Ameria Bridge Construction
Thursday, 22 January 2009
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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