Stabilized Filling
Saturday, 31 January 2009
Hardness Test
Hardness Test
Simply stated, hardness is the resistance of a material to permanent indentation. It is important to recognize that hardness is an empirical test and therefore hardness is not a material property. This is because there are several different hardness tests that will each determine a different hardness value for the same piece of material. Therefore, hardness is test method dependent and every test result has to have a label identifying the test method used.
Hardness is, however, used extensively to characterize materials and to determine if they are suitable for their intended use. All of the hardness tests described in this section involve the use of a specifically shaped indenter, significantly harder than the test sample, that is pressed into the surface of the sample using a specific force. Either the depth or size of the indent is measured to determine a hardness value.
Why Use a Hardness Test?
Easy to perform
Quick - 1 to 30 seconds
Relatively inexpensive
Non-destructive
Finished parts can be tested - but not ruined
Virtually any size and shape can be tested
Practical QC device - incoming, outgoing
The most common uses for hardness tests is to verify the heat treatment of a part and to determine if a material has the properties necessary for its intended use. Establishing a correlation between the hardness result and the desired material property allows this, making hardness tests very useful in industrial and R&D applications.
Hardness Scales
There are five major hardness scales:
Brinell - HB
Knoop - HK
Rockwell - HR
Shore - HS
Vickers - HV
Each of these scales involve the use of a specifically shaped diamond, carbide or hardened steel indenter pressed into the material with a known force using a defined test procedure. The hardness values are determined by measuring either the depth of indenter penetration or the size of the resultant indent. All of the scales are arranged so that the hardness values determined increase as the material gets harder. The hardness values are reported using the proper symbol, HR, HV, HK, etc. indicating the test scale performed.
Five Determining Factors
The following five factors can be used to determine the correct hardness test for your application.
Material - grain size, metal, rubber, etc.
Approximate Hardness - hardened steel, rubber, etc.
Shape - thickness, size, etc.
Heat Treatment – through or casehardened, annealed, etc.
Production Requirements - sample or 100%
Simply stated, hardness is the resistance of a material to permanent indentation. It is important to recognize that hardness is an empirical test and therefore hardness is not a material property. This is because there are several different hardness tests that will each determine a different hardness value for the same piece of material. Therefore, hardness is test method dependent and every test result has to have a label identifying the test method used.
Hardness is, however, used extensively to characterize materials and to determine if they are suitable for their intended use. All of the hardness tests described in this section involve the use of a specifically shaped indenter, significantly harder than the test sample, that is pressed into the surface of the sample using a specific force. Either the depth or size of the indent is measured to determine a hardness value.
Why Use a Hardness Test?
Easy to perform
Quick - 1 to 30 seconds
Relatively inexpensive
Non-destructive
Finished parts can be tested - but not ruined
Virtually any size and shape can be tested
Practical QC device - incoming, outgoing
The most common uses for hardness tests is to verify the heat treatment of a part and to determine if a material has the properties necessary for its intended use. Establishing a correlation between the hardness result and the desired material property allows this, making hardness tests very useful in industrial and R&D applications.
Hardness Scales
There are five major hardness scales:
Brinell - HB
Knoop - HK
Rockwell - HR
Shore - HS
Vickers - HV
Each of these scales involve the use of a specifically shaped diamond, carbide or hardened steel indenter pressed into the material with a known force using a defined test procedure. The hardness values are determined by measuring either the depth of indenter penetration or the size of the resultant indent. All of the scales are arranged so that the hardness values determined increase as the material gets harder. The hardness values are reported using the proper symbol, HR, HV, HK, etc. indicating the test scale performed.
Five Determining Factors
The following five factors can be used to determine the correct hardness test for your application.
Material - grain size, metal, rubber, etc.
Approximate Hardness - hardened steel, rubber, etc.
Shape - thickness, size, etc.
Heat Treatment – through or casehardened, annealed, etc.
Production Requirements - sample or 100%
Thursday, 29 January 2009
Nondestructive testing
The need for NDT
It is very difficult to weld or mold a solid object that has the risk of breaking in service, so testing at manufacture and during use is often essential. During the process of casting a metal object, for example, the metal may shrink as it cools, and crack or introduce voids inside the structure. Even the best welders (and welding machines) do not make 100% perfect welds. Some typical weld defects that need to be found and repaired are lack of fusion of the weld to the metal and porous bubbles inside the weld, both of which could cause a structure to break or a pipeline to rupture.
During their service lives, many industrial components need regular non-destructive tests to detect damage that may be difficult or expensive to find by everyday methods. For example:
Aircraft skins need regular checking to detect cracks;
Underground pipelines are subject to corrosion and stress corrosion cracking;
Pipes in industrial plants may be subject to erosion and corrosion from the products they carry;
Reinforced concrete structures may be weakened if the inner reinforcing steel is corroded;
Pressure vessels may develop cracks in welds;
The wire ropes in suspension bridges are subject to weather, vibration, and high loads, so testing for broken wires and other damage is important.
