foundation system of burj al arab

Why is Workability Required in Fresh Concrete?

When a concrete mix is readily compactable, it is said to be workable mix. Workability defines ease in compaction and resistance against segregation. But expected workability for a specific case depends on available means of compaction, as an example, workability sufficient in case of mass concrete is found quite insufficient for placing in inaccessible, narrow or section with reinforcement congestion.

Thus workability should have a definition irrespective of reference about circumstances prevail for a particular construction. To define this physical property, it is essential to take consideration about mechanism of compaction.

Mechanism of compaction:


The compactive effort is delivered to concrete by whether ramming or vibration. Whatever the method, this process consists of removal of air trapped within concrete until mix has reached close configuration as much as possible for particular concrete mix. The purpose is to overcome two type of friction namely:



• Internal friction

• Friction between constituent particles of concrete

• Surface friction; friction between surface of reinforcement, mold or formwork and concrete itself.

Thus compacting effort should be enough to overcome both types of frictions, additionally some of work done has been utilized to vibrate mold, or to shock and, actually, to vibrate that portion of concrete mix has already been consolidated completely.

Workability:


Interestingly, the effort applied has some portion that is wasted and some portion is useful that are used to overcome surface friction and internal friction as mentioned earlier. Among these, only internal friction is inherent property of concrete mix; so the best definition of workability is amount of internal useful work that are required to achieve full compaction(defined by Glanville et al.)

According to ASTM C125 (Standard Terminology Relating to Concrete and Concrete Aggregate):

Property that determines effort required to use/place a particular quantity of fresh concrete mix least loss of homogeneity.

According to ACI 116R (Cement and Concrete Terminology):

The property of fresh concrete mix or mortar that determines ease, homogeneity or uniformity with which mixing, placing, compaction and finishing of concrete can be done. 



Consistency:


Consistency is another term that defines state of concrete in fresh condition. The ordinary meaning of this word is firmness of structure of a material or ease to flow. Consistency of concrete is often taken as degree of wetness; to some extent, wetness is a measure of workability as wet concrete is more workable than dry one.

According to ACI code:

Consistency is ability of fresh concrete mix or mortar to flow or in one word relative mobility of mix.

Consistency Vs workability:


Concrete having same consistency may not have same workability. A concrete having high consistency essentially more mobile, may not have right workability for particular job. The workabilities for a particular job say roof construction may differ depending on thickness of section; even for same section different workability may require when vibrator is applied instead of hand compaction. Thus instead of only mobility workability is based on assumption of

• Construction type
• Thickness of concrete section
• Compaction techniques
• Obstruction of reinforcement against mobility of concrete

In technical terms we can present many differences between workability and consistency, but by definition they are all qualitative by nature; reflection of personal point of view rather than technical. Same comments can be addressed for additional terms like

• Mobility
• Flowability
• Pumpability


Besides these, a term stability is also used in concrete technology which means cohesion of concrete mix i.e. its resistance against segregation.

How to Mitigate Rejected Pile?

Rejected pile can be repaired by several options. At first we have to assess the pile to detect defects, the pile can be repaired, supplemented, can be replaced or contractor can submitted an alternate solution to keep pile in place. This proposal can be submitted as pile mitigation plan to engineer.

The test method is selected to determine location of defected area, whether this is within core of pile or near the perimeter of surrounding reinforcing cage of pile bar. There have several ways for mitigating pile once it is established that there have sufficient anomalies to reject it. Defective cast in drilled hole piles can be mitigated using following four ways:

• Excavate and recast

• Pressure grout or grouting repair

• Structural bridging

• Supplement/replacement



Excavate and recast of pile:


A rejected pile is repaired in this method by simply removal of defective concrete by mechanical method and replaced it at defective zone of pile. When such repair is conducted with 5 feet from top of pile head, the repair is termed as simple repair. 


Typically this repair method is selected to mitigate defects resulting from not wasting required amount of concrete from top of pile.However, such repair can be used for deeper defects in concrete along the length of pile. Proper shoring is provided to allow access to defective area. Contractor will propose such repair at more than 5 feet below the top of pile; engineer should consult with geotechnical professional to determine the effect on skin friction as a result of accessing to defected area of pile.


Grouting repair:


This type of repair is adopted to mitigate pile that has defects in concrete within it. These repairs, if adopted, can be executed at any depth within pile, but generally, not conducted when defects are located within five feet from the top of pile of interest, as it is considered more effective to conduct basic repair, as discussed above, at these location. Generally three type of procedures are applied in grouting repair techniques; these are

• Permeation grouting
• Replacement grouting
• Compaction grouting



Assessing feasibility of grouting:



Different operations are familiar in replacement and permeation grouting repairs. At first, it has to be ensured that defective area is accessible. The inspection pipes installed usually used in this purpose. The inspection pipe, generally, is removed using high pressure water jet at defective region of pile. Water jet cuts inspection pipe and left into pieces small enough to flush out them through top of inspection pipe. Once inspection pipe at defective area is removed, the next step is to flush this defective area with high pressure water jets. The discharge of flush is then carefully inspected for soil, chips and other contaminants. After this initial operation of washing is completed, the defective area will be evaluated by water flowing testing.

Water flowing test is required to determine nature of defective area to choose whether replacement or permeation grouting is suitable to repair. If injected water can reach into defective area under low pressure and volume of water injected is relatively high, it is considered permeation grout is the right technique for grouting repair.

When defective area of pile is found sufficiently large and permeable, it is suspected to have communication with neighbor inspection pie, and should be checked thoroughly i.e. water injected through an inspection pipe may come out through neighbor inspection pipes to ground surface. Water may flow into soil formation as well if defective area expands to edge of pile concrete. When water cannot reach to defective area of pile, replacement grouting is considered perfect technique for grouting repair. This indicates that defective area located into the pile concrete i.e. this region is embedded into soundconcrete.

After the completion of water flowing test, the defective area are typically flushed with low pressure water flow to flowout any existing loose material. A down hole camera may be used or any other method suitable to ensure no loose materials left during washing.

Permeation grouting:


Objective:

This method usually applied to repair soft tip of pile

• To enhance fractional resistance throughout the length of pile
• To deal with corrosion issues along sides of pile.


Usually this technique is adopted to repair defects associated with presence of excessive amount of settled materials, can’t be removed from bottom of drilled hole before concrete placement. Usually permeation grouting is suggested only when soil exists in discharge of washing of defective area or permeability is ensured in defective area during flowing water test.

Procedure:

The first task is to remove inspection pipe from defective pile at predetermined location ascertained by flowing water test. Then high pressure water jets are used to wash this area to flush out loose materials or any contaminant. The discharge of washing operation is then evaluated. Grout is injected under high pressure which permeates into concrete or soil formation and displaces pore water that may take place in permeable voids. Thus matrix of defective concrete/soil and cement grout is formed.

The grout is usually injected through one of inspection pipes. Thus success of permeation grout mainly depends on permeability of formation i.e. if pore water exists in these formations can be displaced by the pressure of grout. So pore water must be removed either through adjoining inspection pipe or into adjacent soil. Thus grouting technique requires presence of adequate confining pressure for executing grouting operations not returning grout to the ground surface.. This is the cause of not selecting permeation grouting to repair defects within 10 feet from ground surface or defects entirely within pile.

Replacement grouting:


Where to use:

• Typically adopted to repair void remain in concrete or any unconsolidated area. Thus defects related to concrete placement error can be repaired by replacement grouting

• Replacement grouting is usually suggested only when no soil is present in discharge i.e. if water flowing test indicates impermeability of defective area

• Replacement grouting is not suitable if grout finds ways to come out/escape from void area.

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