Fresh concrete tests—such as temperature, slump, air content, and unit weight, in addition to the casting of cylinders or beams—can be performed with concrete sampled at many different stages in the concrete production, transport, delivery, placement, consolidation, and finishing sequence.
For example, concrete can be sampled from the concrete truck before it leaves the plant, immediately upon arrival at the jobsite, before or after water or other admixtures are adjusted in the field, directly from the truck chute, or at the point of placement via pump, conveyor, or concrete bucket. If sampled from point of placement, the choices include before or after depositing the concrete in its final location, before or after consolidation with a vibrator, or before or after the concrete is subjected to a vibratory screed, striked off, or bull floated. Stiffening of the mix makes it tough to collect a sample later in the process, which is why the effects of finishing and curing normally can be assessed only by tests of hardened concrete.
Although a lot of useful information can be obtained from a comprehensive testing program that tracks concrete properties at each stage from the plant to the deck, the cost of testing, the potential for interference with construction operations, and the need for clarity about how and where acceptability is established normally limits the options for where to sample concrete.
A closely related issue is that the concrete producer is most commonly held contractually responsible for the properties of the concrete as delivered to the site, prior to any “field expedient modifications” made to the material by the contractor or the contractor’s subs. The contractor’s forces and equipment have the capacity to remix, lift, drop, shake, vibrate, pressurize, depressurize, wet up, or dry up the concrete—each with its unique effect on concrete properties. For that reason, the only sampling location that isolates the quality of the concrete as delivered is at the point of delivery. Tests on concrete sampled anywhere downstream of the truck chute are likely to be influenced by construction operations thus making multiple parties responsible for the results.
Because of this need for assigining responsiblity and consistency, sampling at the chute is the industry default and should always be included in any sampling scheme. The wide range of sampling options makes for interesting debate, but given the implications of testing costs, the cost of the concrete, and shared responsibility, the sampling location or locations—and the methods for both sampling and testing—have to be included in the specifications (prebid) with absolute clarity.
One way specifiers achieve clarity is to incorporate industry standards for sampling and testing in the specifications. This avoids reinventing and rewriting, promotes uniform practices, and sends an immediate and familiar signal of intent to the concrete producer and contractor. In regard to sampling, the standard most commonly built into specs is ASTM C172, Standard Practice for Sampling Freshly Mixed Concrete. Although applicable to a wide range of industry practices, for the equipment typically used in building work, this standard applies to sampling from the truck chute, and defines the collection of the “composite sample” by combining and remixing two or more individual samples (usually in a wheelbarrow) taken “during discharge of the middle portion of the batch.”
This composite sample broadens the basis for evaluation and avoids nonrepresentative results that can come from sampling a very small slug of concrete. A note in the sampling specification clarifies that, “Sampling should normally be performed as the concrete is delivered from the mixer to the conveying vehicle used to transport the concrete to the forms,” but goes on to say that specifications may require other points of sampling. If no such additional requirement is specified, C172 will lead the test technician and a trusty wheelbarrow to the truck chute.
The specifier is of course free to add clear requirements to contract documents for sampling and testing concrete at any stage in the process, and it intuitively makes sense to sample closer to the final location of the concrete to get the best handle on concrete properties in the finished structure. This is easier said than done, and there are several key issues to keep in mind when deciding to go beyond ASTM C172.
Going above and beyond
Those wishing to go beyond the requirements of ASTM C172 should, first of all, specify point-of-placement sampling only as an addition to sampling at the truck chute. If results at the point of placement are unsatisfactory, diagnosing the problem or determining if the cause is related to the concrete mix or to construction operations is extraordinarily difficult if the baseline is not established at the truck chute. Further, sampling at the truck chute is the only way to unambiguously characterize the concrete as delivered, and the only way to evaluate the concrete producer’s compliance with most standard specifications.
Second, when sampling at the point of placement, many of the detailed procedures of ASTM C172 are difficult, if not impossible, to follow. As a result, individual test technicians have devised workarounds, which, although sometimes ingenious, are nevertheless not standard, vary with each technician, and influence the results of the tests.
A classic example is the practice of securing a sample from the end of a pump hose for testing air content. One popular method is the “clean catch,” executed by holding a plastic 5-gallon bucket under the exit of the pump hose. Important safety tip: At a pumping rate of 60 cubic yards per hour, a 5-gallon bucket is filled with 100 pounds of fresh concrete in 1.5 seconds. OSHA where are you?
