Posts by Kanwarjot Singh

By
Kaushal Kishore, Materials Engineer, Roorkee

Foamed concrete, also called cellular light weight concrete is produced by the mixing of Portland cement, sand including or alone fly ash, water and preformed stable foam. The foam is produced with the help of a foam generator by using foaming agent. The air content is typically between 40 to 80 percent of the total volume. The bubbles vary in size from around 0.1 to 1.5 mm in diameter. Foamed concrete differentiates from (a) gas or aerated concrete, where the bubbles are chemically formed through the reaction of aluminium powder with calcium hydro oxide and other alkalies released by cement hydration and (b) air entrained concrete, which has a much lower volume of entrained air is used in concrete for durability. Curing of foamed concrete unit may be done as per IS: 456-2000. Curing can be speeded up by steam.
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For pipes used for pipe jacking, they should possess the following characteristics:

(i) Pipes should have high concrete strength to withstand the stress induced during the jacking process.

(ii) There is tight tolerance in pipe dimension and the pipe joints are specially designed to provide trouble-free joint details. Two commonly available joints are rebated joint and butt end joint.

(iii) Pipes preferably should have smooth external concrete finishes to reduce the friction between the pipes and surrounding soil.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Since in pipe jacking, the jacked pipes could hardly be jacked in the designed level and alignment and some deviation from the original one is commonly acceptable provided that the deviation are within the tolerance of the Contract. However, in order to avoid damage made to the pipe joints due to overstressing, it is necessary to estimate the stress concentrations resulting from these angular deflections.

Note: Pipe jacking is a trenchless method in which pipes are jacked underground from jacking pits and receiving pits.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

The lubricating fluid serves to reduce the jacking force in the following ways:

(i) It saturates the overcut leading to partial and complete buoyancy of the jacking pipe in the cavity. As such, the contact surface area between the jacking pipe and soils is decreased.

(ii) It stabilizes the cavity by limiting the radial effective stress acting on the jacking pipe.

(iii) The interface friction angle between the soils and the jacking pipe are reduced by the lubricants.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Theoretical line and level of pipelines can hardly be achieved in pipe jacking. As such, the provision of angular deflection is made at pipe joints to accommodate such deviation. Normally, maximum allowable angular deflection at pipe joints in pipe jacking is 0.5^o.

As a rule of thumb, the stress concentration at a pipe joint is about 3 times the joint stress resulting from uniform distribution of stress. As such, for grade 50MPa precast concrete pipe, the allowable uniform joint stress is expected to be one-third of its compressive strength, i.e. 16.67MPa.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Energy is a function of load and speed. Damage may occur on the jacking pipes if it is incapable of absorbing the energy. Pipe jacking requires large energy in the process. Jacking pipes are low speed energy absorbers. To cater for the low-speed-energy absorbing characteristics of jacking pipes, hydraulic jacks should be designed to provide high loads with low speeds.

Otherwise the excessive high speed generated by hydraulic jacks would cause kinetic damage to the pipes.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

By
Kaushal Kishore
Materials Engineer, Roorkee

Cement concrete in India on large scale is being used since the last about 70 years. In the early days the following nominal ratio by volume for concrete were specified.

Cement	:	Sand	:	Aggregate
1	:	2	:	4	Correspond to M-15 Grade
1	:	1.5	:	3	Correspond to M-20 Grade
1	:	1	:	2	Correspond to M-25 Grade

IS : 456-2000 has recommended that minimum grade of concrete shall be not less than M-20 in reinforced concrete work. Design mix concrete is preferred to nominal mix. If design mix concrete cannot be used for any reason on the work for grades of M-20 or lower, nominal mixes may be used with the permission of engineer-in-charge, which however is likely to involve a higher cement content.
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By
Kaushal Kishore
Materials Engineer, Roorkee

Visual inspection is one of the most versatile and powerful of the NDT methods, and it is typically one of the first steps in the evaluation of a concrete structure. Visual inspection can provide a wealth of information that may lead to positive identification of the cause of observed distress. However, its effectiveness depends on the knowledge and experience of the investigator. Broad knowledge in structural engineering, concrete materials, and construction methods is needed to extract the most information from visual inspection.

Before performing a detailed visual inspection, the investigator should develop and follow a definite plan to maximize the quality of the record data. Visual inspection has the obvious limitation that only visible surface can be inspected. Internal defects go unnoticed and no quantitative information is obtained about the properties of the concrete. For these reasons, a visual inspection is usually supplemented by one or more of the other NDT methods, such as by concrete test hammer, ultrasonic concrete tester and partial destructive testing by drilling cores and testing them for compressive strength.
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The New Austrian Tunneling Method originated from rock tunnels and it requires the use of rockbolts and shotcrete swiftly after blasting. Institution of Civil Engineers has renamed it as Sprayed Concrete Lining for construction of tunneling linings by this method in soft ground.

There are some distinct differences in design philosophy between NATM and Sprayed Concrete Lining. In fact, it is not practical to mobilize inherent soil strength through deformation in soft ground. For lower strength of soils in soft ground, there should not be any delay in completing primary support and ground deformation should be reduced as much as possible.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

In the original version of the New Austrian Tunneling Method, it is a tunneling concept with a set of main principles as follows:

(i) Application of thin-spayed concrete lining

(ii) Closure at invert to form a complete ring as soon as possible

(iii) Measurement of deformation until equilibrium is attained

It is a design concept in which the ground (soil or rock) enclosing the opening becomes a load-bearing element through formation of ring-like body. It uses all available means to develop maximum self-supporting capacity to provide support of the underground opening. In essence, it makes use of geological stress of surrounding soils to stabilize the opening. Therefore, NATM is a tunneling philosophy and concept rather than excavation techniques.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.