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Transportation Curriculum Coordination Council |
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Chapter 9
Concrete Operations
Contents
- This Chapter contains an overview of the concreting of the shaft following rebar cage installation and an overview of the Inspector's responsibilities during this phase of drilled shaft construction
- A short quiz is provided at the end of the Chapter.
Learning Objectives
When you have completed this Chapter, you will be able to:
- Describe, in general, the Inspector's role during the placement of concrete
- Describe, in general, the concrete placement methods and process
- Determine theoretical shaft concrete volumes and develop placed concrete volume curves
Concreting Operations
Concreting of the shaft is the final step in the construction process itself. Up until this time, the Contractor has been willing to spend time with the Inspector but often this changes once the concrete is on the way. There are generally time limits, slump requirements, etc., a whole host of issues or potential problems that can occur during this phase. Remember, if concreting goes bad, the shaft is lost and everything the Contractor has done up until this point is essentially lost.
The Inspector needs to perform their duties promptly and efficiently. Speed is of the essence, but do not sacrifice quality and thoroughness.These duties may, depending upon the specifications, include performing standard field concrete tests, monitoring concrete placement and development of the placement curves.
Pictured here is the arrival of the concrete truck. Depending upon the specifications, the Inspector may have concrete sampling and testing to do in addition to placement monitoring.
Concrete Type and Slump
When the concrete arrives on-site, the Inspector may be required to verify the proper mix design is being delivered, that it meets the slump requirements and perform standard field tests. Typically there will be a time limit imposed by the specifications relating to the length of time for concrete placement.
Remember, it is imperative that the hole be clean and this should have been verified by the Inspector before the rebar cage was installed.
Typical concrete field tests the Inspector may be required to perform include:
- Slump
- Air Content
- Temperature
- Test Specimens
Shown above is the typical slumps specified for drilled shaft concrete.
The slump to the left is OK for drilled shafts. Notice the concrete is plastic, not like the one pictured to the right, where the slump is Ok but the concrete not plastic as the segregation of the concrete is visible. 
The photo to the left shows a concrete with too low of a slump. Would be difficult to pump or place through a tremie.
Concrete Placement Methods
A variety of methods or techniques are used by the Contractor to place the concrete. This selection generally depends upon the type of shaft construction being used and the most common one.
Tremie Placement - Gravity-fed tremie placement is generally used for wet shaft construction. In this method, the concrete is introduced into the hole, starting at the bottom, using a water tight tremie (tube). The concrete is fed by pump or bucket into the tremie and falls by gravity and continuously placed until the shaft is full.
Pump-line Placement - This method is similar to the tremie method except that the concrete is "pumped" into the hole, rather than gravity fed. (A pump-line can be used to feed concrete to a tremie in tremie placement).
Free-Fall - In this method, the concrete is placed by free-falling from the top of the shaft to the bottom and is typically used for dry shaft or dry cased shaft construction only. Of importance with this method is that the concrete must be directed to free-fall down the center of the cage and not make contact with the cage or shaft walls. The specifications will often specify the maximum distance concrete may free-fall.
Pictured here is a tremie, with a hopper on top, placed in the shaft for concrete placement.
Pictured here is the pump-line method in use.
This picture shows concrete being placed by the free-fall method.
Concrete Placement Process
The goal of concrete placement is to get the shaft filled with the specified concrete and have no voids or sediment/debris inclusions that effect the structural integrity of the shaft.
During placement by tremie or pump-line, the discharge end is placed near the bottom of the hole and concrete flow started. The concrete, as it rises and fills the shaft, displaces the sediments.
During the pour, whether by tremie or pump-line, the concrete flow must be continuous and the discharge end of tremie or pump-line must be immersed in the concrete a specified distance, typically 5 ft. (1.5 m). If not, and the discharge end breaches (raised above the concrete flow level) the shaft is rejected. The tremie is raised as the concrete level rises, but the required immersion distance maintained.
The placement continues until fresh concrete overflows the top of the shaft.
In this picture, the Inspector is checking the tremie, for material, leaks, etc.
This is a picture of concrete overflowing the shaft.
This picture is looking down inside a cased shaft with the concrete being place by the pump-line method.
Concrete Volumes
So, how much concrete should go into this hole and how do we know if sufficient concrete is being placed? The drilled shaft Inspector deals with two concrete volumes, the theoretical and the actual.
Theoretically, the drilled shaft should take "x" cubic yards or cubic meters of concrete. By calculating the volume of the shaft, we can arrive at "x".
The actual must be determined during the actual placement. By comparing the actual, as it is placed, to the theoretical, the Inspector can get a "feel" for what is happening below the ground surface. For example, if your gas tank was registering empty and holds 16 gallons, but when you gas up you can only get 10 gallons in it, something is wrong. Something is taking up space in the tank or it has collapsed some, assuming the gauge works.On the other hand, you reach 16 gallons, then 18, then 20 gallons, it is probably leaking.
These illustrations show how this relates to a drilled shaft.
With this shape, you would expect the theoretical and actual volumes to be quite close.
But, what if the shaft looked like this below ground surface?
This subject, due to its complexity, is covered in detail in the Drilled Shaft Inspector course.
So, based upon these illustrations, we know that if we compare the actual to the theoretical volumes, during placement, we'll have a feeling for what is happening below ground. This is done with the Concrete Volume Curves.
This is a large part of the Inspector's duties during concreting. The Inspector prepares the Concrete Volume graph, calculates the theoretical and plots it. Then, during concrete placement, the Inspector determines the volume of concrete placed after each load, plots these values, which forms a plot relative to the theoretical.
Theoretical Volume - To calculate the theoretical volume of the shaft:
To obtain the actual volumes, the Inspector needs some basic information such as the volume per truck, the depth in the hole to the top of concrete following each load and the elevation of the top of concrete after each load.
This is best collected in a table form, much like illustrated below. The elevation at the top of cumulative concrete amount after each load is plotted at the corresponding top of concrete elevation.
The plot of the concrete volume curves is typically performed on a provided inspection form. Illustrated below is how the data and curves are developed based upon the collected data.
First the Elevation and Cumulative Volume axis are labeled. Make sure the graph (see graph below) is labeled to include the bottom of shaft and top of shaft elevations and the total cumulative cubic yards.
Next plot the theoretical volume. Plot the Bottom of Shaft elevation at 0 yards and the theoretical volume at the the corresponding Top of Shaft Elevation.
Next plot each cumulative total at the corresponding elevations starting with 0 at the bottom of shaft elevation. Draw in the plot.
Actual plots that:
Parallel the theoretical generally OK
Move away from theoretical generally OK
Cross over or move back towards theoretical generally indicate a problem.
I you have completed Chapter 9 and am ready to take the Quiz
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