Concrete Advice

Polished / Honed

BGC Concrete’s Architectural Range of Columns + Inserts + Pipes Flat Slab/Thickenings

polished mixes will add value to your
Place columns before you pour the slab both Engineer details have to be complied with,
building project. centrally and at the perimeter. Place your its best to have a flat slab. ‘M’ and ‘S’
The success of using this range of mixes order early to ensure they are on site when classification sites with thickenings require
depends upon planning and attention to required, you don’t want to be patching special attention. Construction joints may be
detail, an experienced contractor should unsightly block outs. Columns should be required at ground beams to ensure cracks
always be appointed to achieve the best attached to the footings not slab. Brass or do not emanate at the line of the beam.
results. The contractor should be aware of stainless strips, floor wastes should be fully Structural engineers should be consulted
the limits and details of the mix supplied, secured and in position. Check the height to assess the options of such, its probably
correct ordering and placement of concrete of such inserts, remember the polisher advisable to increase the overall slab
in addition to expert finishing skills are will probably be taking 3-5mm off the slab thickness to provide a consistent thickness
surface. Pipes within the concrete thickness and maintain the desired stiffness to the slab.
required to ensure that a consistent
create a weakness, an area subject to
presentable job is achieved. Remember the
cracking, it is imperative that the concrete
trade skills employed are as important as the Ordering
thickness is uniform, pipes should be placed
mix being supplied, don’t cut corners. The
within the sand pad. Make sure you order enough concrete. Plan
process is not just good luck on the day!
the pour, make an allowance for wastage
To assist your selection process all mixes are and order the full amount required, you don’t
Concrete Grade
coded, no names, just codes. It is imperative want delays in trucks, these lead to cold joints
that you quote the correct code, it will 25 Mpa strength as a minimum, this can be which are unsightly. If it’s a part polished slab,
have the appearance of EX2514A222, the overridden by the engineers requirements, over order the polished concrete requirement,
detail identifies the type of mix, in this case you may need to use a higher grade of adjust the grey concrete quantity to suit, allow
polished, its strength, aggregate size and type concrete for structural reasons eg: 32 a 1 meter overlap to the polished concrete.
and the cement colour. Without the code we Mpa, a mix design will be required. It is The same admixtures (shrinkage reducers)
are unable to supply. BGC Concrete personnel not always possible to use the aggregate should be used in all concrete in the slab.
are available to provide expert advice once you select, not all aggregates comply with The same concrete grade should be used in
you have made your mix selection. AS2758.1 2009 (Aggregates and Rock for the slab, don’t pour 25Mpa in the polished
Engineering Purposes : Part I Concrete area and 20Mpa grey concrete to the
Aggregates) aggregates required in balance. Always pour the polished concrete
Accelerators / Retarders structural elements take precedence over section first to avoid contamination. Codes
aesthetics, check aggregate suitability with must be quoted when ordering. If you have
Precautions need to be taken when pouring
the concrete supplier. poured on site previously, please let the
in hot weather, and to a lesser extent at cold
temperatures. The concrete suppliers aim plant staff know, this allows BGC to check
