Cracking of reinforced concrete slab on grade

[alumpkin]

Gentlemen,

Please comment.

I have a 12 inch thick reinforced concrete slab on grade for a manufacturing plant in South Carolina. The reinforcment is #5@12" c/c each way top and bottom. The 3500 psi mix design is as follows:

cement 537 #
coarse agg (#57) 1046 #
coarse agg (#4) 400 #
Intermediate agg (#89) 696 #
Fine agg 1072#
Water 280#
W/C 0.52

Some mid range water reducer was used at the pour. This slab is on crusher run with no vapor barrier. Poures were approximately 10,000 sf. Diamond plates were used at the cold joints. Control joints were cut every 12'-6". Cuts were 1-1/2" deep which is just above the top reinforcing mat. All pours were interior pours and were covered. The curing compound/ hardener used was the Ashford Formula.
1
We have some random cracking in the slab but not a lot. The German side of the partnership is raising critical concern that our slab is suspect. Cracks are hairline and not structural.

Any thoughts concerning some random cracking for this slab? Why did the cracks not migrate to the saw cuts? As you can see the mix design is well graded mix chosen for its ability to reduce shrinkage and yet we still seem to have some random shrinkage cracking even with a 12'-6" pattern.

 

[dik]

Catch the link...

http://www.union.edu/PUBLIC/CERDEPT/FACULTY/GHALY/CONCRETE/WtSINon.htm

This is an excellent page for mix proportions and you can use it to determine a very good starting point for a mix... the actual mix has to be tinkered with at the batching plant. I've used it for small batching plants located in northern communities.

Dik

 

[JAE]

0.52 seems a bit high to use along with a water reducer. Otherwise, assuming your course aggregate was larger sized (not sure what #57 means) it looks appropriate.

How quickly did the joints get cut? That would be key. Also, are the cracks indicative of shrinkage? or are they alligator cracks - indicative of something else...i.e. bleed water not removed properly before final troweling?

 

[dik]

Also 1-1/2" deep is a little light for a 12" slab... cuts should be 3" deep and likely within 8 hours of finishing the slab.

 

[alumpkin]

Guys,

Several comments for you.

Joints cannot be cut any deeper due to presence of top reinforcing mat. Also, water reducer was used as required since all of the concrete was pumped - can't back trucks over reinforcing mats. Joints were cut the same day - less than eight hours after the pour. I believe the cracks to be shrinkage cracks...hairline width by 12 feet long or less very random in spacing.

 

[dik]

Don't let the cover to the top reinforcing deter you from using a proper depth sawcut. Cutting a 12" slab by 1-1/2" will have little effect. You might consider dropping the top mat to miss the saw blade. Top mat reinforcing can also be coordinated with the sawcutting and dowels used. I've done a lot of slab design, including rigid pavements for industrial buildings, airports and hangers for 747 size airplanes. The distance between sawcuts can be increased; for a 12" slab, I'd likely have used 20' or 25' spacing. The slump can be minimized by using superplasticizers if need be. As noted, the sawcut timing and depth is critical. One of the better improvements for slab on grade construction is the use of the 'Sofcut' saw; it allows cutting even sooner. If you wait too long, the microcracking has already determined where the cracks will occur and sawcutting is just a waste of time.

Dik

 

[alumpkin]

Dik,

Thanks for your reply.

Keep in mind the slab is already poured. Reinforcing was continuous throughout each pour. Sofcut saw was used. In fact they were cutting on one end of the pour while finishing on the other. Typical pour was approximately 200 feet by 50 feet (10,000 sf).

 

[dik]

so the timing of the sawcutting was OK... then the depth of the sawcut might cause the problem... typically a slab should be sawcut 1/4 to 1/3 the depth... That's a fair chunk of concrete... at 12", you'd likely notice the heat generated.

Dik

 

[henri2]

A lot of well thought out and valid opinions have been offered on possible causes of hairline cracking and mitigation methods.

I'd like to chime in with some thoughts on the mix used.

The coarse aggregate designation is based on Table 2 of ASTM C 33.

The mix proportions are based on 1 cubic yard SSD basis.

