17.1 The contractor shall afford the inspector all reasonable access and assistance, without charge, for the procurement of samples of fresh concrete at time of placement to determine conformance of it to this specification.
Section 17 is very similar to Section 15, except that it addresses the contractor rather than the manufacturer. The inspector, which includes the personnel of the testing agency (laboratory) performing acceptance tests, must have the opportunity to sample and test the freshly mixed concrete at the job site. The inspector will not be charged for the concrete, for the down time of the delivery vehicle, or for the idle time of the contractor’s concrete crew while the concrete is sampled and tested. The concrete itself is already paid for by the owner so that is not an issue. The idle time of the delivery vehicle and the crew is a contingency that should be considered when pric- ing concrete and the placement costs. The qualifying term, without charge, means without direct additional charge to oth- ers and at no charge to the inspector or the testing agency.
The question of reasonable access is raised again just as it was in Section 15. And, in this case, reasonable access can depend upon the unloading circumstances. In the context of ASTM C94/C94M, it also revolves around two very specific con- cepts. The first of these concepts is that ASTM C94/C94M applies only to the point of discharge from the mixer or deliv- ery vehicle at the job site. Reference S4 of Section 1.1 states: This specification does not cover the placement, consolidation, cur- ing, or protection of the concrete after delivery to the purchaser.
Section 4.1 reads: The basis of purchase shall be a cubic yard or cubic metre of fresh concrete as discharged from the transpor- tation unit. These two statements make it clear that ASTM
C94/C94M and the manufacturer’s responsibility for the prod- uct quality end when the concrete is discharged from the deliv- ery vehicle. Therefore, in accordance with C94/C94M, the concrete should be tested at the point of discharge. The second specific item is that the concrete samples shall be obtained in accordance with ASTM C172/C172M, which requires that two portions of concrete from the middle part of the load be com- bined into one composite sample to be used for determining the acceptability of the concrete. ASTM C94/C94M makes
exceptions for preliminary slump and air contents, but it makes no exceptions for strength tests and the related tests required when strength tests are made (see Section 17.4).
What does this have to do with reasonable access? The sampling must be done as the delivery vehicle is unloading. It is not difficult to divert the unloading chute away from the pump, power buggy, point of direct placement, concrete bucket, con- veyor, or other point of discharge a couple of times and put some concrete into a wheelbarrow for the inspector and the concrete technician. If the mixer truck can reach the point of discharge, a wheelbarrow should also be capable of reaching it.
Allowing for proper sampling during unloading (discharge) is a contractual obligation of both the contractor and the manufacturer.
There are times when the project specifications require testing the concrete at the point of placement after the concrete is moved through the placement equipment, such as a pump, bucket, or conveyor belt. The samples obtained at the point of placement should be additional samples to those obtained at the discharge from the delivery vehicle to evaluate the effect of the placement equipment on characteristics of concrete. These tests are not a part of ASTM C94/C94M because the placement meth- ods and sampling methods are controlled by the contractor.
Access for remote points of discharge is covered by other por- tions of project specifications because they are beyond the ready-mixed concrete manufacturer’s scope of control. This does cause problems as a practical matter because the Architect/
Engineer (A/E) is interested in the concrete properties in the structure. A process needs to be established prior to the place- ment for the requirements for the concrete at the point of dis- charge, even if they are different from the specification requirements, so that the concrete at the point of placement can comply with those requirements. Items to discuss are changes to the placement methods or equipment to minimize that change, establishing requirements for concrete at the point of discharge to accommodate the anticipated change, or to estab- lish real time communications to address changes in placement methods that will require alterations in concrete characteris- tics. Air content and slump of the concrete, which usually change as it is conveyed from the truck mixer chute to the
location in the structure, depending upon the placement method and subsequent handling and consolidation, are typically the problematic elements. These factors and the sampling methods used will also affect the strength test results of samples obtained at the point of placement.
Changes to fresh concrete characteristics as it moves through a pump have been the subject of much research. Results of the research have indicated that pump pressure, pump line configuration, and pump discharge conditions affect the con- crete properties. Changes to fresh concrete characteristics as the concrete moves through a pump line have been the subject of much research. Research by Ksaibati et al. identified length of boom, slope of the boom, and the free fall drop distance of the concrete as influential factors [1]. A segment of this research focused on the reliability of several sampling techniques. It was found that placing the collection vessel in a selected safe loca- tion and swinging the pump over to the collection vessel pro- duced a significant change in the air content. The least change in air content resulted when sampling at the placement location with no drop, sampling from the concrete immediately after the hose discharge (prior to any consolidation), or to shovel por- tions of the discharged concrete gently into the collection sam- ple container. The key word in the shoveling technique is gently because excessive handling of the concrete proved to produce high losses in air content. Hover [2] delves into some of the var- ious sampling possibilities and discusses why even sampling at the point of placement is not accurate and does not provide the in-place concrete properties the designer is seeking.