Finished machined parts, such as bearings, that have newly been assembled can be tested for missing pieces, such as a ball or roller bearing, or grease within the housing non-destructively with a checkweigher. A roller motor for a conveyor can be tested for the proper level of oil, without disassembling the finished product. Thousand of manufactured products can benefit from this form of testing.
Over the past centuries, swordsmiths, blacksmiths, and bell-makers would listen to the ring of the objects they were creating to get an indication of the soundness of the material. The wheel-tapper would test the wheels of locomotives for the presence of cracks, often caused by fatigue — a function that is now carried out by instrumentation and referred to as the acoustic impact technique.
Use of X-rays for NDT is a common way of examining the interior of products for voids and defects, although some skill is needed in using radiography to examine samples and interpret the results. Soft X-rays are needed for examining low density material like polymers, composites and ceramics.
Methods and techniques
NDT is divided into various methods of nondestructive testing, each based on a particular scientific principle. These methods may be further subdivided into various techniques. The various methods and techniques, due to their particular natures, may lend themselves especially well to certain applications and be of little or no value at all in other applications. Therefore choosing the right method and technique is an important part of the performance of NDT.
Shopping
It is very difficult to weld or mold a solid object that has the risk of breaking in service, so testing at manufacture and during use is often essential. During the process of casting a metal object, for example, the metal may shrink as it cools, and crack or introduce voids inside the structure. Even the best welders (and welding machines) do not make 100% perfect welds. Some typical weld defects that need to be found and repaired are lack of fusion of the weld to the metal and porous bubbles inside the weld, both of which could cause a structure to break or a pipeline to rupture.
During their service lives, many industrial components need regular non-destructive tests to detect damage that may be difficult or expensive to find by everyday methods. For example:
Aircraft skins need regular checking to detect cracks;
Underground pipelines are subject to corrosion and stress corrosion cracking;
Pipes in industrial plants may be subject to erosion and corrosion from the products they carry;
Reinforced concrete structures may be weakened if the inner reinforcing steel is corroded;
Pressure vessels may develop cracks in welds;
The wire ropes in suspension bridges are subject to weather, vibration, and high loads, so testing for broken wires and other damage is important.
Finished machined parts, such as bearings, that have newly been assembled can be tested for missing pieces, such as a ball or roller bearing, or grease within the housing non-destructively with a checkweigher. A roller motor for a conveyor can be tested for the proper level of oil, without disassembling the finished product. Thousand of manufactured products can benefit from this form of testing.
Over the past centuries, swordsmiths, blacksmiths, and bell-makers would listen to the ring of the objects they were creating to get an indication of the soundness of the material. The wheel-tapper would test the wheels of locomotives for the presence of cracks, often caused by fatigue — a function that is now carried out by instrumentation and referred to as the acoustic impact technique.
Use of X-rays for NDT is a common way of examining the interior of products for voids and defects, although some skill is needed in using radiography to examine samples and interpret the results. Soft X-rays are needed for examining low density material like polymers, composites and ceramics.
Methods and techniques
NDT is divided into various methods of nondestructive testing, each based on a particular scientific principle. These methods may be further subdivided into various techniques. The various methods and techniques, due to their particular natures, may lend themselves especially well to certain applications and be of little or no value at all in other applications. Therefore choosing the right method and technique is an important part of the performance of NDT.
Shopping
Monday, 26 January 2009
Sunday, 25 January 2009
Concrete slump test
Concrete slump test
In construction and civil engineering, the Concrete Slump Test (or simply the Slump Test) is an in situ test or a laboratory test used to determine and measure how hard and consistent a given sample of concrete is before curing.
The Concrete Slump Test is, in essence, a method of quality control. For a particular mix, the slump should be consistent. A change in slump height would demonstrate an undesired change in the ratio of the concrete ingredients; the proportions of the ingredients are then adjusted to keep a concrete batch consistent. This homogeneity improves the quality and structural integrity of the cured concrete
The Concrete Slump Test is, in essence, a method of quality control. For a particular mix, the slump should be consistent. A change in slump height would demonstrate an undesired change in the ratio of the concrete ingredients; the proportions of the ingredients are then adjusted to keep a concrete batch consistent. This homogeneity improves the quality and structural integrity of the cured concrete
Purpose
The goal of the Concrete Slump Test is to measure the consistency of concrete. Many factors are taken into account when satisfying requirements of concrete strength, and to make sure that a consistent mixture of cement is being used during the process of construction. The test also further determines the “workability” of concrete, which provides a scale on how easy is it to handle, compact, and cure concrete[3]. Engineers use the results to then alter the concrete mix by adjusting the cement/water ratios or adding plasticizers to increase the strength of the outcome concrete mix.M
Procedure
The Concrete Slump Test has witnessed many technological advances, and some countries even perform the test using automated machinery. The simplified, generally accepted method to perform the test is as follows:
Apparatus
Large pan
Trowel to mix concrete mixture
Steel tamping rod
Slump cone
Ruler
Concrete (Cement, water, sand & aggregates).
Steps
Place the mixing pan on the floor and moisten it with some water. Make sure it is damp but no free water is left.
Place the sand in the pan. Add the cement and mix it with the sand.
