Delayed Concrete Set 3

[concreteworld]

Hi There,

Situation: Concrete post-tensioned beams poured on top of bridge piers. These post-tensioned beams will carry concrete slab that will be deck asphalted for future traffic. During one pour of 300 cubic meters for around 20 such beams, 1 beam web showed delayed concrete setting. This one beam was monitored for setting day-by-day using Schmidt hammer. The results of Schmidt were improving every day so everyone wanted to wait and watch until 14 days. So at the end of 14 days all the concrete surface was hard and Schmidt readings were average 34 except few areas half meter by half meter where it was only 24. There are also random cracks mostly of horizontal nature along the length of the beam web.

The web beam is 500mm thick x 1700mm high x 13 meter long. Post tensioned cables are running along the length of the beam with 100mm cable cover to concrete.

Cores were obtained from the affected areas of concrete due to late setting. The percentage strength achieved in cores is 54% on average of the design strength of 45 Mpa. The age of core at testing is 21 days.

The consultant's demand repair with epoxy injection for strengthening the beam before post-tensioning and undertaking responsibility for the integrity of the structure.

Based on the above info what are the vulnerabilities from structural point of view and stability of the beam during post-tensioning and service period noting that heavy highway traffic loads are expected on this bridge.

attached photo of the web beam.

Thanks in advance

 

[Ingenuity]

Schmidt hammer is not an especially reliable NDT technique to determine concrete strength parameters. I would rely upon your core samples and their tested strengths, and if you need to take more cores to understand the severity and extent of the problem use the Schmidt hammer results ONLY to determine what additional areas should be cored.

Having said the above, by only having approx 25 MPa compressive strength (54% of 45 MPa) does the 'bad' concrete area extend into the PT anchorage zones, such that the local bearing zone is compromised? I presume that there is spalling/bursting end-zone reinforcement detailed and placed, but regardless, with such low concrete compressive strength you have a problem.

Epoxy injection of cracks PRIOR to stressing may too present a problem. Assuming you are using galvanized metal seamed-ducting (with duct couplers), the viscosity of the injection resin is such that it will possibly permeate into the duct, and bond the strands before stressing, especially since the cover to the tendons from the beam side face is only 100mm.

What is the PT configuration? Multi-strand tendons, with single-end stressing, lots of tendons?

You may need to consider some extra NDT like impact echo or ultrasonics to determine the probable extent of the deficiency before deciding if it can be repaired or needs replacement.

 

[Ron]

Epoxy injection will not cure low strength concrete. You can supplement by carbon fiber overlay or remove and replace. Agree with Ingenuity that additional NDT should be done and the methods he mentioned are appropriate. Since the sides of the beam are accessible, a simple velocity check on the concrete and cores would give you a good correlation for strength. As noted, the Schmidt (Swiss) Hammer is not appropriate for strength determination, just consistency and even then only with experienced judgment.

 

[concreteworld]

Thanks for your valuable insights.

Please see attached line sketch of the web beam. The strand is 7-wire high tensile steel. The late setting affect is along the thickness of the beam. The opposite face of the wall is shuttered until post tensioning but we remove one plank for inspection and found localized comparatively smaller affected areas than the visible/accessible face.

Please ignore the Schmidt readings in the attached sketch as they were on day 3 from the cast date which eventually kept increasing and increasing until today with an average of 34 as of 20 days.

Regarding epoxy repair penetrating the steel duct for strands, will it seriously damage the duct if low viscosity epoxy grout is used? Another repair method proposed is saw cutting affected concrete until sound concrete, inserting shear connectors (for anchorage and bond), zinc riching exposed steel, applying bonding compound, shuttering and filling the area with Flowable micro-concrete (high strength 45 Mpa @ 1 day). The problem with saw-cutting and drilling old concrete is chance of damaging the strands, accessibility problems to opposite face of the beam, laborious work.

Kindly review both repair methods and please advise the BEST option.

Thanks for your time.

 

[BigH]

What has been the cause of the delayed set? I would suggest that this needs to be appraised as well; probably, if the cores prove low and the design engineer cannot accept the measured strengths, replacement is likely the only way forward. Ron is correct on the Schmidt Hammer - it is sad that so many think it is the panacea for low strengths. I'll take the SPT 10x over as against the Schmidt Hammer!

 

[concreteworld]

Delayed set has occurred due to retarder overdose. Schmidt hammer was done just to monitor improvement of hardness of concrete and not for accepting strength levels. For strength evaluation core tests were done. We have told the consultants that strengths will continue to develop at later ages due to delayed reaction/hydration because of retarder overdose.

