A1.1 S1 The variation within a batch as provided in Table A1.1
shall be determined for each property listed as the difference between the highest value and the lowest value obtained from the different portions of the same batch. S2 For this specification, the comparison will be between two samples, representing the first and last portions of the batch being tested. S3 Test results conforming to the limits of five of the six tests listed in
Table A1.1 shall indicate uniform concrete within the limits of this specification.
There are several references to uniformity of concrete in Section 12 (Chapter 12) for concrete produced in stationary mixers at a plant (central mixing) or for concrete batched and mixed in a truck mixer (truck mixing). There is an expectation in C94/C94M that during the production process, the separate ingredients of concrete are intermingled and mixed into a homogeneous concrete mixture that has essentially the same characteristics throughout the same load. The process of eval- uating whether this has occurred to some degree of certainty is to perform some tests to evaluate the concrete uniformity within the load. There are several factors that can impact the ability to achieve uniformity of concrete. Among them include the design of the mixer, condition of the mixer, load size rela- tive to rated mixer capacity, speed of mixing, number of revo- lutions at mixing speed, and sequence of loading materials into the mixer. NRMCA conducted a comprehensive series of tests on truck mixers from 1960 to 1970, primarily to evaluate the optimum sequence of loading materials into the mixer [1,2].
Other studies are referenced in these papers. Bozarth et al.
conducted studies of mixing uniformity in central mixing operations [3].
Performing mixer uniformity evaluations is not a trivial exercise. It is not often that these tests are required to be per- formed for qualification of mixers in typical operational mode.
When they are tested the process should be limited to each dif- ferent mixer design. A full capacity load is typically required;
proficient technicians should perform the required tests within prescribed time limits; the disposition of the concrete load can be a problem; and some of these tests are highly correlated, that is an error in one test will propagate to the other measured and
calculated characteristics. Mixing uniformity testing is per- formed for each mixer of a unique design for one rated size by truck mixer manufacturers who belong to the Truck Mixer Manufacturers Bureau (TMMB) and for stationary mixers for those that belong to the Concrete Plant Manufacturers Bureau (CPMB) as part of the requirements of the respective standards of those organizations. Thereby, the evaluation of new equip- ment is covered by these standards in the United States. The operational condition of the mixers, related to build-up of hard- ened concrete or wear on the blades, creates another situation where the potential for obtaining uniformity of mixing can be a concern. For the NRMCA plant certification program and for inspections by state highway agencies, a visual inspection of the condition of the mixer is used. There is some judgment required on the part of the inspector. For central-mixed concrete, NRMCA certification requires mixing uniformity evaluation for only slump and coarse aggregate content tests. This is not required for plants that are shrink mixing their concrete in a stationary mixer and discharging it into a truck mixer. Mixer uniformity testing evaluation might be appropriate when oper- ational changes are proposed, for example, when a shorter mix- ing time is proposed for stationary mixers. Another important affect that is independent of the mixer design and mixer condi- tion is the loading sequence of materials into the mixer. Mixers in excellent condition will not produce homogeneously mixed concrete if the sequence of loading materials is incorrect.
To evaluate the uniformity of concrete, samples from two distinct and separate portions of the load are obtained and the several properties of each sample are measured and compared.
The properties tested from each of the two samples are not required to match each other exactly. The range between the high test value and the low test value may equal but not exceed limits stated in Table A1.1. The key language here is the different portions of the same batch. The high and low test values do not allow testing three or four samples and tossing out the tests with undesirable results. Use the values obtained for each of the two samples. S2 is very specific that two samples will be used—no more and obviously no less. Within ASTM C94/C94M, every reference to the uniformity test indicates that the two samples shall be taken after the discharge of approximately 15 % and 85 %
of the load. Note 15 (following Section 11.4) is explicit that none of the samples shall come before discharge of 10 % of the batch or after 90 % of the batch has been discharged. Mixing unifor- mity requirements are pertinent to truck mixers for both truck- mixed and shrink-mixed concrete or for plant mixers for central-mixed concrete as discussed in Section 12, since these are the methods of mixing the ingredients to produce concrete.
S3 states the limits for mixing uniformity. The limits must be met on any five of the six tests for the equipment being checked to be considered capable of producing a homogeneous batch of concrete. It does not matter which five tests of the six meet the requirements of Table A1.1.
