the section. We say that a plastic hinge has formed at the section.

At this stage area of the compression or tension zone of the section equals A/2, where A is the cross sectional area of the beam.

Total compression on the section = Total tension on the section = f_y.A/2

This means, the neutral axis at this stage is called the plastic moment of resistance (or plastic moment) denoted by M_p and is given by

M_p          = Moment of total compression about the plastic neutral axis + Moment of total tension about the plastic neutral axis.

= f_y.A/2 x Distance of the centroid of the compression zone from the plastic neutral axis +   

   f_y.A/2 x Distance of the centroid of the tension zone from the plastic neutral axis.

= f_y.[Sum of the moments of the compression and tension zones about the plastic neutral  

   axis]

= f_y.Z_p

Where Z_p = Sum of the moments of the compression and tension zones about the plastic neutral axis and is called the plastic modulus.

Thus when a beam section develops a plastic hinge,

Plastic moment of resistance = M_p = f_y.Z_p

We know, the ratio of the moment of inertia of the beam section about the elastic i.e., the centroidal neutral axis to the distance of the most distant edge of the section is the section modulus Z of the beam section.

The ratio of the plastic modulus Z_p to the section modulus Z is called the shape factor or form factor denoted by K_s of the section. This is a measure of the reserve strength the section possesses after the initial yielding.

If M_y = Moment of resistance of the section when the most extreme fibre of the section reaches the yield stress f_y,

M_y = f_y.Z