Hairline crazy pattern cracks in RC retaining wall

One must be careful using undefined acronyms. My field is not civil engineering so I do not know if RC might mean a world standard specific material or construction technique. But I think not, for I come up with Rock Crushed, Reinforced Concrete, Recessed Channel and Rampart Contained for a few plausible acronym meanings that relate to a wall, and I'm not in this discipline.

The concrete may have dried too quickly. Many will put wet burlap sacks on curing concrete to maintain humidity during the curing process. Concrete cures while moist, even under water, if it dries too quickly, it will crack.

Also make sure the cracks you see are not just impressions from the mold.

Hairline cracks are often caused by either improper moisture control as mentioned above or by differential temperatures. Differential curing can cause all sorts of cracking. Sun, wind, rain etc can wreak havoc. You do not state if the cracking occurred before or after backfilling. The inside surface would certainly be the side that would crack and open up once a load was applied. More info would help. Wall dimensions, type of concrete and reinforcement and curing conditions and time frames would tell us a lot.

Dear, cracks observed after backfilling that too 3 months later of wall concreting. is that creep phenomenon ? that concrete must crack when steel takes the load. Grade of concrete is M-40, steel Fy 500. Thanks.

Was this wall designed by an Engineer that is qualified to design walls? If so then were all the specifications on the plans followed? Was a test sample of the Concrete taken by a testing lab when they poured it? Does it meet specs? Have they come out to inspect it since you discovered the cracks?

It could just be results of the curing process and the cracks are only on the surface. But if the cracks are not just surface irregularities then you could have a major problem on your hands when the wall is back filled and then it rains.Even the best designed and built walls without some water. If its a very small wall 3 feet high or less & in an area where if it did fail that it wouldn't do any real damage to people or property I wouldn't be to concerned. But anything over 3 feet can pack a big punch when it gives way.

Yes its expensive to replace or repair right now but it cost even more after you have to come back latter in a built up area to fix it or if someones property gets destroyed because the wall gave way. Fix it while you can and you won't have to worry about it in the near future.

Concrete will crack.

You can limit the cracking and sometimes control it.

The hairline or crazing cracks may be from too much water. Some contractors will add more water in the field to make the pour easier and faster. This changes the slump and will degrade your concrete.

At your next pour, bring out a testing agency to perform slump and air entrainment tests. They can also make test test cylinders to check the 7 and 28 day strength of your concrete.

Do not let the contractor add water on site. The concrete plant has tested the the recipe for the mixture you ordered and anything you add in the field will change the strength and performance of your concrete.

Now for controlling the cracks....add saw cut joints or control joints at 45º angles from every (inside and outside) corner. Typically slabs will also be divided into 25' x 25' sections with saw cut joint as well.

Long walls will need control joint built into them. An engineer would be very helpful to determine the best locations.

Back to your question... excess water may be the culprit and if it is limited to the surface and is not deep enough to expose the re-bar, you may not have a problem. If it is on the outside, I would coat it with a mastic to prevent water from getting to your re-bar. Again the testing agency can provide you more information on the strength and properties of your wall.

Reasons the contractor added water might include... Too high of a slump from the plant, trucks took longer than 90 minutes to get from the plant to the site and place their load. Short handed and needed the concrete to flow easier. Or it maybe the plant gave you the wrong mixture for the job.

concrete cracking FAQ.

Definitely a curing problem. If the forms were removed in less than 72 hours and no curing compound was used this will cause the surface to lose moisture before the cement cures.

Geode Hunter – The contractor should be allowed to add all the water he wants as long as he doesn't exceed the specified water cement ratio and slump. And actually slump isn't that important. Slump is a measure of workability not ultimate strength, but a high slump has been bastardized into representing high water content. You can get a slump of 9 or 10 inches and have a water cement ratio under 0.35 using plasticizers, and still have compressive strength greater than 5000 psi.

Back to rajendrajsh question "whether those are structurally harmful?" If the cracking is surface crazing not a problem, but if they are deeper and the wall is a major structural component there may be issues. I would take a 4 pound mallet and beat the hell out of the wall in the area of the cracking. The cracked areas should flake off and reveal the concrete below. If you can't see the cracks anymore by looking closely with a 10X loop, then you're done. If you can still see the cracks and they are "long and intersecting," you can repair by injection sealing those cracks that are less than 50 mils in width. For cracks that are greater than 50 mils wide you can still repair by injection sealing with epoxy but you should core and test the compressive strength.