A1.2 Coarse Aggregate Content, using the washout test, shall be computed from the following relations:
P = (c/b) × 100 (A1.1)
Where:
P = mass % of coarse aggregate in concrete,
c = saturated-surface-dry mass in lb [kg] of aggregate retained on the No. 4 [4.75-mm] sieve, resulting from washing all material finer than this sieve from the fresh concrete, and
b = mass of sample of fresh concrete in mass per unit volume container, lb [kg].
A1.3 Mass per Unit Volume of Air Free Mortar shall be calculated as follows:
Inch-pound units:
M b c
V V c
G
= −
− × +
A 100
(A1.2).
SI units:
M b c
V V A c G
= −
− × +
100
(A1.3)
Where:
M = mass per unit volume of air-free mortar, lb/ft3 [kg/m3], b = mass of concrete sample in mass container, lb [kg], c = saturated-surface-dry mass of aggregate retained on No. 4
[4.75-mm] sieve, lb [kg],
V = volume of mass per unit volume container, ft3 [m3], A = air content of concrete, %, measured in accordance with
16.1.4 on the sample being tested, and G = density of coarse aggregate (SSD).
The size of each sample should be close to two cubic feet (about 300 lb), which is about the manageable capacity of a wheelbarrow. The six test limits stated in Annex Table A1.1 in the order of appearance within the table and some formula items have been given shorter descriptions and letter designa- tions for purposes of formulas, discussion, and example (see
Table 21.A).
TABLE A1.1 Requirements for Uniformity of Concrete
Test
Requirement, Expressed as Maximum Permissible Difference in Results of Tests of Samples Taken from Two Locations in the Concrete Batch Mass per cubic foot [mass per cubic
meter] calculated to an air-free basis, lb/ft3 [kg/m3]
1.0 [16]
Air content, volume % of concrete 1.0 Slump:
If average slump is 4 in. [100 mm]
or less, in. [mm] 1.0 [25]
If average slump is 4 to 6 in. [100 to
150 mm], in. [mm] 1.5 [40]
Coarse aggregate content, portion by mass of each sample retained on No. 4 [4.75-mm] sieve, %
6.0
Mass per unit volume of air-free mortar based on average for all comparative samples tested, %
1.6
Average compressive strength at 7 days for each sample, A based on average strength of all comparative test specimens, %
7.5 B
A Not less than 3 cylinders will be molded and tested from each of the samples.
B Approval of the mixer shall be tentative, pending results of the 7-day compressive strength tests.
TABLE 21.A Discussion, Abbreviations, and Comments on Tests
Item Notation Units Method of
Determination Concrete density (unit wt.) (air-free) Daf lb / ft3 calculated
Air content A % measured
Slump of concrete 4 in. or less S1 in. measured
Slump of concrete 4–6 in. S2 in. measured
Coarse aggregate content ca % measured
Mortar density (unit wt.) (air-free) M % calculated Compressive strength (average 7 day) Favg psi measured Density (unit weight) of concreteA D lb/ft3 measured Volume of density bucketB, C VD ft3 measured Mass (weight) of density bucketB, C WD lb measured Air-free volume of concreteB AV ft3 calculated
ANot a direct comparative value.
BThese items are only used for calculations.
CDensity bucket and air-meter base will sometimes be the same container and become interchangeable in computations.
Annex A1. ConCrete Uniformity reqUirements (mAndAtory informAtion) 167
The slump tests must be performed first in accordance with ASTM C172/C172M time limits for beginning tests. This does not permit the complete evaluation of one sample at a time. Slump tests should be performed almost in parallel since slump changes with time. Assuming the air content determina- tion will be accomplished by the pressure method, the air-meter measuring bowl (base) is filled next. Before the air content is determined, the concrete density (unit weight) test is per- formed. This concrete density (D) test is critical, and extreme care is essential, especially in the use of the strike-off plate for surface preparation. While there is no limit for difference in the measured density, this measurement is used for three of the test limits. Following careful surface preparation and thoroughly cleaning off excess concrete, the air-meter base is weighed then the air-content test (A) is completed using the pressure method.
The concrete compressive strength cylinders are then molded, and the sample from the concrete density test is washed over a 4.75-mm [No. 4] sieve as the last physical test. The Concrete Manual by the U.S. Bureau of Reclamation [4] suggests the use of the concrete in the pressure air-meter base for quantifying the coarse aggregate content in the concrete sample. The weight of coarse aggregate captured on the 4.75-mm [No. 4] sieve is quantified as a percent of the weight of the original concrete sample.
