16.1 Test ready-mixed concrete in accordance with the following methods:
The purpose of the opening statement in Section 16.1 is to clearly state the relevant standards for testing concrete, the methods of reporting of test results, and the basis for acceptance and rejection of ready-mixed concrete. Concrete should not be evaluated and rejected on the basis of any test method not included in Section 16.1. Each of these test methods has pre- scribed equipment and procedures to produce as much unifor- mity and repeatability as possible in the test results. Other test methods or deviations from the prescribed test methods or practice are not permitted.
16.1.1 Compression Test Specimens—Practice C31/C31M, using standard moist curing in accordance with the applicable provisions of Practice C31/C31M.
Test specimens for compressive strength tests are generally molded at the job site for the purpose of determining whether the concrete as delivered complies with the strength acceptance criteria. For that purpose it is important that standard proce- dures are followed. Standard curing is defined in terms of temperature and moisture while the cylinders are stored at the job site and after they are transported to the laboratory.
Cylinders that have just been molded are very sensitive to the method of handling and storage conditions during the first few hours. Most deviations from standard procedures will result in a lower measured strength of the strength specimens and might cause perfectly acceptable concrete to be rejected due to no fault of the concrete producer.
Two of the most abused practices in concrete testing occur when compression test specimens are molded. The first most often abused practice is that the concrete samples are often obtained improperly such that they are not representative of the load. More discussion on this is in Section 16.1.7, Sampling Fresh Concrete.
The other often abused practice has to do with the proce- dures for initial curing, which are explicit in ASTM C31/C31M. Any movement of cylinders to a storage area must take place
immediately after finishing. The initial curing period is allowed to extend up to 48 hours, but no longer. If the concrete mixture takes additional time to set and harden, the cylinders must not be moved from the storage area until at least 8 h after final set of the concrete. Some high strength concrete mixtures may take longer to set and harden. A commonly acceptable scenario is to transport the cylinders from the job site curing location to the laboratory on the day following molding. The initial curing environment that the test specimens are stored in at the job site shall be maintained at a temperature between 60 and 80 °F and will provide protection against moisture loss from the specimens. Since high strength concrete is more sensi- tive to curing conditions, the temperature range for initial cur- ing of concrete test specimens with a specified strength of 6000 psi or greater should be within a range between 68 and 78 °F.
Specimens shall not be exposed to direct sunlight or radiant heating devices. The storage temperature shall be controlled by use of heating and cooling devices, as necessary, to maintain the temperature and the moist environment. A record of the initial curing temperature should be maintained using a maxi- mum-minimum thermometer. These are not arbitrary proce- dures. Note 6 of ASTM C31/C31M provides some excellent suggestions on good initial curing practices for test speci- mens. Meininger illustrated significant strength differences of 28-day measured strength of test specimens subject to initial curing within the requirements of C31/C31M. Cylinders stored in water at 60 °F had the highest strength, while those stored in air at the same temperature were an average of 6 % lower. Cylinders stored in air at 80 °F on average measured 16 % lower strength than those stored at 60 °F in water [1]. Immersing the test specimens in water at a controlled temperature provides the best and most reliable method of initial curing at the job site.
The purpose of standard curing for acceptance testing is to evaluate the quality of concrete as delivered and to provide an indication of the potential strength when cured in a standard manner. Some might pose the argument that they want to get an idea on the strength of concrete in the structure. For this purpose, strength should be measured by molding an extra set of cylinders to be cured as prescribed for field curing, where the
test specimens are maintained in a curing condition similar to that experienced by the structure. Even field curing will not provide an accurate estimate of the strength in the structure because it is difficult to reproduce the temperature and moisture conditions in the concrete members that have a larger mass. Maturity methods that estimate in-place strength in accordance with Practice C1074 might be used for this purpose.