Add the coarse/fine aggregate and thoroughly mix.
Mix the water and dry cement ingredients thoroughly using the trowel.
Firmly hold the slump cone in place using the 2 foot holds.
Fill one-third of the cone with the concrete mixture. Then tamp the layer 25 times using the steel rod in a circular motion, making sure not to stir.
Add more concrete mixture to the two-thirds mark. Repeat tamping for 25 times again. Tamp just barely into the previous layer(1")
Fill up the whole cone up to the top with some excess concrete coming out of top, then repeat tamping 25 times. (if there is not enough concrete from tamping compression, stop tamping, add more, then continue tamping at previous number)
Remove excess concrete from the opening of the slump cone by using tamping rod in a rolling motion until flat.
Slowly and carefully remove the cone by lifting it vertically (5 seconds +/- 2 seconds), making sure that the concrete sample does not move.
Wait for the concrete mixture as it slowly slumps.
After the concrete stabilizes, measure the slump-height by turning the slump cone upside down next to the sample, placing the tamping rod on the slump cone and measuring the distance from the rod to the ORIGINAL DISPLACED CENTER.
The goal of the Concrete Slump Test is to measure the consistency of concrete. Many factors are taken into account when satisfying requirements of concrete strength, and to make sure that a consistent mixture of cement is being used during the process of construction. The test also further determines the “workability” of concrete, which provides a scale on how easy is it to handle, compact, and cure concrete[3]. Engineers use the results to then alter the concrete mix by adjusting the cement/water ratios or adding plasticizers to increase the strength of the outcome concrete mix.M
Procedure
The Concrete Slump Test has witnessed many technological advances, and some countries even perform the test using automated machinery. The simplified, generally accepted method to perform the test is as follows:
Apparatus
Large pan
Trowel to mix concrete mixture
Steel tamping rod
Slump cone
Ruler
Concrete (Cement, water, sand & aggregates).
Steps
Place the mixing pan on the floor and moisten it with some water. Make sure it is damp but no free water is left.
Place the sand in the pan. Add the cement and mix it with the sand.
Add the coarse/fine aggregate and thoroughly mix.
Mix the water and dry cement ingredients thoroughly using the trowel.
Firmly hold the slump cone in place using the 2 foot holds.
Fill one-third of the cone with the concrete mixture. Then tamp the layer 25 times using the steel rod in a circular motion, making sure not to stir.
Add more concrete mixture to the two-thirds mark. Repeat tamping for 25 times again. Tamp just barely into the previous layer(1")
Fill up the whole cone up to the top with some excess concrete coming out of top, then repeat tamping 25 times. (if there is not enough concrete from tamping compression, stop tamping, add more, then continue tamping at previous number)
Remove excess concrete from the opening of the slump cone by using tamping rod in a rolling motion until flat.
Slowly and carefully remove the cone by lifting it vertically (5 seconds +/- 2 seconds), making sure that the concrete sample does not move.
Wait for the concrete mixture as it slowly slumps.
After the concrete stabilizes, measure the slump-height by turning the slump cone upside down next to the sample, placing the tamping rod on the slump cone and measuring the distance from the rod to the ORIGINAL DISPLACED CENTER.
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Saturday, 24 January 2009
Cable-Stayed Bridge
Radial attachment pattern
Parallel attachment pattern
Cable-Stayed Bridge
Cable-stayed bridges may look similar to suspensions bridges—both have roadways that hang from cables and both have towers. But the two bridges support the load of the roadway in very different ways. The difference lies in how the cables are connected to the towers. In suspension bridges, the cables ride freely across the towers, transmitting the load to the anchorages at either end. In cable-stayed bridges, the cables are attached to the towers, which alone bear the load.The cables can be attached to the roadway in a variety of ways. In a radial pattern, cables extend from several points on the road to a single point at the top of the tower. In a parallel pattern, cables are attached at different heights along the tower, running parallel to one other.
Friday, 23 January 2009
Proctor compaction test
Proctor compaction test
The Proctor compaction test and the related modified Proctor compaction test, named for engineer Ralph R. Proctor (1933), are tests to determine the maximum practically-achievable density of soils and aggregates, and are frequently used in geotechnical engineering.
The test consists of compacting the soil or aggregate to be tested into a standard mould using a standardized compactive energy at several different levels of moisture content. The maximum dry density and optimum moisture content is determined from the results of the test.
Soil in place is tested for in-place dry bulk density, and the result is divided by the maximum dry density to obtain a relative compaction for the soil in place.
History and its origin
Proctor's fascination with geotechnical engineering began when taking his undergraduate studies at University of California, Berkeley. He was interested in the publications of Sir Alec Skempton and his ideas on in situ behavior of natural clays. Skempton formulated concepts and porous water coefficients that are still widely used today. It was Proctor’s idea to take this concept a step further and formulate his own experimental conclusions to determine a solution for the in situ behaviors of clay and ground soils that cause it to be unsuitable for construction. His idea, which was later adopted and expounded upon by Skempton, involved the compaction of the soil to establish the maximum practically-achievable density of soils and aggregates (the "practically" stresses how the value is found experimentally and not theoretically).
