Reinforced Concrete

Exposed aggregate Concrete Pathway

9 Concrete facts for Melbourne Landscapers.

Concrete is without a doubt it is one of the most versatile used materials in landscape construction. In fact, concrete is the second most widely used substance on the planet after water. From in situ concrete walls to pavers to fence posts it has a enormous range of uses. What are the different types of concrete and its uses?  To properly decide on which mix to use, it is important to understand what it is and what factors affect its physical properties.

 

commercial concreting melbourne

Commercial Concreting Melbourne. Reds Landscaping Melbourne using a boom pump to pour architectural concrete walls

What is Concrete?

In technical terms, it is a structural material consisting of a hard, chemically inert particulate substance, known as aggregate that is bonded together by cement and water. It is a composite material as it consists of both a binder and a filler.

What is difference between concrete and cement?

The binder most commonly used is known as Portland Cement. This is a mixture of finely ground limestone (CaCO3) and shale or clay which has been combined together at around 1500℃. In this process, water and carbon dioxide are removed from the mixture (calcination), then calcium silicates are formed. A small amount of gypsum (CaSO4•2(H2O)) is added to regulate the setting.

Concrete with exposed aggregate.

The aggregate, that is the part of the mix made up of several smaller ingredients, will generally be the sand and gravel. The gravel itself will usually be hard stones of a certain size range. Fine aggregates are below about 10mm in size and are often used in small bags of cement mix or for smaller landscaping work. Larger stone aggregates range from 10 to 40mm in size and are commonly used in construction. It is the stone aggregates that give the mix its compressive strength. As the aggregate is around 70% of the mix, it provides much of the bulk and contributes to its dimensional stability. The rougher the surface of the aggregate and the greater the area in contact with the cement paste, the stronger a concrete will be.

River pebbles will not give your concrete the maximum compressive strength, but they do look great in exposed aggregate concrete.

Rounded particles like river pebbles or beach sand will result in lower strength than crushed aggregates. Larger size aggregates lead to relatively lower strength. Where extra strong mix is needed, a little less aggregate can be used.

Aggregate

Course aggregate.

Water

A key ingredient is of course the water. When the water is combined with the cement as paste is formed which binds the aggregate together. Concrete does not harden by drying out, it hardens by a chemical reaction know as hydration. In this reaction, compounds in the cement react with water molecules to form strong chemical bonds. Ideally, the water should be as pure as possible to prevent the occurrence of any side reactions which may weaken or interfere with the chemical reaction taking place. Even small quantities of organic soil compounds result in chemical reactions that seriously affect the strength. In Melbourne access to good quality water is not usually a problem.

 

 

Concrete Mixer

Loading the Concrete Mixer the correct materials in the correct ratios is essential for making strong concrete. The water needs to be as pure as possible.

The other important point for the landscaper is to get the ratio of water to cement correct. The ratio of water to cement is critical if strong concrete is required. If too much water is added, the strength of the mix will be reduced. Excess water above what is required for the chemical reaction will result in pores on the concrete which will reduce the strength especially the tensile strength. Too little will make the it difficult to work, to fill spaces, or create a good connection to the reinforcement. Accurate measurements and thorough mixing of the cement and water will help prevent these problems.

What is it called when concrete hardens?

Concrete sets with a chemical reaction not by drying.

 

It is set by a chemical reaction and not by drying. This means that it will even will set under water. It is important to remember this fact during the curing stage. The two main hydration chemical reactions from the calcium silicates are as follows;
Tricalcium silicate + Water—>Calcium silicate hydrate+Calcium hydroxide + heat
2 Ca3SiO5 + 7 H2O —> 3 CaO.2SiO2.4H2O + 3 Ca(OH)2 + 173.6kJ
Dicalcium silicate + Water—>Calcium silicate hydrate + Calcium hydroxide + heat
2 Ca2SiO4 + 5 H2O—> 3 CaO.2SiO2.4H2O + Ca(OH)2 + 58.6 kJ

Exothermic reactions

Both of these reactions are exothermic, that is, they release heat. This heat will dissipate quickly in thin sections. In thicker sections, the internal temperature is transferred to the outside much more slowly. As the outer surface of the concrete will cool much more rapidly than the inner core, there can be a difference in reaction speed. This can lead to tensile stresses that can crack the surface as a result of this uncontrolled temperature difference across the cross section. For this reason, concrete should not be poured in very cold temperatures. In cases where thermal cracking does occur, it will be at early ages of curing. The heat can also cause moisture to evaporate from the surface of the concrete, making it weaker. This will be the case if there is insufficient water for the chemical reaction. For these reason excessively thick sections should be avoided in a single pour. Wooden formwork and damp hessian covers can help the curing process. Giving your concrete a very light spray of water as it is curing will often improve the strength.

 

What is the correct mix for concrete?

When mixing concrete for footings or foundations use a mix of 3 parts coarse or sharp sand and 3 parts aggregate with 1 part of a high quality cement.

