Determination of Degree of Hydration of Cement

The degree of hydration of cement can be determined by different means. But, unfortunately, the application of these method to commercial cements is by no means simple. These methods are :

a) The amount of chemically combined water.

b) The amount of unhydrated cement present ( using X-ray quantitative analysis ).

c) The amount of Ca(OH)2 present in the paste.

d) The specific gravity of the paste.

e) The heat evolved by hydration.

f) Indirectly from the strength of the hydrated pasted.

g) Thermogravimetric techniques and continuous X -ray diffraction scanning of wet pastes undergoing hydration gives an idea about early reactions.

h) Back-scattered electron imaging in a scanning electron microscope also can help to study on the microstructure of hydrated cement paste.

SEISMIC WAVES

Earthquake

A type of earth movement that locally gives rise to engineering problems is called the earthquake.Whenever the earth is suddenly struck or disturbed vibration are produced. These vibrations are setup or start from a limited area and are propagated outward in all directions. Thus an earthquake may be defined as the passage of these vibrations in the earth.

Earthquake waves

Two types of waves are produced during earthquake. These are

1. Body waves
2. Surface waves
   

1. Body waves

Body waves consist of two waves. These are

a) Primary waves ( P-waves)

b) Secondary Waves (S-waves)

a) Primary waves

1) Nature : These are longitudinal or compressional in nature. Therefore it is known as longitudinal waves or compressional waves.

2) Direction of Particle Vibration : The rock particles vibrates in the direction of propagation of the waves, with a push and pull effect.

3) Speed : It is the fastest waves and therefore first to be recorded at the recording station. It travels with about the same speed as sound through same rock.

4) Example : In granites, P -waves have speed of about 4.8 Km/Sec.

5) Penetration Capacity : These waves are capable of passing through solids as well as liquides.

b) Secondary Waves :

1) Nature: These are transverse or distortional in nature. Therefore it is known as transverse waves, shear waves or shake waves.

2) Direction of Particle Vibration: The rock particles vibrate at right angles to the direction of propagation like light waves.

3) Speed: These travel slower than the P-waves and are second to be recorded.

4) Example: In granites, S-waves have speed of about 3 km/sec.

5) Penetration capacity: These can pass through solids but it is in capable of passing through liquids.

2. Surface Waves

These waves travel along the earth's surface having similarity in behavior with sea waves.These waves also known as Long waves (L-waves).

Surface waves consist of two waves. These are

a) Love Waves

b) Rayleigh Waves

a) Love Waves

Love waves cause surface motions similar to that by S-waves, but with no vertical component. S-waves in associated with effects of Love waves cause maximum damage to structure by their racking motion on the surface in both vertical and horizontal directions.

b) Rayleigh Waves

Rayleigh Wave makes a material particle oscillate in an elliptic path in the vertical plane (with horizontal motion along direction of energy transmission).

The speed of waves in decreasing sequence :


when P-waves and S-waves reach the earth's surface, most of their energy is reflected back. Some of the energy is returned back to the surface by reflections at different layers of soil and rock. Shaking is more severe (about twice as much) at the earth's surface than at substantial depths. This is often the basis for designing structures buried underground for similar levels of acceleration than those above the ground.

REQUIREMENTS FOR STRUCTURAL INTEGRITY

Structure may subject to adverse effect from local damage from severe local abnormal loads in addition to conventional design load, which are not considered to occur in design life. Such loads include explosion due to gas or industrial liquids, vehicle impact, impact of falling objects and local effect of very winds such as tornadoes. Improving redundancy and ductility of structures by providing minor changes in the detailing of the reinforcement , the overall integrity of reinforced concrete structure to withstand such abnormal loads can be enhanced substantially. This is achieved by providing, as a minimum, some continuity reinforcement or tie between horizontal framing members. In the event of damage to a major supporting element or an abnormal loading event, the integrity reinforcement is intended to continue any resulting damage to a relatively small area, thus improving stability.

General integrity to a structure

Improvement of integrity of a whole structure providing proper ties, creates certain differing observation among engineers for a particular framing system. So providing general structural integrity to a structure is a requirement that can not be stated in simple terms. The code, however , does set forth specific examples of certain reinforcing details for cast-in-situ joist, beams, and two-way slab construction.

Cast-in-place Joists and Beams

When a support is damaged, top reinforcement which is continuous over the support, but not confined by stirrups, will tend to tear out of the concrete and will not provide the catenary action needed to bridge the damaged support. Some catenary action can also be provided, providing a portion of the bottom reinforcement in beams continuous over the supports. By providing some continuous top and bottom reinforcement in edge or perimeter beams, an entire structure can be tied together; also, the continuous tie provided to perimeter beams of a structure will toughen the exterior portion of a structure, should an exterior column be severely damaged.

Fig-1 Continuity Reinforcement for Joist Construction.
Notes:

1. Larger of 1/4(+As1) or 1/4(+As2) continuous or spliced with Class A splices

2. Larger of 1/6(-As1) or 1/6(-As2) continuous or spliced with Class A splices


Fig-2 Continuity Reinforcement for Perimeter Beams.


