Building Maintenance — Part 2 MCQs with Explanations

To reduce the cost of paint, fillers are added. If used too much it can weaken the paint. Alumina, magnesia, barite, charcoal, etc are examples of filler.

Soapy patches appear on the painted surface due to chemical reaction of alkalis. This defect is called Saponification.

Cobalt is not a base instead it is a drier. It is used to absorb oxygen from surroundings and harden the paints.

Emulsion paints have polystrene as a base. Emulsion paints are durable and can retain colour for too long. They dry within 15 minutes and hard dry within 2 hours.

If a paint is applied excessively thick then wrinkling occurs. Grinning occurs when under coat and final coat are of different colours. Running occurs if the surface is too smooth; paint run off from such parts. Flaking occurs due to poor adhesion, leading to peeling off of paint.

The Damage caused to anchorages and unbounded tendons are the primary problems in prestressed concrete structures. Good maintenance includes periodical surveillance identification of local damage, deterioration and loss of durability of the structure due to environmental and other load effects.

Environmental effects are one of the reason why rehabilitation of structures may be required. Other reasons may be constructional deficiency, deficiency in design, overloading of structures either due to unanticipated loading or due to accidents and user made changes in the structures during the service life of the structure.

The use of common techniques for rehabilitation of various structures is limited because the problem of rehabilitation is unique for the particular structure. Several new cementations materials and epoxy resigns and compounds have been developed which are highly effective in protecting the basic structure from the destructive effects of severe exposure conditions in the environment.

Maintenance engineer has the primary function of maintaining prestressed concrete structures of various types in a fit and serviceable condition. Investment towards effective periodical maintenance forms only a fraction of the cost to be incurred due to major repairs to rehabilitate the structures and hence it is always advisable to establish a programmed preventive maintenance system to detect any signs of distress in the initial stages itself through inspection procedures and appropriate repairs.

The structural concrete slab panels can be repaired by external bonding of steel plates to the soffit by using epoxy adhesives and hollows.

The inspection more intensive involving examination of structural elements is Major inspection. It even require setting up of special access facilities where required depending upon the importance of the structure. This inspection is generally spaced between 2 and 3 years.

In the form of surface cracks prestressed concrete structures shows visible signs of distress. Spalling of concrete should be special inspection using instruments to users the extent of damage to the structure.

Strain gauges is one of the testing equipments used for inspection. Modern testing equipments which could be of use to the specialized inspection team are listed below: Rebound hammer compressive strength (Schmidt hammer), ultrasonic pulse velocity detection of cracks, snooper-crawler and adjustable ladders, magnetic decor for measuring thickness of concrete cover and for locating reinforcement bars, mechanical extensometer, transparent templates and microscope for reading of crack widths on the surface of the concrete hydraulic jacks, pressure transducers or load cells for measurement of forces etc, electronic strain gauges for measurement of strain in concrete and steel, vibration measuring equipment, electrical resistance meter(for rust pockets).

The pachometer is used to locate the Steel reinforcement. A wide variety of electronic equipment is presently available in the market in many countries for monitoring and inspection of distress in different types of structures and ultrasonic meters are used to determine the degree of deterioration and distress and / or material imperfections that are relevant to the structural integrity of the structure to be inspected and pachometer is used to locate the steel reinforcements and measures the sizes of the bars embedded in concrete to an accuracy of 3m.

Barins snooper vehicle is used to inspect in case of bridge greater than 10.7m in height and which cannot be inspected from beneath due to watery situation. The snooper is mounted on a heavy duty truck with a swiveling platform and the underside of the bridge deck can be inspected by using the hydraulically operated platform.

In the case of dormant cracks wider than about 1m, it is more economical to use grouting and sealing technique. This method involves enlarging the crack along its exposed face and then the crack is cleaned and grouted and the surface is then sealed with a suitable joint sealant and various other techniques widely used for repair of dormant or dead cracks include dry packing, polymer impregnation over lays and surface treatments and autogenous handling.

Three types of sealants are used depending upon their suitability in a given situation. They are: Mastics are asphalts with a low melting point with added fillers or fibers and they are recommended where the total movements will not exceed 15 percent of the width of groove, Thermoplastics comprising of asphalts, pitches and coal tar become liquid or semi viscous when heated and the groove depth to width ratio should 1:1 and the total design movement is of order of 25 percent of groove width and Elastomers include a wide range of materials such as poly sulphides, epoxy poly sulphides, poly urethane, silicones and acrylics and these materials advantages since they can be used without heating.

There are three types of damages present. These are minor damage, moderate damage and several damage. Minor damage requires superficial patching by using epoxy grout or guniting using Shortcrete and the damages and delaminated concrete is removed by hand tools and the surface is cleaned before the application of epoxy grout, moderate damage involves extensive spalling and cracking of concrete is generally applicable as in minor repairs and in Several damage Prestressed concrete girders requires a detailed structural analysis of a design check based on the conditions of the damage and the best engineering assumptions and judgments.

Preloading methods should be considered in making concrete repairs if the loss of prestress is excessive resulting in tensile cracks. This is done in order to restore the equivalent full or partial prestress effect, as per original designs.

By removing the loose concrete, the repair and rehabilitation of damaged or spalled concrete is done. Temporary support is provided to the girder to relieve dead load stresses and the case of pretressed concrete bridge decks, special stress check up is essential before starting the repair works.

In the form of diagonal tension cracks and shear cracks, shear distress is observed near the supports in many cases of heavily loaded girders. Tension cracks and shear cracks may arise due to improper detailing or compaction of concrete near the support zone in such cases holes are drilled diagonally and rebars are placed and grouted to arrest the shear cracks.

Bridge girders located in zones of very severe exposure conditions suffer spalling. extensive spalling of concrete exposing the reinforcements and in such cases the unsound or loose concrete around the girders is removed and repairing against the loss of concrete section is done by jacketing the girders using a steel form.

Crack propagation in the concrete girders can be arrested by the principles of post tensioning and tensile cracking in the girder are arrested by including compression using tension ties forced to the sides of the girder at locations of tensile cracks.

Links and dowels are used in some cases of repairs of girders damaged by collision when the damage is severe. Links and dowels are used along with wire mesh tied to the reinforcements and the entire damaged part is repaired using mortar or non shrink grout.

The corrosion of reinforcement due to extreme exposure is common for structure located in costal zones. The high tensile strands may get damaged resulting in loss of prestresss in the girder.

Post tensioning rods in conjugation with jacking can be used for restoring strength in case of damaged prestressed concrete I girders.

In restoration of strength by adding external reinforced concrete the damages concrete is first repaired by applying preload and the concrete a corbels are constructed with the required conventional steel reinforcement and it shows adding external reinforcement concrete to restore the strength of the damaged girder.

Metal sleeve jacket is added in method 3 to do the restoration. In this method preload is applied before the repair of the damaged concrete and removed after the completion of repairs and a metal sleeve jacket is installed around and beyond the damaged area and the gap between the metal sleeve girders is filles with epoxy grout by pressure injection.

600°C is the correct answer to “When exposed to fire, concrete has very little strength left after:”. This question is standard in competitive examination syllabi for Building Maintenance.