(07) 3219 2777
Cement & Concrete Association of Australia
Information Note September 1995
In September, 1995, the Cement & Concrete Association of Australia released a document titled "Concrete as a Termite Barrier - Pouring Slabs to AS 2870. It is quite informative, and many of the points raised are still valid today. The document follows:
CONCRETE
FROM THE CEMENT AND CONCRETE ASSOCIATION OF
AUSTRALIA
The Association is a national
non-profit organisation sponsored by the cement industry in Australia to provide
information on the many uses of cement and concrete. Since the information provided is intended for
general guidance only and in no way replaces the service of professional consultants on particular projects, no liability can be
accepted by the Association for its use.
Concrete as a Termite Barrier
- Pouring Slabs to AS 2870
For years people have been spraying chemicals under concrete slabs to
protect houses from attack by subterranean termites. On the 30th June, 1995, the
two organochlorines, heptachlor and chlordane, which were used due to their long
life, were banned as a result of health issues. A number of short term chemicals
(eg dursban, byflex), have since been approved, but their effective life is
limited, and re treatment under slabs is difficult. Note that at this stage, the
use of reticulation systems have not been approved as a means of re treatment,
as their capability to last the life of the structure is yet to be confirmed.
For the first time, concrete slabs have been recognised as .part of an
effective' termite barrier system, to minimise the risk of termite infestation
and damage to new buildings. To complete the system, service penetrations need
to be protected, and some form of treatment around the perimeter of the slab
also has to be provided.
For penetrations, either Termimesh or Granitgard are acceptable, whilst
for slab edges, four options are available: Termimesh, Granitgard, edge exposure
and vertical horizontal chemical barriers. Chemical barriers may also be used to
protect service penetrations, but a complete horizontal barrier is required, and
with the difficulty to retreat, this is not seen as a viable option at this
Stage. These requirements are all outlined in the new Australian Standard AS
3660.1 - 1995.
The new code, AS 3660.1, specifies that to be
regarded as part of the barrier system, the slab has to be constructed in
accordance with Australian Standard AS 2870 -Residential Slabs and Footings. What does this require the
Builder/Designer/Engineer to comply with?
2.
Control of Drying Shrinkage Cracking
The intent of requiring slabs to be constructed in accordance with AS
2870, is to ensure that the concrete has been properly placed (without addition
of extra water), compacted by mechanical vibration, cured, and that the correct
cover has been set to the reinforcement. A few simple quality issues, aimed at ensuring
that the long term drying shrinkage of the concrete is controlled.
Controlling the drying shrinkage of the concrete, and hence the cracking
associated with it, is important, because we understand that the termite species
found in N.S.W. require at least a 1 mm wide crack all the way through the slab,
to be able to gain entry through the slab, and into the structure above.
Generally, drying shrinkage cracks are the only ones of constant width, which
extend the full slab depth.
With the adoption of good concreting practices, the reinforcing fabrics
given in the standard slab designs contained in AS 2870, are sufficient to
control the width of drying shrinkage cracks to less than this 1 mm. Note that
if alternative types of reinforcement are used, the Designer needs to consider
control of possible crack widths.
One issue requiring clarification, and one which is often questioned, is
the acceptable crack widths nominated in Table A2 of AS 2870. This Table, which
lists damage categories for slabs, in terms of crack widths, states that crack
widths up to 2 mm are acceptable. Cracks between 1 and 2 mm in width (Category 2
damage), we estimate only occur in approximately 5% of slabs. The Cement and
Concrete Association is currently investigating crack widths in housing slabs to
re affirm this. The Standards Committee responsible for AS 3660.1, considered
that the likelihood of having a small percentage of cracks wider than 1 mm,
combined with an active termite colony at the same location, was sufficiently
small to still regard concrete slabs as able to minimise the risk of damage from
termites.
Note that if designs outside the standard ones in AS 2870 are nominated
by an Engineer, due regard to the control of drying shrinkage crack widths needs
to be considered by the Designer.
3. Quality Issues
The Residential Slabs and Footings Code, is a
document that basically lists standard construction details and techniques for concrete slabs and footings. While
it does not directly cover the above quality issues, in Clause 1.4, it refers to
Australian Standard AS 3600 -Concrete
Structures. The quality issues that effect the durability and strength of
concrete slabs, are given within this Standard.
Essentially, there are four main areas that need
to be considered with house slabs: (a)addition of water on site, (b)compaction,
(c)curing, and (d)provision and placement (cover), of reinforcement. There are
many other factors that effect the strength of concrete, such as type of cement,
aggregates, admixtures and temperature and so on. These are generally taken care
of by ordering one of the standard strength grades of concrete; either N20, N25,
N32, N40 or N50.
