Integrated Buildings & Structures

Greetings to everyone,

I just got a question from one of my students asking for good options to solve a situation he had in building construction. As you can see in the pictures attached, there are rudimentary spacers, and they did not work as they should; the result, the mesh is at the bottom of all the slabs.

Some solutions include shooting grout to add a concrete cover, slab perforations to pour a fluid concrete top-down with a previous wood form assembly. Nevertheless, the best solution for me is to demolish and to build the slab using proper reinforcing spacers and to check any deflections or abnormalities during construction.

Please let me know your thoughts or if you know any similar cases and how do you solve it.

Regards,

Andres Guzman


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Andres Guzman Ing., M.ASCE
Associate Professor
UNIVERSIDAD DEL NORTE
Barranquilla
------------------------------

Greetings Andres, 

After viewing the the attached photos I totally agree with what you said to demolish the slab and recast with the required reinforcement. In addition, your student must be present before and during the casting of the slab to observe good practice and workmanship is deployed.

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Malik Daniyel M.ASCE, RCOP
Design Consultant
MD Designs & Consultants
Bridgetown Barbados
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Dear Malik, thank you for your comment. My student was contracted for a design review, and this contract turned out to a vulnerability study. I suggested to him to change the agreement because the responsibilities are different.

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Andres Guzman Ing., M.ASCE
Associate Professor
UNIVERSIDAD DEL NORTE
Barranquilla
------------------------------

Original Message:
Sent: 11-21-2019 06:34
From: Malik Daniyel
Subject: Visible slab reinforcement - suggestions?

Greetings Andres,

After viewing the the attached photos I totally agree with what you said to demolish the slab and recast with the required reinforcement. In addition, your student must be present before and during the casting of the slab to observe good practice and workmanship is deployed.

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Malik Daniyel M.ASCE, RCOP
Design Consultant
MD Designs & Consultants
Bridgetown Barbados
------------------------------

Greetings Andres,

Your student should have his contractual agreement changes base on the observations. I'm not sure if he was contracted for a section of the building or the overall assessment but protocol should be establish with having all documents/ drawings with specifications etc but as I previously said I'm unaware of his contractual agreement.

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Malik Daniyel M.ASCE, RCOP
Design Consultant
MD Designs & Consultants
Bridgetown Barbados
------------------------------

Original Message:
Sent: 11-21-2019 12:27
From: Andres Guzman
Subject: Visible slab reinforcement - suggestions?

Dear Malik, thank you for your comment. My student was contracted for a design review, and this contract turned out to a vulnerability study. I suggested to him to change the agreement because the responsibilities are different.

------------------------------
Andres Guzman Ing., M.ASCE
Associate Professor
UNIVERSIDAD DEL NORTE
Barranquilla
------------------------------


Original Message:
Sent: 11-21-2019 06:34
From: Malik Daniyel
Subject: Visible slab reinforcement - suggestions?

Greetings Andres,

After viewing the the attached photos I totally agree with what you said to demolish the slab and recast with the required reinforcement. In addition, your student must be present before and during the casting of the slab to observe good practice and workmanship is deployed.

------------------------------
Malik Daniyel M.ASCE, RCOP
Design Consultant
MD Designs & Consultants
Bridgetown Barbados

The quality of formwork is definitely not what we expect in the USA!  In my opinion your number one concern is protection if the reinforcing.provided there aren't some embedments and lap splice concerns.  There may be some coatings available provided there aren't some embedment and lap splice concerns.  I'd recommend sandblasting the surface and applying shotcrete concrete over a bonding agent to achieve your cover.

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J Warner P.E., M.ASCE
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Dear James, thank you for your suggestion. I will do research sandblasting and shotcrete services in the building area. Those are not standard services.

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Andres Guzman Ing., M.ASCE
Associate Professor
UNIVERSIDAD DEL NORTE
Barranquilla
------------------------------

Original Message:
Sent: 11-21-2019 08:11
From: James Warner
Subject: Visible slab reinforcement - suggestions?

