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Archive for March, 2007
pervious concrete
Posted by Sandy from Mackinaw City, MI, US on March 28, 2007
Will pervious concrete work in northern climates where temperatures stay below freezing for several months?
Yes, if it is designed properly for permeability and base thickness. The success of the system depends on how well the base drains before and during a freezing cycle.
Follow ACI 306R (Cold weather concreting) recommendations while placing pervious concrete in cold weather, such as to cover the pavement with thermal blankets to protect it from freezing during the first few days after placing etc.
Three year old driveway crumbling away
Posted by John from Creighton, NE, US on March 27, 2007
I’m not sure this pertains to your field of expertise….my driveway is three years old and starting to break up. In some spots it is worse than others. The top eighth inch is flaking off in one area and in another area you can push a screwdriver in a quarter inch or so. In the latter area it retains a “wet” look like after it rains and it stays damp for days or even weeks after. I have pictures I could email if that would help solve my problem. I would like to find out what caused it and how to repair it. Thank you.
It is difficult to comment on such problems without visiting the actual site. But reading your question it seems the problem in your case seems to be due to one or more of the following reasons:
1. Bad quality concrete (low grade, low strength, high porosity, more water in the concrete mix).
2. Insufficient pavement thickness.
3. Improper sub base or less compacted subgrade soil.
4. Improper finishing of the concrete surface (excessive bleeding or formation of laitance on the concrete surface).
I would recommend you to call an engineer or a qualified contractor to come and have a look at your driveway.
Type G admixtures
Posted by Frank from Abu Dhabi, OT, AE on March 25, 2007
Bruce,
We have a mix design using type G admixture to improve the slump/workability and retardation of settting time for marine piling works (ie barged concrete). Is it acceptable to redose on site with the same Type G admixture to increase not only the slump but the time allowed for placing even though the specifications require a maximum time of 1.5 hours?
A Type G admixture can be used for re-dosing in specific cases. The factors influencing would be the amount of admixture added while re-dosing, the slump of concrete before re-dosing and target slump. Re-dosing will cause retardation, the extent of retardation would depend on the ambient temperature. At higher temperatures re-dosing will have less influence on retardation.
I would advice you to contact the admixture company from whom you are buying the admixture, provide him the mix design details and ask their opinion about this matter.
If the purpose of re-dosing is just to increase the slump and you are not looking for a longer slump retention, the best option would be to re-dose using a Type F admixture.
Use of slag in road building
Posted by Sandra from Ann Arbor, MI, US on March 24, 2007
Many years ago, when my house was being built, slag was used by the builder on a particularly soggy area of the approach to the building site, to allow vehicular access. It was extremely effective. Our unpaved driveway is long (1/8 mi.) and we have a chronic problem with potholes. I would like to try using slag to fill these areas. What are your thoughts on the use of slag in road building and maintenance, and where can I buy slag in the vicinity of Ann Arbor, MI?
I don’t think I can give an accurate answer to this question with the information provided. It would appear that the builder used something other than the slag we use in concrete. My best guess is that he used something like crusher run from an aggregate supplier. Perhaps a call to someone in Michigan to find out their defination of “slag” would help.
Efflorescence on exterior & interior basement walls
Posted by Robert from Cottleville, MO, US on March 20, 2007
Hi, my name is Robert Sims
I have a villa that is 6 months old. Foundation poured in July or August 2006. We now have efflorrescence on both the exterior and interior walls. The builder brought in a Technical Service Engineer to take a look at it and give me a report on it.
He checked the compressive strength in numerous places with a Swiss Impact Hammer. The results were 4710psi. He gave the build instructions on cleaning it up and sealing the walls.
A couple of questions that weren’t answered are;
Can this cause damage to the rods or metal ties in the foundation that would cause weakening in years to come?
Can this cause corrosion in the concrete itself which would be a problem later.
My electrical box is attached to the wall in question, should this be removed and cleaned and sealed behind. Can this eat into the metal and wiring?
Can this re-occur?
Do you know of a company in my area to use for a second opinion?
Thank you for any assistance you can give me in this matter.
God Bless
Bob
If the wall is made of concrete blocks, then there shouldn’t be any issue of corrosion, but if they are made of reinforced concrete, efflorescence can cause corrosion.
Efflorescence occurs when free calcium hydroxide from the concrete is transported by water through the capillaries to the surface of the concrete. On the surface calcium hydroxide reacts with carbon dioxide in the air to form an insoluble material calcium carbonate (white deposit).
The migration of calcium hydroxide to the surface creates channels through which harmful materials like chlorides; contaminated water can enter the concrete and react with the steel to cause corrosion.
During corrosion the steel rusts and expands, which may lead the concrete to spall.
Efflorescence if present should be cleaned and the concrete surface should be sealed with a penetrating sealer (coating). There are products available for this in the market. Do a google search for “Stopping efflorescence” for more information.
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