our records and ensure that the code being
is to design mixes which allow the concrete Curing
quoted and concrete supplied are correct.
to remain ‘plastic’ for the same time period,
Curing benefits all characteristics of Always order sufficient concrete to complete
whether in summer or winter. To do this the
concrete, it lessens moisture loss, improves the job, plus loads are to be avoided as the
supplier uses accelerators and retarders.
the hydration process, the concrete will time to batch (remember these are custom
Firstly accelerators, the only real issue here
shrink less, have higher strength, improved loads requiring the plant to individually
is that you have enough manpower to place durability and long term performance. In batch, they are not produced automatically
and finish the concrete. Secondly retarders, hot conditions, compromised hydration due through the batch plant) and deliver (plus
they present problems that need to be to inadequate curing, resulting in a lower loads), particularly in the hot summer
understood, yes you will be able to place the stength and a relatively ‘soft’ surface, can months may result in a cold joint. Not all
concrete however as the surface dries out lead to aggregates popping out during batch plants produce exposed/polished
the body of the concrete remains plastic. the polishing process. This will affect the mixes the plus load issue is exacerbated by
This leads to surface cracking unless it is appearance of the concrete, a high quality the distance to site. Further, it is difficult to
managed, referred to as plastic shrinkage curing product should be used (chlorinated batch small quantities, ie: 0.2m3 to achieve
cracking. The stresses which occur due rubber), better still continual wetting for an accurate match, it is better to divide
to concrete shrinkage are greater than the 7 days would be most advantageous in the job quantity into equally sized loads
speed the concrete is gaining strength, producing a hard surface for polishing. If whenever possible for a consistent finish. As
the stresses cannot be overridden by aggregate popping is experienced during you can see planning and ordering are two
strength gain. Moisture loss at the surface the polishing process, the slab (if relatively key elements in achieving a successful job.
is a problem, shade and wind breaks will new) should be treated with a fluorosilicate,
help, the use of aliphatic alcohol sprayed this will improve the surface hardness to
on the surface will also prevent moisture such an extent that the polishing process Polythene
loss, this needs to happen until the body can be completed successfully. Please Ensure it is secure, taped and most
of the concrete is set and the concrete be refer to manufacturers data sheets, any importantly you use two layers, this will allow
finished. If moisture is lost at the surface surface membranes will need to be removed concrete to slip on the ground, there is less
and finishing commences too early the before application as the fluorosilicate will resistance against shrinkage, consequently
concrete will appear to be spongy, the body need to penetrate the slab. Curing is often less likelihood of cracking. Two storey
of the concrete is still reaching initial set, the overlooked by concreters, it is perhaps residences with thickenings anchor
surface has set. Once finished the concrete the most important aspect of concrete concrete to the ground profile, two layers of
must be cured (refer separate section). placement particularly in hot environments, polythene will afford some movement at the
strength can be compromised by up to 20%. ground/concrete interface.

