At Beton we review project specifications for various reasons. Sometimes it’s to develop concrete mixture proportions that will help a contractor meet the requirements. Sometimes it’s to come up with a thermal control plan. Unfortunately, sometimes it’s because something went badly wrong on a job, and we need to figure out why. Here are some things I wish specifiers knew.
Codes, guides, and reports
Specifications incorporate codes by reference. For example, ACI 318, Building Code Requirements for Structural Concrete, would be cited in a specification for a building. But engineers may not realize that the American Concrete Institute publishes many other documents that aren’t codes. They can be very useful as guides to best practices, but they don’t belong in a specification. A specification may refer to them, but not incorporate them.
Why is that? A code employs mandatory language, that is, “thou shalt” and “thou shalt not”. It’s clear whether a design or practice complies with it in any given situation. Also, many authorities have adopted the code as the law governing construction in their jurisdiction, so it has the force of law.
Neither of these is true of a guide or a report. These documents aren’t in mandatory language because they provide background on the how or why of a particular practice. They often provide more than one alternative, leaving it up to you to decide which one you prefer. You can’t do all of them, so there’s no way for you to follow it as you would a code. Also, you may have good reason to do something else entirely. That’s perfectly fine with a guide or report, but you can’t ignore code provisions.
As a result, it doesn’t make sense to incorporate guides or reports in specification language. Unfortunately, it happens all the time. In the case of ACI 318, Building Code Requirements for Structural Concrete has a parallel commentary alongside it. The commentary is not in mandatory language, as it provides explanations and guidance. It’s not part of the code and doesn’t belong in a specification.
Specifications also incorporate standards by reference. Like codes, standards are in mandatory language. However, the engineer may not know all the particulars.
A standard sets out the requirements for a test method, practice, or material. ASTM C1077 is a standard practice for labs for acceptance testing of concrete. It covers the licensing, certification, and experience of the personnel, calibration of the equipment, and reporting and record keeping procedures.
We’ve written extensively about test methods in this blog. They tell you how to conduct the test and what to measure. However, they don’t say what’s acceptable. For example, ASTM C39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, tells how to determine compressive strength. It doesn’t say what’s high enough—or too high. The specification must state what strength(s) it requires at what age(s).
Materials standards do specify minimum and/or maximum limits. However, some materials standards cover more than one type, grade, or class of that material. The specification must indicate which ones are acceptable. For example, the specification might reference ASTM C618, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete. ASTM C618 covers both Class F and Class C fly ash, so the specification should indicate which one(s) the engineer wants.
Also, some materials requirements are optional. For example, ASTM C150 Type II cement has an optional requirement for sulfate resistance. If you need that, call it out in the specification.
Quite a few materials standards aren’t adequate for use with high-performance concrete. For example, ASTM C618 specifies a maximum of 6.0% for loss on ignition (LOI), a measure of the residual carbon. Carbon adsorbs surfactant admixtures, especially air-entraining admixtures, making them less effective in the concrete. Because of this, a specification might require a lower LOI, say 3%, when the concrete needs to withstand cycles of freezing and thawing.
Performance v. prescriptive
Prescriptive specifications lay out every detail about what materials to use in what proportions, and how to batch, mix, place, consolidate, and cure the concrete. Performance specifications leave the means and methods to the contractor and suppliers. Performance specifications have the potential to produce the desired results more efficiently.
Another example is ASTM C33, Standard Specification for Concrete Aggregates. The aggregate grading it specifies isn’t adequate for self-consolidating concrete, or for minimizing cracking. For these and other applications, the aggregate suspension method uses a more precise aggregate grading for better performance.
ASTM C33 also falls short in identifying aggregates susceptible to alkali-aggregate reactions. ASTM C1778, Standard Guide for Reducing the Risk of Deleterious Alkali-Aggregate Reaction in Concrete, provides a more thorough, nuanced approach. However, neither standard identifies aggregates susceptible to popouts.
Aggregates are almost always local, as the cost of shipping them long distances quickly outstrips their value. You don’t have to live in a state to know the local aggregates, but it helps. In reviewing the specifications for the floor slab of a big-box store, I noticed that there was no provision to mitigate aggregate popouts. The designer was a specialist in slabs on ground. I called him to express my concern about popouts, which I knew would be unacceptable for exposed concrete in that application. I explained how our local aggregates behave and advised him how to prevent popouts. He was grateful for the advice, knowing I’d saved him from an expensive problem.
When it comes to specifying aggregates, the best source of information is often the state highway department. The highway department places more concrete than any other entity. They build roads and bridges all over the state, so they need to characterize their aggregates thoroughly. The information is available free on their website, and their specifications are readily available as well. Depending on your application, you may need to modify their requirements. For example, the acceptable number of popouts per square yard will be lower in a floor than on a highway. But at least you’ll know what to look out for.
National specifications for local projects
In a previous job, I worked on a number of big-box stores. These stores often use essentially the same specification all over the country. That can be problematic, as site conditions vary from place to place.
In addition, the capabilities of contractors and concrete suppliers aren’t the same everywhere. Big-box stores have a uniform look regardless of location; it’s part of their branding. But colored concrete, for example, may be too sophisticated for a mom and pop ready-mixed concrete supplier to produce reliably. And in some places, that’s all you have. Here in Minnesota, contractors and ready-mixed concrete producers in the Twin Cities can handle just about anything. But outside the immediate area that’s not necessarily the case.
Another problem I’ve seen with writing specifications from a central office is a lack of appreciation for exposure conditions. Having spent most of my adult life in the Midwest, I’m all too familiar with cycles of freezing and thawing. But someone in Arkansas or Texas may not be. They probably know to specify air entrainment, but may not appreciate how to maintain a good air-void system through pumping, consolidation, and finishing. Or how important it is to provide good drainage and good curing. In the Midwest, we deal with both hot– and cold-weather concrete construction. Each can have negative effects on concrete, particularly its durability.
If you’re specifying a project in an area you’re not familiar with, it’s helpful to work with a local design professional. They can advise you about local conditions and practices. One benefit of participating actively in national organizations such as the American Concrete Institute is that you develop a network of professional contacts around the country. That way you know whom to call.