By addressing each of the four key quality
issues listed above, the concrete slab will conform to the requirements of AS
2870, and thus be able to function as part of the termite barrier system.
4.
Water/Cement Ratio
The water to cement ratio is the amount of water (litres or kg), divided
by the weight of cement
The addition of water on site, so as to increase workability, and make it
easier for the concreter to place and finish, will cause increased shrinkage,
and a loss of concrete strength. As the graph on page 3 indicates, an increase
in the w/c ratio from 0.5 to 0.6, decreases the strength by about 30 per cent.
Note that for a typical mix having say 300 kg of cement per cubic metre, this
only requires the addition of 30 litres of water per cubic metre to reduce the
strength by 30 %
In terms of shrinkage, water in a concrete mix occupies the spaces
between the cement, sand and aggregate particles. The more water that is added,
the further apart the particles in the concrete mix are forced. With the long
term drying out of this water, increased cracking will occur as the particles
tend to draw closer to each other, and fill the voids. According to the
Australian Standard for the manufacture of concrete, premixed concrete suppliers
are required to deliver concrete that has less than one millimeter of shrinkage
per metre length of slab. However, typical values of between 0.6 and 0.65 mm per
metre are obtained. Country plants using Type SL or shrinkage limited cement,
should obtain even better results. If water is added at the client's
instruction, the manufacturer is no
longer responsible for the performance of the concrete, either in terms
of its shrinkage, or strength. That is, if cracks wider than 1mm occur due to
excessive shrinkage caused by the addition of water, the premixed concrete
supplier would not be responsible.
The addition of water also increases bleeding. This is the rise of free
water to the surface of the concrete at the concrete finishing stage. It results
from the settlement of particles in suspension. While excessive bleeding does
not effect the ability of the slab to prevent entry of termites, it can produce a number of other problems:
Excessive Bleeding can Result in:
A) Weaker surface layer of concrete-
If surface finishing is commenced prior to the
evaporation of all bleed water, the excess water on the surface is mixed with
the cement paste from the screeding process. This substantially increases the
water/cement ratio near the surface, and results in a surface, which even though
finished off well, may be very weak and easily worn or broken away. Finished
concrete slabs that have "powdery" surfaces, are examples of this
problem. Note that the other major contributor to powdery surfaces is lack of
proper curing. Refer Section 6.
In some instances (eg hot/windy days), the rate
of evaporation of water from the surface of the slab, can be greater than the
rate of bleeding. This drying out of the surface can give the impression that
bleeding has ceased, and finishing should be commenced. The finishing operation
produces a dense, compact surface layer, which allows a much reduced rate of
evaporation. Any further bleed water, not able to work its way through the
compacted surface layer, can accumulate and form a thin film underneath this
layer. The result is that the top 2 to 3 millimetres can ''flake'' off, as the
water film prevents adequate bonding of the surface layer to the concrete
substrate. This immediately destroys whatever surface finish has been achieved.
Examples have occurred where 20 mm of concrete was found to be
"drummy", and subsequently ''flaked off". While this is unusual,
repairs can be costly. Note that finishing off before bleeding has ceased
(indicated by a sheen on the surface), can produce the same result. It is very
important to ensure that bleeding has ceased prior to finishing, and that on
hot/windy days the concrete is adequately protected.
This is caused by the rising bleed water
carrying small particles of lime to the surface, resulting in a white staining
due to the formation of calcium carbonate. This can be removed by washing with a
mild hydrochloric acid solution (1 part acid to between 10 and 20 parts water).
Note, it is important to thoroughly pre wet the surface initially, to prevent
acid soaking in, and to thoroughly rinse the acid off afterwards. Note that a
slight roughening of the surface may result, due to the acid solution etching
the surface.
If the concrete is not vibrated or compacted
properly, or it contains an excess of water, the settlement of particles can
cause voids to form under the reinforcing wires or bars. Settlement of particles
occurs because the specific gravity of the cement, sand and aggregates, is
greater than that of water. As the heavier particles settle "to the
bottom of the slab or beam, the water is displaced to the surface. Voids under
the reinforcement have a significant effect on the bond strength between the
hardened concrete and the reinforcement, and can thus seriously reduce the
structural performance of the reinforced concrete member.
5. Compaction
Clause 19.1.3 of AS 3600 states that,
"concrete shall be handled, placed and compacted so as to - in part (d), completely
fill the formwork to the intended level, expel entrapped air, and closely
surround all reinforcement, tendons, ducts,
anchorages and embedments." While the code does not state how entrapped air
is to be expelled (which is not the code's function), the requirement that it
must be done is there.
