Although developing standards through consensus can be a grueling process, such effort is rewarded with heightened awareness and some agreement among concrete producers and purchasers. A case in point is the recent passage of a new specification for mixing water by ASTM Subcommittee C09.40 that is responsible for ASTM C 94, Specification for Ready Mixed Concrete. Two standards were approved: ASTM C 1602/C 1602M-04 Standard Specification for Mixing Water Used in the Production of Hydraulic Cement Concrete and ASTM C 1603/C 1603M-04 Standard Test Method for Measurement of Solids in Water. In addition, a revision to C 94 removes the requirements for water within the specification and makes reference to C 1602, as it does for all other ingredient materials used to produce concrete.

The initiative was intended primarily to address the reuse of process water (wash water) from ready mixed production as mixing water in new concrete. Statistics that support this need are astounding. Conservative estimates indicate that the volume of returned concrete nationwide exceeds 15 million cubic yards and the volume of process water generated exceeds 8 billion gallons annually. The National Ready Mixed Concrete Association estimates an annual cost of $60 million to industry producers for compliance with environmental regulations.

When the issue first surfaced in the early '70s on the West Coast, several NRMCA members collected data on the characteristics of wash water in settling ponds. Thus established were grounds for a revision to C 94 in 1978 that permitted the use of wash water as mixing water in concrete with certain controls to protect the purchaser of ready mixed (as well as the producer). No change in the specification has been introduced since then.

On the operational side, requirements in compliance with environmental regulations have increased 10 fold, abetted by recent enforcement construing infractions of the Clean Water Act as criminal violations. The volume of recycled water generated at concrete production facilities traditionally has been higher by far than that which can be reused and/or processed for discharge economically. Recently, however, innovations in equipment and chemical admixtures have enabled concrete producers to better manage process byproducts and achieve “zero-discharge.” Many ready mixed producers in the U.S. and Canada have committed the time and financial resources to accomplish this aim. In Europe and Japan, it's standard operating procedure.

Accordingly, standards were lagging behind industry practice. In about 1995, an effort was initiated to revise the water provisions. Following several failed ballots on revisions to C 94, subcommittee focus shifted to developing a separate specification for mixing water in concrete. Such a standard was finally approved in October 2004.

ASTM C 1603 establishes a performance basis for the qualification of water for use in concrete. Distinguishing sources of water as potable, nonpotable, or deriving from concrete production operations, it establishes qualification requirements by mandating a certain frequency of tests, depending on the intended use. Combined water comprises one or more of the sources used in combination. Criteria apply to the total mixing water in concrete, whether obtained from an individual source or a combination of sources.

Primary or mandatory criteria qualify the proposed water for effects on concrete strength and setting time. Like the requirements of C 94, these criteria stipulate that “test” water should not reduce strength by more than 10 percent nor affect set time, as compared to a concrete mix with tap water, beyond 60 minutes earlier or 90 minutes later. While earlier provisions required such tests to be conducted on mortar and cement pastes, the current standard allows qualification trials to be performed on either lab or field concrete batches.

Optional criteria relate to water chemistry in terms of the concentration of chlorides, sulfates, alkalis and total solids. Their status is optional insofar as the purchaser must specifically invoke one or more of these items when the concrete is ordered. Unchanged from previous requirements, these criteria also apply to the total mixing water.

As indicated in Figure 1 (page 57), testing frequency for density of water and for other regulated criteria is differentiated by water source. The reference to Table 1 in C 1602 relates to mandatory criteria for strength and setting time, while Table 2 lists criteria for the optional requirements discussed above. Potable water is permitted for use without any testing. Nonpotable water includes stormwater or water from streams and other natural sources that cannot be consumed by humans. Water from concrete production represents process or wash water.

The major source of contention in gaining approval of the standard was establishing testing frequencies for the qualification of water for use in concrete. The result constitutes the best consensus attainable within ASTM. Testing frequencies for wash water are governed by the density of combined water proposed for use. Density less than 1.01, for example, is typical of clarified water from washout ponds that a producer can use for mixing concrete; experience and research have shown that density at this level has little bearing on concrete properties, so the testing frequency is low. Density of 1.03 represents a combined-water solids content of approximately 50,000 ppm (5 percent) by weight, e.g., wash-water slurry from reclaimed concrete diluted with clear water. If water with a higher concentration of solids is used, testing frequencies are increased to ensure constant monitoring of the effects of process water on concrete properties. Universally adopted is the premise that, if a higher level of solids is qualified by performance testing, water with solids at or below that level is acceptable.

ASTM C 1603 is a test method that provides procedures for measuring the density and the solids content of water. The standard also provides guidance on blending two sources of water to stay within a target density or solids-content range.

Now that the standards are in place, producers and purchasers will require some time to fully understand their intent and how to use them. Standards continue to evolve as needs change, and these standards will follow suit. Establishing a precedent, however, was the most difficult step. The ASTM Subcommittee is now working on an issue that C 94 does not address — a means to reuse returned concrete by batching on top of it. Now that will take a long time to accomplish!

Colin Lobo is Vice President of Engineering for the National Ready Mixed Concrete Association. This article was reprinted from Concrete infocus magazine, Winter 2005, with permission from NRMCA, www.nrmca.org

GUIDE FOR TESTING FREQUENCY AS RELATED TO WATER SOURCE USED IN MIXING WATER — ASTM C160

Water Sources	Combined Water Density, g/ml	Testing Frequency
		Density, C 1603	Table 1	Table 2
Potable	N/A	N/A	N/A	N/A
N/A Non-PotableA	N/A	N/A after 4 tests, annually	3 months;	6 months
Concrete ProductionA	<1.01		6 months; annually after 2 tests
	1.01 - 1.03	Daily	Monthly; 3 months	6 months
	> 1.03		after 4 tests, weekly; monthly after 8 tests
A Testing frequencies apply to the combined mixing water when it is wholly or partially composed of the listed source as defined in Section 3.
(Table reprinted, with permission, from C1602/C1602M-04 Standard Specification for Mixing Water Used in the Production of Hydraulic Cement Concrete, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA19428.)