History of concrete

What have the Romans ever done for us? The Romans are widely credited for the spread of building technologies including concrete throughout Europe. It was the Roman’ Empires’s engineering abilities that enabled them to built an enormous empire throughout Europe and through parts of North Africa and the Middle East. The concrete architecture of the Romans is famous amongst fans of history.

Durable Roman Concrete has lasted centuries.

The Roman  formula for quality concrete

It was know to the Romans as “opus caementicium”. Opus meaning a fortification, composition or a piece of work and caementicium meaning quarried or unhewn stone. The Romans developed their recipe in the third century BC. The ingredient the Romans used was volcanic dust known as pozzolana. This volcanic dust included fine particles of alumina and silica which created the chemical reaction enabling the setting. To this they added a mixture of lime or gypsum, brick or rock pieces and water. Usually the mix was a ratio of 1 part of lime for 3 parts of volcanic ash.

Concrete Pantheon Rome

The Pantheon in Rome was constructed entirely in concrete.

 

 

Concrete Dome of the Pantheon in Rome.

Concrete Dome of the Pantheon in Rome.

Roman Waterproof Concrete

Roman builders discovered that adding crushed terracotta to the mortar created a waterproof material which could be then be used with cisterns and other constructions exposed to rain or water. Recently, it has been found that the Roman mix used in seawall construction has better endurance to seawater than the modern stuff. This was mostly due to one of the minerals of the volcanic rock phillipsite, reacting with the seawater to form aluminous tobermorite which reinforced the concrete over time. After the fall of the Roman empire the technology for making concrete was lost for many years.

 

Assyrians Babylonians and Egyptians.

Among the ancient Assyrians and Babylonians, clay was often used as the bonding material. The Egyptians developed a substance more closely resembling modern concrete by using lime and gypsum as binders. Lime (calcium oxide), was derived from limestone, chalk, or (where available) oyster shells. (Pozzolans are actually a broad class of siliceous or siliceous and aluminous materials.)

Portland cement

In 1824 an English inventor, Joseph Aspdin, burned and ground together a mixture of limestone and clay. As the chemistry of concrete was not fully understood at the time, the proportions of the ingredients were developed by trial and error. This mixture, called Portland cement, has remained the dominant cementing agent used in concrete production. It is named Portland cement as it is an attempt to imitate the limestone from Portland in Dorset on the jurassic coast of England. Portland Limestone formed slowly over the last 150 million years or so as tiny grains of sediments and clays infused the limestone grew and compacted. This gives it both its unique physical properties when grown up for cement, but also its attractive appearance. Portland Limestone has been used in many of the iconic London buildings such as Saint Paul’s Cathedral and the palace of Westminster. As a building material Portland Limestone was popularised by architect Sir Christopher Wren.

Reinforced Concrete.

One of the drawbacks of concrete, despite its great compressive strength, is its lack of tensile strength. This is largely due to its natural porosity. Plain unreinforced concrete does not easily withstand stresses such as wind action, earthquakes, and vibrations and other bending forces and is therefore unsuitable in many structural applications. Low tensile strength also means low strength in bending or when used as a beam.

The Strength of Steel

Steel when compared with concrete has great tensile strength. The solution is to embed the steel into the concrete. This is usually achieved with the use of steel mesh reinforcement. The reinforcing steel, normally takes the form of rods, bars, or mesh. The reinforcement bars are often coined along the surface to give them a good connection to the concrete. The addition of tightly bound reinforcement bars makes the concrete section into a true composite beam. For this reason, the reinforcements must have good overlap.

commercial concreting melbourne. Concrete retaining wall footings.

Commercial Concreting Melbourne. Concrete retaining wall footings with steel reinforcement.

 

 

Joseph Monier

Reinforced concrete is usually attributed to Joseph Monier, a Parisian gardener who made garden pots and tubs of concrete reinforced with iron mesh.This was patented in 1867. In reinforced concrete, the tensile strength of steel and the compressional strength of concrete render a member capable of sustaining heavy stresses of all kinds over considerable spans. Despite the strength of reinforced concrete, efforts should be make to minimise the loads on garden retains walls. This can be achieved by adequate agricultural drainage near the wall. It is important to remember that a cubic metre of water weighs a tonne. Plant selection near the retaining wall is also important plants should be chosen that do not have an invasive root system. For your existing trees, consider the use of a tree root barrier.

Common Problems with Concrete

Concrete Cancer

If the steel reinforcement inside the concrete becomes exposed to water and air, weak carbonic acid can form. This can cause the steel reinforcement to form iron oxide or rust. The Iron oxide, Fe2O3, has a higher volume than the original steel. The expansion of the reinforcement can cause the concrete to crack leading to even more water ingress and rust.

Concrete cancer Elwood Victoria

Pre-cast concrete wheel stop buffers Elwood Victoria. As the steel reinforcement rusts it expands allowing more water in. Melbourne bayside suburbs like Elwood are particularly susceptible.

 

When designing or constructing concrete structures, the reinforcement bars need to be located away from anywhere water can accumulate. This includes the counter bores for fasteners. In Melbourne bayside suburbs like Elwood, salt water can accumulate in counterbores and seep through to the reinforcement via the concretes natural porosity, or through microcracks.

