|[ Top ]||Contents Page|
The Idiots' Guide to Highways Maintenance
Copyright © 2000/14, C.J.Summers
THE BASICS OF CONCRETE
|What is concrete||Time taken to place concrete after batching|
|Constituents of concrete||Addition of water|
|Cement||Specifying concrete strength|
|Water||Measuring concrete strength|
|Additives||Sample Concrete Mixes, for guidance only|
WHAT IS CONCRETE
Concrete is the product of mixing, aggregate, cement and water.
The setting of concrete is a chemical reaction between the cement and the water, not a drying process.
This reaction is called hydration, it evolves heat as does any chemical reaction, and the process is irreversible.
There is an initial set when the concrete will cease to be liquid but have little strength (e.g. 6 to 24hrs. old), thereafter the concrete will gradually gain strength over time until it achieves the strength required.
Differing mix proportions and cement types will achieve required strengths in differing time spans.
CONSTITUENTS OF CONCRETE
Cement, Aggregate and Water, (and sometimes additives).
Aggregates are usually distinguished between fine and coarse aggregate.
Aggregates are classed as inert materials, such as washed natural sand (fine); and natural gravel, which can be crushed to produce the appropriate size and grading of aggregate, and similarly crushed, quarried stone (coarse).
The aggregate must have a minimum inherent strength requirement for structural concrete, the coarse aggregate must not be weaker than the concrete paste.
All aggregate must be "clean", i.e. not contaminated with organic matter or clay/silty soils and overburden during extraction and storage.
Basically a material made by heating limestone and a suitable clay to produce a clinker rich in calcium silicates.
This clinker is ground to produce a fine powder, this is cement.
By using different clinkers, grinding them to differing degrees of fineness and the use of additives many different types of cement are produced with varied properties in their use, e.g. rapid hardening cement, sulphate resisting cement, etc..
Generally speaking the more cement in a mix the stronger more durable the concrete produced will be, but this does have to be related to other factors, primarily the amount of water used in the mix, i.e. water/cement ratio.
Water is an extremely important part of concrete, and drinking quality water is usually required, or water from an approved source free from impurities.
The most commonly used additive is a "foaming" agent to produce air entrained concrete, mainly for carriageway concrete, but also other exposed situations.
Another common use of an additive is to increase the workability of concrete without adding extra water and thus increasing the water/cement ratio and decreasing the strength of the concrete.
TIME TAKEN TO PLACE
CONCRETE AFTER BATCHING
From the time of adding water to the cement the chemical reaction has begun and you only have a limited amount of time to place and compact the concrete, this is usually specified as 90 minutes.
The delivery ticket of the load of concrete will be stamped with the time of batching.
ADDITION OF WATER
Given a set amount of cement and aggregate there is an optimum amount of water to be added to produce a chemical reaction to give the maximum obtainable strength, too little or too much water will produce a weaker concrete.
Unfortunately as in all things, life is not that simple, and the workability of the concrete has to be considered when placing concrete, especially in difficult situations.
These situations can be areas of high density of reinforcing bars, complicated formwork design, or where the concrete needs to be suitable for pumping.
In these situations water content is increased to make the concrete more workable, BUT this increase in water content is calculated at the design stage and the cement content is increased accordingly to retain the strength of the mix.
For every designed concrete mix with a specified strength there is a set WATER:CEMENT RATIO which must be retained in order to achieve the designed strength.
SHOULD NEVER BE ADDED TO CONCRETE, ESPECIALLY ON SITE, WITHOUT
THE CONSIDERATION OF ALL THE ABOVE POINTS,
An on site simple test for determining workability is the SLUMP TEST.
This consists of a conical mould 300mm. high, with an opening at the top of 100mm. diam., and at the bottom of 200mm. diam..
The mould is filled with concrete in 4 layers and rodded to remove air voids, with the smaller orifice uppermost.
The "slump" is the difference in height between the height of the mould and the height of the concrete column with the mould removed.
The workability of the concrete will depend upon the situation into which the concrete is being placed.
Low workability, i.e. stiff concrete, is needed for carriageway concrete which is laid by a "paving train".
High workability concrete is needed in situations of high density of reinforcing steel to enable the concrete to flow around all the reinforcing without leaving any voids.