A variation on the theme is to hold the bowl of a pressure air meter under the hose. This gets filled in 0.6 seconds, less than a heartbeat. One way to get around this problem is to let the concrete splat on the deck (losing air on impact) and then dig it up with a shovel and place it in the air meter. This can be done before or after insertion of a vibrator, which can make a significant difference in the results given the capacity of an immersion vibrator to reduce air volume. (As discussed later, a reduction in air volume does not necessarily imply a reduction in freeze/thaw durability.) Further, ASTM has no guidance on the effect of placing concrete and then reexcavating it for air tests.
A second workaround is to bring the test technican’s wheelbarrow as close as possible to the point of placement, fill it from the pump, and then proceed with standard testing. Because it is usually difficult to get the wheelbarrow close to the actual point of placement, it is almost always necessary to change the pump boom configuration and to slow the pumping rate to the barest trickle. And because changing boom angles almost always affects the impact of pumps on air content and air loss is generally greater at a slower pumping rate, the sample collected in the wheelbarrow is not likely to represent the concrete actually placed in the structure. Comparable problems are encountered when evaluating the effects of concrete buckets and conveyors on air content.
Point of placement testing
So why are these sampling details important to the specifier? Because it is exactly these details that have to be spelled out in the specs if point-of-placement testing is required, due to the common inability to fully comply with ASTM C172 anywhere but at the truck chute. Writing a job-specific sampling protocol is challenging given the wide range of construction methods and equipment that could be used, the variable impact of each, and the lack of consensus on how this sampling should be done. Science fair project here we come!
An even tougher challenge for a specifier requiring point-of-placement testing is to decide how to modify the typical acceptance criteria, developed on the basis of many years of testing and acceptance at the truck chute. It is common knowledge that acceptance criteria for concrete strength based on cylinders taken at the chute are significantly different than acceptance criteria based on cores taken from the hardened concrete in place. There are many reasons for this difference, but the bottom line is that the average strength of concrete sampled at the chute generally needs to be 10% to 15% higher than the specified f´c. In contrast, the average strength of cores drilled from the point of placement can be up to 15% less than f´c. That’s a big difference!
Air content testing
In like manner, it makes sense to consider a modified acceptance criteria for air at the point of placement, but here’s where things get complicated. Freeze/thaw resistance depends on both the cumulative volume of air bubbles in concrete and on the average bubble size (the smaller the bubble, the more protection it provides per unit volume of air). For this reason, knowing the total air content is only part of the story, especially since the smaller the average bubble size, the less total air is needed.
When relatively high air volumes, such as 6%, are specified, and if a good air entraining admixture is used and the concrete is well mixed, there is a margin for tolerable loss of air during handling, placing, and even vibrating. Further, pumping, dropping the concrete from a bucket or conveyor, and vibrating normally remove the larger and less effective air bubbles so the loss in durability is often negligible even though the loss in air volume may be significant. Most of the time a drop of 1.5% to 2% from typically specified at-the-chute values does not compromise freeze/thaw durability (although deicer-scaling resistance of flatwork is more difficult to achieve and less tolerant of air loss).
But here’s the rub: Until more is known about the size of the air bubbles, it’s unclear how much air loss can be tolerated. If devices such as the Air Void Analyzer are used onsite to estimate air bubble size, a rational value for tolerable air loss can be computed. Alternatively, the ASTM C457 microscopical analysis test can be used to evaluate hardened concrete to predict freeze/thaw durability and determine after the fact if the point-of-placement air content is satisfactory. The Canadian A23 “Methods of Test and Standard Practices for Concrete” includes in-place requirements for the air voids with required air content greater than or equal to 3%. This is strong support for acceptable air content at the point of placement of less than the normally specified values.
The specifier also must consider the cost implications of any extra testing and the safety implications for all personnel involved. Trying to grab a concrete sample right at the center of the placing and finishing can be dangerous, especially when concrete buckets are swinging or concrete pump hoses are involved. The deadly event called “hose whipping” is most likely when pumping air-entrained concrete.
For those who are responsible for complying with specs, the first piece of advice is to read them! Given that there are cost implications of sampling for the concrete producer, the contractor, and the testing company, it is important to determine the sampling requirements at the time of bidding. Find out what the specs require and then be prepared to comply. But also use your familiarity with the specs to know the difference between that which you are required to do and that which you are requested to do. And finally, if you are the test technician sampling concrete at the point of placement, stay alert and be careful out there!