A bit of info on concrete - care of Mr Google.HY-TEC CONCRETE...

A bit of info on concrete - care of Mr Google.

HY-TEC CONCRETE

NORMAL CLASS CONCRETE

Products ranging from 20 to 50 MPa compressive strength at 28 days witha design slump that has a point of acceptance from 20 to 120 mm are availablein both 10 mm, 14 mm and 20 mm aggregate sizes.

20 MPa and 25 MPa

Commonly used for house slabs, driveways, footings and footpaths.

Residential slabs and footings

A default slump of 100 mm has been adopted by the premix concreteindustry to reduce the uncontrolled addition of water on site and make placingand compacting of concrete easier for concreters.

32 MPa, 40 MPa, 50 MPa

Higher strength is commonly used for concrete that will experiencegreater loads and traffic. This may be specified by engineers or builders tosuit the load requirements which the concrete must support during its life.

SPECIAL CLASS CONCRETE

High strength 65 MPa, 80 MPa, 100 MPa

Commonly used for projects requiring high strength concrete with optionsin design slump between 140 and 200 mm. 65 MPa concrete is available in 10mm, 14 mm and 20 mm aggregate sizes while 80 and 100 MPa are onlyavailable in 10 mm and 14 mm aggregate size. High strength concreteis usually specified by engineers in applications such as high risebuildings.

Block fill

7 mm and 10 mm flowable product ranging from 20 to 40 MPacompressive strength at 28 days with options in design slump between 140mm and 230 mm. Block fill product has been designed to pump through a twoinch rubber hose line making it easier when placing into block work walls.

Shotcrete

Shotcrete is designed as a 7 mm and 10 mm aggregate sized sprayedproduct ranging from 20 to 50 MPa compressive strength at 28 days, with adesign slump of 60 mm. Ideal for spraying pool walls, embankments, structuraland retaining walls.

ENVIRONMENTAL CONCRETE

Conservation of non-renewable resources in the construction industry hasled to the use of alternative raw materials being utilised in concreteproducts. Raw materials such as fly ash, ground granulated blast furnace slag,bottom ash, air cooled slag aggregates, recycled water and concrete aggregatesare currently being utilised in concrete design in an effort to further supportsustainable development. As an example, The Ark project, a high rise commercialbuilding in Sydney, was constructed using concrete made with industrialbi-products such as fly ash, ground granulated blast furnace slag and aircooled slag aggregates to successfully achieve specific 'Green' criteria

Concrete strength

Many factors influence the rate atwhich the strength of concrete increases after mixing. Some of these arediscussed below. First, though a couple of definitions may be useful:

The processes of 'setting' and 'hardening' are often confused:

Setting is thestiffening of the concrete after it has been placed. A concrete can be 'set' inthat it is no longer fluid, but it may still be very weak; you may not be ableto walk on it, for example. Setting is due to the formation of ettringite andearly-stage calcium silicate hydrate. The terms 'initial set' and 'final set'are commonly used; these are arbitrary definitions of early and later set.There are laboratory procedures for determining these using weighted needlespenetrating into cement paste.

Hardening is the process of strength growth and may continue forweeks or months after the concrete has been mixed and placed. Hardening is duelargely to the formation of calcium silicate hydrate as the cement continues tohydrate.

The rate at which concrete sets is independent of the rate at which it hardens.Rapid-hardening cement may have similar setting times to ordinary Portlandcement.

Measurement ofconcrete strength

Traditionally, this is done bypreparing concrete cubes or prisms, then curing them for specified times.Common curing times are 2, 7, 28 and 90 days. The curing temperature istypically 20 degrees Centigrade. After reaching the required age for testing,the cubes/prisms are crushed in a large press.

The SI unit for concrete strengthmeasurement is the Mega Pascal, although 'Newtons per square millimetre' isstill widely used as the numbers are more convenient. Thus 'Fifty Newtonconcrete,' means concrete which has achieved 50 Newtons per square millimetre,or 50 Mega Pascals.

While measurements based on concrete cubes are widely used in the constructionindustry, the European standard for cement manufacture, EN 197, specifies atest procedure based on mortar prisms, not concrete cubes. For example, acement described as conforming to EN 197-1 CEM I 42.5 N would be expected toachieve at least 42.5 MPa at 28 days using the specified mortar prism test.Whether 'real concrete' made from that cement will achieve 42.5 MPa in concretecube tests depends on a range of other factors in addition to any intrinsicproperties of the cement.

