High strength Concrete

[Waqar Saleem]

Dear Colleagues,

Salam, i have a high strength concrete on a project specified C90/105, it is around 13ksi, concrete has some irregular cracks on top and bottom both sides, concrete depth is 1m and it has 4 layers of reinforcement D32@150 both sides. I checked the design mix trial tests, it is achieving 87.5Mpa in just three days, is it right? mix design has 1.5 air entrainment, what is the purpose of this? normally we compact and reduce voids, concrete is also self compacting, is it possible for a SCC to entrain some air?

 

Thank You

[Dik]

I assume your mix design is 90 MPa concrete (@28 days).  I don't know what the concrete object that has the cracking is. For that strength gain, there is likely a very small portion of fly-ash, so there is likely a lot of cement powder.  What is the size of your aggregate; you could have a lot of shrinkage. 1.5% air is not unusual. You can have that amount with normal mixing.  This is normally less than 1%. In our environs exterior  concrete usually has about 6% or 7% for frost resistance. A lot of our concrete has fly-ash for economy.  I've not used self compacting concrete, but have often spec'd superplasticisers. I always spec mechanical vibration, even for piles and caissons.

Dik 

[Waqar Saleem]

Thank You Dik, Slab has cracks, its 1000mm thick, and in octagonal shape, please see the document attached, its an initial draft. Mix design is also attached. 

@Badar (BAZ) @UmarMakhzumi Kindly have a look.

Initital report for foundation cracks.pdf Mix design C-90 105.pdf

[Dik]

Could the issue be plastic shrinkage cracking?

https://www.sciencedirect.com/topics/engineering/plastic-shrinkage

[Badar (BAZ)]

I am not in a position to comment on the part related to properties of concrete.

There can be number of reasons for it. Shrinkage is most obvious one. Do they have reasonable spacing of temp and shrinkage rebars, if any?

Are cracks wide enough to lead to yielding in rebars?

Have you established the structural adequacy to eliminate the possibility of structural cracks?

[Dik]

First choice due to the high strength requiring lots of cement paste.

[Waqar Saleem]

Salam, Thank you Badar, thickness is 1000mm and it has 4 layers of Dia 32 rebars in both directions, and a smaller size mesh at the top also of Dia 16, my perception is also of temp/shrinkage cracks, I collected the data, first crack appeared one week after pouring of concrete, cracks continue to appear and randomly, scanned and depth is 18cm at top and 10com at the bottom, at top cover is 8 cm, i dont understand why this much cover is provided.

[Muhammad Imran Zafar]

Dear Waqar

please check the pouring methodology and measures adopted to control temperature for mass concreting, if any. Then you can calculate the temperature rise during hardening of concrete and compare the difference of atmospheric temperature and concrete temperature. As very high cement content is anticipated so stringent temperature control requirements should have been adopted. By this you would be able to figure out whether these are temperature shrinkage cracks or otherwise. Moreover please calculate the crack width. As cover is high therefore there is a possibility that you may end up in calculating the crack width equivalent to whatever is visible there in the slab.  Pattern of cracks you highlighted indicates the temperature shrinkage nature of cracks however calculations are needed to confirm the anticipation.

[Waqar Saleem]

Dear Imran sb, Thank You, very much, we are on it. Slab is octagonal and pouring is done 4 portions first and then a gap and 4 are poured, temp is monitored by embedded sensor and readings are okay, a canopy is set above the slab and heaters are placed to maintain the temperature difference normal, scanning results are received, 50 readings, top/bottom 25/25, 50 percent are crossing into the core area or below the top bar, mostly are top side cracks, crack depths are max 20cm, cover to top rebar is 8cm. one thing i observed that mix design recommends 3 days strength 60/90 but in actual testing it is 87/90 in 3 days, rate of hydration is more then recommended or observed during trial mixes, please some data/example to calculate crack widths. Thank You

[Muhammad Imran Zafar]

Dear Waqar please follow ACI 224 "Control of Cracking in Concrete Structures". It can easily be downloaded and lucidly furnish guidelines. You may also consult Handbook of concrete engineering by Mark Fintel

[Muhammad Imran Zafar]

Dear Waqar

you may use ABAQUS for crack analysis. By simulating temperature gradient you may also use ETABS for crack width calculations