In the early 1930s, he finally created a solution for determining the maximum density of soils. He found that in a controlled environment (or within a control volume), the soil could be compacted to the point where the air could be completely removed, simulating the effects of a soil in situ conditions. From this, the dry density could be determined by simply measuring the weight of the soil before and after compaction, calculating the moisture content, and furthermore calculating the dry density. Ralph R. Proctor went on to teach at the University of Arkansas.
Soil compaction
Compaction is the process of increasing the bulk density of a soil or aggregate by driving out air. For any soil, for a given amount of compactive effort, the density obtained depends on the moisture content. At very high moisture contents, the maximum dry density is achieved when the soil is compacted to nearly saturation, where (almost) all the air is driven out. At low moisture contents, the soil particles interfere with each other; addition of some moisture will allow greater bulk densities, with a peak density where this effect begins to be counteracted by the saturation of the soil.
Different tests
The original Proctor test, ASTM D698 / AASHTO T99, uses a 4-inch diameter mold which holds 1/30th cubic foot of soil, and calls for compaction of three separate lifts of soil using 25 blows by a 5.5 lb hammer falling 12 inches, for a compactive effort of 12,400 ft-lbf/ft³. The "Modified Proctor" test, ASTM D1557 / AASHTO T180, uses the same mold, but uses a 10 lb. hammer falling through 18 inches, with 25 blows on each of five lifts, for a compactive effort of about 56,000 ft-lbf/ft³. Both tests allow the use of a larger mold, 6 inches in diameter and holding 1/13.333 ft³, if the soil or aggregate contains too large a proportion of gravel-sized particles to allow repeatability with the 4-inch mold. To ensure the same compactive effort, the number of blows per lift is increased to 56.
The California Department of Transportation has developed a similar test, California Test 216, which measures the maximum wet density, and controls the compactive effort based on the weight, not the volume, of the test sample. The primary advantage of this test is that maximum density test results are available sooner, as evaporation of the compacted sample is not necessary.
There is also a test (ASTM D4253) which uses a vibrating table using standard vibrations for a standard time to densify the soil. This test method prevents particle breakage, but is only usable for granular soils. The test method also includes a method to determine the minimum density of the soil; density of soils in place are compared against the maximum and minimum to obtain a relative density.
The Proctor compaction test and the related modified Proctor compaction test, named for engineer Ralph R. Proctor (1933), are tests to determine the maximum practically-achievable density of soils and aggregates, and are frequently used in geotechnical engineering.
The test consists of compacting the soil or aggregate to be tested into a standard mould using a standardized compactive energy at several different levels of moisture content. The maximum dry density and optimum moisture content is determined from the results of the test.
Soil in place is tested for in-place dry bulk density, and the result is divided by the maximum dry density to obtain a relative compaction for the soil in place.
History and its origin
Proctor's fascination with geotechnical engineering began when taking his undergraduate studies at University of California, Berkeley. He was interested in the publications of Sir Alec Skempton and his ideas on in situ behavior of natural clays. Skempton formulated concepts and porous water coefficients that are still widely used today. It was Proctor’s idea to take this concept a step further and formulate his own experimental conclusions to determine a solution for the in situ behaviors of clay and ground soils that cause it to be unsuitable for construction. His idea, which was later adopted and expounded upon by Skempton, involved the compaction of the soil to establish the maximum practically-achievable density of soils and aggregates (the "practically" stresses how the value is found experimentally and not theoretically).
In the early 1930s, he finally created a solution for determining the maximum density of soils. He found that in a controlled environment (or within a control volume), the soil could be compacted to the point where the air could be completely removed, simulating the effects of a soil in situ conditions. From this, the dry density could be determined by simply measuring the weight of the soil before and after compaction, calculating the moisture content, and furthermore calculating the dry density. Ralph R. Proctor went on to teach at the University of Arkansas.
Soil compaction
Compaction is the process of increasing the bulk density of a soil or aggregate by driving out air. For any soil, for a given amount of compactive effort, the density obtained depends on the moisture content. At very high moisture contents, the maximum dry density is achieved when the soil is compacted to nearly saturation, where (almost) all the air is driven out. At low moisture contents, the soil particles interfere with each other; addition of some moisture will allow greater bulk densities, with a peak density where this effect begins to be counteracted by the saturation of the soil.
Different tests
The original Proctor test, ASTM D698 / AASHTO T99, uses a 4-inch diameter mold which holds 1/30th cubic foot of soil, and calls for compaction of three separate lifts of soil using 25 blows by a 5.5 lb hammer falling 12 inches, for a compactive effort of 12,400 ft-lbf/ft³. The "Modified Proctor" test, ASTM D1557 / AASHTO T180, uses the same mold, but uses a 10 lb. hammer falling through 18 inches, with 25 blows on each of five lifts, for a compactive effort of about 56,000 ft-lbf/ft³. Both tests allow the use of a larger mold, 6 inches in diameter and holding 1/13.333 ft³, if the soil or aggregate contains too large a proportion of gravel-sized particles to allow repeatability with the 4-inch mold. To ensure the same compactive effort, the number of blows per lift is increased to 56.