Alternatively, it may be desirable to use a larger unit-weight container and possibly a different air content determination method such as the volumetric air content by ASTM C173/
C173M. When separate containers are used for the density and air content measurements, the air content is determined imme- diately after the slump test, and the cylinders should be molded before the ASTM C138/C138M density (unit weight) test. For the determination of coarse aggregate content, it is acceptable to use a separate portion of the concrete sample. The original weight of the concrete should be determined before it is washed over the sieve. The concrete sample used for this purpose should be at least 20 lb to obtain reliable information. Obtaining the sample from the wheelbarrow for this test should be in full scoops without trying to dribble mortar to achieve some target weight as this can result in the sample being nonrepresentative.
The pressure air-meter measuring bowl (base) with a typi- cal volume of 0.25 cubic feet (ft3), cannot be used for the density (unit weight) determination of concrete when the nominal maximum size of aggregate exceeds 1 in. For this evaluation wet-sieving as addressed in ASTM C172/C172M should not be done. A larger density (unit weight) container should be used.
The Concrete Manual published by the U.S. Bureau of Reclamation, 7th and 8thEditions [4,5] contain the same numerical calculation example when removing large aggregate by wet sieving for the mixing uniformity test. The presentation methods differ slightly, but each are identified as Appendix, Designation 26.
A1.2 and A1.3 contain formulas that will be used in an example of uniformity testing.
Batch Characteristics: The batch size for either stationary mix- ers or truck mixers should be the maximum volume proposed for production and not necessarily at the maximum rated mixing capacity of the unit. It is a common practice, particu- larly in truck mixers located in hilly or mountainous terrain, to size mixers larger than the usual maximum load size. The extra volume is needed to prevent spills on uphill pulls. Using a smaller than normal maximum batch size, for example, less than 80 % of the normal batch size, is not adequate to evaluate the mixing capabilities of the unit. Mixing times for stationary mixers should be carefully recorded on the uniformity test cal- culation sheet. The minimum time required to achieve unifor- mity is an important part of this test if this is the purpose of the evaluation. With truck mixers, it is recommended that at least 70 revolutions, and preferably the full 100 mixing revolutions permitted, be used unless a special project limits the drum to fewer revolutions.
Unless the uniformity tests are strictly for a project or geo- graphic area not requiring air entrainment, it is recommended that an air-entrained mixture be used because the ability to generate the required quantity of air throughout the mixed concrete is an important part of this evaluation.
EXAMPLE 21.A Uniformity test calculations and evaluations.
Having one person to do all the recording of test results and to maintain the timing for the testing schedule is helpful. At least two certified technicians are essential. If the testing technicians work as a team, the most efficient procedure is to allow each technician to perform the same tests on each sample and thereby reduce the variation of tests performed by multiple technicians.
Slump: This is one of the six tests that has a variable requirement depending on the level of slump. When the average value of the two slump tests is 4 in. or less, the maximum allowable difference between the test results is 1 in. When the average value of the two slump tests is between 4 and 6 in., the allowable difference is 1.5 in. When the average slump value is greater than 6 in., the batch is not acceptable for unifor- mity testing. At high slump, segregation is more likely, and obtaining representative samples for this and the other tests becomes more difficult. There are currently no provisions for testing uniformity of self- consolidating concrete.
Example:
Sample No. 1 slump = 3.00 in.
Sample No. 2 slump = 4.25 in.
Average slump (S1) = 3.625 in. (3⅝ in.) Difference = 1.25 in. > 1 in. NG
The average slump value of 3.625 in. is less than 4 in., thus the uniformity requirement is a maximum difference of 1 in. The measured difference is 1.25 in., which is greater than permitted, and slump range failed this uniformity requirement.
Air Content: The pressure meter has been found to produce the more consistent results of any of the air content tests. Therefore ASTM C231/
C231M is the preferred test method if the type of aggregate in the con- crete permits it to be used. ASTM C231/C231M cannot be used to test the air content of concretes made with lightweight aggregates, air-cooled blast furnace slag, or aggregates of high porosity.
Example:
Test No. 1 air content = 4.5 % Test No. 2 air content = 5.1 % Difference = 0.6 %
The difference of 0.6 % on the air content between the two samples is less than the allowable difference of 1.0 %. The air-content test meets mixing uniformity requirements of Table A1.1.
