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HALF-SLAB SLIPPAGE PROBLEM

HALF-SLAB SLIPPAGE PROBLEM

(OP)
We are a retailer of Prestressed Concrete Strands 9.5mm (ASTM A416). Our customer experienced slippage on their Precast Half Slab and bonding problem with concrete due to oiled Strands. Our supplier informed us that they applied Water-Soluble Oil and it will be dried by natural air drying for 2-3days. After several days out in the open-air, there is still oil/grease residue on the strands as seen in attached image and our customer has lost confidence of using the remaining stocks of strands which is 88 metric tons (16 coils).

I would like to receive your insights and solution on how to remove the oil, what type of chemical to apply to it and speed up the process to remove it totally to salvage the remaining affected stocks of strands.

RE: HALF-SLAB SLIPPAGE PROBLEM

JBP00:

This can be a serious problem - both the strand slippage/deficient bond issue, and the removal of the residual oil.

In my area of practice, about 10 years ago one of the local precaster's experienced strand bond slippage and it was due to the residual coating that was left-over by the manufacturing process. I understand the strand was rejected and remaining stock was returned.

What is the country of origin of the strand?

The US-based TRANSPORTATION RESEARCH BOARD authored a document in 2008 entitled "Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete". It is very detailed on matters related to strand surface, and may address part of your problem. Free downloaded copy is available here: Link

Part copy of paragraph from page 7 of the above document:



I would be placing the onus on the manufacturer to directly address the issue - bypass the supplier and get answers from the manufacturer.

In the past, have any of your other customers experienced similar issue with the same strand manufacturer?

RE: HALF-SLAB SLIPPAGE PROBLEM

Ingenuity... there may not be any contractual relationship with the manufacturer, only the supplier.

Dik

RE: HALF-SLAB SLIPPAGE PROBLEM

dik, probably true, however, if the manufacturer does not step up to the plate they will not only have a irate supplier, but an end-user that will never purchase such a brand again...along with telling all precast competitors of the issue.

Too many players in this situation: user-retailer-supplier-manufacturer.

When I used to purchase strand in large quantities (typically from Japan and the US manufacturers) it was always preferable to go as close to the manufacturer as possible. Most times you have to go through a distributor - but we often had a direct link to the manufacturer to check on production/supply delays.

When we used to purchase product by Florida Wire and Cable in Jacksonville. We purchased directly from them. Made it easier to get resolution on a 100t consignment that rolled over in containers due to inadequate tie downs.

RE: HALF-SLAB SLIPPAGE PROBLEM

(OP)
Ingeniuty,

Thank you for your response. Our Manufacturer is from China and we have sourced to them with 4 shipments before and only now our customer(end-user) has experienced this issue. Our Manufacturer informed us that we shall apply kerosene and wipe cloth after to clean the residue.

May I ask if you have a swiftly way to remove the oil residue from the strands?

RE: HALF-SLAB SLIPPAGE PROBLEM

Does not kerosene leave an oily residue... not positive, but seem to recall. Acetone may be a better choice... but, if you have miles of the stuff... can you dip the rolls in a solvent?

Dik

RE: HALF-SLAB SLIPPAGE PROBLEM

(OP)
Our manufacturer informed us that we shall apply kerosene and wipe it after to dry it out. If we dip the whole coil in solvent(acetone), should we wipe it off after or just let it dry?

Dik,

Which solvent do you think we can dip it into and not wipe it off after to reduce work? Thank you.

RE: HALF-SLAB SLIPPAGE PROBLEM

Quote (dik)

Does not kerosene leave an oily residue... not positive, but seem to recall.

dik: I think you are correct.

JBP00: If I was the end-user I would REJECT the strand. Period. The effort to clean and the risk/liability that it was indeed satisfactory, is too great, in my opinion.

Quote (JBP00)

Our Manufacturer informed us that we shall apply kerosene and wipe cloth after to clean the residue.