The California Department of Transportation has developed a similar test, California Test 216, which measures the maximum wet density, and controls the compactive effort based on the weight, not the volume, of the test sample. The primary advantage of this test is that maximum density test results are available sooner, as evaporation of the compacted sample is not necessary.
There is also a test (ASTM D4253) which uses a vibrating table using standard vibrations for a standard time to densify the soil. This test method prevents particle breakage, but is only usable for granular soils. The test method also includes a method to determine the minimum density of the soil; density of soils in place are compared against the maximum and minimum to obtain a relative density.
Thursday, 22 January 2009
Bridging by Segmental Box Girder
Bridging by Segmental Box Girder
SEGMENTAL BOX GIRDERS SYSTEM
Introduction.
Segmental box girders (segments) system is used for building superstructure for bridges / other structure in replacement of conventional construction via pre-cast beams and cast-in-situ decks. Segments will be pre-cast at another location preferably at outskirts of city. New setup of casting shall be done specifically to carry out the segments casting works. Segments are cast using specific moulds as the shape and dimension are also specific. Moulds may be made out of cut and weld steel sections and other accessories to complete the moulds. The cast segments later shall be brought to site and erected at specific location via launching girder system (or other method) by continue joining segments to each other to form the completed decks in between of two piers. Post tensioning in form of stressed groups of strand wires shall keep the segments together and provide the strength required to support the required loading on the deck. Further works can then be carried out as per requirement prior opening to users.
Advantages of using segmental box girders system (comparing to conventional method)
The segments system reduces the environmental disturbance compare to the conventional method by carrying out the concreting works further away from the construction site where is usually located at city centers.
Quality can be maintained since the casting of segments shall be carried out at control environment as compared to carry out at site.
Aesthetic of using segments system is better due specific shape can be cast if so required by clients.
Since segmental box girders are pre-cast elements, casting of segments can proceeds while piling works, pile-caps, piers, pier caps construction are proceedings at site. Therefore, shorter duration of construction is achievable.
Corrosion of Steel In Concrete
What is Corrosion of Steel?
ASTM terminology (G 15) defines corrosion as “the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties.” For steel embedded in concrete, corrosion results in the formation of rust which has two to four times the volume of the original steel and none of the good mechanical properties. Corrosion also produces pits or holes in the surface of reinforcing steel, reducing strength capacity as a result of the reduced cross-sectional area.
Why is Corrosion of Steel a Concern?
Reinforced concrete uses steel to provide the tensile properties that are needed in structural concrete. It prevents the failure of concrete structures which are subjected to tensile and flexural stresses due to traffic, winds, dead loads, and thermal cycling. However, when reinforcement corrodes, the formation of rust leads to a loss of bond between the steel and the concrete and subsequently delamination and spalling. If left unchecked, the integrity of the structure can be affected. Reduction in the cross sectional area of steel reduces its strength capacity. This is especially detrimental to the performance of tensioned strands in pre-stressed concrete.
Why Does Steel in Concrete Corrode?
Steel in concrete is usually in a non-corroding, passive condition. However, steel reinforced concrete is often used in severe environments where sea water or deicing salts are present. When chloride moves into the concrete, it disrupts the passive layer protecting the steel, causing it to rust and pit.
Carbonation of concrete is another cause of steel corrosion. When concrete carbonates to the level of the steel rebar the normally alkaline environment, which protects steel from corrosion, is replaced by a more neutral environment. Under these conditions the steel is not passive and rapid corrosion begins. The rate of corrosion due to carbonated concrete cover is slower than chloride-induced corrosion.
Occasionally, a lack of oxygen surrounding the steel rebar will cause the metal to dissolve, leaving a low pH liquid.
How to Prevent Corrosion.
Quality Concrete and Concrete Practices The first defense against corrosion of steel in concrete is quality concrete and sufficient concrete cover over the reinforcing bars. Quality concrete has a water-to-cementitious material ratio (w/c) that is low enough to slow down the penetration of chloride salts and the development of carbonation. The w/c ratio should be less than 0.50 to slow the rate of carbonation and less than 0.40 to minimize chloride penetration. Concretes with low w/c ratios can be produced by (1) increasing the cement content; (2) reducing the water content by using water reducers and superplasticizers; or (3) by using larger amounts of fly ash, slag, or other cementitious materials. Additionally, the use of concrete ingredients containing chlorides should be limited. The AI 318 Building Code provides limits on the maximum amount of soluble chlorides in the concrete mix.
Another ingredient for good quality concrete is air entrainment. It is necessary to protect the concrete from freezing and thawing damage. Air entrainment also reduces bleeding and the corresponding increased permeability due to the bleed channels. Spalling and scaling can accelerate corrosion damage of the embedded reinforcing bars. Proper scheduling of finishing operations is needed to ensure that the concrete does not scale, spall, or crack excessively.