Chapter 8 on air-entrained concrete has a lengthy discus- sion concerning pumping and sampling at the end of the pump line. Note that ASTM C172/C172M does not include sampling procedures at the discharge end of a pump, or any other con- veying equipment, because such sampling is beyond the scope of ASTM C94/C94M, and C172/C172M exists in support of C94/
C94M.
Section 17.1 requires reasonable access and assistance.
What does assistance mean? This does not mean that the con- tractor is expected to furnish people to carry testing equipment and cylinders, although there are times and places where this type of assistance is both helpful and appreciated. What reason- able assistance means is cooperation in diverting the discharge chute at the proper times to allow good sampling techniques, a clear working area near the curing box suitable for performing tests, a location for truck mixers to be spotted for preliminary tests, an electrical outlet for a temperature regulated curing box, space on the job site for the testing and curing of test specimens, and water to wash off testing equipment and for other needs.
Cooperation between contractor, manufacturer, inspector, and testing technician is the bottom line goal. All work for the owner, and all want or should want a successful project. This spirit of cooperation will assist in producing good representa- tive samples to more accurately reflect the quality of the con- crete being delivered to the project. Without this cooperation, any nonconforming test results will become questionable.
17.2 Tests of concrete required to determine compliance with this specification shall be made by a certified technician in accordance with Practice C1077.
Some tests at the job site may not be acceptance tests to determine compliance with project specifications. The manufac- turer may send a technician to measure slump or air content on several loads during the beginning of a project or as things change to adjust plant-added batch water or the air entraining admixture dosage. Other quality control activity may require monitoring or testing to gain information about the product being delivered and address questions raised regarding the prod- uct in a timely fashion. For these purposes, personnel who per- form these actions or tests to the manufacturer’s satisfaction will not need to comply with certification requirements. When acceptance tests are performed to determine specification com- pliance, a higher level of proven proficiency is required.
Considering that the cost value of a load of ready-mixed concrete is between U.S. $800 and $1100 or more, the certification requirements are justified. Before a load is rejected due to noncompliant test results, all parties need to be sure that the tests are performed correctly and that the test results are accurate. It is important to know that the concrete is of acceptable quality, through accurate testing, before it is permitted to be placed into the project structure.
Technicians certified to be ACI Concrete Field Testing Technicians, Grade I, for example, have been tested for a knowl- edge and proficiency of the following ASTM Test Methods and Practices. The same is true for other recognized certification programs.
ACI Publication CP-1(12) [3] provides detailed guidance on testing procedures, sample questions for each procedure, grading requirements, and some helpful supplementary infor- mation. It should be noted that certification of technicians, by itself, does not assure that accurate results are obtained.
Certification does not assure people’s behavior, and there are often short cuts or violations of standardized procedures by testing technicians to get the job done quickly. Representatives of the manufacturer, such as the driver, are often advised to observe and document such erroneous practices.
ASTM C1611/C1611M is the method to measure slump flow of self-consolidating concrete and is listed in Section 17. This method is not covered by the ACI Field Testing Certification program. Evaluating the proficiency of a technician to perform
C31/C31M Making and Curing Specimens in the Field C138/C138M Density, Yield, and Air Content (Gravimetric) C143/C143M Slump of Concrete
C172/C172M Sampling Freshly Mixed Concrete C173/C173M Air Content by the Volumetric Method C231/C231M Air Content by the Pressure Method C1064/C1064M Temperature of Freshly Mixed Concrete
Sampling and TeSTing FreSh ConCreTe 143
this test might be appropriate on a project that has a significant amount of self-consolidating concrete involved.