A 3-ton coil of 1/2" dia strand is 12,000 feet long, so 9.5mm would be even longer. Using a wipe-cloth to remove any residual kerosene or any other applied liquid/cleaner, will take a long time to execute, and it is only practical to clean it after the strands are laid in the bed (but prior to stressing) - but very laborious.

I have de-greased and re-greased many UNbonded strand tendons using a custom-made automated system, but your issue is very different.

If the oil is indeed 'water-soluble oil' then clean and wipe with water will work, BUT, you run the risk that water will fill the interstices between the outer 6-wires and the central king-wire, and may promote future corrosion. One of the reasons why the manufacturing process uses drying processes.

7-wire strand manufacture is a complex one. Don't mess with the end-product.



RE: HALF-SLAB SLIPPAGE PROBLEM

(OP)
Ingeniuty,

Thank you for you advice. Since the coils cannot be returned to China, we shall degrease the coils as our customer wants to solve this issue as soon as possible. May you advice us how to degrease the coil without harming the strands and not take too much labor?

RE: HALF-SLAB SLIPPAGE PROBLEM

Low concentration sodium hydroxide bath followed by clean water bath and then followed immediately by heating to remove moisture? I agree with Ingenuity that this is going to be a lot of work to even come close to guaranteeing that the final wire will perform without any corrosion issues and have all the oil removed with an easy to implement process.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: HALF-SLAB SLIPPAGE PROBLEM

Can anyone suggest a means of salvaging the coil(s) of strand? Is it possible to dip in a solvent to remove the oil? provide sufficient corrosion resistance and maintain any certification?

Dik

RE: HALF-SLAB SLIPPAGE PROBLEM

dik, that's why I suggested the sodium hydroxide (aka lye or caustic soda, essentially oven cleaner) as probably the most effective industrial strength degreaser I can think of that would require little manual labor. I'm no chemist but I believe sodium hydroxide reacts with most water soluble oils and fats by turning them into soap. This way the oils the sodium hydroxide removes are trapped in solution and wont re-adhere to the wires. The other advantage is sodium hydroxide is available on an industrial scale and would be much cheaper than trying a solvent like acetone (which may not fully clean the strands depending on the oil).

Sodium hydroxide is corrosive though so some small trial batches should be done to establish the minimum concentration required to remove all the oil and verify no corrosion concerns exist following the treatment. Then a hot water bath to remove any remaining sodium hydroxide solution or maybe an acid bath as this would help pickle the steel and neutralize the sodium hydroxide. Then a propane heater to heat dry the coils and you're done.

It's the most effective way I can think of to degrease a large quantity of strand with minimal manual labor, rapid cleaning, and a controllable process to keep degradation of the wires to a minimum.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: HALF-SLAB SLIPPAGE PROBLEM

Were you able to resolve this problem. If so, what was the solution?

Dik

RE: HALF-SLAB SLIPPAGE PROBLEM

(OP)
Hello Dik,

We have tapped degreasing firms that also supply rust preventive Oil. They informed me that dipping process with caustic solution is not possible since the strands should be uncoiled and not applicable on this process and will not penetrate the inside of the coil. The strands cannot be uncoiled as per our customer.

The degreasing firm have sampled to me wiping with cloth dipped with solution (alkaline or CL I am not sure) and it removed the oil totally. Although this process is laborious, I think this is the only Solution.

Ingeniuty,

May you inform me your automated solution for degreasing?
"I have de-greased and re-greased many UNbonded strand tendons using a custom-made automated system, but your issue is very different."

Regards.

RE: HALF-SLAB SLIPPAGE PROBLEM

Quote (JBP00)

May you inform me your automated solution for degreasing?

Unfortunately I cannot as I am in the process of applying for a patent on the equipment and technique I developed. Sorry.

RE: HALF-SLAB SLIPPAGE PROBLEM

(OP)
Ingeniuty,

Thank you for the response, this technique your are applying for a patent can degrease the strands even the coil is intact?