The strength/grade of concrete is specified and measured in newtons/sq. mm., meganewtons/sq. metre or even megapascals, in fact the numerical figure will be the same in each case.
E.g. a strength of 20 newtons/sq.mm. is the same as 20 meganewtons/sq.metre.
The strength/grade of concrete is normally specified by stating the strength you wish the concrete to achieve after a period of 28 days.
The specifications governing the design, use and testing of concrete have undergone tremendous changes in the lat few years, I will not go into this topic on this page other than to say you may like to be aware of the introduction of,
BS 8500-1:2002:Concrete - Complementary British Standard to BS EN 206-1
Part 1 : Method of specifying and guidance for the specifier
BS 8500-2:2002:Concrete - Complementary British Standard to BS EN 206-1
Part 2 : Specification for constituent materials and concrete
These are British Standards that have been published to help you understand, the current standard for concrete, which is,
BS EN 206 - 1 : Concrete : Part 1 : Specification, performance, production and conformity
and it is likely that you will need the help of BS 8500, even then it may prove difficult to understand BS EN 206. It is not a "user friendly" document.
MEASURING CONCRETE STRENGTH
The strength is measured by crushing concrete cubes to failure and recording this strength.
Concrete cubes are made from fresh concrete sampled at the time of pouring by placing correctly sampled concrete into a steel mould and compacting to remove air voids.
The concrete is allowed an initial "set" period of 24 hours, the mould is then stripped and the cube is cured in water at a temperature of 20 deg.c for 28 days prior to crushing.
If you wish to strike shuttering before 28 days, extra cubes will be required to determine that the in-situ concrete has achieved the appropriate strength at the time you wish to strike the shuttering.
This is usually an arrangement agreed by the contractor, the concrete supplier and the engineer.
CONCRETE MIXES, FOR GUIDANCE ONLY
(but you will be able to understand them)
Below are a number of different types of concrete mixes showing batch weights and cement contents when a particular source of clean crushed river gravel and sand where used, so they are only an indication of concrete composition and will not be suitable for all aggregates / sand / cement.
I have included this item because of what I believe is the lack of basic concrete information that is readily available to young engineers and engineering technicians wishing to improve their working knowledge of concrete.
These mixes are in fact concrete mixes produced to comply with the Specification for Road and Bridge works of 1963, i.e. still current in 1969 to 1972 while I was working on the M6 Link for Owen Williams and Partners.
Back in this period, when I was beginning my career in highways materials, specifications and British Standards were written in such a way that basic information regarding concrete could be obtained from them, and they were my original learning tool.
It is my opinion that this is no longer the situation with current specifications and standards hence the publication of BS 8500, Parts 1 & 2, and I would not regard these as particular user friendly for a "beginner".
Basic concrete mixes are basic concrete mixes, and these are examples of basic concrete mixes.
I know things have moved on, and there are now many types of cement, admixtures and fibres that produce enhanced concrete for particular uses, and that you do need to take into account the density of the aggregates, and the workability of the produced concrete, the source/chemical composition of the aggregate, etc., etc..
But remember this is "The Idiot's Guide to Highways Maintenance", and this is some basic information to assist those who want to know a little bit more about concrete for basic on site uses, and possibly minor structures.
For most uses you will be buying your ready mixed concrete from an established supplier to an appropriate, modern, specification, but it does not hurt to have some knowledge of what you are buying and how it was produced.
I have used the original broad descriptions (e.g. A¾) used in this specification to describe the types of concrete, each type having a particular use with regard to required strength, workability and cost to produce.
These mixes were used in the Longford Viaduct and the Bedworth Viaduct, and the many other large structures on Contract 11, and the last time I looked they were all still standing, so I have confidence to reproduce them here.
NOTE : I have quoted the quantities in LBS. and GALLONS (the original measures) as well as converting them to metric quantities, and S.I. units for strength, although I remain with meganewtons not megapascals, they are the same figures.
If you do wish to use these quantities as a basis for determining your own batching weights, you MUST note that these figures will only give you a cubic yard of mixed concrete NOT a cubic metre, and if the density of your aggregates are different to the aggregates used in these mixes you may get slightly less or slightly more than a cubic yard.
To save you looking it up, 1 cubic yard of concrete = 0.7646 cubic metres.