The quality of formwork is definitely not what we expect in the USA!  In my opinion your number one concern is protection if the reinforcing.provided there aren't some embedments and lap splice concerns.  There may be some coatings available provided there aren't some embedment and lap splice concerns.  I'd recommend sandblasting the surface and applying shotcrete concrete over a bonding agent to achieve your cover.

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J Warner P.E., M.ASCE
------------------------------

​It kind of depends if the location of the re mesh in the depth of the slab was critical to the strength to resist moments from floor loadings.  If there are slab negative moments over the beams and the mesh is in the bottom, tension cracks may open on the top.  If not, then methods to just add cover may work, otherwise demo.

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Randall Bernhardt P.E., F.SEI, F.ASCE
Senior Consultant
St. Peters MO
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Dear Randall, that is an interesting question. That is why I asked the student for the structural drawings. He told me that this slab is reinforced with a double mesh, and it is critical regarding the pictures. The mesh is not for temperature changes and concrete contraction; it is intended to bear tensile loads. I will keep suggesting to demolish those slabs.

------------------------------
Andres Guzman Ing., M.ASCE
Associate Professor
UNIVERSIDAD DEL NORTE
Barranquilla
------------------------------

Original Message:
Sent: 11-21-2019 09:14
From: Randall Bernhardt
Subject: Visible slab reinforcement - suggestions?

​It kind of depends if the location of the re mesh in the depth of the slab was critical to the strength to resist moments from floor loadings.  If there are slab negative moments over the beams and the mesh is in the bottom, tension cracks may open on the top.  If not, then methods to just add cover may work, otherwise demo.

------------------------------
Randall Bernhardt P.E., F.SEI, F.ASCE
Senior Consultant
St. Peters MO
------------------------------

I won't address the resolution to the existing problem, but to address the root cause that the reinforcing mesh was just "crushed" by the mass of the fluid concrete.  If indeed this is mesh reinforcement, which it appears to be from the photos, spacers or chairs are not effective unless they are placed at very close intervals.  The spacing must be checked before the concrete placement to ensure that the reinforcement does not move when walked on.  Remember, the fluid concrete is going to try and push down the reinforcement as it is placed!  Another method which requires skill and experience is during the placement of the concrete, the mesh must be pulled up into the fluid concrete to the proper level in the slab geometry - generally use a round bar with a hook at the end of it and a circular ring at the top of the rod so the ironworker ("rod buster" in the USA) can hold it in his gloved hand and keep pulling up the mesh into the slab.  This is generally used for slabs on grade, but it takes skill to ensure that the finished product is not similar to the photos of the elevated slab.  Once the mesh is in the correct level, it won't go anywhere - yes there is concrete above it trying to push it down, but also concrete below it to support the mesh at the right level.  Spacers / chairs can be used with larger diameter reinforcement to keep it as the prescribed level at larger intervals, but if they are to be used at all, the reinforcement must be adequately supported so it does not move when inspected.

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Michael Kozinetz Aff.M.ASCE
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Original Message:
Sent: 11-21-2019 12:33
From: Andres Guzman
Subject: Visible slab reinforcement - suggestions?

Dear Randall, that is an interesting question. That is why I asked the student for the structural drawings. He told me that this slab is reinforced with a double mesh, and it is critical regarding the pictures. The mesh is not for temperature changes and concrete contraction; it is intended to bear tensile loads. I will keep suggesting to demolish those slabs.

------------------------------
Andres Guzman Ing., M.ASCE
Associate Professor
UNIVERSIDAD DEL NORTE
Barranquilla
------------------------------


Original Message:
Sent: 11-21-2019 09:14
From: Randall Bernhardt
Subject: Visible slab reinforcement - suggestions?

​It kind of depends if the location of the re mesh in the depth of the slab was critical to the strength to resist moments from floor loadings.  If there are slab negative moments over the beams and the mesh is in the bottom, tension cracks may open on the top.  If not, then methods to just add cover may work, otherwise demo.

