CA-B Or ACQ - Any Difference For Ground Contact?

karma_carpentry

I've been asked to build a freestanding terraced raised bed for a landscaper, and I found both CA-B and ACQ landscape timbers (6x6-8' rough-sawn white pine).

Anybody got a clue which treatment would be longer lasting in ground contact, and also which would be less toxic if the homeowners plant edibles in there?

BTW I also plan to coat the inside of the box with Redgard to reduce contact with plant roots and leaching of the preservative into the soil.

Tom R

Can't say I know the answer, - - but they do have different (ground contact) 'pressure ratings', - - even amongst themselves.

I 'think' you need .60 (at least for the ACQ).

I believe what you buy off the shelf at HD 'n all is usually only .25 or somethin' like that.

I don't think they should be near 'edibles' at all . . .

__________________
http://www.tr-built.com

BuiltByMAC

In addition to the Redgard (paint?), you might think about lining the inside of the box w/ a plastic or rubber membrane to further prevent the possibility of chemicals from the lumber interacting with the food being grown...

Maybe build the box on gravel to prevent the timbers from sitting on water-soaked dirt, the gravel would promote drainage away from the timber...

from http://www.fpl.fs.fed.us/rwu4723/pre...aqs/types.html

Copper Azole (CBA-A and CA-B)
Copper azole is another recently developed preservative formulation that relies primarily on amine copper, with co-biocides to protect wood from decay and insect attack. The first copper azole formulation developed was copper azole-Type A (CBA-A), which contains 49% copper, 49% boric acid, and 2% tebuconazole. More recently, the copper azole-Type B (CA-B) formulation was standardized. CA-B does not contain boric acid and contains 96% copper and 4% tebuconazole. Wood treated with either copper azole formulation has a greenish-brown color and little or no odor. The copper azole formulations have been evaluated with in-ground stake tests and demonstrated efficacy against attack by decay fungi and insects. The formulations are listed in AWPA standards for treatment of a range of softwood species. Minimum retentions of CBA-A in the wood are 3.3 kg/m3 (0.20 lb/ft3 ) and 6.5 kg/m3 (0.41 lb/ft3) for wood used above ground, or in ground contact, respectively. Minimum retentions of CA-B in the wood are 1.6 kg/m3 (0.10 lb/ft3 or 3.4 kg/m3 (0.21 lb/ft3) for wood used above ground or in ground contact, respectively. Although listed as an amine formulation, copper azole may also be formulated with an amine-ammonia formulation. The ammonia may be included when the copper azole formulations are used to treat refractory species, and the ability of such a formulation to adequately treat Douglas-fir has been demonstrated. The inclusion of the ammonia, however, is likely to have slight affects on the surface appearance and initial odor of the treated wood. The copper azole treatments do increase the rate of corrosion of metal fasteners relative to untreated wood, and hot-dipped galvanized or stainless steel fasteners are recommended. Because copper azole has only been recently developed, relatively few treating facilities are currently using this preservative.

Alkaline Copper Quat (ACQ)
Alkaline copper quat (ACQ) is one of several wood preservatives that has been developed in recent years because of environmental or safety concerns with CCA. The fungicides and insecticides in ACQ are expressed as copper oxide (67%) and a quaternary ammonium compound (quat, 33%). Multiple variations of ACQ have been standardized or are in the process of standardization. ACQ type B (ACQ-B) is an ammoniacal copper quat formulation, ACQ type D (ACQ-D) is an amine copper quat formulation, and ACQ type C (ACQ-C) is formulated with either ammonia or amine and a slightly different quat compound. Wood treated with ACQ-B wood has a dark greenish brown color that fades to a lighter brown and may have an ammonia odor until the wood dries. Wood treated with ACQ-D has a lighter greenish brown color and little noticeable odor. Wood treated with ACQ-D weathers to a brown color. Wood treated with ACQ-C varies in appearance between that of ACQ-B and ACQ-D, depending on the formulation. Stakes treated with these three formulations have demonstrated efficacy against decay fungi and insects when exposed in ground contact. The ACQ formulations are listed in AWPA standards for a range of applications and many softwood species, although the ACQ-C listings are limited because it is the most recently standardized. Minimum retentions of 4 kg/m3 (0.25 lbs/ft3) or 6.4 kg/m3 (0.4 lbs/ft3) are specified for wood used above ground or in ground contact, respectively.
The multiple formulations of ACQ allow some flexibility in achieving compatibility with a specific wood species and application. When ammonia is used as the carrier, ACQ has improved ability to penetrate into difficult-to-treat wood species. However, if the wood species is readily treated, such as southern pine, an amine carrier can be used to provide a more uniform surface appearance. All the ACQ treatments do accelerate corrosion of metal fasteners relative to untreated wood, and hot-dipped galvanized or stainless steel fasteners are recommended. The number of pressure treatment facilities using ACQ is increasing. In the western United States, the ACQ-B formulation is primarily used because it allows better penetration in difficult-to-treat western species, although ACQ-D is being increasingly used. Treating plants in the remainder of the country generally use the ACQ-D formulation. Use of the more recently standardized ACQ-C formulation is expected to increase in both parts of the country.
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After reading through that, I don't see one as being significantly better than the other to use for ground contact.


Mac