Monitering water added to concrete on-site 2

[HeavyCivil]

I have a few years of field experience around concrete but never wearing the engineer / inspector hat. I understand the implications of water added to concrete on-site.

I calculate the amount of water that can be added to the mix based on the ticket and target or max w/c that we have specified. The problem is that many of the trucks I've been dealing with have broken meters or inaccurate ones due to pressure in the tank. The redimix company is well respected and very professional - not a cobb job outfit.

I cannot justifiably send a truck back because I don't know how much water was added, but I cant justifiably be comfortable with the mix either when all I have is the operator's word that he "added 10 gallons".

Any solutions to this?

 

[msquared48]

One final comment here Hokie...

I understood you to say that you have never seen the condition. OK. Point taken, but that does not mean that it cannot occur. After all, as far as I know, the only way to test for this condition is destructive testing. That is not likely to occur except in the lab, unless someone pays the bill. It's just basic physics.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Mike,
Destructive testing can show whether segregation has occurred, but not whether it is due to excessive vibration.

 

[msquared48]

Point Taken.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

cntw1953...to answer your question as to whether you need to worry about w/c if strength is ok....YES.

The w-c ratio controls strength, but also controls durability. You can and often do achieve compressive strength requirements without achieving adequate durability for the application. This is prominent in slabs on grade and pavement applications.

Retempering and adding water after initial mixing often results in more bleed water. This leads to a reduced w-c ratio at the surface, but not necessarily deeper in the concrete. The primary strength capacity of the mass of concrete is relatively unaffected; however, the surface durability is compromised and the result can be crazing, dusting, and spalling.

hokie66...sometimes, depending on how much effort the client is willing to pay for, we can determine indications of excessive vibration through petrography. It produces more and elongated water gain voids.

To revisit VTEIT's dilemma....I firmly believe that only the QA representative of the ready mix supplier should modify the mix in the field. Do not accept any liability for changing the mix. That's the problem of the supplier.

 

[hokie66]

Ron meant to say more bleed water leads to INCREASED w/c ratio at the surface.

 

[Ron]

hokie66...thank you!! That's correct. Should have proofed my post!!

 

[cntw1953]

"Retempering and adding water after initial mixing often results in more bleed water. This leads to a reduced w-c ratio at the surface, but not necessarily deeper in the concrete. The primary strength capacity of the mass of concrete is relatively unaffected; however, the surface durability is compromised and the result can be crazing, dusting, and spalling."

Many times I have seen the finisher spread dry cemment to control excessive bleeding, any comments on this practice, if "... The primary strength capacity of the mass of concrete is relatively unaffected..."?

 

[DRC1]

Messing with the surface, be it retempering or adding cement will lead to a suface that may experience problems. That said, some retempering is needed to finish. The most often cause of bleed water after excessive retempering, is exssive finishing or trying to finish to early. This is often due to addiotives such water reducing agents or retarders that are part of the mix design and are difficult to adjust for daily site conditions. In addition, WRA's are considerably more difficult to finsh than moderate slump mixes with out WRA leading to excessive tempering.

 

[Ron]

Adding cement to the surface during the finishing process is called "dry shaking". This form of dry shaking is not good. It leads to scaling and dusting. Avoid the practice.

Another problem that can be caused by dry shaking is blistering. This is where a power trowel is being used and the surface is being burnished, but water gets trapped just below the surface creating a separation plane below the finishing "crust". These blisters will flake off. There are other causes of blistering, but this is one that can be easily avoided.

 

[cntw1953]

DRC1 & Ron, thanks.