Exposed Aggregate Concrete Advice

To assist you in planning your exposed aggregate concrete projects, I've included the text I wrote for Cemex in 2010 and I later updated for Holcim when they purchased the company.

If you'd like the PDF of the printed booklet just drop me an email request. I will be happy to send this to you.

The PDF copy has colour photos as well as a checklist to ensure you end up with concrete that is totally fit for purpose.

Exposed Aggregate - Best Practice Placement Guide
Plan ahead, hire a professional, enjoy the results

To assist you in the preparation and placement of your ConceptTM concrete project, CEMEX has produced this leaflet with the aim of providing you with all the necessary facts about our ConceptTM product and to aid in specifying the job with your concrete installer or grano contractor.

At CEMEX we believe in helping you understand our products and asisting you in achieving a completed project that will provide years of trouble-free use and enjoyment. After all, one of our major goals is to provide you with a quality product that will not only perform to expectation, but also look great for years to come.

Whilst we strictly control the quality of our concrete product, there are many factors that you should consider in planning your ConceptTM project. This leaflet aims to help you with your ConceptTM project and ensure that your ConceptTM concrete remains looking great for many years into the future.

Thankyou for choosing CEMEX ColouraveTM and we trust that the information that follows will be of practical assistance to you.

Preparation is the key

Like any concrete slab, CEMEX recommends that a structural engineer designs your ConceptTM paved areas to ensure the slab thickness, steel reinforcement and compressive strength of the concrete will be suitable for your needs.

Ensure the area where you are about to lay your ConceptTM concrete has been graded and compacted and a compaction certificate is obtained. Please be aware that areas alongside buildings, retaining and pipework are usually poorly compacted and settling and water action in and around soakwells can produce settlement cracks in your ConceptTM project.

It is recommended that any area to be concreted, be well watered prior to your installer coming onsite.

In simple terms … a better prepared site will result in a better finished job.

Reinforcement

Whilst offering very high compressive strengths, concrete is a product that is weak in tensile strength. Therefore, it is essential to use steel reinforcing to give concrete additional strength in tension and at the same time, assist in the control and movement of cracks.

When placing the steel mesh, bar chairs designed to hold the steel reinforcement above the ground must also be used. The use of commercial bar chairs is essential as they will hold the reinforcing mesh in the upper part of the concrete, where it will best assist in crack control.

The reinforcing mesh should be a minimum 20mm from any concrete surface so it is not in contact with the atmosphere. Concrete is highly alkaline and steel will not rust in concrete, provided that it is fully encased by the concrete. To ensure the edges of concrete paving have no air holes, it is useful for the installer to compact paving edges simply by tamping along these areas prior to floating or trowelling the surface.

In Honed or Exposed Aggregate Concept™ projects, CEMEX suggests the use of F62, or heavier, reinforcing mesh.

If you are using Polished Concept™ concrete as your house slab or suspended slab, please ensure your structural engineer and architect or designer are aware of your choice. CEMEX recommends the use of F82 slab mesh in all Polished CourPave™ ground slabs and your design team is always welcome to call on CEMEX’s specialist architectural products staff.

Mechanical vibration is required for Polished Concept™ floors but should be avoided for Exposed Aggregrate or Honed projects.

Cracking

Whilst it is not desirable, cracking can occur in concrete products and cracks will impact on the appearance of your ConceptTM project. There are many causes of cracking, all of which can be controlled to produce a successful project.

Plastic shrinkage cracking occurs when the concrete is still plastic or wet. Surface cracks of varying lengths appear at random over the surface of the concrete caused by excessive "bleeding" of moisture from the concrete. This is most likely to occur where there is high temperature, humidity or wind. Protecting the concrete surface from drying out will assist in preventing this type of cracking.

CEMEX recommends that installers apply aliphatic alcohol to the concrete surface after initial screeding or floating. The aliphatic alcohol applies a thin film surface membrane to your concrete and will slow the evaporation rate and keep moisture within the concrete at optimal levels. Additionally, wind barriers should be erected around the project.

If possible, avoid pouring ConceptTM concrete on days of high temperature or high wind. However, if it is critical to pour concrete on such days, it is essential to prevent rapid drying of your ConceptTM concrete.

On warm to hot days, your concrete is susceptible to rapid drying after final placement. Normal “pond” curing is not suitable to either Polished floors or Exposed Aggregate projects. In such circumstances it may be necessary to apply a “same-day” acrylic sealer to your finished concrete. Please seek advice from your CEMEX recommended installer or from your local CEMEX staff.

Using a water vapour barrier (plastic) under the concrete is essential where practical, and can help avoid such cracking. It can also assist in reducing movement restraint due to friction between the concrete and the sub-grade (or soil) below your concrete. Additionally, it will also help in protecting your concrete from possible discolouration from underground water, dampness or waterlogging.

Adding excess water to the mix should be strictly avoided, as this practice will alter the colour, finish and strength of your concrete. CEMEX Concept™ is designed to be installed at a “slump” of 80-95mm and adding water onsite may increase the slump beyond this range.

Sprinkling water on the surface during or after finishing should also be avoided.

Control joints

As concrete loses moisture over time it shrinks. If the concrete is restrained from shrinking by such things as footings or existing walls then internal tensile forces will be created in the concrete which can cause the concrete to crack. This type of cracking can be reduced by providing sufficient strength in the concrete to resist the tensile forces (by the addition of adequate reinforcement) and remove restraints on the concrete (allow it to move) by the use of control and isolation joints.

Control joints are purposely created planes of weakness in concrete to ensure that anticipated cracking due to shrinkage will take place at predetermined and aesthetically appropriate areas. To be effective, control joints need to be deep enough so that it is the weakest part of the surrounding concrete.

