Hardened bags of concrete aren't as good for buiding retaining walls because their shape has already set. The beauty of using concrete bags is that they naturally conform with surrounding bags before hardening. The resulting structure is therefore very sturdy because of the natural way in which they will lock together. However, if the hardened bags are interspersed among non-hardended bags, then they can be salvaged. Just fyi, when concrete mix is properly mixed with water and allowed to cure, it becomes rock hard. However, concrete bags allowed to naturally cure through slow absorbtion of water will not be as strong as standard concrete and is normally somewhat crumbly by comparison. Just my observation. However, since stacked walls function based upon gravity and rarely hold back hydrostatic pressure, the actual concrete psi strength probably isn't as important as a poured unit which will hold back water. By the way, gravity stacked concrete block retaining walls are built all the time and their durability is unquestioned. Once the walls exceed 4' in height, it is normally required that such walls be engineered with steel re-inforcing or geotextile fabric integrated into the soil that is being "retained." Most such walls are only 8" - 12" wide. A bag of concrete mix can be much wider (depending upon the size and the orientation of the stack). From afar, such walls are not unattractive. Close up, however, the walls are not as attractive as engineered retaining blocks as the finish was not "finished" but rather remain as they were inside the bags. The pleats of the bags will show through into the finished product when the paper deteriorates. However, in a natural environment with landscaping foliage taking the focus off the wall, I personally think they are attractive. As far as cost is concerend, there is no additional cost for form materials or the labor to erect and remove the forms. The actual cost of the concrete is not more than the cost of poured concrete. Around here, a yard of ready-mix delivered is approximately $100/yd, give or take. The concrete pump is another $350. Compare that to the cost of a sack of 80# mix at approximately $2.39 or so. The 80# mix equates to 2/3 cubic feet to which you would need 41 bags to make a yard. You'll need to pay the lumberyard (or masonary supply), whatever, to deliver. But it saves you the $350 cost of the pump and of course you are saving the form materials. Since we are addressing this in the tractor forum, I think it is safe to presume that most folks here have a way to transport the bags (loader, forklift attachment, 3pt. forks) directly to the site of the wall. All the labor that is required is to load onto tractor and unload directly onto the wall being built.