The latter part of standard curing is the curing in the labo- ratory at 73.5 ± 3.5 °F with free water on the cylinder surface at all times. This curing is equally important. Most laboratories are conscientious about maintaining this prescribed environ- ment, so laboratory curing does not usually present the chal- lenge associated with standard curing in the field.
16.1.2 Compression Tests—Test Method C39/C39M.
C39/C39M states requirements for equipment and proce- dures to be used for testing concrete test specimens. Standard size test specimens can be either 4 in. diameter by 8 in. height or 6 in. diameter by 12 in. height. The smaller size specimens tend to be easier to handle and likely subject to less damage and are more popular in some areas. The ends of the cylinders are capped with sulfur mortar or alternative capping material in accordance with C617/C617M or by using neoprene rubber pad caps in a retaining ring in accordance with C1231/C1231M. The bearing ends of the cylinders should be smooth and perpendicular to the axis of the cylinder to ensure that the load is uniformly distrib- uted along the cross-section of the test specimens. The cylinders should be tested to failure.
Storage of cylinders during initial curing does not always take place in a level spot as required, and if the ends of test cyl- inders are not prepared properly for testing, the measured strength will be lower. Another problem that sometimes arises, especially with neoprene pad caps, is an edge breaking off of the cylinder, which may cause a sudden drop in the load, and an overzealous technician terminating the loading of the compres- sion machine to reduce the cylinder debris in and around the testing machine. Often the applied load will immediately recover after the momentary drop and continue to bear a higher load until true failure of the specimen. Compression cylinder tests must be carried to complete failure, and the type of failure should be recorded on the test report as required by ASTM
C39/C39M.
Note in Section 18.2 that the average of at least two test specimens obtained from the same sample and tested at the same age is required to represent a strength test result. Due to possible reduced precision of 4 by 8 in. cylinders, ACI 318 Building Code Requirements for Structural Concrete [2] requires the average of three specimens to be used for the test result. It is rare for two or more specimens, even from the same concrete sample and handled in the same manner, to have identical mea- sured compressive strengths. There is a limit to such differences, however, and these are provided by the precision statement in
ASTM C39/C39M. For 6 by 12 in. cylinders molded in the field, the coefficient of variation is 2.9 % of the average compressive strength. At this time there is no equivalent precision statement for 4 by 8 in. cylinders prepared in the field, but it can be assumed to be similar to marginally higher than that of the 6 by 12 in. specimens. Based on the determined within-test preci- sion, the acceptable range between two companion cylinders is 8.0 % of their average strength. This acceptable range is estab- lished for a 95 % confidence level. If three companion cylinders are used, the acceptable range between their measured strengths should be less than 9.5 % of the average. The acceptable range in this context means that when compressive strengths of com- panion sets of cylinders are observed, the range or difference between the highest and lowest strength in a set should not exceed the acceptable range more often than one time in twenty.
An occasional occurrence where this range is exceeded should not be a cause for concern, but when this range is exceeded frequently, the testing procedures from the process of molding specimens, initial curing, final curing, and strength testing procedures may be deficient. ACI 214R Guide to Evaluation of Strength Test Results of Concrete provides additional informa- tion on the acceptability of test results and provides ranges of coefficient of variation of testing that can be used to judge the performance of a laboratory for acceptance testing of strength [3].
16.1.3 Yield, Mass per Cubic Foot—Test Method C138/
C138M.
ASTM Test Method C138/C138MTest Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete is used in the determination of the density of concrete and cal- culating the yield or volume of concrete delivered when the total mass (weight) of the concrete load is known. The density of a small sample of about 0.25 ft3 is measured to represent the estimated volume of a capacity load in the range of 10 to 12 yd3. The multiplier from 0.25 ft3 container to determine to the 27 ft3 (1 yd3) volume of concrete introduces a multiplier of 108 and an order of magnitude higher for the capacity load. Thus any error in the test result is significantly magnified and demands a care- ful and accurate performance of the test procedure. The calibra- tion of the sample container should be accomplished to one-thousandth of 1 ft3. ASTM procedures do not require duplicate verifications of the container volume, but duplicate verifications can increase the accuracy and confidence in the determined volume. The mass of concrete filled and
EXAMPLE 16.A Acceptable range of compressive strengths.