Testing for the density of lightweight concrete can some- times be confusing, depending on the wording of the project specification. A specification may refer to a density of the fresh concrete (C138/C138M) or it may specify the density as the equi- librium density. As discussed in Chapter 6, the desired equilib- rium density should be correlated to the density of freshly mixed concrete in the mixture development phase and mea- sured at the job site in accordance with C138/C138M. This is typ- ically done by the calculation method addressed in ASTM C567. If the equilibrium density needs to be determined by measure- ment, this should be done with 6 by 12 in. cylinders. The 4 by 8 in. cylinders, that have become so popular, are not acceptable for this density test. The only basis for acceptance of the density of lightweight concrete on the job site should be by ASTM C138/
C138M. There should be no attempt to prepare cylinders at the job site to determine compliance with the specified equilibrium density.
17.3 Samples of concrete shall be obtained in accordance with Practice C172/C172M, except when taken to determine uniformity of slump within any one batch or load of concrete (11.4, 12.3.3, 12.5.1, and 13.4).
ASTM Practice for Sampling Freshly Mixed Concrete (C172/C172M) is a short document that is a must-read for every concrete technician. It is the most commonly violated practice of acceptance testing of concrete. Whether the violations are due to ignorance of the standard or attempting to follow the easiest method, violations still occur repeatedly, thus bringing test results into question. The following statement in the Significance and Use section of ASTM C172/C172M speaks volumes:
This practice is intended to provide standard requirements and procedures for sampling freshly mixed concrete from dif- ferent containers used in the production or transportation of concrete.
Why is obtaining a concrete sample so painstakingly described? Think in terms of a 10-yd3 load of concrete as it arrives at the job site. This size load represents a U.S. $800 to
$1100 cost value. Furthermore, the load may be chosen for acceptance testing for the day’s pour, or it may be to represent the 150-yd3 pour. A sample size of 1 to 2 ft3 is obtained in a wheelbarrow and tested to determine if the load is acceptable or not. A 10-yd3 load has a volume of 270 ft3. The fate of this load is thus determined by a sample that represents approximately 0.5 % of the total load and 0.03 % of 150 yd3 pour (or 15 loads).
The worst case scenario when the 28-day strength test fails the acceptance criteria, work stops while in-situ testing is performed or cores are taken. When these tests prove the problem was with the original tests and not with the concrete, the total cost of such
a scenario is very significant and some of it cannot be quantified.
The Scope of ASTM C172/C172M gets right to the point:
This practice covers procedures for obtaining representative samples of fresh concrete as delivered to the project site on which tests are to be performed to determine compliance with quality requirements of the specifications.
The key phrase here is representative samples. The sample must represent the entire load, not just an isolated segment. All stakeholders, including the ingredient material suppliers, the manufacturer, the contractor, the A/E, and the owner want a representative sample. ASTM C172/C172M is really about one thing: obtaining a truly representative sample of the concrete as delivered.
The initially discharged concrete may be affected by the loading sequences of materials, especially the cement, quantity of tail water (last metered or weighed water), the number of mixing revolutions, and total drum revolutions. If the batching sequence places the cement at the end or near the end of the loading process, and only a small quantity of tail water is used, the initially discharged concrete may have a high proportion of cement and produce test strengths in excess of what is represen- tative of the load. If an excessive amount of tail water is used, and the mixing is not sufficient to incorporate this into the complete load, the initial discharge may have a higher water content and thereby a higher slump, lower air content, and lower strength.
A series of unpublished tests by Daniel [4] compared com- pressive strength tests of samples obtained from initial discharge of a load to the strength results of samples obtained in accor- dance with ASTM C172/C172M. Approximately 75 % of the strength tests from the initial discharge samples were higher and approximately 25 % were lower than the strength tests of com- posite samples obtained from the middle of the load in accor- dance with ASTM C172/C172M. The standard deviation of the strength test results from the initial discharge was approximately 40 % higher than that of the standard samples. These are signifi- cant differences and explain why preliminary samples from the initial discharge and single portions as samples are not permit- ted to be used for acceptance strength testing.
ASTM C172/C172M discusses sampling from stationary mixers, paving mixers, revolving-drum truck mixers, revolv- ing-drum agitators, open-top truck mixers, agitators, nonagi- tating equipment, and other types of open-top containers. The central theme in the procedure for each is collecting two or more distinct portions of the concrete then compositing these into a single test sample. The most common of the sampling proce- dures is from a revolving-drum truck mixer, which reads by col- lecting two or more portions taken at regularly spaced intervals during discharge of the middle portion of the batch. This instruction is emphasized by a later statement: do not obtain samples from the very first or last portions of the batch dis- charge. A note defines first and last portions as 10 %. This
indicates that samples can be obtained from the middle 80 % of the load. Prudent practice is to focus on the middle two-thirds of the load. Another requirement is that the first and last por- tions of the composite sample should be obtained within a 15-min time span.
