boral tn adding water
DESCRIPTION
water proofingTRANSCRIPT
Nothing is easier than adding excess mixing water to pre-mixedconcrete at building sites. And nothing is more likely to makerepairs to a concrete construction necessary, or more likely todamage a contractor’s reputation for efficiency and reliability.
As supplied from Boral trucks, batched concrete is carefullyproportioned and mixed to produce strength according tospecifications. Less than half the water it contains is needed forhydration of cement. The rest of the water is there to maketransporting and workability easier (by providing lubricationbetween sand/aggregate particles) and to ensure that there will besufficient inherent water in the curing process.
The plant supplying the mix will adjust the water contentfractionally to meet summer or winter conditions of transportingand placing.
“Wetness” of concrete is directly related to its setting strength –the 28-day compressive strength of concrete is reduced by about1.5 MPa for each additional 20mm of slump produced by addingwater.
To put this another way, each additional 10 litres of water per cubicmetre will reduce the strength of concrete by about 2.5 MPa.
Water should only be added to the concrete to maintain thespecified slump under the supervision of a Boral representative.Excess water added on site will damage the concrete propertieswith potentially disastrous results.
Adding Water to Plant BatchedConcrete
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Boral Concrete
Technical Notes
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Cementitious binder (i.e. cement & fly ash) needs lessthan half its own weight of water to turn concrete intodurable stone. The “wetter” this cementitious paste is, the weaker it is. The table below shows how strengthdecreases as water content of a mix increases.
Approx. w/c ratioCharacteristic compressive
strength (MPa)
0.35 0.40 0.50 0.55 0.65
5040322520
Advantages• Easier placing
Disadvantage• Lower compressive strengths.• More time and labour needed for finishing, and for stripping and cleaning
forms.• Cracking – with too much water there will be low tensile strength, high
shrinkage.• Dusting and scaling. Bleeding of excess water brings too many fines to
the surface of floors.• Sand streaks. Excess water bleeding up the sides of forms washes out
cement paste and leaves an unsightly streaked surface.• Mud pies. Too much water in concrete placed on grades causes earth to
mix with the concrete, slows the set, reduces strength sharply.• Permeability. Voids left as excess water evaporates invite water to seep
through walls and floors.• Dead losses – costly repairs, or, in extreme cases, removal and
re-building, at contractor’s expense.• Increase in setting times.
EFFECT OF TOO MUCH MIXING WATER
Figure 1: Characteristic Compressive Strength approx. w/c ratios.
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When water is added here
To make concrete run there
Do not add extra water to flow the concrete from one point to another. If flowing concrete is required use Easyplace.
When water is added to increase slump, strength decreases.
30
25
20
15
1050 100 150 200
SLUMP mm
STRENGTH MPa
The strength here
Be ready to place ready-mixed concrete as it arrives. Prolonged mixing in the truck agitator makes concrete stiffer, harder to work.
Use vibrators to compact mixes having slumps of 80mm or less.
For easier placing, specify concrete with plasticisers or, best of all, Easyplace.
Make sure that enough people are available on site to transport, place and compact fresh concrete.
GOES THERE
When water is added here
To make concrete run there
Do not add extra water to flow the concrete from one point to another. If flowing concrete is required use Easyplace.
When water is added to increase slump, strength decreases.
30
25
20
15
1050 100 150 200
SLUMP mm
STRENGTH MPa
The strength here
Be ready to place ready-mixed concrete as it arrives. Prolonged mixing in the truck agitator makes concrete stiffer, harder to work.
Use vibrators to compact mixes having slumps of 80mm or less.
For easier placing, specify concrete with plasticisers or, best of all, Easyplace.
Make sure that enough people are available on site to transport, place and compact fresh concrete.
GOES THERE
When higher-slump mixes appear to be desirable for difficultplacements, advice about mix designs, setting strengths andsafety factors is always available from Boral Technical staff.
FOUR WAYS TO MINIMISE TEMPTATION TO ADD EXCESS WATER
The ever-present site problem is that all members of placingteams quickly learn about the labour-saving effect that “a little”added water has on workability of concrete.
The site supervisor can’t tie knots in the hose(s) needed forcleaning equipment. So, “a little” water may be added to manybarrow or buggy loads of concrete during a pour.
With this unofficial system operating, it doesn’t take long to add100 litres of water to a truck-load (say 5m3) of plant-mixedconcrete. The effects will be:
(a) an increase of about 80mm of slump over the slumpspecified.
(b) a reduction in compressive strength of about 5MPa.
(c) uneven strength throughout the concrete mass.
The only answer is to make at least one responsible member of each placing team fully aware of the harm excess water can cause.
Perhaps the most dangerous of all practices is the use of extrawater to help concrete “flow” along elevated forms to lowerpoints. In this case, if shores/toms are removed after a nominalcuring period, severe structural cracking and/or collapse are morethan possibilities.
Generalinformation:please email us [email protected]@[email protected]@[email protected]@[email protected] for further information.
Customer Service:at Boral we are committed to excellencein service so please contact QLD 1300 30 59 79NSW 1300 55 25 55VIC 133 006TAS 03 6336 1366SA 08 8425 0400WA 08 9333 3400NT 08 8947 0844for further information.
Boral website:Visit the Boral website for the fullrange of Boral productswww.boral.com.au or for specificconcrete information visitwww.boral.com.au/concreteproducts
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