------------------------------
Randall Bernhardt P.E., F.SEI, F.ASCE
Senior Consultant
St. Peters MO

Welded wire fabric often ends up at the bottom of the slab. Mr. Kozinetz pointed out that there are procedures that the slab crew uses to try and achieve proper placement within the slab, however the construction workers foot traffic often caused the problem which is certainly excessive. Based on the photographic evidence presented, I question chairs were really used. We try to use larger diameter bar reinforcing, #3 or #4 bars at larger spaces so construction workers can put there feet between the bars so the foot traffice does not push the reinforcing down onto the form.
​​

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Matthew Beck P.E., M.ASCE
Engr
Beck Engineering Pc
Bondville VT
------------------------------

Original Message:
Sent: 11-21-2019 18:00
From: Michael Kozinetz
Subject: Visible slab reinforcement - suggestions?

I won't address the resolution to the existing problem, but to address the root cause that the reinforcing mesh was just "crushed" by the mass of the fluid concrete.  If indeed this is mesh reinforcement, which it appears to be from the photos, spacers or chairs are not effective unless they are placed at very close intervals.  The spacing must be checked before the concrete placement to ensure that the reinforcement does not move when walked on.  Remember, the fluid concrete is going to try and push down the reinforcement as it is placed!  Another method which requires skill and experience is during the placement of the concrete, the mesh must be pulled up into the fluid concrete to the proper level in the slab geometry - generally use a round bar with a hook at the end of it and a circular ring at the top of the rod so the ironworker ("rod buster" in the USA) can hold it in his gloved hand and keep pulling up the mesh into the slab.  This is generally used for slabs on grade, but it takes skill to ensure that the finished product is not similar to the photos of the elevated slab.  Once the mesh is in the correct level, it won't go anywhere - yes there is concrete above it trying to push it down, but also concrete below it to support the mesh at the right level.  Spacers / chairs can be used with larger diameter reinforcement to keep it as the prescribed level at larger intervals, but if they are to be used at all, the reinforcement must be adequately supported so it does not move when inspected.

------------------------------
Michael Kozinetz Aff.M.ASCE
------------------------------


Original Message:
Sent: 11-21-2019 12:33
From: Andres Guzman
Subject: Visible slab reinforcement - suggestions?

Dear Randall, that is an interesting question. That is why I asked the student for the structural drawings. He told me that this slab is reinforced with a double mesh, and it is critical regarding the pictures. The mesh is not for temperature changes and concrete contraction; it is intended to bear tensile loads. I will keep suggesting to demolish those slabs.

------------------------------
Andres Guzman Ing., M.ASCE
Associate Professor
UNIVERSIDAD DEL NORTE
Barranquilla


Original Message:
Sent: 11-21-2019 09:14
From: Randall Bernhardt
Subject: Visible slab reinforcement - suggestions?

​It kind of depends if the location of the re mesh in the depth of the slab was critical to the strength to resist moments from floor loadings.  If there are slab negative moments over the beams and the mesh is in the bottom, tension cracks may open on the top.  If not, then methods to just add cover may work, otherwise demo.

------------------------------
Randall Bernhardt P.E., F.SEI, F.ASCE
Senior Consultant
St. Peters MO

Indeed that formwork is inadequate. I fully agree with James, chipping or sandblasting the soffit plus a bonding agent before shotcreting will do the job.

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Napoleon De La Colina Ing., A.M.ASCE
Eng Mgr
ICM - Ingenieria Civil Montajes
LIMA
------------------------------

Hello Andres,

I am currently a student, graduating May 2020, but I worked as a concrete laborer for 18 years.  I have never seen such a subpar concrete job. The improper installation of the reinforcement is obviously not the only issue.  The photos indicate that the concrete contractor was not experienced and there was nobody around to correct that.  I expect that most specifications were not followed and there was no quality control.  Knowing that, you know that care was not taken to clean the deck or any area before concrete was placed which puts debris into the concrete and weakens bonding.  It appears that they had some type of plastic on top of whatever they used to form.  I have never seen anything like that and am curious as to why anybody would ever think that was a good idea.  The only reason I could think is that the decking and forming work was inadequate and they put plastic over it so the concrete would not leak out.  

