This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.
Initially I will be talking about the process that we will be going about in order to get the project of our car park complete to construction. For our car park we have all discussed it as a group and decided to go for a Hybrid construction process for many different reasons which I will be talking about further in this report. I will also be talking about the concrete that is going to be used in the construction of the car park and also that during the construction of this project we will be meeting British standards and also that make sure our design of the car park is realistic.
Hybrid Construction: we chose Hybrid construction because a major factor involved with this type of construction is that it would save us a lot of time and also it would be very cost effective as hybrid construction combines all the benefits of precasting (eg. quality, form, finish, colour, speed, accuracy, prestressing), with all the benefits of in-situ construction (e.g. economy, flexibility, mould ability, thermal mass, continuity, durability and robustness). It also provides the benefits of precast concrete quality combined with the structural stability of an in-situ concrete frame. It has proved to be a practicable alternative to conventional construction and more flexible in design than alternatives.
The main materials that are going to be used in the construction of the car park are going to be re-in forced concrete and steel. To meet British standards there are going to be a number of tests carried out on the concrete before being used on site. We will be talking about British standards further in our report and also how British standards came in play during the construction of our project. We will talk about the testing that is going to be carried out on each specimen before it is bought to the construction site and also the tests that may have to be carried out on site to ensure quality control and another major factor which is safety.
We will show tables where the group feel is necessary and also will show 3D illustrations of what out car park may look like but at the same time making sure we show all the specifications from the British standards that may be needed. We have ensured that this report is done as a group and each group member will contribute to the report equally and all input from each group member will be taken into consideration.
The Piled Foundations
The integrity of the pile shaft is of paramount importance, and the concreting mixes and methods that have evolved for bored piles are directed towards this, as opposed to the high-strength concrete necessary for pre-cast piles or structural work above ground.
This prerequisite has led to the absorption of highly workable mixes, and the total collapse mix for tremie piles has been mentioned. In order to ensure that the concrete flows between the reinforcing bars with ease, and into the interstices of the soil, a high-slump, self-compacting mix is called for. A minimum cement content of 300 kg/m3 is generally employed, increasing to 400kg/m3 at slumps greater than 150mm, with a corresponding increase in fine aggregate content to maintain the cohesion of the mix.
Generally, Conventional structural grade concrete with 28-day strengths ranging from 4000 to 7000 lb/in2(27.6 to 48.3 MPa) is used for cast-in-place concrete piles. Some special types of piles require special concrete or grout mixes. For filling normal-size pile shells, especially under difficult placement conditions, a reduced-coarse-aggregate concrete is often specified. Difficult placement conditions include piles longer than 15m, piles driven on a batter steeper than 1:4, and pile shells containing heavy reinforcing cages. A typical reduced-coarse aggregate mix includes 800 lb of coarse aggregate per cubic yard (475kg/m3) with a corresponding increase in the sand and cement content.
Materials including cement, sand, coarse aggregate, water, and admixtures, should be inspected for conformity with the specifications and accepted practice.
Cement: Type IV cement should not be used for pile concrete. Type III or high-early cement may be permitted for cast-in-place concrete test piles to get a fast strength gain. Types II or V may be specified for sulphate exposure.
Cement remaining in bulk storage for more than 6 months or cement stored in bags for a period longer than 3 months should be retested before it is used to ensure that it meets the requirements.
Cement should be inspected for contamination by lumps caused by moisture. Cement bags should be inspected for rips, punctures or other defects. If cement is to be batched by bag, the weights of the bags should be spot-checked and should not vary by more than 3 percent.
Sand: Sand should be clean, sharp and well graded-free of silt, clay or organic material. The specific gravity and/or fineness may be specified for special mixes such as reduced coarse aggregate concrete.