Density of Concrete (Unit Weight): Using the base section of the pres- sure air meter, determine the density by measuring the mass of the meter base while empty and then again after being properly filled and the top surface prepared, but prior to completing the assembly of the top section for the air content determination. The base of the pressure air meter typically has a volume of approximately ẳ ft3 and will weigh approximately 10 lb. Scales are required to be accurate to 0.1 lb or to within 0.3 % of the test load, whichever is greater, at any point within the range of use. For a 10-lb weight, this is 0.1 lb versus 0.03 lb.
The 0.1 lb is the required minimum accuracy of the scale. Older field scales may not meet the accuracy criterion. Predetermine (calibrate) the volume of air meter base in accordance with procedures described in ASTM C29/C29M.
Example:
Density of Concrete (air-free): The air content of the two samples has already been determined. By calculating the concrete density of each sample on an air-free basis, the effect of the different air contents of each sample is removed from this calculation so that the composition of the samples in terms of density can be compared.
The density of concrete on an air-free basis can be calculated using:
= − ×
D D
100 A 100
af (1)
where:
Daf = the density on an air free basis, D = the measured density, and A = the air content in percent.
From previous calculations:
A difference of only 0.58 lb/ft3 is acceptable relative to the allow- able difference of 1.0 lb/ft3. This is the second of three uniformity ranges that is within allowable limits.
Item Sample No. 1 Sample No. 2
Concrete + WD (density measure) (lb) 45.22 44.86
WD (density measure) mass (lb) 9.68 9.68
Concrete = b (lb) 35.54 35.18
VD = Volume of base (ft3) 0.25 0.25
Density of concrete (unit weight) D = b ÷ VD
D = 35.54 ÷ 0.25 (Sample No. 1) (lb/ft3) 142.16
D = 35.18 ÷ 0.25 (Sample No. 2) (lb/ft3) 140.72 Note: Scale readings in 0.1 lb increments are acceptable.
Sample No. 1 Sample No. 2
Density, D (lb/ft3) 142.16 140.72
Air content, A % 4.5 5.1
Density on an air-free basis, Daf (lb/ft3) −. × . 142 16 100 4 5 100
−. × . 140 72 100 5 1 100
148.86 148.28
Difference (lb/ft3) 148.86 – 148.28 = 0.58
Coarse aggregate content (%): Use the density (unit weight) sample to determine coarse aggregate content. It does not matter that the ASTM C231/C231M air test contaminated the sample with water because orig- inal weight of the concrete has been measured with the density test.
Using a hose wash the mortar from the concrete sample over a 4.75-mm [No. 4] sieve to retain the coarse aggregate on the sieve. The 4.75-mm sieve represents the separation point between coarse and fine aggregate. The coarse aggregate retained on the 4.75-mm sieve must be collected meticulously, while the portion passing the 4.75-mm sieve may be wasted. An 8-in. diameter sieve may be used, but a larger tray sieve used for coarse aggregate sieve analysis is sturdier and easier to use. Wipe the retained coarse aggregate surface dry, and then deter- mine the mass (weight). Alternatively, the wet material may be weighed while suspended in water and the saturated surface dry (SSD) mass computed using a previously determined SSD relative density (sp gr).
This will more likely need to be performed in a laboratory. The coarse aggregate as a percent of the original concrete sample mass is deter- mined by the formula provided in Section A1.2. See Fig. 21.A.
Example:
The difference of 2.2 % is less than the allowable difference of 6.0 %.
The coarse aggregate content meets the mixing uniformity requirements.
Mortar Unit Weight (air-free): This is a means of determining differ- ences in mortar properties of the two samples. The coarse aggregate has been removed in the calculation by the previous determination, and the unit weight is corrected for the measured air content, thereby resulting in the unit weight of air-free mortar, that is, cementitious materials, fine aggregate, and water. Bloem, Gaynor, and Wilson [6]
suggest, based on considerable testing, that differences of more than 1 lb/ft3 in this determination indicates substantial variations in the mortar composition and recommended that a difference of 2 lb/ft3 in the air-free mortar density between two samples represented varia- tions in mixture composition in the same load that are unacceptable.
It should be recognized that the determination of air-free mortar Sample No. 1 Sample No. 2 Mass of SSD aggregate retained on
4.75-mm (No. 4) sieve, c
(lb) 16.63 15.70
Original mass of concrete, b (lb) 35.54 35.18 Percent Coarse Aggregate,
P = c/b × 100 % (16.63 ÷
35.54) × 100
(15.70 ÷ 35.18) × 100
46.8 44.6
Difference % (46.8 – 44.6) = 2.2
FIG. 21.A Washing coarse aggregate from a concrete sample.