Xypex explained.
Xypex contains a bit more cement which improves the concrete slightly, but it would be cheaper to ask for more cement in the readymix concrete.
Its speciality is it retards the initial set of the concrete substantially.
As practitioners ourselves we fail to see the benefit.
The Xypex BBA Certificate.
My comments
OPC is a reactive crystalline admixture
Contains PC, which may not be OPC, presumably.
More cement without more water would always be expected to increase strength, water tightness and durability of concrete.
The control concrete was about 3 times as resistant to water permeability as typical structural concrete (source: Concrete Society) and the additive improved that to about 5 times. Still penetrable.
This is an improvement of about 50%. So it is still permeable.
Who does site trials?
This clause seems to exclude the Xypex from the equation.
If their base mix was pure OPC, its initial set would be a lot less than 170 minutes. So it isn't. It will probably be a blend with flyash.
Increasing the initial setting time from nearly 3 hours to over 5 might have advantages for some but it would increase the strain on some formwork and increase the risk of the surface being affected by rain.
This is C35A from BS 8007 to which Xypex is added. They claim water tight. It is already water tight.
|
|
These quotes in dark red are all from the Xypex product certificate on the BBA web site
here.
But we have changed the order in which they appear.
This Agrement Certificate Product Sheet relates to Xypex Admix C-1000 NF, an admixture used to provide watertight concrete.
This certificate ... has to be read, considered and used as a whole document - it may be misleading and will be incomplete to be selective
1.1 Xypex Admix C-1000 NF is a reactive crystalline admixture for incorporation in concrete to enhance the watertightness and durability in its hardened state.
1.2 The product is supplied as a powder consisting of blended Portland cement and proprietary chemicals.
4.1 Xypex Admix C-1000 NF is satisfactory for use in concrete mixes at an addition rate of between 1.0 and 1.5% by weight of cement to provide watertight concrete for basements, swimming pools, tunnels and culverts, without the requirement for additional applied protection.
6 Water penetration.
6.2 Tests showed that concrete containing the product (1.25% wt/wt PC) showed a water permeability (1) of 1.99x10–13 m/s-1 compared with 3.26x10–13 m/s–1 for the control concrete.
(1) The specific effect of the product on water penetration for a particular mix and site conditions should be evaluated through site trials prior to use.
7 Water vapour permeability
7.2 Test results for concrete containing the product (1.25% wt/wt PC) showed a water vapour permeability(1) of 440x10–12 [g/m (Ns)–1] compared to 690x10–12 [g/m (Ns)–1] for the control concrete.
(1) The specific effect of the product on these properties for a particular mix and site conditions should be evaluated through site trials prior to use.
7.3 Concrete made with a high water/cement ratio can have a water vapour permeability above 3000x10–12 [g/m (Ns)–1]. The permeability of concrete is strongly dependent on the exact mix design and the figures given in this Certificate
indicates the levels that can be obtained using the product.
11.2 The setting time of concrete mixes containing the product will be retarded when compared to equivalent plain concrete. The amount of retardation will depend on the concrete mix design used and ambient temperature during placing and curing.
(from table 4, initial set of control concrete 170 minutes and with Xypex 315 minutes)
19.2 For Grade 3 (where control of water vapour is required), it will be necessary to provide a mix with a sufficiently low vapour permeability in combination with an
adequate section thickness (see sections 7.2 and 7.3).
20.2 The concrete must have a minimum cement content of 325 kg/m–3 and be batched with a maximum water/cement ratio of 0.5.
|
|
There are some other figures on the Xypex BBA certificate. Are they a deliberate but cloaked warning? What of?
From
Table 1 Effects of Xypex Admix C-1000 NF
on the properties of fresh wet concrete
|
Property
|
Control concrete
|
Concrete containing Xypex Admix C-1000 NF
|
|
Slump (mm)
|
|
|
|
0 min
|
65
|
60
|
|
30 min
|
30
|
35
|
|
|
At 0 minutes the gypsum in the cement is reacting and about to shut the rest of the future reactions down for about two hours.
If the slump has reduced after 30 minutes we don't know when it reduced, only that it was within 30 minutes. Usually before the concrete has arrived and is discharged.
A 35mm slump is of little use to anyone. Workers spreading it by hand would want water adding. A pump operator would insist that water was added. Once water is added the benefits of the Xypex in making the concrete slightly more watertight would be lost.
|
But the reduction in slump applies to both the control and the Xypex concrete. What is the significance? Why is this information on the certificate? What could be in the Xypex, which is only about 4kg per m³, that would reduce the loss of slump from 35mm to 25mm? Flyash?
The Concrete Society has an explanation of slumps and consistence class here. The deviations allowed are huge compared to the changes reported on this certificate. So why are they there?
See the chart below.
BBA certificates have figures for water permeability.
The Concrete Society had the idea of plotting them all on a chart. We have done that as well. But we have also included some best guesses. For instance, only the Caltite certificate has no figures for compressive strength.
Some of the control concretes were better than concretes with admixture in them.
Xypex is in the middle showing a fairly modest improvement, which might be just from adding a little bit more cement.
|
|
|
|
|
|
Please note. The author acknowledges all the trademarks it mentions on this site.
|
|
|
|
