Subject: Stay up to date
Occasionally something that seems to be rather simple can turn into a bigger deal. Such was the case recently. I received a call from a contractor friend with what I thought at the time was an easy question. He asked if there was any reason not to substitute a lightweight concrete masonry unit (CMU) for a “standard/regular” weight unit.
Since I knew what unit he was referring to, the short answer was no. There is no reason not to ask for a substitution. Anyone that knows me knows that he tugged on my chain with the use of the words standard/regular. There are NO standard or regular weight CMU. For that matter, there are no heavyweight units.
ASTM uses the density of the material used to manufacture CMU as the breakdown of the units. In Table 2 of ASTM C 90, the density of the material for the CMU is broken down as Lightweight @ less than 105 lb. per cubic foot, Medium weight @ >105 lb. to < 125 lb. and Normal weight @ > 125 lb. All three are required to have a minimum compressive strength of 2000 lb. Since this is rather simple, I asked him to send in a pre-bid request for information for approval.
Since there is a higher cost for the lightweight (LW) unit, some people think that will just run up the cost of the masonry. This contractor does not see it that way. He believes that since the unit is considerably lighter, the masons will lay more units per day. The lighter unit also makes it easier for the mason tenders to handle them. This leads to less chipping and loss of units. In time he received his answer, which permitted to use the LW units, IF.
Well the IF is what opened the door to all of the other issues. The construction manager (CM) or design team responded that IF the compressive strength of the concrete used to make the block was at least 3250 psi and IF we used type M mortar we could make the change. So now it is as clear as mud what the problem. They are trying to use Table 2 from TMS 602, but the specifications referenced an un-dated Code and Specification by the wrong name. According to Table 2, you can use Type M or S.
We then received copies of the project specification and the structural notes on the project to determine why we still have no clarity. They are trying to use Table 2 from TMS 602, but the specifications reference an un-dated Code and Specification by the wrong name. Switching to the structural notes only made it worse. The structural notes referenced ACI 530-05/ ASCE 5-05 and ACI 530.1-05/ASCE 6-05. The current and correct reference is to TMS 402/602-16. Six more years and they would match Rip Van Winkle.
The project documents also called for normal weight CMU, which is not necessary or maybe not available without additional cost. Another issue was with the grout. They called out a 3/8″ aggregate and a slump of 8″-10″. Slump is now 8″-11″. We haven’t used 3/8″ aggregate in years since we have an aggregate that causes the grout to be between a course and fine grout. Once again let me say that you should know your masonry business because you may be the only expert on the project. Or at least you will know that IF a frog had wings, it wouldn’t bump its butt every time it hopped.
Remember to Raise the line and come on around the corner.