The correct amount of steel will help keep cracks tight. ACI 224 helps the design engineer to minimize the formation of cracks that could be detrimental to embedded steel. In general, the maximum allowable crack widths are 0.007 inch in deicing salt environments and 0.006 inch in marine environments.
Adequate cover over reinforcing steel is also an important factor. Chloride penetration and carbonation will occur in the outer surface of even low permeability concretes. Increasing the cover will delay the onset of corrosion. For example, the time for chloride ions to reach a steel rebar at 2 inches from the surface is four times that with a 1 inch cover. ACI 318 recommends a minimum of 1.5 inches of cover for most structures, and increases it to 2 inches of cover for protection from deicing salts. ACI 357 recommends 2.5 inches of minimum cover in marine environments. Larger aggregates require more cover. For aggregates greater than ¾ inch, a rule of thumb is to add to the nominal maximum aggregate size ¾ inch of cover for deicing salt exposure, or 1 – ¾ inch of cover for marine exposure. For example, concrete with 1 inch aggregate in a marine exposure should have a 2 – ¾ inch minimum cover.
The concrete must be adequately consolidated and cured. Moist curing for a minimum of seven days to 70°F is needed for concrete with a 0.40 w/c ratio, whereas six months is needed for a 0.60 w/c ratio. Numerous studies show that concrete porosity is reduced significantly with increased curing times and, correspondingly, corrosion resistance is improved.
Modified Concretes and Corrosion Protection Systems – Increased corrosion resistance can also come about by the use of concrete additives. Silica fume, fly ash, and blast-furnace slag reduce the permeability of the concrete to the penetration of chloride ions. Corrosion inhibitors, such as calcium nitrite, act to prevent corrosion in the presence of chloride ions. In all cases, they are added to quality concrete at w/c less than or equal to 0.45.
Water repellents may reduce the ingress of moisture and chlorides to a limited extent. However, ACI 222 indicates that these are not effective in providing long-term protection. Since good quality concrete already has a low permeability, the additional benefits of water repellents are not as significant.
Other protection techniques include protective membranes, cathodic protection, epoxy-coated reinforcing bars, and concrete sealers (if reapplied every four to five years).
How To Limit Corrosion
Use good quality concrete air-entrained with a w/c of 0.40 or less.
Use a minimum concrete cover of 1.5 inches and at least 0.75 inch larger than the nominal maximum size of the coarse aggregate.
Increase the minimum cover to 2 inches for deicing salt exposure and to 2.5 inches for marine exposure.
Ensure that the concrete is adequately cured.
Use fly ash, blast-furnace, slag, or silica fume and/or a proven corrosion inhibitor.
ASTM terminology (G 15) defines corrosion as “the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties.” For steel embedded in concrete, corrosion results in the formation of rust which has two to four times the volume of the original steel and none of the good mechanical properties. Corrosion also produces pits or holes in the surface of reinforcing steel, reducing strength capacity as a result of the reduced cross-sectional area.
Why is Corrosion of Steel a Concern?
Reinforced concrete uses steel to provide the tensile properties that are needed in structural concrete. It prevents the failure of concrete structures which are subjected to tensile and flexural stresses due to traffic, winds, dead loads, and thermal cycling. However, when reinforcement corrodes, the formation of rust leads to a loss of bond between the steel and the concrete and subsequently delamination and spalling. If left unchecked, the integrity of the structure can be affected. Reduction in the cross sectional area of steel reduces its strength capacity. This is especially detrimental to the performance of tensioned strands in pre-stressed concrete.
Why Does Steel in Concrete Corrode?
Steel in concrete is usually in a non-corroding, passive condition. However, steel reinforced concrete is often used in severe environments where sea water or deicing salts are present. When chloride moves into the concrete, it disrupts the passive layer protecting the steel, causing it to rust and pit.
Carbonation of concrete is another cause of steel corrosion. When concrete carbonates to the level of the steel rebar the normally alkaline environment, which protects steel from corrosion, is replaced by a more neutral environment. Under these conditions the steel is not passive and rapid corrosion begins. The rate of corrosion due to carbonated concrete cover is slower than chloride-induced corrosion.
Occasionally, a lack of oxygen surrounding the steel rebar will cause the metal to dissolve, leaving a low pH liquid.
How to Prevent Corrosion.
Quality Concrete and Concrete Practices The first defense against corrosion of steel in concrete is quality concrete and sufficient concrete cover over the reinforcing bars. Quality concrete has a water-to-cementitious material ratio (w/c) that is low enough to slow down the penetration of chloride salts and the development of carbonation. The w/c ratio should be less than 0.50 to slow the rate of carbonation and less than 0.40 to minimize chloride penetration. Concretes with low w/c ratios can be produced by (1) increasing the cement content; (2) reducing the water content by using water reducers and superplasticizers; or (3) by using larger amounts of fly ash, slag, or other cementitious materials. Additionally, the use of concrete ingredients containing chlorides should be limited. The AI 318 Building Code provides limits on the maximum amount of soluble chlorides in the concrete mix.