Factors affectingconcrete strength

There are many relevant factors; someof the more important follow:

Concrete porosity: voids in concrete can be filled with air or withwater. Air voids are an obvious and easily-visible example of pores inconcrete. Broadly speaking, the more porous the concrete, the weaker it willbe. Probably the most important source of porosity in concrete is the ratio ofwater to cement in the mix, known as the 'water to cement' ratio. Thisparameter is so important it will be discussed separately below.

Water/cement ratio: this is defined as the mass of water divided bythe mass of cement in a mix. For example, a concrete mix containing 400 kgcement and 240 litres (=240 kg) of water will have a water/cement ratio of240/400=0.6. The water/cement ratio may be abbreviated to 'w/c ratio' or just'w/c'. In mixes where the w/c is greater than approximately 0.4, all the cementcan, in theory, react with water to form cement hydration products. At higherw/c ratios it follows that the space occupied by the additional water abovew/c=0.4 will remain as pore space filled with water, or with air if theconcrete dries out.

Consequently, as the w/c ratio increases, the porosity of the cement paste inthe concrete also increases. As the porosity increases, the compressivestrength of the concrete will decrease.

Soundness of aggregate: it will be obvious that if the aggregate inconcrete is weak, the concrete will also be weak. Inherently weak rocks, suchas chalk, are clearly unsuitable for use as aggregate.

Aggregate-paste bond: the integrity of the bond between the pasteand the aggregate is critical. If there is no bond, the aggregate effectivelyrepresents a void; as discussed above, voids are a source of weakness inconcrete.

Cement-related parameters: many parameters relating to thecomposition of the individual cement minerals and their proportions in thecement can affect the rate of strength growth and the final strength achieved.These include:

• alite content
• alite and belite reactivity
• cement sulfate content

Since alite is the most reactivecement mineral that contributes significantly to concrete strength, more aliteshould give better early strengths ('early' in this context means up to about 7days). However, this statement needs to be heavily qualified as much depends onburning conditions in the kiln. It is possible that lighter burning of aparticular clinker could result in higher early strength due the formation ofmore reactive alite, even if there is a little less of it. Not all alite iscreated equal!

For a particular cement, there will be what is called an 'optimum sulfatecontent,' or 'optimum gypsum content.' Sulfate in cement, both the clinkersulfate and added gypsum, retards the hydration of the aluminate phase. Ifthere is insufficient sulfate, a flash set may occur; conversely, too muchsulfate can cause false-setting.

A balance is therefore required between the ability of the main clinkerminerals, particularly the aluminate phase, to react with sulfates in the earlystages after mixing and the ability of the cement to supply the sulfate. Theoptimum sulfate content will be affected by many factors, including aluminatecontent, aluminate crystal size, aluminate reactivity, solubilities of thedifferent sources of sulfate, sulfate particle sizes and whether admixtures areused.

If this were not complicated enough, the amount of sulfate necessary tooptimize one property, strength for example, may not be the same as thatrequired to optimize other properties such as drying shrinkage. Concrete andmortar may also have different optimum sulfate contents.

This fascinating area is discussedfurther under ' cement-relatedconcrete strength variability .'

In addition to the compositional parameters considered above, physical parameters are also important, particularly cement surface area and particle size distribution.

The fineness to which the cement is ground will evidently affect the rate at which the cement hydrates and therefore the rate of strength growth; grinding the cement more finely will result in a faster reaction. If a cement manufacturer finds that his strengths are decreasing, often the first thing he will do to rectify the problem is to grind the cement more finely.

Fineness is often expressed in termsof total particle surface area, eg: 400 square metres per kilogram. However, ofas much, if not more, importance is the particle size distribution of thecement; relying simply on surface area measurements can be misleading. Someminerals, gypsum for example, can grind preferentially producing a cement witha high surface area. Such a cement may contain very finely-ground gypsum butalso relatively coarse clinker particles resulting in slower hydration.

A more detailed lookat concrete strength

We've just looked at some of the mainfactors that affect concrete strength. Of course, there are many more, somerelating to intrinsic problems with the cement, some of which are quite subtle.Others relate to how the cement is used, an obvious example being that there isinsufficient cement in the mix but there are many others that are rather lessobvious.

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