Exceptions to the composite sample requirement are stated in Section 17.6 when preliminary tests for slump and air content are made to permit job-site adjustments to the load. If a load is being tested for uniformity of slump or for complete uniformity, the requirement to use composite samples is also waived by Section 17.3.
The section references to specialized testing (not for accep- tance of concrete testing) are as follows:
11.4 Slump test on mixer or agitator to determine the prob- able degree of uniformity,
12.3.3 Sampling for uniformity tests of stationary mixers, 12.5.1 Sampling for uniformity of concrete produced in truck
mixers, and
13.4 Slump test on nonagitating equipment to determine the probable degree of uniformity
As indicated earlier, the scope of ASTM C172/C172M is lim- ited to ready-mixed concrete as delivered to the job site. Issues related to sampling at two locations, that is, the point of dis- charge and the point of placement, are not addressed in this standard because it is not the concern of ASTM C94/C94M. Hover discusses the differences in strength tests of samples taken from the truck discharge and of cores taken at the point of placement and found that higher test results were obtained for samples obtained at the point of discharge from the truck chute [2].
17.4 S1 Slump or slump flow, air-content, density, and temperature tests shall be made at the time of discharge at the option of the inspector as often as is necessary for control checks. S2 In addition, these tests shall be made when specified and always when strength specimens are made.
S1 is a reference to tests for control and not acceptance tests for the various tests listed. In Section 17.5 a recom- mended frequency of testing is expressed for strength tests.
Section 17.4 recognizes the need for more frequent testing on the fresh concrete properties at any time during the project.
Typically this might be done earlier during the project or for the first few loads of the day. These are supplemental tests used to evaluate the concrete when it is delivered and not 28 days later when strength tests become available. The intent is to verify that the characteristics of the concrete are in line with what is required by the project or the specification, and if this is determined accurately, potential problems can be recognized and addressed. Each of the tests listed often have stated limits in the specifications and can subject these loads to rejection. However, the important part of this clause states
that these tests are for control checks, and the intent is to fix a problem if there is one as the primary goal of a quality control activity. If there are no specification limits or criteria for either of these measured tests, there is likely some control criteria of the approved mixture that can identify a potential problem. Most such checks are for indications of excessive water, inadequate air content, or to check the density for pos- sible batching or testing problems.
This section indicates that the inspector may have any or all of these tests performed any time it is desired. The inspec- tor may be directed by the owner or his representative to do so.
Typically, all tests would not be performed on each load, but it is not uncommon for one of these tests to be conducted on each load. If more than a few loads are evaluated, that would be a very comprehensive inspection of fresh concrete, and the question would be whether such extensive testing is really justified. At the option of the inspector does not require any justification. However, it should be recognized that this exer- cise can be costly to the manufacturer because it ties up trucks while inspections and testing take place, thus requiring extra trucks on a project. Delays in discharging the concrete during these tests will change the workability characteristics of the concrete and may change the processes for consolidation and finishing of the concrete. If it is known ahead of time when these tests will be performed and at what frequency, it is best, for all concerned, to state it in the specifications.
What types of circumstances could lead to checking every load of concrete for a particular property of the fresh product?
Some possibilities are listed as follows:
1. Super flat floors, especially when constructed with laser screeds, require large volumes of concrete at a consistent slump and no entrained air for the best results. Trying to test every load will adversely impact the construction process.
2. Pumping concrete, particularly with a high coarse aggregate content, requires a consistent slump.
3. Air content can change dramatically after a long haul on a hot day. If traffic conditions, and therefore delivery times, are variable, the air content also may be variable, as would be the slump.
4. Temperatures can become particularly important if weather conditions are borderline and the project specifications con- tain specific temperature restrictions for either hot or cold weather for all or for mass sections of concrete.
5. Density of concrete may be the acceptance criterion on a lightweight concrete project. If the density of the fresh con- crete is not consistent, it may be prudent to continue to mea- sure fresh densities until the condition is solved and densities are stabilized.
6. Highly variable slumps may lead to checking slumps on every load until the cause is determined. The cause often proves to be the same trucks with high slumps because wash water is not being discharged prior to batching concrete or otherwise controlled as required by Sections 9.3 and 14.2.9.