Notes:

1. Larger of 1/4(+As1) or 1/4(+As2) continuous or spliced with Class A splices

2. Larger of 1/6(-As1) or 1/6(-As2) continuous or spliced with Class A splices

Fig-3 Continuity Reinforcement For Beams Without Closed Stirrups

The following specification should follow to improve integrity of the overall structure :

a) In one-way slab construction, at least one bottom bar shall be continuous or shall be spliced over the support with a class-A tension splice. At non-continuous supports, the bars may be terminated with a standard hook.

b) Beams at the perimeter of the structure shall have at least one-sixth of the tension reinforcement required for negative moment at the support and one-quarter of the positive moment reinforcement required at mid-span made continuous around the perimeter and tied with closed stirrups. Closed stirrups need not be extended through any joints. The required continuity may be provided with top reinforced spliced at mid-span and bottom reinforcement spliced at or near the support with class-A tension splices.

Application Of Sand Drains

Sand drains

Sand drains is a process of radial consolidation which increase rate of drainage in the rate of drainage in the embankment by driving a casing into the embankment and making vertical bore holes. These holes is back filled with suitable grade of sand.

Process of construction of drains

The driven casing is withdrawn after the sand has been filled. A sand blanket is placed over the top of the sand drains to connect all the sand drains. To accelerate the drainage, a surcharge load is placed on the sand blanket. The surcharge is usually in the form of dumped soil.

Mechanism of consolidation

The pore water pressure is increased by the applied surcharge load in the embankment. The drainage occur in the vertical and horizontal directions. The horizontal drainage occure because of sand drains. The sand drains accelerate the the process of dissipation of excess pore water created by the surcharge.

Spacing of drains

The drains are generally laid either in a square pattern or a triangular pattern. The spacing (s) of the drains is kept smaller than the thickness of the embankment (2H) in order to reduce the length of the radial drainage path.

Zone of influence

The zone of influence of each drain in a triangle pattern is hexagonal in plan, which can be approximated by an equivalent circle of radius R, where R = 0.525 S. In case of a square pattern, the radius of circle of influence R is equal to 0.554 S. The radius of the sand drain is represented by r_w.

Theory of sand drains

The theory of sand drains was given by Rendulic (1935) and Barron (1948). Later, Richart (1959) summarized the theories. Depending on the type of strain, there are two cases.

1)  Free strain case.

2)  Equal strain case.

1)  Free strain case

If the surcharge load placed over the sand blanket is flexible, free strain case occurs. In this case, there is uniform distribution of surface loads, but the settlement at the surface is uneven.

2) 2) Equal strain case

This case occurs when the surcharge applied is rigid, such as heavy steel plates. In this case, the settlements are uniform, but the distribution of pressure is non-uniform.

Limitation of sand drain application

Following consideration is not included in design of sand drains:

1) 1) Secondary consolidation is not taken into account in the design of sand drains. In fact, the sand drains are ineffective in controlling the secondary consolidation for highly plastic and organic soils.

2) 2) In case of deriving equation for effectiveness of sand drains, it is not considered that the excess pore water pressure developed, actually in soil where sand drains are exist, is generally less than that of the case having no sand drains. Sand drains tend to act as weak piles and reduce the stresses in the clay.

3) The typical design parameter for sand drain may vary as below :

a) Spacing of sand drains, S = ( 2 ~ 5) m

b) Depth of sand drains, 2 H = (3 ~ 35) m

c) Radius of sand drains well, r_{w = (0.2} ~ 0.3) m

d) Thickness of sand blanket = (0.6 ~ 1) m

STRUCTURE IMPORTANCE CATEGORY

Based on the level of necessity of remaining safe and functional during any post disaster period e.g. after a cyclone, or an earthquake, buildings, structures and related equipments are classified into five structure importance categories such as

Each building or structure shall be placed in one of the structure importance categories and provided with a structure importance coefficient for design against wind earthquake induced forces.  

Essential Facilities :

1. Hospital and other medical facilities having surgery and emergency treatment area.

2. Fire and police stations.

3. Tanks or other structures containing, housing or supporting water or other fire-suppression materials or equipment required for the protection of essential or hazardous facilities, or special occupancy structures.

4. Emergency vehicle shelters and garages.

5. Structures and equipment in emergency-preparedness centres, including cyclone and flood

shelters.

6. Standby power-generating equipment for essential facilities.

7. Structures and equipment in government communication centers and other facilities required for emergency response.

П) Hazardous facilities

Structures supporting or containing sufficient quantities of toxic or explosive substances to be dangerous to the safety of the general public if released.

Ш) Special occupancy structures

1) Covered structures whose primary occupancy is public assembly with capacity >300 persons.

2) Buildings for schools through secondary or day-care centers with capacity >250 students.

3) Buildings for colleges or adult education schools with capacity >500 students.

4) Medical facilities with 50 or more resident incapacitated patients, not included above.

5) Jails and detention facilities.

6) All structures with occupancy > 5,000 persons.

7) Structures and equipment in power-generating stations and other public utility facilities not included above, and required for continued operation.

ІV) Standard occupancy structures

All structures having occupancies or functions not listed above.

V) Low risk structures

Buildings and structures that exhibit a low risk to human life and property in the event of failure, such as agricultural buildings, minor storage facilities, temporary facilities, construction facilities, and boundary walls.