Assuming a SSG SG of 2.65 for aggregates, the mix proportions can be expressed as follows:

Cem = 537 lb (5.71 sacks)
#4(1-1/2 in agg) = 400 lb (12.4% of combined aggregate absolute vol)
#57(1 in agg) = 1046 lb (32.5 % of combined agg absolute vol)
#89(3/8 in agg...pea gravel) =696 lb (21.7% combined agg absolute vol)
FA (sand) = 1072 lb (33.4% combined agg absolute vol)
Water = 280 lb or 33.6 gal
Entrapped Air approx 1.3% of total volume (27 cubic feet)

Mix is 1-1/2 in MSA (max size agg) non-air-entrained

A few questions:

1. What are FM of sand, and FM of combined aggregate?

2. Any aggregate grading info available?

3. Has this mix been utilized for similar application (i.e. pump mix for SOG)..and if so, were there similar problems?

4. Have you considered the possibility that the amount of water indicated in the mix was exceeded? This is possible if there are no batch plant inspector at the batch plant, and inspector/technician at the jobsite to monitor operations.

In addition to environmental factors and construction practices, mix proportions and batchng operations can also play a role in contributing to these types of cracks

 

[mitchelon]

Why does the mixture has an ability to reduce shrinkage? The first cracks to appear are typically temperature related. You will not see a significant amount of shrinkage cracks in the early stages of hydration. Although it is not a mass concrete structure, I would expect to see an increase of temperature cracks in a 12 in slab (when compared to a 4 in). Why was 3500psi used and not 4000-5000? It is true that a lower w/c ratio will increase creep and shrinkage stresses, but a high w/c ratio requires more water and the more water a mixture has, the more it will shrink. Shrinkage is just not one of those things you can easily control using conventional concrete mixtures in reinforced concrete. Are you just having problems to sell the product to the owner? This happens a lot as they don’t understand that concrete just cracks. If a beam is designed to crack under service loading conditions, why can’t a slab on grade be cracked?

 

[miecz]

Agree with dik that a 1-1/2" saw cut will have little effect in a 12 inch slab, particularly if you didn't interrupt the reinforcing at the joint. With joints at 12'-6" O.C., I would have stopped the rebar 2" short of each joint.

 

[concretedoc]

If the reinforcement is continuous through the joint, random cracking is likely regardless of the shrinkage characteristics of the concrete.

The following excerpt is from Section 3.2.4 of ACI 302.1R-04,"Guide for Concrete Floor and Slab Construction:"
"Reinforcing bars or welded wire reinforcement should be
discontinued at any joints where the intent of the designer is to let the joint open and reduce the possibility of shrinkage and temperature cracks in an adjacent panel. Where the reinforcement is continued through the joint, cracks are likely to occur in adjacent panels because of restraint at the joint (WRI/CRSI 1991). When used in sufficient quantity, reinforcement will hold out-of-joint cracks tightly closed. Some designers prefer partial discontinuance of the reinforcement at contraction joints to obtain some load-transfer capacity without the use of dowel baskets."

You can discontinue the steel at the joints to force cracking at that location, or use enough continuous steel to let the concrete crack where it wants to but keep the cracks tightly closed. Your ratio of reinforcement area to gross concrete area is greater than 0.004 so that should keep the cracks tightly closed. It will also probably restrain joint movement enough to cause out-of-joint cracking in some of the panels.

The same rationale applies to concrete slabs placed on elevated metal decking. Many engineers use enough reinforcing steel, don't waste money on joint cutting, and just let the concrete crack.

 

[henri2]

mitchelon, you are right about when shrinkage cracks appear...i.e. after concrete has hardened. Plastic cracks appear prior to hardening of concrete.

"Many factors affect plastic shrinkage cracking, in particular the evaporation of water from the surface of freshly placed concrete. Other factors also influence the likelihood of plastic shrinkage cracking such as water-cement ratio, fines content, member size, admixtures, and on site building practices." ---PJ Uno, CI Materials Journal, Vol 95, Issue 4, July 1 1988

 

[alumpkin]

Gentlemen,

Thanks for your responses. Just a couple of comments.

This mix design is very similar to the Home Depot and Lowes unreinforced floor designs. Because this mix provides a well graded aggregate the cement paste can distribute more evenly that other mixes.

This 3500 mix breaks well over 4000 and we saw no need to go to the 4000 mix.

Currently, we are checking batch tickets for this particular pour to determine if any field influence was used to modify the mix.