I agree with you that demolition is the solution.  One must expect that this structure is inadequate.  Even if this structure was properly designed, it is apparent that it was improperly constructed.  I would have no faith that the contractor installed the required amount of steel.  I expect that they said they did, but tried to cut costs not thinking of the harm they could cause.  The obvious incompetence also indicates that they did not know how to handle the concrete.  I would bet that any coring sample would show that the concrete is not the expected strength.  It appears the contractor was inexperienced, so pours were likely delayed resulting in concrete trucks waiting and people adding excessive amounts of water to each truck to maintain workability.  It is unlikely that they would spend any money for superplasticizer if they are producing this work. 

This structure is dangerous.  I hope your student is able to convince others of this fact.  Thank you for the post and showing me something that I could never have imagined or believed if I did not see the pictures.  <u5:p></u5:p>

Good luck!<u5:p></u5:p>

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Michelle Calcagno S.M.ASCE
Chicago IL
------------------------------

Hi Michelle,  
I agree with you 100%

------------------------------
Malik Daniyel M.ASCE, RCOP
Design Consultant
MD Designs & Consultants
Bridgetown Barbados
------------------------------

Original Message:
Sent: 11-23-2019 02:05
From: Michelle Calcagno
Subject: Visible slab reinforcement - suggestions?

Hello Andres,

I am currently a student, graduating May 2020, but I worked as a concrete laborer for 18 years.  I have never seen such a subpar concrete job. The improper installation of the reinforcement is obviously not the only issue.  The photos indicate that the concrete contractor was not experienced and there was nobody around to correct that.  I expect that most specifications were not followed and there was no quality control.  Knowing that, you know that care was not taken to clean the deck or any area before concrete was placed which puts debris into the concrete and weakens bonding.  It appears that they had some type of plastic on top of whatever they used to form.  I have never seen anything like that and am curious as to why anybody would ever think that was a good idea.  The only reason I could think is that the decking and forming work was inadequate and they put plastic over it so the concrete would not leak out.  

I agree with you that demolition is the solution.  One must expect that this structure is inadequate.  Even if this structure was properly designed, it is apparent that it was improperly constructed.  I would have no faith that the contractor installed the required amount of steel.  I expect that they said they did, but tried to cut costs not thinking of the harm they could cause.  The obvious incompetence also indicates that they did not know how to handle the concrete.  I would bet that any coring sample would show that the concrete is not the expected strength.  It appears the contractor was inexperienced, so pours were likely delayed resulting in concrete trucks waiting and people adding excessive amounts of water to each truck to maintain workability.  It is unlikely that they would spend any money for superplasticizer if they are producing this work. 

This structure is dangerous.  I hope your student is able to convince others of this fact.  Thank you for the post and showing me something that I could never have imagined or believed if I did not see the pictures.  <u5:p></u5:p>

Good luck!<u5:p></u5:p>

------------------------------
Michelle Calcagno S.M.ASCE
Chicago IL
------------------------------

Why is nobody talking about utilizing fiber reinforced concrete in lieu of welded wire and/or rebar for any slab on grade and slab on metal deck?  The advancement with macro synthetic fiber has allowed this type of replacement.  As discussed by everyone, the traditional steel is never in the correct location (upper 1/3rd of the slab and uniform so chaired every 2 feet) where as the fiber is throughout the mix and is exactly at the location where the crack is when it appears.  #justsaying #frcs

As for this particular slab is ​​situation is very dangerous and should be taken out and replaced.

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Rich Gardner Aff.M.ASCE
Midwest Technical Sales Manager
Fiberforce By ABC Polymer
Chesterton IN
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Andres,

Can you please tell us the city where this was built?


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Michelle Calcagno S.M.ASCE
Chicago IL
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