Please advice a suitable repair for this case, technically sound and not too high in cost.

Thanks for your responses.

 

[Ingenuity]

In you sketch attachment you state: "Late setting of concrete due to manual batching" so you are dealing with a retarder overdose situation. Check with your retarder manufacturer/supplier on the long-term effects of overdosing on strength gain.

A quote from UK Cement Admixtures Association: Link

"As a general rule, if concrete contains an overdose of a retarding admixture and has not set hard in 5 days, then it may not gain useful mechanical strength."​

Another paper on the subject is: Link

You need to ask yourself if the delayed-set area will have any long-term effects (notably compressive and possibly tensile strength, durability, creep, serviceability etc) compromised, and if so, then it must be removed.

Epoxy injection will only address crack repairs. It will not make the concrete have increased compressive strength, as Ron stated.

From your sketch it appears you have 3 PT tendons. I assume they are either 12.7 or 15.2mm diameter strands, and probably 12 or more strands per tendon. The duct will be probably 70mm diameter or larger - so easy to locate using NDT.

It is a rather a simple task to locate the PT tendons via subsurface radar (aka GPR), mark the tendon on the web face in crayon for all to see so that any concrete remedial work will be very cautious in this area.

No need to saw cut the beam. First use a 15-20 lb chipping hammer and remove defective concrete. [Hydro demolition is often used in the USA for controlled-concrete removal whereby only 'defective' concrete is removed, but in this instance the area/volume is too small, so not recommended IMO]. Work cautiously around the PT ducts, removing concrete in a controlled manner. Given the thickness of the beam (500mm), work may have to be from both faces of the web, assuming that defective concrete is indeed prevalent across the full web-width.

Once all the defective concrete has been removed then oil-free compressed air will be required to clean the chipped concrete interface. I would suggest self-consolidating concrete (SCC) be used (with vertical forms designed for the required temporary form pressures). This will ensure flow of the concrete to all part of the chipped area, especially around the PT ducts. SCC is pumped under pressure and given the volume of concrete repair required, hand-patching is NOT appropriate. Shotcrete is another option, but it is a specialty that may not be available in your area.

Best to submit a means-and-method/procedure to the consulting engineer prior to commencing any work, so that all parties are in agreement of the proposed methodology.

Good luck.

 

[concreteworld]

3 cores were obtained from the affected areas of concrete due to late setting. Test results show 53% of design strength (45 Mpa) at almost 21 days. Also some cores obtained over the cracked concrete show cracks penetrate upto a depth of 300mm. Based on all the above, the Consultants have decided to remove/demolish and replace full length of the beam that is going to be post-tensioned.

Demolition contractor is working on the beam and we have told him to be careful enough not to damage or disturb the strand duct along the beam but our concern is the stability of the strand during demolition, reconstruction with new concrete. Some of secondary steel reinforcement will have to be removed to facilitate demolition works.

Please put your comments on the above with any other precautions that have to be taken during and after replacement work.

 

[Ron]

As Ingenuity noted, you should be careful what you put back for the repair. I would not recommend manual batching in the field with ANY admixture. Cement, water and aggregates are ok for field batching, but not the addition of admixtures unless you have already done several control mixes to see that you get proper distribution of the admixture. If you must do field batching, make sure the admixtures (which are typically water soluble) are put into the water prior to adding the water to the mix. Do not put the admixtures into the mix any other way. Small mixers are not efficient enough to get good distribution of the mix constituents without a lot of time in the mixer which most construction people don't have the patience for!

Also pay attention to the type of cement you are using. If it is Type I portland cement, either by US/Canadian Standards or Type I in the UK, this is acceptable. Don't use a cement with added blast furnace slab as this will considerably delay the strength gain.

 

[Ingenuity]

...be careful enough not to damage or disturb the strand duct along the beam but our concern is the stability of the strand during demolition, reconstruction with new concrete. Some of secondary steel reinforcement will have to be removed to facilitate demolition works.

If the tendons profiles/location are clearly marked on both side of the beam web then the chipping/demo crew should be able to productively demo the concrete without any damage to the tendon duct.

The tendon profile was probably achieved via secondary rebar (may be U-bars) that straddled the beam stirrups. If some of the secondary support rebar needs to be removed to achieve the demo, then it is important to provide direct temporary tendon support in the areas until such time that the rebar can be replaced. Usually the tendon is profiled with vertical offsets and secondary rebar at 1m centers, so if this is maintained throughout the demo process your should be okay. Multistrand tendons are HEAVY - it they drape UNsupported over more than 1m or so, the ducting can crush over the support bars, or the seams open, and if undetected, concrete can enter any duct seams that are split during concrete placement and make subsequent stressing problematic.