We write into our specs that no water will be added -- in the field-- that increases the slump over 7". We also will specify the design mix.

if the weather is hot, the plant will adjust their mixtures to compensate. If the concrete truck has been delayed and the concrete has stiffened or began to setup (typically 90 minutes from the time it was batched), then it should be returned to the plant for disposal of its contents.

Less water in the will yield a stronger, more durable concrete; more water will give an freer-flowing concrete with a higher slump.

If the concrete needs to flow easier-- say through a pump-- then there are plasticizers that will reduce the slump for a small amount of time before returning to the deign standards.

I am not advocating exceeding the specified slump or any other criteria. I merely sought to point out that a high slump is not an indication that concrete will not ultimately achieve it's design strength or greater. Those who are unfamiliar with the fundamentals of mix design often get confused on this because higher slump can be achieved by adding water. However; if the water cement ratio as specified in the mix design is not exceeded, the concrete will meet it's design strength regardless of the slump.

It is also possible to exceed the water cement ratio without exceeding maximum slump, resulting in lower strength concrete. That is why the water cement ratio should be verified when concrete is delivered, and recalculated with the addition of water at the placement location.

If you would like some help learning how to compute water cement ratio. I would be happy to help.

Geode Hunter

I normally will not enter into a debate between commentors but I feel very compelled to join this discussion. P Moon is entirely correct in everything he is telling you and I hope you will either take him up on his offer to help or seek guidance from some other qualified source. An excellent source of information can often be your admixture supplier. As an owner of a precast concrete company, a general contractor, and a card carrying ACI certified concrete testing technician as well as other pertinent experience and qualifications I have some room to speak on this subject. Your sweeping statements, including specifying not adding water to exceed a slump, and to presume the redi mix company doesn't use Delvo or some other retarder to allow longer transit time are concerning to someone who can be on the receiving end of this misguided rule making. Many of us in the industry have long advocated performance based specifications and this concept is finally taking hold. No offense to engineers in the office, but if the designers would simply tell us what the desired performance is, we suppliers and contractors can offer specifications that will make more sense and usually offer superior results. A good submittal process should ensure you receive desired results.

Remember, we are all learning something everyday. Or at least we should be.

Although technical papers, like, ASTM C 94 allow the addition of water on the jobsite, it also states a need to record the amounts and test the new mixture.

It seems the pecking order, when something fails (concrete in this instance) is: Architect --Engineer--contractor--supplier--subcontractor. And he added water to the design mix? Well there you are.

1 gallon of water, added to the design mix, per cubic yard can:

•• Increase slump 1 inch

•• Decrease compressive strength 150 to 200 psi

•• Waste about 1/4bag of cement

•• Increase shrinkage by 10% (shrinkage is cracking)

Who onsite knows exactly how much concrete is still in the truck, knows what the current cement to water ratio is,

Use the appropriate design mix for the project, hire an onsite testing firm to check the first or second truck load of the day or every 50 yards of larger pours, and designate one person who will take responsibility for field changes or rejection of the batches as required to meet the goal of the project.

Extra water does not have to be the only culprit here. What was the air temperature when the pour was made? How long was the concrete in the truck after batching? Did the contractor over work the material?What was the air content of the mix?

Since you are worried about the strength of your concrete, are you planning on further tests? A penetrometor test would be the cheapest before going on to more invasive tests.

Hello Rajendra

There are many reasons for development of hairline cracks. Most likely your hairline cracks may be due to shrinkage or quick drying. Please forward some pictures showing the hair-line cracks you have mentioned. Its too difficult to decide if your hairline cracks are structurally safe or not without the pictures.

Was this retaining wall designed by a qualified structural Engineer? Stresses in individual bars can be calculated to estimate safe crack widths at the design stages. But now, since the walls are done, please go with thin / lean mix or HILTY expansion grout to save your re-bars from progressive corrosion due to water and air.

Br\\

Fareed

fareed_siddique@yahoo.com