HALF-SLAB SLIPPAGE PROBLEM 3

[JBP00]

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.

 

[JBP00]

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

 

[dik]

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

 

[Ingenuity]

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.

 

[rapt]

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

 

[JBP00]

Ingeniuty,

I will ask the end-user these questions. With regards to them measuring the slippage, once they cut the concrete and lifted it with a crane they measure the strand slippage on how many mm it will slip on the inside as attached.

JBP

 

[JBP00]

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

 

[Ingenuity]

JPG00

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.

 

[JBP00]

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

 

[Ingenuity]

So the bed is about 120m long.

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

 

[rapt]

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.

 

[dik]

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

Dik

 

[JBP00]

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

 

[dik]

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

 

[rapt]

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!