Another ingredient for good quality concrete is air entrainment. It is necessary to protect the concrete from freezing and thawing damage. Air entrainment also reduces bleeding and the corresponding increased permeability due to the bleed channels. Spalling and scaling can accelerate corrosion damage of the embedded reinforcing bars. Proper scheduling of finishing operations is needed to ensure that the concrete does not scale, spall, or crack excessively.
The correct amount of steel will help keep cracks tight. ACI 224 helps the design engineer to minimize the formation of cracks that could be detrimental to embedded steel. In general, the maximum allowable crack widths are 0.007 inch in deicing salt environments and 0.006 inch in marine environments.
Adequate cover over reinforcing steel is also an important factor. Chloride penetration and carbonation will occur in the outer surface of even low permeability concretes. Increasing the cover will delay the onset of corrosion. For example, the time for chloride ions to reach a steel rebar at 2 inches from the surface is four times that with a 1 inch cover. ACI 318 recommends a minimum of 1.5 inches of cover for most structures, and increases it to 2 inches of cover for protection from deicing salts. ACI 357 recommends 2.5 inches of minimum cover in marine environments. Larger aggregates require more cover. For aggregates greater than ¾ inch, a rule of thumb is to add to the nominal maximum aggregate size ¾ inch of cover for deicing salt exposure, or 1 – ¾ inch of cover for marine exposure. For example, concrete with 1 inch aggregate in a marine exposure should have a 2 – ¾ inch minimum cover.
The concrete must be adequately consolidated and cured. Moist curing for a minimum of seven days to 70°F is needed for concrete with a 0.40 w/c ratio, whereas six months is needed for a 0.60 w/c ratio. Numerous studies show that concrete porosity is reduced significantly with increased curing times and, correspondingly, corrosion resistance is improved.
Modified Concretes and Corrosion Protection Systems – Increased corrosion resistance can also come about by the use of concrete additives. Silica fume, fly ash, and blast-furnace slag reduce the permeability of the concrete to the penetration of chloride ions. Corrosion inhibitors, such as calcium nitrite, act to prevent corrosion in the presence of chloride ions. In all cases, they are added to quality concrete at w/c less than or equal to 0.45.
Water repellents may reduce the ingress of moisture and chlorides to a limited extent. However, ACI 222 indicates that these are not effective in providing long-term protection. Since good quality concrete already has a low permeability, the additional benefits of water repellents are not as significant.
Other protection techniques include protective membranes, cathodic protection, epoxy-coated reinforcing bars, and concrete sealers (if reapplied every four to five years).
How To Limit Corrosion
Use good quality concrete air-entrained with a w/c of 0.40 or less.
Use a minimum concrete cover of 1.5 inches and at least 0.75 inch larger than the nominal maximum size of the coarse aggregate.
Increase the minimum cover to 2 inches for deicing salt exposure and to 2.5 inches for marine exposure.
Ensure that the concrete is adequately cured.
Use fly ash, blast-furnace, slag, or silica fume and/or a proven corrosion inhibitor.
Wednesday, 21 January 2009
Tuesday, 20 January 2009
Monday, 19 January 2009
PILE WALL
DEFINITION: PILE WALL IS A RETAINING WALL, EITHER DESIGNED AS CANTILEVER WALL OR ANCHORED WALL. PILEWALL IS USUALLY MADE OF CAST-IN-PLACE BORED PILES WITH DIAMETER, LENGTH AND REINFORCEMENT ADEQUATE TO THE HEIGHT OF THE RETAINED SOIL AND THE LATERAL FORCES ACTING ON THE PILE WALL.
PILEWALLS CAN BE MADE OF CONTIGUOUS PILES, TANGENT PILES, SECANT PILES, OR SIMPLY ALIGNED PILES WITH GAPS BETWEEN THE PILES FORMING THE PILEWALL.
PILES CAN BE PLACED ON A SINGLE LINE, OR ON TWO OR MORE PARALLEL LINES, DEPENDING UPON SOIL CONDITIONS AND DESIGN REQUIREMENTS.
SOIL ANCHORS, GENERALLY POST-TENSIONED, ARE APPLIED AT ONE OR MORE LEVEL OF THE PILEWALL, IF DESIGN LOADING CONDITIONS CALL FOR IT.
ANCHORS APPLIED TO PILE WALLS CAN BE OF THE FRICTION TYPE, DRILLED, INSERTED INTO SUBSOIL, GROUTED AND POST-TENSIONED, WITH THE “ACTIVE” LENGTH OF THE ANCHORS DESIGNED TO BE BEYOND THE POTENTIAL SLIPPAGE AREA OF THE SOIL FORMATIONS RETAINED BY THE PILE WALL, OR CAN BE ANCHORED TO DEAD-MEN PLACED AT A DISTANCE FROM THE PILEWALL SUFFICIENT TO MAINTAIN THEM OUTSIDE THE AREA OF POTENTIAL SOIL SLIPPAGE.
PILES FORMING THE PILE WALL CAN HAVE DIAMETERS RANGING FROM 300mm [ 0.3M ] TO 2500mm [ 2.5M ] AND ABOVE, DEPENDING UPON THE HEIGHT OF THE RETAINED SOIL, THE SOIL TYPE, THE PRESENCE OF WATER TABLE AND THE SEISMIC CONDITIONS OF THE GEOGRAPHICAL LOCATION OF PILEWALL.
