Pavement cracking unexplained 1

[AdmiralK]

A project contractor has prepared subgrade with 6% lime stabilization, hydrated and mixed according to spec. Proctor tests surpass 98.5 at optimum moisture content. Pavement is 3000 psi plant tested mix with 7" depth using #4 bars on 18' x 30" spacing (local standards for streets). Concrete was placed by mix truck (less that 4 mile haul) had a slump of 5-1/4 inches with entrained air to spec. Placement was by by hand and small spreader (cable towed vibrating truss plate type) using additional hand held random vibrators and surface fogging. Expansion redwood joints with dowels placed 12" apart on 40 foot spacing were pre placed and tied. Forms were steel rails. Control joints on 20' spacing were cut within 18 hours using diamond saw to 1/2 " depth. We see hairline cracks that penetrate through the pavement within 3 hours of saw cutting. Inspectors saw cracks develop while control joints were being cut. Contractor used retardant spray coating for moisture control immediately after finishing with first a bull float and then a dragged burlap broom finish. Cylinder breaks at 7, 14, 21 and 28 days were normal and borings through crack areas confirmed the crack was through the entire pavement thickness, though breaks still exceed 3000 psi. We are at a loss to explain. Any ideas? Anyone? Thanks

 

[BigH]

Pattern of cracking? Temperature of concrete at time of placement? Ambient temperature at time of placing? Wind velocity? It is quite likely that you have plastic cracking due to loss of surface water by evaporation. See Neville's tome on Properties of Concrete - has some good discussions on cracking. I am sure Ron will be able to add more - there is another very recent thread that discusses this as well as giving a site to determine if plastic cracking may be a problem. I've seen a lot of this in very hot climates even with curing membrane placed soon after placement. When we pushed water curing even with membrane, the cracking was negated.

 

[BigH]

See thread167-158845 .

 

[GeoPaveTraffic]

If the pavement was cracking during sawing, you sawed to late. That is the only answer.

 

[AdmiralK]

Temperature was 80 deg with 2-4 knot wind, sunny day, humidity about 80%. Cracks are parallel to crack control joints and appear about mid way between expansion joints. Most run from side to side of each lane (lanes are poured in 14 day intervals so one lane is always accessible). In one pour of 600 feet we counted 56 cracks in a section of 14 expansion joints. Measured surface temperature with a pyrometer at six hours after completion and noted it was 133 deg on new surface and 124 on old roadway. By 2pm each day ambient temp is low to mid 90's with winds 5-10 knots and humidity near 75%. Tried a water flood at 3 hours after pour and kept it wet for 20 hours. In that section we developed a longitudinal crack rather than transverse as happened in all the others. Usually full sun on work. We had geo-tech on site taking samples and cores as well as marking cracks. We also found that other jobs in the area are having the same problem. Only discrepancy I noticed was they used a vibratory roller rather than sheeps foot to compact sub grade, yet lime content, dispersion and Proctors all passed.
Thanks for the reply, I look forward to any more advice.

 

[looksatstars]

Cracking thru the entire 7" slab with rebar? wow something is definately wrong.
Sounds like its more a problem with the subgrade heaving and the concrete being too brittle.
I would recheck the specs on subgrade and actual insitu properties of the subgrade.
Check us posted on what happens, it's interesting to say the least.

 

[Ron]

GeoPaveTraffic nailed it yet again....sawcuts were too late. Also, the sawcut depth is too shallow and the control joint spacing is on the outside range. For a 7" slab, the minimum depth should have been 1.4" and preferably over 1.5". For the conditions you noted, those sawcuts should have been performed in the first 4 to 8 hours after placement. The joints should be on the order of 15 to 18 feet, though 20 feet is within the "rule of thumb" guidelines.

The slump was also too high for a pavement, unless you got the slump from the addition of a plasticizer. Pavements have extreme exposure conditions and high abrasion. For those reasons, you need a very durable concrete. That's why the water-cement ratio should be kept as low as practicable for pavement applications.