Coarse Aggregate: The specifications may permit gravel or crushed stone. The use of crushed-rock-aggregate requires more cement and sand for comparable workability. Lightweight aggregates are not recommended, and slag aggregates generally are not used. Alkali-reactive aggregates or aggregates from shells, friable sandstone, clay or micaceous rock, or cherts should not be permitted. Aggregates should be uncoated and free of silt, clay, organic material, and chemical salts. The specific gravity of the coarse aggregate may be specified. Aggregates should be well graded, with a maximum size of 20mm and the amount of undersize aggregates (5mm) should be held uniform and within 3 percent.
Wet Concrete - Requirements
Most concrete designs for in-situ foundations aim to produce fresh concrete with the following characteristics:
- Flow able
- Compacting under its own weight (particularly for deep foundations)
- Minimal tendency for segregation and bleeding
- Able to be placed so that a continuous monolithic concrete is formed
In order to achieve the high workability and cohesion that are very frequently required, the total mass of fine material in concrete is very important in these concretes. BS EN 1538 suggests that this should be 400-500 kg/m3. Fine material is defined as less than or equal to 2-63 micrometers, which includes cement, pfa and ggbs as well as the fine end of natural sand. The standard allows a maximum water/cement ratio to 0.60 and allows water-reducing, plasticizing and superplasticising admixtures to be used to control bleeding or segregation.
To produce flow able concrete suitable for tremie placement and requiring no subsequent compaction, the following design guidelines are useful:
- Design target slumps of 150-225 mm
- Design maximum free w/c ratio of 0.50
- Use coarse aggregate/sand ratio of 1.0 to 1.2
- Use of natural rounded gravel aggregate
- Use of composite cements
- Use of water-reducing, plasticizing or super plasticizing admixtures.
For shallow in-situ concrete foundation construction, usually low-slump concrete is preferable - e.g., in massive pours built up in layers. The performance requirements are also typically less onerous, since the nature of the elements, their positioning, geometry and orientation, are generally less problematic with respect to concrete placement. The best and most common method of compacting shallow foundation concrete is probably to use internal poker vibrators positioned with appropriate spacing and working in uncompacted concrete no deeper than the length of the metal casing of the vibrator.
Specification and Measurement of fresh properties
There are several methods available to measure the workability of concrete. These include the slump test (BS 1881: Part 102), the compacting factor test (BS 1881: Part 103) and the flow test (BS 1881: Part 105). Although the slump test is a simple and useful test for checking uniformity of fresh concrete, the high tolerances associated with the results can be unacceptable, particularly where highly workable, flowing in-situ concretes for tremie placements are concerned. Additionally the slump test measures the ‘yield' of the concrete rather than its flowability - which is frequently the most important characteristic of such concretes. The flow test, which provides a direct measure of the mobility of the fresh concrete - is considered a useful method for measurement and specification of workability for in-situ foundation concrete.
The fresh properties for Self Compacting Concrete (SCC) that have to be specified and measured put it well outside the scope of current tests for workability. There are no established reliable texts that can quantitatively assess parameters such as segregation resistance (stability) and passing ability (resistance to blocking) of fresh SCC mixes.
The most common method for assessing workability is the slump flow test in which the spread of the sample contained within a standard slump cone is measured instead of the slump proper. The time taken for the concrete to spread 500mm may also be determined.
Unfortunately, the test is useful only for operatives with a substantial experience of the behaviour of SCC mixtures - the numerical results alone are of little practical value. Again it is important to discuss methods of measuring workability and the acceptable tolerances with the contractor and concrete supplier.
The period over which the concrete retains suitable workability - important in many foundation applications particularly for avoiding cold-joints in large pours - may also be specified. Workable time may be modified by the incorporation of admixtures.
Cohesiveness, ie resistance to bleeding and segregation, is an important characteristic in high workability concretes. However, it is generally covered in specifications by a simple blanket-statement and, where in-situ foundation applications and concretes are concerned, more detailed specification may be necessary.