As with any new concrete mix you must take cubes and crush them at appropriate dates to determine the real strength of the concrete you have designed / produced, only then can you consider using it in the works /structure, and you should expect to attain the Preliminary strengths in your trials to ensure the Works strength for normal production.
The cement used in these "standard" mixes was OPC (Ordinary Portland Cement), and the strengths quoted are for 150mm. (6 inch) cubes crushed at 28 days.
And do not forget to weigh your cubes and determine the density, as density is a good indication of a well designed concrete, and if you have crushed as many concrete cubes as I have, you will have noted that for each "mix", cube making and curing being constant, the cubes with the highest density will give the highest strengths.
Surface Dry Batch Weights for a number of Classes of Concrete
|Concrete Class||Strength (Mn - lbs/in²)||Cement (kgs/lbs)||Sand (kgs/lbs)||Aggregate||Agg. /Cement Ratio||Water/Cement Ratio|
|Preliminary||Works||20mm.- 5mm.||40mm.- 20mm.|
|A¾||38.5 / 5600||29.0 / 4200||327 / 720||408 / 900||975 / 2150||-||4.25||0.52|
|B¾||34.5 / 5000||26.0 / 3750||281 / 620||458 / 1010||975 / 2150||-||5.1||0.49|
|C¾||27.5 / 4000||20.5 / 3000||218 / 480||517 / 1140||975 / 2150||-||6.85||0.62|
|E1½||N/A||N/A||168 / 370||472 / 1040||454 / 1000||630 / 1390||9.3||0.78|
|Y¾||52.0 / 7500||41.5 / 6000||340 / 750||395 / 870||975 / 2150||-||4.1||0.51|
|Y⅜||52.0 / 7500||41.5 / 6000||371 / 820||1279 / 2820||840 / 1850||-||5.7||0.45|
The amount of "free" water added to the above aggregate is based on the water cement ration and the moisture contents of the aggregates, especially the sand.
That is why batching weights are initially given as "surface dry", you then determine the total amount of "free" water by applying the water cement ration to the quantity of cement.
E.g. the water cement ratio for the concrete mix below is 0.45,
water = 0.45
Therefore:- weight of water = weight of cement x 0.45 = 371 x 0.45 kgs. = 167kgs./litres (near enough for the purposes of demonstration)
To better indicate this I include below a reproduction of a batching chart for Y⅜ concrete used at a concrete batching plant, i.e. as the moisture content of the aggregate, usually the sand, increases the added water decreases.
However at times of heavy rainfall and with smaller aggregate the coarse aggregate can retain sufficient water that it needs to be taken into account, especially with high strength concrete.
This indicates the importance of knowing the moisture content of the sand in the stockpiles, and often in the different levels of the stockpile.
|Sand Moisture Content %||10mm. Crushed Gravel||Sand||Added "free" Water||Cement|
|0||840kgs. / 1850lbs.||1271kgs. / 2801lbs.||168litres / 36.5galls||371kgs. / 820lbs.|
|1||"||1284kgs. / 2830lbs.||155litres / 34.0galls.||"|
|2||"||1297kgs. / 2860lbs.||141litres / 31.0galls.||"|
|3||"||1309kgs. / 2885lbs.||130litres / 28.5galls.||"|
|4||"||1320kgs. / 2910lbs||118litres / 26.0galls.||"|
|5||"||1334kgs. / 2940lbs.||105litres / 23.0galls.||"|
|6||"||1347kgs. / 2970lbs.||91litres / 20.0galls.||"|
|7||"||1359kgs. / 2995lbs.||77litres / 17.0galls.||"|
|8||"||1372kgs. /3025lbs.||66litres / 14.5galls.||"|
|9||"||1383kgs. / 3050lbs.||52litres / 11.5galls.||"|
|10||"||1397kgs. / 3080lbs.||40litres / 9.0galls.||"|
For information on a range of items, publications and specifications relating to concrete, its production and testing, press HERE
I try to refer to as few commercial sites as possible in compiling my site, but when a site offers particularly useful information about a subject I make an exception.
For further information on the cement production, types of cement, and concrete production, press, HERE and, HERE
For information on concrete road pavements, press HERE
[ Top of Page ]