Assume a set of two 28-day 6 by 12 inch cylinders tested at 3360 psi and 3600 psi.
Average strength is 3480 psi.
The acceptable strength range is 3480 × 0.08 = 278 psi.
The actual range is 3600-3360 = 240 psi < 278 psi, OK.
Practices, test Methods, and rePorting 133
consolidated in a standard manner is divided by the known volume of the container to obtain the mass per unit volume or density of the concrete.
From Section 4 of ASTM C94/C94M, density (unit weight) samples for yield determinations are required to come from the middle portion of three different loads and the measured densi- ties averaged. Again, accuracy and uniformity are prime con- siderations. See Fig. 16.A.
The most common source of error with the concrete density test is in the strike-off procedure. Inadequate work with the strike-off plate or failure to clean concrete adhering to the outside of the container results in a higher reported density and a lower calculated yield. Improper strike-off procedures may compress the concrete rather than trim the top. The use of a field technician certified by ACI or an equivalent certification program is essential to this yield determination procedure. Calculation of yield is illustrated in Example 16.B.
16.1.4 Air Content—Test Method C138/C138M; Test Method
C173/C173M or Test Method C231/C231M.
Each of these test methods presents a different procedure and different equipment for measuring the air content of con- crete and each will provide a slightly different result.
ASTM C138/C138M is primarily the test procedure to measure the density and calculate the yield of the concrete. It is not typically used as the basis for acceptance or rejection of concrete for air content. It is more common in trial batch evalu- ation in the laboratory. It is becoming common in the testing of lightweight concrete for consistency of unit weights and air content. It is used more as a measure of the consistency between loads of concrete than for the actual air content, but is an important part of field testing for this purpose. Measurement of the density and comparison of it to the density of the mixture as designed provides a quick indication of a potential problem.
This method is useful in the laboratory for development of concrete mixtures. When the batch masses (weights1) of the ingredients in the load are recorded and known, the air content by the gravimetric method can be used. The difference in the actual volume of concrete and the volume of the ingredients batched is assumed to be the volume of air. To determine the volume of the ingredients, the relative densities (specific gravities1) of each material in the batch should be known. The manufacturer usually will have this information. The density (unit weight) of a sample of concrete obtained from the middle of the load is measured as described in the previous section.
Example 16.B demonstrates determination of the gravimetric air content.
How much does the calculated air content vary as differ- ences in density (unit weight) determinations vary? The precision statement of ASTM C138/C138M states that for a sin- gle operator, two properly conducted tests on the same sample may differ by 1.85 lb/ft3, and for a multi-operator test, the results should not differ more than 2.31 lb/ft3. From an inter-laboratory study for developing these precision estimates, the standard deviations for these two cases are 0.65 lb/ft3 and 0.82 lb/ft3, respectively.
EXAMPLE 16.C Allowable variations in gravimetric air content.
Assume a change in the measured concrete density of
Example 16.B from 143.75 to 141.9 lb/ft3 (difference of 1.85 lb/ft3).
The new calculated air content becomes:
A = [(150.36 – 141.9) ÷ 150.36] × 100 = 5.6 %
FIG. 16.A Equipment for density tests (ASTM C138/C138M).
EXAMPLE 16.B Yield and gravimetric air content determination.
Material
Mass (Weight) lb/batch
Mass (Weight) lb/yd3
Relative Density (Sp Gr)
Absolute Volume ft3/yd3
Cement (lb) 4,210 421 3.15 2.14
Fly ash (lb) 930 93 2.51 0.59
Coarse agg.