JBP

RE: HALF-SLAB SLIPPAGE PROBLEM

No, it is for existing/installed strands within a building floor systems that utilize UNbonded (greased and sheathed) 7-wire strands.

RE: HALF-SLAB SLIPPAGE PROBLEM

(OP)
Hello everyone,

One of degreasing firm here informed me that it is possible to dip the coils in an emulsifiable degreaser. I would like to ask for your insights regarding the penetration of the degreaser for the whole coil on degreasing bath.

TYPICAL CHARACTERISTICS:
Specific Gravity @ 15.6 °C 0.797
Appearance Clear
Flash Point, TCC, °C 40.6
Color, ASTM + 28
Appearance of Emulsion Milky White


Regards,
JBP

RE: HALF-SLAB SLIPPAGE PROBLEM

Based on the information you provided, we can offer no opinion (speaking on behalf of everyone).

Dik

RE: HALF-SLAB SLIPPAGE PROBLEM

I'll join with dik; that doesn't tell me anything particularly useful about the degreaser.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

RE: HALF-SLAB SLIPPAGE PROBLEM

(OP)
Hello Everyone,

I have a new development regarding our problem on the slippage. We have a new brand from another mill of PC Strands supplied to our customer who has slippage problem; the new strands has no visible and touchable oil in it which our customer agreed. After they casted and tested new strands, there is still slippage occurrence.

Regards,
JBP

RE: HALF-SLAB SLIPPAGE PROBLEM

What is the concrete mix? can you take a small length of strand and cast it into a concrete sample so the embedment is a couple of feet and take it to a lab and do a tension test to see what sort of resistance you have?

Sounds like the problem is more than just the strand.

Dik

RE: HALF-SLAB SLIPPAGE PROBLEM

Quote (JBP00)

After they casted and tested new strands, there is still slippage occurrence.

The nature and technique of pre-tensioned strands embedded in concrete is that some slip will occur - the transfer length of strand is 50 x nom. diameter of strand - however, excessive slip is the real cause for concern. Researchers talk about values between 0.5mm and 1.5mm. Pretty small magnitudes and typically field measurements are not of such precision.

A few questions:

1. How is the precaster measuring the slip? Things done in the field are often not especially 'accurate' - so it may be important to know both the magnitude of the slip and how it is measured.

2. How is the precaster releasing the stressing force from the bed to the concrete - gradually or sudden?

3. What is the length of the precast segments being manufactured? Are the slabs/planks especially short in length?

I also agree with dik - conduct a few strand/concrete bond tests using the same concrete and strand as used in production.

RE: HALF-SLAB SLIPPAGE PROBLEM

There is a problem with doing strand pull out tests. They work differently to pretensioned strand development. With pretensioned strand development, there is a wedging effect as the strand it released and "expands". Doing a tension pullout test will result in very different figures as the reverse happens and the diameter reduces slightly, effectively reducing bond, so the transfer/development length increases by about a factor of 2.5 according to the Australian code.

For normal strand, Australian code would suggest a minimum transfer length of 60db but could be higher depending on the jacking stress. If you assume a linear loss in force and 60db transfer length, the slip would be (all in SI) .7 * 1850 / 2 * 9.5 * 60 / 200000 = 1.85mm! As the first .1Ldt is assumed to have no force in it, this would increase to about 2mm

RE: HALF-SLAB SLIPPAGE PROBLEM

(OP)
Dik,

Our customer reported to us that Test results of the 11 castings show that they reach only 60% of the designed capacity thus they cannot deliver these to their customers.

Please be informed that we received the slippage report from them 14 days after the first slippage occur and now they are claiming for us the cost of the failed castings. We contested the claim since we were not aware that there is a slippage on the first place and they still cast the strands because they want to eliminate certain suspects like they said they changed Form Oil, Concrete mixture etc. and they least expected the strands to be the cause since they used this same brand of strands for almost 4 months already.