Prior to concrete replacement, the tendon profile should be re-checked that it matches the PT shop drawings or consultant's design drawings - in vertical and horizontal profile. Incorrect profiles and/or acute angle changes of the tendon/s can cause significant issues (bursting, spalling, cracking etc) during/after stressing.

If you have to do any chipping/demo work in close proximity to the PT anchorages (say within a horizontal distance of 1.5D, D = beam depth) you may need to take some special precautions.

Given that this is now a 'repair' project, it is important that this 'repair' does NOT have to be repaired again, ever! So stay on top of the demo crew, and the crew doing the PT, and the crew doing the concrete placement. Emphasize team work and it the results shall be productive with long-term durability.

Good luck.

“…structural engineering isn’t rocket science. Evidently, it is considerably more difficult.”
Norbert J. Delatte, Jr., PhD, PE. 2009. Re Harbour Cay Condominium Collapse, March 27, 1981.

 

[hokie66]

Why not just provide new tendons and be done with it? I know, the cost of material. But the cost of not getting it right is what is being dealt with now, and that can't be allowed to happen again.

 

[hokie66]

Further to the last, you should be able to reuse the prestressing steel with new ducts.

 

[Ingenuity]

hokie66,

I understood that only a internal segment of the beam is to be demolished and the anchorage zones unaffected. I may be wrong.

But...having re-read the OP's recent post he/she indicates that the full beam length is to be demo'ed, so I think you are correct.

If the beam is 'short' and therefore only single-end stressed, they may have used swaged dead-ends, making tendon removal more difficult - but then again, often in bridges dead-ends are simply live-end anchorages without stressing at that anchorage, so strands are easily removed.

If the strands can be removed and temporarily stored, then I agree, replace with new ducting and re-install the same strands.

Good idea.

“…structural engineering isn’t rocket science. Evidently, it is considerably more difficult.”
Norbert J. Delatte, Jr., PhD, PE. 2009. Re Harbour Cay Condominium Collapse, March 27, 1981.

 

[concreteworld]

The total length of the beam is 26 meters and only half portion of 13 meters is affected by late concrete set. The dead end of the strands are at the end of 26 meters and live end near the affected part as shown in the attached line sketch. The two web beams are now demolished.

But the designer has also noticed few areas of weak concrete in the diaphragm wall which is 2meters thick and with heavy reinforcement. Cores have been taken to verify concrete quality upon which further decisions will be taken wheater or not repair/demolishing works have to be undertaken. Two cores taken today have shown internal cracks 2mm width. This has complicated the situation. All works are halted due to cracks found in diaphragm walls also. Replacing these diaphragms will be a very laborious and costly affair.

Please advise...

Thanks again

 

[Ingenuity]

Thanks for clarifying the partial demo length.

The area where the PT beam intersects with the end diaphragm is where your PT live-end anchorages are located, so the concrete must be sound - as is it should in all areas - but especially so in this zone.

At 2m thick, the quality of the concrete through its full-thickness will need to be evaluated, and probably ultrasonic NDT or impact-echo NDT may be best (less $ and quick results) unless you want to take lots of long core samples. Do you have access to a local company that has experience with NDT of concrete structures?

It is wise to have it determined why the 2mm width cracks are present - was it due to the poor quality & over-dosed concrete, restraint to shrinkage, plastic settlement, etc.

If it is deemed that the concrete is of satisfactory standard, AND there are acceptable explanations why and what initiated the cracks, then the cracks may be repaired rather simply using epoxy pressure injection - keeping in mind the caution to avoid filling up the PT duct with resin, that we mentioned before. Again, epoxy injection will repair cracks, it won't improve low-strength or low-quality concrete, as Ron stated previously.

Whilst demolishing and replacing the affected diaphragms is a "very laborious and costly affair" it is significantly more difficult (orders of magnitude greater) to have to do it later after the bridge is in service, when further distress manifests.

“…structural engineering isn’t rocket science. Evidently, it is considerably more difficult.”
Norbert J. Delatte, Jr., PhD, PE. 2009. Re Harbour Cay Condominium Collapse, March 27, 1981.

 

[Ron]

Ingenuity...did you live in NE Florida at one time?

 

[Ingenuity]

No, Ron. I have lived in Sydney, Honolulu, Guam and Toronto. Never actually been in the state of Florida...yet!