Prior to placing it is difficult to asses the cohesivity of fresh concrete and its tendency to segregate. At this time there is no accepted and satisfactory method of testing for, or specifying, this property which is generally assessed by qualitative judgement. An indication may be obtained from careful observation of the flow-test. The risk of segregation during placement may be reduced by suitable handling and placement techniques. Until satisfactory tests are developed it may be prudent to undertake site trials to establish the tendency for concretes to segregate during placing.
Where necessary, the tendency for concrete to bleed may be measured. ASTM C232 (1992) is a test for accessing bleed by measuring the proportion of bleed water produced by a small sample from a test concrete. This test is however impractical for most site conditions.
A European test (prEN480-4) is currently in preparation and, although this test is intended for use in the assessment of admixtures for concrete, it is equally applicable to concrete not containing admixtures.
Self Compacting Concrete (SCC)
In recent years concrete technology has led to the development of concretes that combine additions and admixtures to produce a variety of ‘high-performance concretes'. Such concretes have been designed specifically to exhibit high performance in one or more areas. For example, high workability, low heat, high strength and high durability. SCC is designed to achieve full compaction under its own weight without additional vibration.
As it is a relatively new technology there is not very much information out there regarding its use, it has been successfully implemented in Japan and used for many projects, but in the UK it is relatively new.
SCC can be readily manufactured from conventional concreting materials, but requires some changes to mix proportioning to achieve the required properties. The key properties of SCC are an adequate rate of flow under self-weight no segregation during flow and no blocking when passing through reinforcement. The first requires a low yield stress, without excessive plastic viscosity, while the second requires a reasonably high viscosity. Optimisation of this property is necessary. The no-blocking condition requires sufficient volume of the mortar fraction to lubricate the coarse aggregate hence there is a need for high mortar content.
The principle differences between SCC and conventional concrete are:
- Lower coarse aggregate content, typically 750 to 900kg/m3
- Lower sand content in the mortar, typically 40-50 percent by volume
- Restriction of the water content to between 45-55 percent by volume of the paste.
At current there is no agreed method to measure practically properties such as flow and resistance to segregation. There are several techniques available but all require specialist assistance to interpret the results so the currently best and universally best test available to judge the workability of SCC is the slump flow test.
Vebe Test: BS EN 12350-3:2000
This test determines the strength of fresh concrete by means of the Vebe time. If the maximum size of aggregate exceeds 63mm this test is not applicable. If the Vebe time is less than 5s or more than 30s, the concrete has a consistency for which the Vebe test is unsuitable.
The fresh concrete is compacted into a slump mould, the mould is lifted clear of the concrete and a transparent disc is swung over the top of the concrete and carefully lowered until it comes in contact with the concrete. The slump of the concrete is recorded. The vibrating table is started and the time taken for the lower surface of the transparent disc to be fully in contact with the grout (the Vebe time) is measured.
Place the Vebe meter on a rigid horizontal base, ensuring that the container is firmly fixed to the vibrating table by means of the wing nuts. Dampen the mould and place it in the container. Swing the funnel into position over the mould and lower into the mould. Tighten the screw so that the mould cannot rise from the bottom of the container.
From the sample of concrete obtained, the mould should be filled in 3-layers, each approximately one-third of the height of the mould when compacted. Compact each layer with 25 strokes of the compacting rod, ensuring that the strokes are uniformly distributed over the cross-section of each layer. Compact the concrete throughout its whole depth, taking care not to strike the base. Compact the second layer and the top layer throughout its depth, so that the strokes just penetrate into the underlying layer. In filling and tamping the top layer, heap the concrete above the mould before tamping is started. If necessary, add further concrete to maintain an excess above the top of the mould throughout the compacting operation.
After the top layer has been tamped, loosen the screw and raise the funnel and swing it out of the way and tighten the screw in the new position. Ensure that the mould does not rise or move prematurely and concrete is not allowed to fall into the container.
