concrete cylinders failed after 28 and 56days

[syedyh58]

Dear sir,

we have the waffle slab having the required concrete strength 38Mpa and the columns and the shear wall 50Mpa
we have crushed the concrete cylinders of slab after 28 and 56days but we could'nt reached the required strength as per ACI code .
then we have taken the concrete core sample from different location 3 sample from main beam and 3 from ribs beam and all are failed atleast a single sample not reached to 75% of design strength as 38Mpa we got maximum value out of 6 sample 24Mpa. again taken 4 sample from different location all are main beams , all cores are failed ,single sample not reached to required strength as 75% of design strength got maximum value 23.73Mpa.
In such scenario can you suggest us what should we go to strengthen the concrete slab......is there any solution or we have go for demolished the slab (area of slab 1214.45Sqm )

 

[JAE]

Review Chapter 5 of ACI 318 (I think there is a 318M for your units).
Also read this FAQ - faq507-1575
Specifically notice in the middle section of the FAQ where the next steps are:

[blue]1. With the lower f'c derived from the strength tests above, the structural engineer should review the design of the affected portion of the structure to determine if the lower f'c is acceptable. If the lower f'c is acceptable, then no further efforts are required and the concrete can be accepted. Steps still should be taken by the contractor to increase the compressive strength of the concrete for future mixes.
2. If the lower f'c is found to be critical to the performance of the structure, then further testing should be performed using drilled core samples.[/blue]

If your calculations show that the lower f’c is not working then the FAQ states this:

[blue]If the cores don't meet the required levels the following are all options to consider:
1. Perform a load test of the structure in the area under consideration (per ACI Chapter 20)
2. Provide additional structural framing to strengthen the portion of the affected structure to meet the required load carrying capacity.
3. Accept the concrete if acceptable to the owner.
4. Reject the concrete and remove and replace the portion that is considered below strength. [/blue]


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[msquared48]

Well, as JAE so aptly pointed out, your options are kinda set in concrete.

How would you reinforce a waffle slab to keep an Architect happy?

Mike McCann, PE, SE (WA)

 

[Ron]

I would suggest that before you embark on a reinforcement scheme that you consider a full scale load test. Check the load vs. deflection and the recovery response. This will give you a good idea of the capability of the system, without depending only on limited concrete strength day, even though I would certainly consider your concrete results to be low.

 

[hokie66]

Is this a waffle pod slab on the ground or a suspended slab? The approach in deciding acceptance would be different.

 

[bb29510]

tear the slab out