Assuming the depth of your Concept™ is 100mm, CEMEX recommends that control joints be no further spaced than 2.5 metres. When planning your project with your installer you should aim to achieve square rather than elongated shapes with your control joints.

Control joints should run from all hard corners such as brick piers, soakwell covers or the corner of a building. If there is a rectangular cutout in your paving, such as a garden bed, control joints should run from each of the 4 corners.

Any circular cutout shape in your paving should also have 4 separate control joints radiating from the shape. Try to avoid sharp triangular shapes as concrete may crack across narrow shapes. In the case of curves or garden beds, control joints should take the shortest path from the curve to the closest wall or pier.

Should you be laying Concept™ alongside an existing concrete project such as a kerb, please ensure that any existing control joints continue through to your newly laid Concept™.

When laying concrete with a random pattern finish, continue to observe the normal rules relating to control joints. Remember, a joint will encourage your concrete to crack at this point, and if a joint stops in a random pattern, the concrete may crack beyond the point at which the random pattern joint stops. So it is important to utilise full-length or full-width control joints even in random pattern finishes.

CEMEX recommends that all joints be cut a minimum 25% of the concrete depth. These joints may be cut by trowel and jointing tool whilst the concrete is plastic or by saw, no later than 24 hours after the concrete is laid.

Alternatively, your installer may supply commercial jointing products. CEMEX sells and recommends the Australian designed “Crack-A-Joint”, which not only controls surface cracking but it looks great and is available in several colours.

Concrete will crack if it is restrained from moving by bonding with hard surfaces such as brick walls, existing concrete surfaces, columns and plumbing fittings. It is essential to construct filled isolation joints around and along hard surfaces and CEMEX stipulates the use of a commercial foam joint. Again this product is available from your CEMEX sales and display centre.

Colour

Environmental conditions can affect the colour and finish of concrete. Concrete placed in shaded areas can appear different to concrete placed in direct sunlight. Concrete placed in cold conditions can appear different to concrete placed in warm conditions. To reduce variation it is recommended that ConceptTM Concrete be poured under the same environmental conditions.

Concrete finished inconsistently can also result in colour variation, due to light reflecting differently from the surface, changing the apparent colour and tone. Indeed many factors will impact on the final appearance and serviceability of your ConceptTM project including natural variation, placement methods, contamination and the condition of your sub-grade.

For example, sub-grade areas containing mud, standing water, or soils with uneven absorptive properties may cause surface discolouration and cracking.

CEMEX has a comprehensive quality system in line with Australian Standards, which includes extensive raw material testing, strict control over batching and mixing procedures. However, the materials used in the manufacture of your ConceptTM product are subject to natural variation of colour, texture, shape and size. Such natural variations can influence the overall colour and appearance of your finished project.

At CEMEX, we employ a comprehensive quality system. We do all we can to ensure you receive a first-class product but please be aware, our ConceptTM range is a natural product and CEMEX cannot guarantee your project will be an exact match to sample panels, previous projects or existing concrete.

Environment

The process of exposing aggregate (and grinding Polished or Honed concrete floors) will produce environmentally sensitive waste materials and cement slurries and care should be taken to dispose of all waste in a responsible manner.

Please talk to your installer and once again, plan ahead.

If you are pouring concrete on a sloping site, please remember that the slurry will wash downhill. For this reason, please plan ahead with your installer so that if there is an expansive area, the top part of the job is completed on one day and the lower area completed the next. This will enable your installer to was down the initial section without affecting the lower section.

When working on a sloping site it is always best practice to pour concrete working up the slope. This stops the concrete from slipping down the slope whilst it is still plastic.

Playing it Safe

Care needs to be taken to avoid on-site contamination, which may result from other concrete products, site materials walked on to the concrete, wind blown materials, wildlife and pedestrians.

Ensure your contractor uses only CEMEX recommended retarders when exposing the aggregate in your concrete. CEMEX staff will be happy to confirm these details for you. Though CEMEX stocks retardant products, your installer should include the cost of this product in his quote or contracted price.

The retardant is applied to slow the chemical bonding of the concrete at the surface level and is “hosed” off to reveal or expose the top portion of the concrete aggregates (or stones). Some installers may vary the time of retardant exposure or even use different retardant products to purposely create varying textures, patterns or stencil effects in your Concept™ project.

Your installer may or may not provide a final sealer to protect your Concept™ concrete. Please check with your installer.

It is an easy process to seal your Concept™ project yourself, and you’ll find more information in the CEMEX Concept™ product brochure, or simply consult your local CEMEX sales and display centre.

To achieve the best from your project, CEMEX recommend that only reputable, experienced concretors place and finish its CEMEX ConceptTM concrete range. CEMEX will not be responsible for poor workmanship, so to achieve the best from your project, contract only an experienced, CEMEX-trained ConceptTM concretor.

As you are now aware, ConceptTM concrete uses natural materials, which may vary slightly, but other things also affect the finish and colour of concrete; such as site preparation, environmental conditions (including sunlight, temperature, wind and shade) as well as placing, finishing, curing and sealing.

When you visit your local CEMEX display centre, our staff will provide you with a list of local ConceptTM projects to help in your planning and product choices. We will also provide a list of CEMEX recommended ColoupaveTM installers.

Whilst CEMEX does everything within its control to ensure the consistency and quality of the ConceptTM range, please remember that many factors affecting the final quality of your project will always be out of the direct control of CEMEX.

So please read this leaflet carefully. If you plan your project carefully, you will be informed and aware of these factors .... and be able to enjoy the many benefits of your ConceptTM project for many years to come.

You can download the original Readymix Exposed Aggregate leaflet here.











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