I'm curious as to why you have 14 expansion joints in 600 feet! Generally you would only have 2 or 3 expansion joints in that length of pavement. For control joints, you would have about 40 in that length, for unreinforced concrete....which brings up another question...why are you reinforcing the slabs? Just because it is a "city standard" doesn't mean it's right (after all, they are letting you place a pavement with a 5-1/4" slump...that's not right either!) That tends to promote cracking as well, because of the weird stress distributions set up during initial curing.

It's not just another slab on grade.

 

[GeoPaveTraffic]

Thanks Ron.

This is what I do all day, every day.

The saw cuts should be at least 1/3 the thickness of the slab. Distance between saw cuts should be on the order of 15 feet. At 20 feet cracks will occur between saw cuts, but much later, maybe 10 years.

If no water reducing agent is used, then 5.5" slump is too high, it should be on the order of 4" with a w/c of ~0.40.

The curing compound and flooding of the slab are not relavent to the problem that you are seeing. However, they are good for the long term durability of the slab.

All saw cuts should be made as soon as possible to avoid cracks. I would expect that, as Ron noted, the cuts could have been made within 6 hours with a soft cut saw.

With respect to expansion joints; why do you have them? Concrete does not generally grow after initial set. Yhou might want to look at ACPA's web site about jointing. The address is

http://www.pavement.com.

 

[BigH]

Good show Ron and GeoPaveTraffic! When glancing, I missed the 18 hours for saw cuts. Still, this reminded me of the jobs I saw in India with many many cracks.

 

[AdmiralK]

Thanks to all of you who replied. In somewhat of an order here is more:
1. The County establishes the standards for all street construction and they require expansion joints on a maximum of 40' spans. The engineers (not us) will routinely just copy the spec in their P&P drawings and let the GeoTech do the pavement design. All concrete streets in southeast Texas that we know of require rebar reinforcement because of the high shrink swell characteristics of unstable clay. While many question expansion joints the powers that be continue to require them unless thickness of pavement exceeds 11 inches or has double steel reinforcing (AKA the Interstate highways which use continuous reinforcing and monolithic pours).
2. The saw cut comments are very helpful - we see mostly inexperienced mexican labor who work as shown rather than trained and they have no clue as to anything other than what the superintendent directs them to do. (FYI the Project Engineer's duties cease with plan approval and no construction phase services are used - the County uses it's own inspectors to monitor the job(s)). Slump standard for the pavement design was 4" to 6". I don't know what temperature the mix was when the test was done, however I did watch them record that.
3. As a follow up the Contractor is proposing to saw these cracks and epoxy them. It appears they would be better off sawing and sealing with silicone or other material designed for sealing operations. Do you have any response to that?

 

[BigH]

Just another point - we had some longitudinal cracking near "saw" cut joints when constructing a toll plaza concrete pavement in sections (usually 40m by 4m). In this case, the saw cut wasn't directly over the abuting construction joint - so the pavement cracked at the abuting construction joint. They were off by some 25 to 50mm. Another instance where good attention to detail is needed in sawcutting - not only in timing.

 

[AdmiralK]

Thanks again to all who responded. Based on your information we have concluded the following:
Saw cuts were too late in the process and too shallow to induce the proper control;
Use of curing membrane was not sufficient for the evaporation rate considering the temperature and sun influence;
The addition of fly ash did not contribute to the failure, in fact it strengthens the mixture;
Water flood or burlap blanketing is needed for 24 hours to 48 hours after the pour;
Crack control needs to be on the order of 25 times the pavement thickness with expansion joints (only because they require them) to be on 60 foot centers.
Slump standard need to be reduced to 4 to 5 inches rather than 4 - 6 inches.
Thanks again.

 

[Ron]

AdmiralK...just a couple of points on your summary.

Fly ash addition does not necessarily strengthen the mix. In fact, depending on the percentage, it will reduce the early strength gain (which can help reduce cracking, by the way!).

When you specify the slump, specify it as a number, such as a 4-inch slump. If you also specify that delivery will be in accordance ASTM C94, then your slump range is set. For a 4-inch slump, ASTM C94 provides an acceptable range of +/- 1-inch. If you specify greater than 4-inch slump, the range jumps to +/- 1-1/2 inches.