Compaction is a two-stage process. First, the
aggregate particles are set in motion and consolidated to fill the form and give
a level top surface. In the second stage, entrapped air is expelled. This
description of the process is true whether compaction is carried out by rodding,
tamping and similar manual methods, or when vibration is applied to the
concrete. The latter, by temporarily 'liquefying' the concrete, is generally
much more efficient than hand-tamping or rodding, and hence is almost
universally applied on construction sites; typically with the use of poker
vibrators.
It is important to recognise the two stages in
the compaction process because, with vibration, initial consolidation
of the concrete can often be achieved relatively quickly. The concrete liquefies
and the surface levels, giving the impression that the concrete is compacted.
Entrapped air takes a little longer to rise to the surface. Compaction must
therefore be prolonged until all the air has been expelled, ie until air bubbles
no longer appear on the surface.
Most concrete arrives on site with between 5 to
20 % entrapped air in it, as a result of batching and mixing. As much of this as
possible must be removed by proper vibration. In reality, about 1.5 to 2 percent
of air will remain in the final concrete, despite thorough vibration. This
entrapped air exists as minute bubbles of air trapped in pockets within the
aggregates. Entrapped air up to 2 percent has very little effect on the
compressive strength. However, as the graph below illustrates, each 1 % of air
voids in excess of this level gives approximately a 5 % loss in compressive
strength. Thus if concrete is not vibrated, a strength loss up to about 20% to
25 % could occur.
Now it is often argued that a 100 mm thick
ground slab does not need vibration due to the amount
With respect to termites, the main issue is the
honeycombing that may result with poor compaction. This makes it easier for the
termites to work their way through the slab, or around physical/chemical barrier
systems (especially if the concrete strength ends up being considerable lower
than designed).
In terms of compaction, the difference between a
good and inadequate job is only a few minutes.
6. Curing
The setting and hardening of concrete involves a
series of chemical reactions between the cement
and water in the concrete, resulting in the
growth of crystals. As these crystals grow, they mesh and lock together, binding
individual particles of cement and aggregates together. It is this binding, or
cementing together of the ingredients that make up the mix, that gives concrete
its strength. Without water being present, the crystals can not continue to
grow, and achieve their required size/length. As the crystals are thus smaller,
they can not interlock as effectively, and lower concrete strengths result.
Curing is the process whereby water is retained
in the concrete, to allow the hydration process, or
Curing is dealt with in Section 4 of AS 3600 -Design for Durability. Depending on the location
of
For exposure classification Al (residential
slabs), Clause 4.4 of AS 3600 states that concrete shall
The graph on the next page indicates the effect
of various degrees of curing on the strength of
In terms of controlling the long term drying
shrinkage cracking, curing ensures that the crystal
Further information on liquid membrane curing
compounds for concrete can be found in AS 3799 -
1990 -Curing
Compounds.
Remember the concrete must have water to gain
strength, and must be kept continuously wet for the minimum curing period. This is particularly so with concrete mixes using blended
7. Cover to
Reinforcement/Durability
Reinforcement in a concrete ground slab is used to control the width of
drying shrinkage cracks, as well as for control of temperature related stresses.
Its location within the slab is important if it is to
perform satisfactorily.
One of the primary functions of concrete in a reinforced concrete
structure is to protect the
Min. f'c
Classification
Description
25Mpa A2 Above ground exterior environments more than 50 kms from the coastline in non-industrial and temperate climatic zone Applies to areas out as far as Bourke and Cobar.
32 MPa
B1
Above ground exterior environments from 1 to 50 kms from the
coastline.
Also, members in fresh water, and those in industrial areas more than 50 kms
from the coastline
40 MPa
B2
Above ground exterior environments within 1 km of the coast.
Also members permanently submerged in sea water.
50 MPa
C
Surfaces of members in sea water tidal or splash zones.
Combined with the above minimum concrete strengths for durability, Table
4.10.3.2 gives the
Note that as AS 2870 and 3600 are referred to in the Building Code of
Australia, the requirements
8. Plastic Shrinkage Cracking
Concreting during warm and windy conditions can
often result in cracking of the surface of a
Note that below a rate of 0.5 kglm2/hr,
no problems with surface drying shrinkage cracking should be experienced.
Between 0.5 and 1.0 kglm2/hr, precautions should be considered. Above
1.0 kglm2/hr, conditions exist that promote cracking (either high
wind, low humidity, and high
Effect of concrete and air temperatures, relative humidity, and wind
velocity on the rate of evaporation of surface moisture from concrete.
9.
S
W/C ratio 0% -30% reduction possible depending on quantity of water added.