FOR PROPER AND ECONOMICAL DESIGN OF PILE WALLS AND GENERALLY OF ANY RETAINING WALL, IT IS VERY IMPORTANT TO OBTAIN COMPLETE INFORMATION ON ALL PREVAILING SITE CONDITIONS THAT WILL AFFECT THE PILEWALL DURING ITS SHORT TERM CONDITIONS [CONSTRUCTION PERIOD] AND DURING LONG TERM CONDITIONS [SERVICE CONDITIONS] OF THE PILE WALL .
PILEWALL IS GENERALLY EXECUTED AS A TOP-DOWN TYPE OF CONSTRUCTION, BY INSTALLING THE PILES AHEAD OF GENERAL EXCAVATION, IN THIS WAY THE PILES THAT WILL EVENTUALLY FORM THE PILEWALL ARE INSTALLED ON UNDISTURBED SOIL, AND THE EXCAVATION ON ONE SIDE OF THE PILE WALL WILL PROCEED IN STAGES, DESIGNED TO KEEP THE SYSTEM RETAINED SOIL-PILEWALL IN EQUILIBRIUM.
Sunday, 18 January 2009
Bored Piles
Bored piles are cast in place cylindrical piles excavated either by use of rotary equipment operated augers , buckets, under static drilling fluid or large drill bit (for hard rock) with reverse circulation, with chisel grab and casing oscillator for bouldery ground, with large diameter DTH hammers and compressed air (drilled piles), among others.
Most common large diameter bored piles, are installed through an overburden of cohesive or cohesionless soil strata, with or without water tale, down to firmer ground, to achieve the design bearing capacity by skin friction, base bearing or both, to serve as foundation piles for residential, commercial, institutional buildings, industrial complexes or infrastructures.
Bored piles installed in common soil with the presence of water table, generally require the use of a short temporary steel casing and a drilling fluid as static suspension to provide support to the surrounding soil while excavating the pile and until complete backfill of the pile excavation with concrete, in order to prevent cave-in of the excavation and destabilizing the surrounding soil formation.
The preparation and handling by most effective drilling fluid, Bentonite Mud, is a sophisticated technology by itself and requires a complete set up of dedicated equipment and (basic) field laboratory.
The most common diameters of bored piles range from 0.6 meter to 2.0 m meters, likewise length can range from few meter to sixty or more meters, depending upon design loads and soil parameters.
Bored piles can be heavily reinforced if required by design, rebar cages usually are prefabricated in segments with length and weight depending upon available commercial lengths of rebars and available lifting equipment. Splicing of rebar cages can be done by lap splice, welded lap splice or mechanical threaded couplers. Casting is done by pouring concrete with the design strength and slump as required, through watertight segmental Tremie Pipes, starting from the pile bottom and letting the tremie pipe bottom end remain at least 3 meters submerged in concrete until the completion of pouring, to guarantee the pile continuity and the final good quality of the concrete cast.
Drilling fluids, if needed, can be water, a suspension of bentonite (bentonite mud), a suspension of polymers, depending upon soil type, soil conditions, presence and elevation of water table, chemical properties of water table (Ph, Salinity).
Steel casings can be temporary, in which case the wall thickness is usually big enough toallow many uses, are provided with collars for easy handling by vibro hammers and diameter slightly larger than bored piles’ nominal diameter, to allow easy passage of drilling tools. Permanent casings, if needed, are sacrificial casings and as such the wall thickness is as small as allowed by the need to drive the casing through the ground.
Bored piles are commonly employed for bridge foundations, on land and water, because the versatility of bored piles design and execution allows the construction of practically any needed diameter, including the very large diameters, and the pile reinforcement can be provided as heavy as needed by seismic design and the codes (it is not uncommon to have double wall rebar cages in order to accommodate all the needed bars, however care shall be taken to leave sufficient space between bars for concrete to low through).
Bored piles are also used to form retaining walls (see PileWall.com), as contiguous pile wall or secant pile wall or aligned pile wall, with or without post tensioned soil anchors as tie back .
Bored piles testing is usually done in two (2) stages, first stage testing to verify design assumptions and achievable design load , is done before starting the execution of the working piles: test piles are installed in the proposed construction area, as per design, and tested, first for integrity and continuity by P.I.T. (Pile Integrity Test) then for load bearing capacity , either by Static Load Test or by Dynamic Load Test PDA, (Pile Dynamic Analysis). At times the Designer might require a Pull-out test and a lateral load test.
Once the design pile capacity has been confirmed, Bored Piles construction for the working piles starts and quality control is then done on representative piles. Quality Control consists of testing the material used for the bored piles, i.e. reinforcing bars and concrete, then testing of the piles at random with PIT and PDA, and predetermined piles with static pile load test. Predetermined piles can also be tested using the cross-hole ultrasonic test, by inserting instruments through vertical pipes installed within the reinforcing steel cage all throughout the bored pile length.
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