(SSD)
(lb) 17,970 1797 2.69 10.70
Sand (SSD) (lb) 13,500 1350 2.62 8.26
Water (Total) (lb) 2,830 283 1.00 4.54
TOTAL 39,440 3944 26.23 ft3
D = measured density (unit weight)A = 143.75 lb/ft3 Y = Actual Yield = 39440 ÷ (143.75 × 27) = 10.2 yd3
T = Theoretical Density (air-free basis) = 3944 lb ÷ 26.23 ft3 = 150.36 lb/ft3
A = Air Content = [(T - D) ÷ T] × 100 = [(150.36 - 143.75) ÷ 150.36]
× 100 = 4.4 %
A Value determined by field test.
1 Two deprecated terms, weight and specific gravity, are contained in parentheses in this discussion. The accepted terms, mass and density, are shown along with the deprecated terms. Other such pairs of terms or phrases are used throughout the text. While specific gravity is in common usage, the accepted term is relative density. The accepted term for unit weight of concrete or mass per unit volume is density.
The acceptable difference for tests performed by the same operator for measured concrete density allows for an acceptable difference of (5.6 – 4.4) = 1.2 % in air content. Had the test been performed by two operators on the same sample of concrete, the acceptable difference in air content for this example would be (5.9 – 4.4) = 1.5 %.
Note that 1.5 % is 50 % of the allowable tolerance for air as specified in Section 8.2. Testing errors (or inaccuracies) reduce the margin of error available for the manufacturer. Correct and careful testing procedures are once again demonstrated to be essential.
ASTM C173/C173M measures the air content of concrete by displacing the air after a large amount of water is incorpo- rated into the test sample by agitation inside the test apparatus.
It is referred to as the volumetric method for air content deter- mination. It is also referred to as a Roll-a-Meter test. During the performance of the test, the agitation generates foam that presents a problem when obtaining a reading. To minimize the foam, a quantity of isopropyl alcohol is added with the water before the sample is agitated and rolled. The method does take some time and needs physical effort to ensure that all the air in the concrete sample is extracted and measured. This is the test method normally used for lightweight concrete due to the porous nature of lightweight aggregates.
An inter-laboratory study to develop a new precision state- ment has not been accomplished. The current precision state- ment indicates that the multi-operator coefficient of variation is 11 % of the measured air content. The acceptable difference of test results obtained by two operators on the same sample is 32 % of their average air content. Two items are noted here. The acceptable difference in two test results is approximately one third of the test value, and the acceptable difference is expressed in terms of coefficient of variation and not an air content percentage. Since the precision is expressed as a coefficient of variation, the precision estimate increases as the value of the measured air content increases. Examples of the acceptable differences changing with varying air contents follow:
1. Measured air content = 4.4 % --- 0.32 % × 4.4 % = 1.4 % 2. Measured air content = 5.0 % --- 0.32 % × 5.0 % = 1.6 % 3. Measured air content = 6.0 % --- 0.32 % × 6.0 % = 1.9 %
The base of the roll-a-meter is too small to qualify as an acceptable measure for determining the density of concrete, as is possible with the sample container used for the pressure method,
C231/C231M. ASTM C138/C138M does not permit the use of den- sity measures with a volume less than 0.2 ft3. See Fig. 16.B.
The ASTM C231/C231M test method uses air pressure to measure the air content in fresh concrete. A Type A meter uses a cylinder of water above a bowl of concrete (see Fig. 16.C).
Applied air pressure on the water column compresses the air in the concrete sample, lowering the height of water in the cylin- der. The measured drop of the water height provides a measure of the air content. A Type A meter is not common; most tests are performed using a Type B pressure meter.
A Type B meter uses a closed air chamber that is pressur- ized to a predetermined level (see Fig. 16.D). The pressurized air is then released into the container of concrete. The air pres- sures equalize in the concrete bowl and in the air chamber. The change in air pressure in the pressurized chamber is related to the air content in the concrete. The gauge is calibrated to pro- vide a reading in terms of air content in the concrete sample.