T 08 6220 4700 | PO Box 7223 Cloisters Square Perth WA 6850 | concrete@bgc.com.au | bgcconcrete.com.au
Pumping Sequence Slump
As polished mixes contain a higher Plan the pour sequence, pour the polished Pour the concrete at the correct slump,
percentage of coarse aggregates difficulties section first, do not allow contractors to don’t pour the concrete too wet, water =
can be experienced when pumping, walk in the polished area once screeding is shrinkage, if the contractor wants to pour
especially when long lengths of rubber hose completed. Contamination of black sand, >80slump you must look at using a super
are being used. We highly recommend the yellow sand, grey concrete, etc. on boots will plasticiser (SP). (You can increase the slump
use of Viscosity Modifiers in the concrete impact on the final finish. without adding water.) The addition rate of
which will assist, these products are a MUST SP must be controlled, the higher the initial
if white sand is being used. It is important slump, the less SP that will be required to
that the correct cement is used as a slurry, Shrinkage Reducers achieve the desired final slump. Too much
ie: for white concrete a use white cement. Every effort needs to be made to avoid SP or pouring at an excessive slump can
If you are pouring two mixes on the same cracking BGC recommend the use of result in segregation of the mix and ‘sinking’
day it is imperative that the pump is fully shrinkage reducers which are capable of the aggregate, additional polishing will be
washed out, including the pump hopper of achieving up to a 40% reduction in necessary to achieve an even appearance.
before the new mix is introduced to shrinkage. Shrinkage reducers in our
avoid contamination. opinion are a MUST, a 25Mpa mix typically
Stairs
shrinks 7mm in 10m, this can be reduced to
4mm, going a long way to avoid cracking. Fully vibrate they are difficult to patch,
Reinforcement
All polished mixes are priced including especially the risers take extra care,
Use a high grade of mesh (SL82), keep the shrinkage reducers. don’t rush.
correct cover and make it secure, you don’t
want the mesh too close to the surface, it
may be exposed by the polishing process. It Slab Detail Toppings
is better to go overboard with re entrant bars - Check and double check dimensions Polished concrete can be used as a topping
than not supplying enough. Overlap and tie before the pour. there are however a unique set of problems
the mesh. which should be considered.
- The mesh is secure and level.
Always comply with minimum engineering - Cracks in the base slab can be reflected
requirements if heavier than SL82. If - You have doubled re entrant bars at
through to the polished slab. To minimize
electrical underfloor heating is being corners (other than where a control joint
the likelihood of such repair the crack first,
installed, this must be secured, if tied to the is to be located) at all pipes, steel columns
stitching with epoxy, and mesh over the
reinforcement there must be enough cover and penetrations, narrow sections of
crack before pouring.
to ensure that the wires are not exposed slab and risky areas, which may require
during the polishing operations. additional steel to be in position. - Use an epoxy to bond the old concrete
to the new topping, you don’t want the
- Steel columns in the slab are generally
topping delaminating. Epoxy should
clad with pressings, make sure expansion
Screeding be applied to a dry concrete surface
foam is used around the columns and that
progressively as the concrete is placed.
The Contractor needs to take extra care to the columns are in position on the footing
Epoxy’s have a limited life (20 minutes),
ensure that the surface has a consistent before you pour.
thus personnel need to be available to
appearance when polished. If finished
- Full height windows, ensure that the edge mix and place the epoxy as a separate
uniformly this will be achieved, the slab
board is straight and true, close the cavity operation ahead of concrete placement.
should be vibrated (refer separate section).
Screed marks can occur when the screed is 10-15mm at the location of full height
- Aggregate size and selection, may be
pushed into the concrete surface resulting windows to avoid unsightly patching at the
restricted by the topping thickness, as a
in lines in the polished slab, similarly feet slab edge once completed.
rule of thumb the aggregate size should be
marks can sometimes be seen in a slab, - Don’t rely on the contractor to form up a maximum of 1/3 of the topping thickness.
the contractor must fill feet marks with a special slab details, it’s often worthwhile
uniform mix on stepping backwards out of asking a form worker to place step down
the concrete. The identified issues are likely forms, steps, drains, grates etc – it will Trials
to be exacerbated if pouring at a high slump, ensure that such forms are secure and If you are unsure about the mix appearance
rectification through additional polishing accurate making life easier for the polisher. or selecting a mix outside of BGC Concrete
may be necessary. Architectural Range, samples can be
- Floor heating, make sure that the correct
arranged. There is a charge for this service.
cover is achieved, pipes or wires can be
Seeding exposed during the polishing operation if
Expensive aggregates can be hand seeded, they are too high in the slab. They should be Vibrator Use
be aware this is a subjective issue, supervise securely tied to the mesh and held down.
Vibrators will provide better compaction,
the contractor, or seed the slab yourself - Protect the floor during construction, let removing voids. Their use should be
to the desired level. Make sure you have all the trades know It is a polished floor, if consistent throughout the pour. The vibrator
enough aggregate to seed the slab before two storey don’t let the form worker nail the should enter the concrete surface vertically
you start pouring, divide the aggregate props in position, they should be placed and in a set pattern to ensure the whole area
into portions and place it at locations carefully on a protective plate (ie: fibre is covered. Do not drag vibrators horizontally
around the slab to ensure that each area is cement) you need to minimise any damage through the concrete surface, they will leave
covered and that you don’t run out. Seeded to the floor. trails in the concrete void of aggregate,
aggregates should be soaked 1 hour before resulting in an inconsistent finish.
using. Always comply with the safety
regime on site, wear PPE as required and
ensure scaffold is in place on all elevated
work areas and above all ask the builders
permission to access the site, you will, most
likely be accompanied.

T 08 6220 4700 | PO Box 7223 Cloisters Square Perth WA 6850 | concrete@bgc.com.au | bgcconcrete.com.au