One of the biggest selling points with the ReCon Block is the long term durability of wet cast air entrained concrete as compared to the smaller dry cast concrete block products. The Federal Highway Administration issued a report in 2008 documenting the problems with the durability of the small dry cast block products in areas of the U. S. where there are significant freeze thaw cycles combined with road de-icing chemicals.
As a ReCon Licensee, you need to make sure that (a) you are promoting the durability of wet cast concrete, and (b) you have in place a durable concrete mix design for the ReCon block that you are producing. As ReCon (and big block in general) has become more and more accepted by specifiers, some questions have arisen regarding what is a proper “big block specification”. Some specifiers use the ReCon spec “as is”; others amend it to include provisions more appropriately designed for the small dry cast block products; and others take a spec from another big block product which may vary from the ReCon spec. Obviously the customer can select a specification that “works for them”. Our job is to try to keep the specification “relevant”.
As you know, long term durable concrete is a function of your inpidual mix design. Key variables include:
- Cement content
- Water / Cement ratio
- Resulting psi
- Air content
- Quality Aggregates
Neither ReCon nor its competitors have focused on each of these variables in its specification. For example, ReCon has focused on just psi and air content. In addition, ReCon has referenced a durability outcome via ASTM 1372 for freeze thaw testing (which a number of ReCon’s big block competitors also reference). Nobody (to-date) has specified a minimum cement content or a maximum water / cement ratio.
As of late, questions have arisen regarding ASTM 1372 and also the minimum unit weight per cubic foot of concrete (145 pcf) contained in the ReCon spec. We have examined these issues and we have decided to modify the ReCon spec as it appears on our website (and eventually in our Design and Construction Manual on the “next printing”).
ReCon’s current specification as it relates to “quality concrete” references the following:
- Minimum 28 day compressive strength of 4000 psi.
- Air Entrainment 4.5 -7.5 percent by volume
- Weight of concrete at a minimum of 145 pcf
- ASTM 1372 Section 7, 8, and 9.
ReCon’s new specification as it relates to “quality concrete” will reference the following:
- No Change…the minimum 28 compressive strength shall remain at 4000 psi.
- Air entrainment
- Current target for air entrainment by volume is 6% plus or minus 1.5% creating a range of 4.5% to 7.5%.
- New target for air entrainment by volume will be 6.5% minus 1% or plus 2%.
- In zones where freeze thaw and road salts are an issue, sufficient targeted air entrainment is critical. Some of the air entrainment can be lost via the vibration process. Thus we want to keep the minimum at time of air-pot testing to no less than 5.5% (as opposed to the 4.5% previously specified).
- Unit Weight of concrete
- Currently set at 145 pcf
- We are going to eliminate this requirement. Weight of concrete is not necessarily a measure of quality of concrete. Some of our licensees are producing a very high quality concrete with a weight that is slightly less than 145 pcf. A weight less than 145 should not be a reason for concern to a customer, if other quality measures are in place. Thus the change is being made.
- Caveat…for those of you that do have a mix design that weighs less than the 145 pcf, you should know that our ReConWall software assumes a weight of 145. You should consider informing your design engineer that your concrete has a lesser weight, which could in turn have a slight impact on both gravity and geogrid wall designs. The software allows the engineer to change the unit weight of concrete.
- Freeze Thaw Testing
- Currently ReCon references ASTM C-1372 Sections 7, 8 and 9.
- This standard is for dry cast block, not wet cast block. Nonetheless, a number of the competing “big wet cast block licensors” still reference 1372 and this standard has and will find its way into specifications. The actual test requires 100 freeze thaw cycles in water, generally with samples cut from block.
- ReCon is going to change the reference to ASTM C-1372 Section 7 ONLY, which deals with “Finish and Appearance”, not freeze thaw testing.
- The proper reference spec for freeze thaw testing of wet cast concrete is ASTM C666. At this time, we are not going to add this requirement to the specification. However, this is the ultimate performance specification on freeze thaw durability of wet cast concrete. It is a measure of quality that you should voluntarily consider, to set yourself above the competition and provide a answer to those specifiers that may ask for 1372 testing results.
ReCon will be sending to you shortly a second article providing suggestions for each of you to consider in “raising the bar” on quality, and eliminating some of the competition. Stay tuned for more and feel free to call with questions.
ReCon Wall Systems, Inc.