Must know - for Students
What are the functions of different components of a typical expansion joint?
In a typical expansion joint, it normally contains the following components: joint sealant, joint filler, dowel bar, PVC dowel sleeve, bond breaker tape and cradle bent.
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Joint sealant: it seals the joint width and prevents water and dirt from entering the joint and causing dowel bar corrosion and unexpected joint stress resulting from restrained movement.
Joint filler: it is compressible so that the joint can expand freely without constraint. Someone may doubt that even without its presence, the joint can still expand freely. In fact, its presence is necessary because it serves the purpose of space occupation such that even if dirt and rubbish are intruded in the joint, there is no space left for their accommodation.
Dowel bar: This is a major component of the joint. It serves to guide the direction of movement of concrete expansion. Therefore, incorrect direction of placement of dowel bar will induce stresses in the joint during thermal expansion. On the other hand, it links the two adjacent structures by transferring loads across the joints.
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PVC dowel sleeve: It serves to facilitate the movement of dowel bar. On one side of the joint, the dowel bar is encased in concrete. On the other side, however, the PVC dowel sleeve is bonded directly to concrete so that movement of dowel bar can take place. One may notice that the detailing of normal expansion joints in Highways Standard Drawing is in such a way that part of PVC dowel sleeve is also extended to the other part of the joint where the dowel bar is directly adhered to concrete. In this case, it appears that this arrangement prevents the movement of joint. If this is the case, why should designers purposely put up such arrangement? In fact, the rationale behind this is to avoid water from getting into contact with dowel bar in case the joint sealant fails. As PVC is a flexible material, it only minutely hinders the movement of joint only under this design.
Bond breaker tape: As the majority of joint sealant is applied in liquid form during construction, the bond breaker tape helps to prevent flowing of sealant liquid inside the joint.
Cradle bar: It helps to uphold the dowel bar in position during construction.
What if on-site slump test fails, should engineers allow the contractor to continue the concreting works?
This is a very classical question raised by many graduate engineers. In fact, there are two schools of thought regarding this issue.The first school of thought is rather straightforward: the contractor fails to comply with contractual requirements and therefore the engineer could order suspension of the Works. As per general contractual obligations, the contractor is not entitled to any claims of cost which is the main concern for most engineers.The second school of thought is to let the contractor to continue their concreting works and later on the contractor is asked to prove that the finished works comply with other contractual requirements e.g. compression test.This is based upon the belief that workability is mainly required to achieve design concrete compression strength.In case the compression test also fails, the contractor should demolish and reconstruct the works accordingly.In fact, this is a rather passive way of treating construction works and is not recommended because of the following reasons:
- Workability of freshly placed concrete is related not only to strength but also to durability of concrete. Even if the future compression test passes, failing in slump test indicates that it may have adverse impact to durability of completed concrete structures.
- In case the compression test fails, the contractor has to deploy extra time and resources to remove the work and reconstruct them once again and this slows down the progress of works significantly. Hence, in view of such likely probability of occurrence, why shouldn’t the Engineer exercise his power to stop the contractor and save these extra time and cost?
In concrete compression test, normally 150mmx150mmx150mm concrete cube samples are used for testing. Why not 100mmx100mmx100mm?
Basically, the force supplied by a concrete compression machine is a definite value. For normal concrete strength application, say below 50MPa, the stress produced by a 150mmx150mmx150mm cube is sufficient for the machine to crush the concrete sample. However, if the designed concrete strength is 100MPa, under the same force (about 2,000kN) supplied by the machine, the stress under a 150mmx150mmx150mm cube is not sufficient to crush the concrete cube. Therefore, 100mmx100mmx100mm concrete cubes are used instead to increase the applied stress to crush the concrete cubes. For normal concrete strength, the cube size of 150mmx150mmx150mm is already sufficient for the crushing strength of the machine.
What are the major problems in using pumping for concreting works?
In pumping operation, the force exerted by pumps must overcome the friction between concrete and the pumping pipes, the weight of concrete and the pressure head when placing concrete above the pumps. In fact, as only water is pumpable, it is the water in the concrete that transfers the pressure. The main problems associated with pumping are the effect of segregation and bleeding. To rectify these adverse effects, the proportion of cement is increased to enhance the cohesion in order to reduce segregation and bleeding. On the other hand, a proper selection of aggregate grading helps to improve the pumpability of concrete.
What are the different types of concrete pumps commonly used in the industry?
Direct-acting, horizontal piston type- with semi-rotary valves set to permit always the passage of the largest aggregate particles. This type of pump can cover a horizontal distance of 1000 m and a vertical distance of 120 m. The concrete is fed in by gravity and is also partially sucked in during the suction stroke. The valves open and close with definite pauses so that concrete moves in a series of impulses, but the pipe always remains full. These pumps are capable of pumping 130 m3 of concrete per hour with 8 inch pipes.