The pressure air meters are based on Boyle’s law, which states: If the temperature remains constant, the volume of gas (air) is inversely proportional to the pressure. Boyle’s law is repre- sented as V1 P1 = V2 P2.
At this time, there is no published precision statement for Type B meters. A completed study using the Type B pressure
FIG. 16.B Equipment for volumetric air content measurement (ASTM C173/C173M).
FIG. 16.C Type A pressure meter for air content measurement (ASTM C231/C231M).
Practices, test Methods, and rePorting 135
meter indicates an acceptable range of two test results to be 1.42 % for an air content of around 5 %. The standard devia- tion in this study for multiple operators indicates a value of 0.51 %, and 0.63 % for two separate batches with total air con- tent of 5 % and 11 %, respectively. The acceptable range of two such tests performed by different operators would be 1.4 % and 1.76 %. In this same study the standard deviation for sin- gle-operator precision was determined to be 0.40 % for the batch with a total air content of 11 %. Since it was not expected to have a lower precision with a higher air content, Subcommittee C09.60 is planning a revised inter- laboratory study with the hope of obtaining precision estimates in line with expectations. Specifiers need to realize that the produc- tion of air-entrained concrete and its testing is not an exact science, and it can be difficult to comply with the specification requirements.
With the Type B pressure meter, the most common prob- lem is not determining the required aggregate correction factor and failing to use it. This correction accounts for air within the aggregate porosity that is not filled with water. Since the air content of interest, which provides freeze-thaw resistance to concrete, is that which is in the cement paste, the aggregate correction factor is subtracted from the measured air content.
Because the test method measures the air within aggregate particles, it cannot be used for concrete containing very porous aggregate or lightweight aggregate.
The bowl in which the concrete sample is placed has a minimum capacity of 0.20 ft3, and most Type B meters are 0.25 ft3 in size. Thus, these containers are large enough to mea- sure the concrete density according to ASTM C138/C138M when the concrete contains 1 in. nominal maximum size coarse aggregate or smaller. When the same sample is used for con- crete density (unit weight) and air content, the density must be performed first, and the specimen top must be prepared with a strike-off plate, not just a strike-off bar.
16.1.5 Slump—Test Method C143/C143M.
The slump tests provide a measure of consistency of the concrete that is somewhat crude, but quick and is an often specified test method (see Fig. 16.E). Chapter 7 of this text has a discussion of the test. The major performance problems with the test include improper measurement of the slump and non- uniform consolidation of the concrete in the mold by rodding that often results in a lopsided slumping of the con- crete when the mold is removed.
After removal of the mold, the slump measurement is taken at the displaced original center of the top surface of the fresh concrete specimen and not from the low point of the original top or the high point of the original top. Improper selection of the point of measurement can lead to errors in the slump value reported.
The precision statement of ASTM C143/C143M indicates increasing variability as the slump value being measured increases. The precision statement is expressed at different slump ranges. For single operator precision, the acceptable dif- ference between two measurements on the same sample is a range of 0.65 to 1.13 inches for a measured slump of 1 to 8 in.
For precision when there are multiple operators, the acceptable difference between measured slumps should not exceed a range between 0.82 and 1.50 in. for the same measured slump levels, respectively. Except for low slump paving concrete, it is safe to say the typical range between acceptable slump test results is approximately 1 to 1.5 in. This range of acceptable test results is about 50 % or more of the specification tolerance of the speci- fied slump permitted by Sections 7.1.1 and 7.1.2.
16.1.6 Slump Flow—Test Method C1611/C1611M.
Because slump does not adequately describe the consis- tency of self-consolidating concrete (SCC) an alternate field measurement was developed by a modification of the
FIG. 16.D Type B pressure meter for air content measurement (ASTM C231/C231M).
FIG. 16.E Slump cone and measurement of slump (ASTM C143/
C143M).