 

concrete cancer in a precast buffer

Concrete cancer in a precast car park buffer. Reinforced concrete should be designed and constructed so that the steel reinforcement is not too close to areas where water can accumulate.

 

Commercial landscaped gardens , or pebble ballast roofs, on flat concrete are particularly susceptible to cancer. For these reasons, it is essential that the roof is properly waterproofed and rooftop garden beds have adequate drainage.

 

Concrete cancer in a footpath crossover.

Concrete cancer is a likely cause of this failure in a pedestrian crossover. Note how close the steel reinforcement has been placed to the top surface allowing pooled water to seep through to the steel reinforcement.. An early indicator of this failure are the grid pattern of cracks in the concrete surface.

 

Understanding the nature of the materials used in residential or commercial landscaping is key to creating a quality Melbourne landscaping solution that can be enjoyed for years to come.

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Exposed aggregate Concrete Pathway

Landscaping Concrete

Landscaping Concrete must to be the correct grade with the correct reinforcement for the job it is selected for. If this is not the case, cracking can occur. To properly decide on which mix to use, it is important to understand what it is and what factors affect its physical properties. There are many different grades of concrete available, so it important to pick the correct one for the job. Coupled with this, there are different grades of steel reinforcement to choose from.

Pre-mixed Multipurpose Concrete for garden walls

Pre-mixed Multipurpose Concrete for garden walls

 

Garden Paving and Concrete Path Construction

Bending stresses are not normally a problem with garden paving when a properly prepared sub base has been created. Steel reinforcement will however, help to prevent cracks opening in the pavement.

Generally, the path should be at least 100mm thick N25 concrete. N25 means that the concrete will achieve a compressive strength of 25 mPa after 28 days. If you are in a part of Melbourne with a reactive clay soil, you need to ensure there is sufficient thickness of roadbase under the paving to cope with the expansion and contraction of the soil.

Preventing Cracks in landscaping concrete.

Even the strongest concrete will be weak in tension or bending and even properly cured concrete will have microcracks. Therefore, to minimise cracking steel reinforcement must be used. For pathways, this should be at least SL 72 using saddles to keep it in the top 1/3 of the concrete.  SL72 means that the bars are each 7mm in diameter with grids of 200mm. The path should be laid on 100mm thick class 3 roadbase , if it is for public spaces. If there is any possibility of a vehicle driving on the path, then the path needs to be built like an exposed aggregate concrete driveway. This will be the case with any vehicle crossovers in the path design. In these cases the concrete needs to be at lease 125mm thick N32 (32mPa) concrete with SL92 mesh laid on at least 100mm of class 2 roadbase.

Thermal Expansion and Contraction of Concrete

To allow for thermal expansion of concrete, saw cuts and isolation foam are essential for preventing cracks. It is important that the landscape architect as well as the concrete contractors keep this in mind.

A concrete path needs an allowance for expansion.

 

Landscaping concrete. Steel reinforcement helps to prevent large cracks opening up in your concrete.

For paving, the steel mesh should be placed about 30mm from the top surface. When reinforcement steel is placed too near the surface, it can corrode. Expansion results as steel is converted to iron oxide through corrosion. This expansion can crack the concrete surface and accelerate the concrete cancer.

Concrete block garden retaining wall with coping next to the alternating exposed aggregate path.

Concrete block garden retaining wall with coping next to the alternating exposed aggregate path.

When the crack is caused by corroding steel, corrosion is typically visible at the slab surface. In the case of retaining walls, the wall is in effect a cantilever beam with the soil applying pressure to the wall. Steel reinforcement will help increase bending strength of the wall.

Melbourne Landscaper - Concrete in-situ steps Mill Park Leisure centre.

Melbourne Landscaper – Concrete in-situ steps Mill Park Leisure centre.

landscaping concrete

Architectural Landscaping Concrete in Melbourne

Modern concrete is now available with an enormous range of colours and textures. There are some available that can mimic the appearance of stone, but at a much lower cost to the landscaper. There are also some techniques by landscape architects to break up a large expanse of concrete by using alternating contrasting colours.

 

 

 

Exposed aggregate Concrete Pathway

Exposed aggregate Concrete Pathway using alternating coloured concrete. The appearance is also softened by the mulched garden bed with plantings of native grasses. These architectural concreting techniques provide a very cost effective solution but maintain the aesthetic appeal.

Commercial Landscaping

In commercial landscaping all the concrete must be to the grade specified by the architect and poured exactly to the drawing. Test samples are kept by the concrete supplier for each batch for testing after the appropriate cure time.

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Contact us

For help with the design  and development or your landscaping ideas, contact one of our experienced Landscape Gardeners.

We can help with small garden design all the way up to  Commercial Landscape design.

Our specialities include fast growing screening plants, plant health and horticulture, garden lighting  and in-situ concreting.

By Callum O’Brien – Landscaper Melbourne

 

 

© 2020 Reds Landscaping and Civil quality commercial landscaping Melbourne

 

For more information on concrete

 

How to minimise thermal cracking

 

How it is made and the scientific principles.

Read more