Squeeze pumps or peristaltic pumps- are the ones that use vacuum pumping. These pumps can cover a distance of 90 m horizontally and 30 m vertically, and are capable of pumping 20 m3 of concrete per hour using 3 inch pipes.
What are the requirements of a Pumping Concrete?
- Concrete mixture should neither be too harsh nor too sticky; also, neither too dry nor too wet.
- A slump between 50 and 150 mm is recommended (note that pumping induces partial compaction, so the slump at delivery point may be decreased).
- If the water content in the mixture is low, the coarse particles would exert pressure on the pipe walls. Friction is minimized at the correct water contents. The presence of a lubricating film of mortar at the walls of the pipe also greatly reduces the friction.
- High cement content in concrete is generally beneficial for pumping.
- Water is the only pumpable component in the concrete, and transmits the pressure on to the other components.
- Two types of blockage to efficient pumping could occur: (1) Water can escape from the mixture if the voids are not small enough; this implies that closely packed fines would be needed in the mixture to avoid any segregation. The pressure at which segregation occurs must be greater than that needed to pump concrete. (2) When the fines content is too high, there could be too much frictional resistance offered by the pipe. The first type of blockage occurs in irregular or gap-graded normal strength mixtures, while the second type occurs in high strength mixtures with fillers. In order to avoid these two types of failure, the mixture should be proportioned appropriately.
- Other mixture factors that could affect pumping are the cement content, shape of aggregate, presence of admixtures such as pumping aids or air entrainment. Air entrainment is helpful in moderate amounts, but too much air can make pumping very inefficient.
- When flowing concrete is being pumped, an over-cohesive mixture with high sand content is recommended. For lightweight aggregate concrete, pumping can fill up the voids in the aggregate with water, making the mixture dry.
What is the indication of shear slump and collapse slump in slump tests?
There are three types of slump that may occur in slumps test, namely, true slump, shear slump and collapse slump.
- True slump refers to general drop of the concrete mass evenly all around without disintegration.
- Shear slump implies that the concrete mix is deficient in cohesion. Consequently, it may undergo segregation and bleeding and thus is undesirable for durability of concrete.
- Collapse slump indicates that concrete mix is too wet and the mix is deemed to be harsh and lean.
In erection of false-work, for a rectangular panel inside a false-work should it be braced along the two diagonals?
When a rectangular panel is subject to an eccentric load or a lateral load, it tends to deform into a parallelogram with one diagonal shortening and the other elongating. Theoretically, it is sufficient to brace along one of the diagonals (the one in tension). If one diagonal is only allowed to brace inside the rectangular panel, it should be not braced in the diagonal in compression because under severe lateral loading the diagonal may buckle leading to failure of structure. However, in actual situation lateral loads may come from both sides of the panel and hence it should be braced in both diagonals.
In carrying out compression test for concrete, should test cubes or test cylinders be adopted?
Basically, the results of compression test carried out by using cubes are higher than that by cylinders. In compression test, the failure mode is in the form of tensile splitting induced by uniaxial compression. However, since the concrete samples tend to expand laterally under compression, the friction developed at the concrete-machine interface generates forces which apparently increase the compressive strength of concrete. However, when the ratio of height to width of sample increases, the effect of shear on compressive strength becomes smaller. This explains why the results of compression test by cylinders are lower than that of cubes. Reference is made to Longman Scientific and Technical (1987).
What is the function of rebate in a typical construction joint?
Construction joints are created on sites to facilitate the construction process. However, if improperly constructed, the completed construction joints will leave an uneven scar on the concrete surface and affect significantly its appearance. To avoid this, a rebate is formed during the first pour of one side of construction joint. After the other pour is concreted, it will hide the uneven joint inside the rebate.
Can grout replace concrete in normal structure?
The mixture of cement and water alone cannot replace concrete (Longman Scientific and Technical (1987)) because:
- Shrinkage of grout is several times that of concrete with the same mass.
- The effect of creep of grout is far more than that of concrete.
- Heat of hydration of cement with water is more than normal concrete and this leads to the problem of severe cracking.
Which type of bar reinforcement is more corrosion resistant, epoxy-coated bars, stainless steel bars or galvanized bars?
Based on the experiment conducted by the Building Research Establishment, it was shown that the corrosion resistance of galvanized steel was the worst among the three types of bar reinforcement. For galvanized steel bars, corrosion started to occur when a certain chloride content in concrete (i.e. 0.4% by cement weight) was exceeded. However, for epoxy-coated bars, they extended the time taken for cracking to occur when compared with galvanized steel bars. The best corrosion resistant reinforcement among all is stainless steel. In particular, austenitic stainless steel stayed not corroded even there was chloride contamination in concrete in the experiment. Reference is made to K. W. J. Treadaway (1988).
Can a concrete structure be completely free of expansion joints and contraction joints?
Consider that the concrete structure is not subject to the problem of differential settlement. For contraction joints, it may be possible to design a concrete structure without any contraction joints. By using sufficient steel reinforcement to spread evenly the crack width over the span length of the structure, it may achieve the requirement of minimum crack width and cause no adverse impact to the aesthetics of the structure. However, it follows that the amount of reinforcement required is higher than that when with sufficient contraction joints.
