INTRODUCTIONNZS 3121:2015 Water and aggregate for concrete has a dedicated section on the use of water from sources other than public supply. This reflects the widespread use of recycled water as a result of tighter environmental controls on wastewater discharge from ready-mix concrete plants.

Typical rural low volume ready mixed plants operate a recycling system that settles the solids from the fines out of suspension and then allows reuse of the clear wash water. The solids that have settled are periodically removed and allowed to dry prior to disposal to landfill. For larger plants the amount of solid material to be disposed of is prohibitive, and a recycled wash water system is typically used.

Such systems agitate the wash water to maintain a suspension of cement and aggregate fines along with admixtures, pigments etc., termed ‘grey water’ or slurry. Its reuse in concrete manufacture, in comparison to using fresh water, is more technically demanding and requires additional control processes to ensure uniformity of the concrete. Agitation systems need to keep the solids material from settling out and forming a solid deposit at the bottom of the holding tank.

The consistency of recycled wash water is controlled by regular monitoring of its Specific Gravity (SG) and is adjusted by diluting the grey water with clear wash water or fresh water. The SG typically can vary during the day or over a weekend from 1.07 to 1.20. Grey water from freshly recycled concrete will include cement particles that are still hydrating. This will then be mixed with partially hydrated or fully hydrated cements of varying ages, which can bring about unexpected setting properties of fresh concrete mixes. This is related to the chemistry of the grey water, and the fine particles acting as nucleation points for hydration.

This effect on setting times, which also affects the finishing properties of concrete mix, is exacerbated in warmer weather. This can be overcome with the use of set-retarding admixtures. Also the use of hydration stabilising admixtures can be used to overcome the effect on setting times and even halt hydration completely for up to two or more days.

A research project carrying out a first stage evaluation of the fresh and hardened state properties using recycled slurry, was undertaken by James Mackechnie, at Canterbury University, under the fellowship arrangements existing with the Cement & Concrete Association of New Zealand.

The full research report is titled Influence of recycled water on fresh properties of concrete (September 2004).

KEY POINTSThe following are key points that emerged from the report:

1. Specific Gravity (SG) of the LiquidSG of liquid in the holding tank will vary during the day/over a weekend etc. Values measured ranged from 1.07 to 1.20. Tests were carried out on concretes containing 1.07 to 1.16 SG washwater. Town water has SG 1.00.

2. WashwaterImmediately washwater was substituted for town water the initial setting time for the concrete was shortened see Fig 1. The initial set was used as the indicator of the end of any open time for finishing. 17.5 MPa mix initial set time was reduced by max 2 hours in the winter and 1½ hours in the summer. A 40 MPa had shortened open time by up to 1½ hours in winter and 1 hour in the summer. A self levelling concrete had shortened open time by 1 hour in winter.

3. Delivery/Placing Open TimesOpen times were considerably reduced for summer and winter conditions irrespective of washwater dosage i.e. 17.5 MPa reduced from nearly 8 hours to 5 hours, 40 MPa reduced from 6½ hours to 5 hours.

CONCRETE NZ READYMIX TECHNICAL NOTE 2WATER AND AGGREGATE FOR CONCRETE JUNE 2018

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4. Slump LossesOnly a limited study was carried out within the test programme but the rate of slump loss between mixesusing recycled washwater and tap water was very similar e.g. Tap 120 mm slump decreased to 60 mm in1 hour. Washwater 120 mm slump to 75 mm in 1 hour.

5. StrengthThe 28 day strengths of concrete appear to be not significantly effected by the SG levels of the washwater. It isnot possible to carry out a full statistical evaluation because of the limited number of results.

PRACTICAL IMPLICATIONSIn theory every time the SG of the washwater changes, the mix quantities need to be changed to account for the change in order to maintain w/c ratio and yield. This is not seen as warranted on the majority of New Zealand plants.

However some understanding and adjustment is needed if the industry is going to be able to supply concrete with consistent fresh properties to the concrete placing industry.

Water adjustments required for SG variations are higher than permitted changes at the delivery position and hence must be dealt with at the plant.

The Specific Gravity can be measured by weighing a litre of water. Town water will weigh a 1000 grams, washwater will weigh over the 1000 grams e.g. 1050 to 1150 grams having an SG 1.05 to 1.15. Overseas experience has been that no significant finishing problems in the fresh concrete properties occur when the SG is 1.07, hence the adoption of the 1.07 threshold.

The following recommendations are made:

1. Concrete plants using clear washwater only i.e. no continuous agitation/stirring.Annual check, and only deemed as 'clearwater' if SG < 1.01.

2. Concrete plants using agitated washwater but without detailed frequent SG measurement.AM: Check that washwater has SG 1.07 or less (mix designs based on 1.07).PM: Check that washwater has SG 1.07 or less. If more than 1.07, use proportion of town water e.g.:

• SG 1.10, 30% town supply. 70% washwater.• SG 1.15, 50% town supply, 50% washwater.

3. Concrete plants using agitated washwater but with detailed SG measurement.Continuous working slurry washwater/town supply based on actual SG.

17.5 MPA WINTER AND SUMMER MIXES 40 MPA SUMMER AND WINTER

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CONCRETE NZ READYMIX TECHNICAL NOTE 2WATER AND AGGREGATE FOR CONCRETE JUNE 2018

IMPURITIESImpurities in recycled water may adversely affect the properties of concrete or the durability of concrete, by interfering with the hydration of the cement or with the cement aggregate bond, or by excessive air entrainment. NZS 3121:2015 Table 1 gives examples of suitable test methods and suggested limits for harmful impurities.

CONTROL TESTS ON RECYCLED WASH WATERNZS 3121:2015 Table 2 repeated below gives recommendations for frequency of testing for SG. The Standard uses the term ‘agitated wash water’, the equivalent term used in this technical note is ‘greywater’

RECYCLED WASH WATER SG TEST REQUIREMENTS

WASH WATER INTIAL TEST FREQUENCY

NO. OF CONSECUTIVE COMPLIANT RESULTS

RDUCED TEST FREQUENCY

CONTROL REQUIREMENT

Clearwatera 6-monthly 2 12-monthly SG < 1.01

Agitated wash waterb Monthly 4 3-monthly 1.01 ≤ SG < 1.03

Agitated wash waterb Weekly 8 Monthly 1.03 ≤ SG < 1.07c

a. Recycled water where solids are allowed to settleb. Recycled wash water where solids are kept in suspensionc. Recycled wash water with SG > 1.07 shall be diluted with potable water

Source: NZS 3121:2015 Water and aggregate for concrete.

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CONCRETE NZ READYMIX TECHNICAL NOTE 2WATER AND AGGREGATE FOR CONCRETE JUNE 2018