And now we supplied to them another brand and non-oily strands and still slippage occurs. We are having the benefit of the doubt if the two brands we supplied is the real cause of slippage.

JBP

RE: HALF-SLAB SLIPPAGE PROBLEM

JPG00

Quote (JBP00)

Test results of the 11 castings show that they reach only 60% of the designed capacity

Eleven (11) panels were destructively load tested, and they failed at 60% of design? Flexural or shear failure?

Find out the length of the panels.

RE: HALF-SLAB SLIPPAGE PROBLEM

(OP)
Ingeniuty,

Once casting has a total sqm of 146.4.

B. CROSS SECTIONAL PROPERTIES of slab

Width = 1,196mm
Height of plank = 60mm
Height of rib = 70mm

JBP

RE: HALF-SLAB SLIPPAGE PROBLEM

So the bed is about 120m long.

What are the typical panel lengths after they cut them from the bed?

RE: HALF-SLAB SLIPPAGE PROBLEM

JBP00

I think your company really needs to hire a consultant who understands the design aspects and can check all of these the design of these precast elements.

This is getting too complex to be sorted out with questions on Eng-Tips.

RE: HALF-SLAB SLIPPAGE PROBLEM

Can you dip them in a container under a partial vacuum to displace air?

Dik

RE: HALF-SLAB SLIPPAGE PROBLEM

(OP)
Hello All,

Thank you for the insights that you've shared to us. I have a new development on our issue on the strands. Since we have supplied another source from another mill to our customer who experienced slippage even though it has lesser oil and dry; the second supplier informed us that they have questions on the production process of our customer. Please see summary below from our customer:

1. Tension Method: Pretension, products are pre-stressed concrete slabs and inverted T-beams(t-joist).

2. Force Applied to Strands: Depends on the type of product; it was about 64950 N or about 15Mpa from the Stressing machine gauge.

3. Measuring of slippage: Thru a caliper / Visual

4.When the concrete hardens when do you release the force on the PC Strand? How do you release the stressing force from the bed to the concrete?: When the concrete hardens to more than 4,200 psi(29Mpa). This is determined when the maturity set is reached and thru compression test on concrete cylinder samples. De-stressing is carried out by means of a hand tools(Bolt cutter) by cutting one strand at a time.

Additional Response:
Our customer produces pre-stressed concrete slabs on a 1.2m width x 132m long steel bed with fixed anchors at both ends. The production process begins by cleaning the casting bed.
When the bed is ready, the required strand wires are pulled from active end to the passive end. After strands are placed in their correct places in the stressing abutment and fastening of the grips is checked, form oil is now applied. Afterwards, the strands are now stressed one strand at a time using a Stressing Jack. The casting is done thru Elematic Slipformer machine that runs from the active to the passive end of the bed. Curing starts immediately after casting and it is being controlled by a Maturity Control system. When the desired maturity of the concrete is met and samples have attained the required releasing strength then strands are now detensioned. The slab is then marked and cut according to required size. Cutting is done using a slab saw. The cut slab is lifted up and move to the Slabshifter and eventually to the stockyard.

Sorry for the long post, I just want to share and eventually receive another response from you guys.

JBP

RE: HALF-SLAB SLIPPAGE PROBLEM

Most precast operations I've encountered release at full strength, but, the precast is steam cured. It is then released and 'cut to fit'... Strand usually relaxes and 'pulls back' int the concrete approximately 1 to 2 mm.

Dik

RE: HALF-SLAB SLIPPAGE PROBLEM

How much slippage is there at the ends?

How long is each panel after cutting/

Have you hired a consultant/expert to check the capacity of the panels? If they are all failing at 60% of the expected capacity, there are 2 possibilities, either

- the panels are defective
or

the design is wrong and the panels are failing at the correct capacity.

You need to get this checked by an expert!

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