For expansion joints, the consequence of not providing such joints may be difficult to cater for. For example, a concrete structure has the coefficient of thermal expansion of 9x10-6 /oC and a Young’s modulus of 34.5 kN/mm2. With an increase of temperature of 20oC and it is restricted to free expansion, then the structure is subject to an axial stress of 6.21MPa. If the structure is very slender (e.g. concrete carriageway), buckling may occur. Therefore, the structure has to be designed to take up these thermal stresses if expansion joints are not
provided. However, for water retaining structures, most of them are not affected by weather conditions because they are insulated from the water they contain internally and soil backfill that surround them. Therefore, it is expected that a smaller amount of thermal movement will occur when compared with normal exposed concrete structure. Consequently, expansion joints may be omitted in this case with the view that the compressive stress induced by thermal expansion toughens the structure to limit the development of tensile stress.
Does the presence of rust have adverse impact to the bond performance of bar reinforcement?
In fact, the presence of rust in bars may not have adverse impact to the bond performance and it depends on the types of bar reinforcement under consideration. For plain round bars, the rust on bars improves the bond performance by the formation of rough surfaces which increases the friction between steel and concrete.
However, for deformed bars, the same theory cannot apply. The presence of rust impairs the bond strength because corrosion occurs at the raised ribs and subsequently 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. On the contrary, the bond between concrete and plain round bars derives from the adhesion and interface friction. With such differences in mechanism in bonding, the behaviour of bond between deformed bars and plain round bars in the presence of rust varies. Reference is made to CIRIA Report 147.
What is the difference 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. 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. They are used as bedding for concrete kerbs in roadwork.
What is the purpose of skin reinforcement for deep beams?
In BS8110, it states that secondary reinforcement should be provided for beams exceeding 750mm deep at a distance measured 2/3 depth from the tension face. Experimental works revealed that at or close to mid-depth of deep beams, the maximum width of cracks arising from flexure may be about two to three times larger than the width of the same crack at the level of surface where the crack originally forms.
The presence of crack is undesirable from aesthetic point of view. Moreover, it poses potential corrosion problems to reinforcement of deep beams. To safeguard against these crack formation, skin reinforcement is designed on the sides of deep beams to limit the formation of flexural crack widths. Though the principal function of skin reinforcement is to control crack width, it may be employed for providing bending resistance of the section.
References:
1. http://www.theconcreteportal.com,
2. Practical Civil Engineering Works by Vincent T. H. CHU
3. Properties of Concrete by AM Neville
4. Concrete international-ACI publication
EOM
BUILT EXPRESSIONS
MAY 2013
CODES
IS Codes Used For Structural Steel
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Indian Standard
CODE OF PRACTICE FOR
GENERAL CONSTRUCTION, IN STEEL Is:
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800-2007
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Indian Standard
CODE OF PRACTICE FOR USE OF
COLD-FORMED LIGHT GAUGE STEEL
STRUCTURAL MEM’BERS IN GENERAL
BUILDING CONSTRUCTIONIS, :
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801 - 1876
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Indian Standard
SPECIFICATION FOR
RECTANGULAR PRESSED STEEL TANKS IS:
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804 - 1967
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Indian Standard
CODE OF PRACTICE FOR
USE OF STEEL IN GRAVITY WATER
TANKS
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805 - 1988
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Indian Standard
DIMENSIONS FOR HOT ROLLED STEEL
BEAM, COLUMN, CHANNEL AND
ANGLE SECTION IS
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808 : 1989
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Indian Standard
SPECIFICATIONFOR
COLDFORMEDLIGHTGAUGESTRUCTURAL
STEELSECTIONS
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811-1987
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Indian Standard
HIGH STRENGTH BOLTS
STRUCTURES
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4000-1992
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Indian Standard
SAFETY CODE FOR ERECTION OF
STRUCTURAL STEELWORK
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7206 - l974
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Indian Standard
TOLERANCES FOR
FABRICATION OF STEEL STRUCTURES
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7215-1974
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Indian Standard
CRITERIA FOR DESIGN OF STEEL BINS FOR
STORAGE OF BULK MATERIALS
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9178 (Part 1) - 1979
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Indian Standard
CRITERIA FOR DESIGN OF STEEL BINS FOR
STORAGE OF BULK MATERIALS
PART II DESIGN CRITERIA
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9178 (Part II) - 1979
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Indian Standard
PRACTICE FOR DESIGN,
, TESTING AND INSTALLATION
OF UNDER-GROUND/ABOVE-GROUND
HORIZONTAL CYLINDRICAL STEEL STORAGE
TANKS FOR PETROLEUM PRODUCTS IS
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10987: 1992
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Compiled By: Dr. G. Sarangapani, Professor & Head, Department of Civil Engineering, NIE, Mysore