ACI-318 and interesting core results
ACI-318 and interesting core results
(OP)
10k PSI concrete in a 15” x 24” column with 8 # 7 verts.
The mix =
Type I = 495
Slag = 270
Flyash = 135
Natural sand = 992
Stone # 7 (3/8” max) = 1930
33 gal water
153 oz high range water reducer
slump 6 to 8 post SP
air 1 to 3
W/C = .31
One set of 6 test cylinders (6”x12”)
Two 7 day breaks = 6110 and 6190
Two 28 day breaks = 9140 and 9190
Two 56 day breaks = 9860 and 9920
3 Cores tested at 73 days = 9730 and 8180 and 7230
average 8380 psi and 84% of fc
one single core was 72% of fc
Technically the core results failed because the average of the 3 cores was under 85% of fc and one single core was under 75% of fc. Additionally, I think I recall seeing something in the EOR specs about requiring cores to be 90% of fc for fc mixes over 6000psi or 7000psi. I’ll have to look that up.
The point of this post:
ONE core 14” long was drilled from the 15” face. The core was subsequently sawed into THREE pieces to get the ACI-318 required 3 compression tests.
The core nearest the outside of the column was the highest break and the core nearest the inside of the column was the lowest break. Upon coring, I had expected the cores breaks to be within a hundred psi of each other or so, but obviously this did not happen.
ACI-318 5.6.5.2 — If the likelihood of low-strength concrete is confirmed and calculations indicate that load-carrying capacity is significantly reduced, tests of cores drilled from the area in question in accordance with “Method of Obtaining and Testing Drilled Cores and Sawed Beams of Concrete” (ASTM C 42) shall be permitted. In such cases, three cores shall be taken for each strength test that falls below the values given in 5.6.3.3(b).
Two questions:
Does the taking of ONE long core and then sawing it up to get three compression tests satisfy the intent of ACI-318-5.6.5.2.
Also, how does one interpret the core results? The cylinder breaks appear to be dead on the money with those tight numbers but the cores had a difference of more than 2000 psi. Could this be a phenomenon of the vibrating technique used during concrete placement, of a moisture gradient between the exterior and the interior of the column, or of a thermal condition in the interior of the column causing micro cracking at the aggregate?
Techmaximus





RE: ACI-318 and interesting core results
What was the Diameter of the cores, the L/D ratio and the condition of the cores when tested? (i.e. wet or dried?)
RE: ACI-318 and interesting core results
Techmaximus
RE: ACI-318 and interesting core results
The one core as three specimens is a stretchy argument. The intent is to use discrete specimens from various locations representing the same placement, but not essentially the same piece cut into 3 pieces. That can badly skew your evaluation. Suppose you had three specimens from different locations and the one in the area that you cut into 3 pieces was the only low one.
Don't expect the in-situ strengths to be the same as the cylinder strengths. That's one reason for the 85% acceptance criterion.
RE: ACI-318 and interesting core results
The core breaks were:
3 Cores tested at 73 days = 9730 and 8180 and 7230
The cores had a wide difference of more than 2000 psi and this is the discrepancy I am interested in. Could this be a phenomenon of the vibrating technique used during concrete placement, of a moisture gradient between the exterior and the interior of the column, or of a thermal condition in the interior of the column causing micro cracking at the aggregate?
Techmaximus
RE: ACI-318 and interesting core results
Just a few thoughts to consider:
First, I would send a copy of your first post to the ACI Technical Services Dept. and ask for their opinion, afterall they wrote the code.
Secondly, I seem to recall, but haven't looked yet, that the direction of the core has some influence on the results. Test cylinders are poured vertical and tested vertical, while your core was horizontal. This is common in core testing of columns, but I do believe I've read that there is an effect to be expected.
Third, as Ron stated and you may already know, there is a real difference to be expected between cylinder tests and core tests. Cylinder tests are really more of a check on mix consistancy than actual field strength.
Fourth, I've never heard of taking a long core and sawing it into the three tests. I also agree with Ron that the intent of the code is three independant cores.
Lastly, I would expect strength variation thru the thickness of a member as well as along it's length, particulaly in a column where the lower levels are more consolidated than the upper. Although, I have to admit 2,500 psi high to low seems like a lot!
RE: ACI-318 and interesting core results
We put computerized loggers in concrete to measure maturity and estimate strength. The most conservative estimates always come from concrete closest to the surface. See engius.com for detailed explanations.
Fred J. Croen, RSM
Engius, LLC
Boston, MA
www: engius.com
RE: ACI-318 and interesting core results
3 Cores tested at 73 days = 9730 and 8180 and 7230
ONE core 14” long was drilled and then cut into 3 pieces for 3 compression tests.
The core nearest the outside of the column was the highest break and the core nearest the inside of the column was the lowest break.
It appears that my core achieved the reverse of what you would have expected as far as in-situ strength being a function of time and temperature.
BTW, the core was from the mid-height on the column. What is your opinion on the one core being sawed into 3 pieces for 3 compression tests?
Techmaximus
RE: ACI-318 and interesting core results
1. Taking core specimens: "A core specimen taken perpendicular to a horizontal surface shall be located, when possible, so that its axis is perpendicular to teh bed of concrete as originally placed and not near formed joints or obvious edges of a unit of deposit. A specimen taken perpendicular to a vertical surface, or perpendicular to a surface with a batter, shall be taken from near the middle of a unit of deposit when possible and not near formed joints or obvious edges of a unit of deposit."
2. From section 6.9.1: "..results of two properly conducted tests of single cores by the same operator on the same sample of material should not differ from each other by more than 9% of their average."
3. There is nothing stated in C42 about using a single core and cutting it. However, it "seems" as though cutting a single core might go against the inherent assumption that you are getting three discreet samples to average from across a sample area of your element. One long core at one location doesn't really accomplish this.
RE: ACI-318 and interesting core results
Fred J. Croen, RSM
Engius, LLC
Boston, MA
www: engius.com
RE: ACI-318 and interesting core results
RE: ACI-318 and interesting core results
Techmaximus
RE: ACI-318 and interesting core results
Based on responses, I’m going to recommend that long cores cut up in to multiple compression tests not be performed. I am thinking that this is a function determined by the driller, a subcontractor of the contractor or the concrete supplier. The driller gets paid by the inch, not by the hour. Obviously more inches can be obtained in fewer hours and much less effort with one long core rather than with three separate cores of the correct length.
Techmaximus
RE: ACI-318 and interesting core results