Concrete in Landscape Construction
Concrete is without a doubt it is one of the most widely used materials in landscape construction. From sleeper retaining walls to pavers to post reinforcements to it has a wide range of uses. In this blog we will discuss some of the different types and its uses of in landscape design. To properly decide on which mix to use, it is important to understand what it is and what factors affect its physical properties.
What is it?
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. 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.
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.
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.
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.
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.
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
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.
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 Roman formula
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.
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.)
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.
On 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. Steel on the other hand 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 overlap.
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.
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. 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.
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.
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