I appreciate Mr. Neuber's comments and concerns (April 2006, p. 56). Even though Mr. Neuber has attended some P2P meetings, it appears that he somehow has some misconceptions about the P2P Initiative. Mr. Neuber, however, makes several comments that I don't think anyone disagrees with and that are clearly in line with what the P2P Initiative is trying to rectify. This is why we need his support for the P2P Initiative. Through the American Society of Concrete Contractors, the P2P Steering Committee has established a separate task group to specifically work with contractors, which has been very positive. The P2P initiative also clearly states that it is working toward performance-based specifications as an “alternative” and is not trying to force this on anyone.
To paraphrase and agree with Mr. Neuber, the National Ready Mixed Concrete Association (NRMCA) is here to protect its members. That's the reason an association exists. An important part of this member service is to help members control their product and support their business activities. The P2P Initiative, however, is broader than just the concrete supplier. It is an attempt to improve the way concrete is specified. A specification is a set of instructions written to the contractor. The P2P alternative is a means of getting away from the details of the concrete mixture and the means and methods of construction in a specification and to leverage the expertise of those who have it. It is a means of eliminating inherent specification conflicts that do not allow efficiencies in the use of materials or the process of construction.
Mr. Neuber is accurate that the producer and the contractor need to work as a team. Partnering has been a goal of the P2P Initiative from the beginning. There also seems to be a presumption by Mr. Neuber that moving toward P2P will result in worse mixture designs. If the team is functioning well, that would not be possible.
Many of the issues that Mr. Neuber discusses relative to floor slabs should not be specification items that are dictated by the design professional but items that should be worked on together by the floor contractor and the concrete producer. To comment on some of these:
- A 2-inch maximum size aggregate, when available, is difficult to handle relative to segregation and batch-to-batch consistency, and in some cases impossible to pump.
- Concrete producers will stockpile aggregates that are in general use unless a project has sufficient significance and volume to justify otherwise. The same goes for aggregate storage and bins. It is unlikely that a producer, say in a rural area, will take on considerable capital investment for an occasional floor slab job.
- The concept of oversanded mixtures has evolved because of demands from the flatwork contractor for ease of placement and finishing.
- Mr. Neuber comments on the use of well-graded lower paste-content mixtures with large aggregates. Conceptually, this should work well when practical. Increasing the aggregate size automatically reduces the paste content. Including an intermediate size will get closer to a “well-graded” mixture. In many areas, the local materials do not allow one to get to an “optimum” grading economically and allowance should be made for that. Most of the cited experience on these aspects have been subjective without much technical data showing relative measurable performance differences from a “gap-graded” mixture. An article by Mc-Call, et al, in the March 2005 issue of Concrete International concluded that there was no significant difference in drying shrinkage between “well-graded” and “two-aggregate” concretes in Florida. NRMCA has conducted limited studies, supported by similar tests done by producers in different regions, with similar preliminary conclusions. Harrison had some great ideas in his March 2004 article in Concrete International but lacked any technical data to support his recommendations. In summary, there needs to be some measurable and verifiable performance measure that indicates specification compliance. Is the performance requirement shrinkage, finishability, workability, pumpability, setting time? Then, if possible, it should be in the specification or worked out between the contractor-supplier team. In many cases, it is difficult to assign quantitative values and clearly these do not fit in a specification. The requirements in a specification cannot be subjective and preferably should not delve into the details of the mixture composition for perceived performance that may or may not work. Qualified producers can then experiment with their available resources to deliver the needed performance.
Mr. Neuber indicates that NRMCA should better spend its time guiding the industry on better mix designs. NRMCA conducts training and certification programs for all job functions within the ready mixed concrete industry. It administers more than 50 training programs touching 1500 people annually. This includes technical training programs, such as the biannual technical short course where concepts of mixture proportioning are taught and attendees answer exams towards an NRMCA certification—Level 2 and Level 3. However, mixture development for specific projects is still a matter of knowing the local materials and developing mixtures that are very application specific.
Purchasers of concrete should insist on getting their material from companies that have demonstrated a commitment to quality, such as those with NRMCA-certified or DOT-inspected plants and with certified tech services and quality control people, plant operators, and drivers. We recognize that there will be many producers and contractors who will not be qualified or do not want to bid on performance-based projects and for that reason, P2P is clearly being proposed as an alternative. An important part of the P2P initiative is to establish a quality standard for those producers that can and will deliver a product based on performance. This, as is the P2P Initiative, is a work in progress. I invite Mr. Neuber to join with the ASCC and NRMCA to ensure that we get it right.
Clarification from Joe Neuber
My April article had a quote pulled out that I think emphasized the wrong point. It stated that what we are asking the ready-mix producer to provide is “more demanding and more costly to produce.” Discussions within ACI Committees 302 and 360, of which I am a member, indicate that overall, the mixes we are looking for are not more demanding or costly to the normal concrete producer. My comments about it being more demanding and costly were directed at the small ready-mix producer who has an older three-bin plant with limited storage space for aggregate. In that case it can be more demanding and thus more costly. But for the majority of producers it is not.
I review mix designs for producers all over the country and have found that I usually get some resistance in the beginning. But if it comes down to a matter of whether or not they get the job, they can usually find a way to produce a better slab mix at the same cost as their regular 4000 psi slab mix.