Strike off the concrete level with the top of the mould with a sawing and a rolling motion of the compacting rod. Remove the mould from the concrete by raising it carefully in a vertical direction, using the handles. Perform the operation of raising the moulds in 5s to 10s by a steady upward lift with no lateral or torsional motion being imparted to the concrete.
If the concrete shears, collapses, or slumps to the extent that it touches the wall of the container, record the fact.
If the concrete has not slumped into contact with the wall of the container, and a slump has been obtained, record the fact.
Swing the transparent disc over the top of the concrete, loosen the screw and lower the disc very carefully until it just comes into contact with the concrete. Provided there has been a true slump, when the disc just touches the highest point of the concrete, tighten the screw. Read and record the value of the slump from the scale. Loosen the screw to allow the disc to easily slide down into the container to rest fully on the concrete.
If there has not been a true slump, ensure that the screw is loosened to allow the disc to slide down into the container to rest on the concrete.
Start the vibration of the table and the timer simultaneously. Observe the way the concrete is remoulded through the transparent disc. As soon as the lower surface of the disc is fully in contact with the cement grout, stop the timer and switch of the vibrating table. Record the time taken to the nearest second.
Carry out the entire operation, from the start of the filling, without interruption, and complete within 5 minutes.
It is important to note that the consistence of a concrete mix changes with time, due to hydration of the cement and, possibly, loss of moisture. Tests on different samples should, therefore be carried out at a constant time interval after mixing; if strictly comparable results are to be obtained.
The time read from the stopwatch, to the nearest second should be recorded. This is the Vebe time, expressing the consistency of the mix under test.
Concrete develops strength by forming hydrates and the hydration will continue for many years provided there is water available for hydration and there is cement/additions available to react. The rate and magnitude of concrete strength development depends on:
- The basis for comparison
- The temperature and temperature history
- Cement type, class and source
- The type, source and amount of addition
- The water/cement or water/binder ratio
- The consistence
Hydration of cement is an exothermic reaction and in large foundation structures the heat dissipation is low and the temperature within the section can rise significantly. If not controlled this may have undesirable circumstances, most notably thermal cracking. High temperatures produced during early hydration may also reduce the ultimate strength.
In-situ concrete foundations can wither be cured below ground or in conventional formwork. There are several factors that may affect the properties of concrete that has been cured below ground, which include rate of heat dissipation, bentonite effects, moisture migration between soil and concrete.
Concrete placed below ground may develop strength at a faster rate than the same concrete placed above ground. Strength development characteristics are affected by curing temperature, especially at an early age, so the increase in strength gain may reflect the generally higher temperatures developed in foundations.
Testing - Compression Test
The most common test performed for concrete is for compressive strength. There are several reasons for this, (i) it is assumed that the most important properties of concrete as directly related to compressive strength; (ii) concrete has little tensile strength and is used primarily in compression; (iii) Structural design codes are based on compressive strengths; (iv) the test is relatively simple and inexpensive to perform.
This uses a 150mm cubic mould, which is filled in three layers, rodded 35 times with a 25mm square rod or compacted with a vibrator. The cube is tested at right angles to the position casted and therefore requires no capping or grinding. The loading rate is 33lb/in2/s.
Compression tests assume a pure state of uniaxial loading; however this is not the case, because of friction forces between the load plates and the specimen surface. The affect is to restrain the specimen from expanding. As specimen length to diameter ratio decreases the end effects are more important resulting in higher apparent compressive strengths. The use of rubber or lubricant between the specimen and the loading plate can induce lateral tensile load at the end of the specimen. This will induce vertical splitting and reduce apparent strength.
A hard or stiff plate will concentrate stress at the outer edges whereas a softer plate will have higher stress at the centre. These same concepts of hard and soft are applicable to the testing machines themselves. A soft machine will release the stored energy of its deformation to the specimen as it fails whereas a hard machine will not.
As l/d decreases below a value of 2 the strength increases. At ratios above 2 the effect is more dramatic. Also, this phenomena is significant in high-strength cement.
Specimen size is important for the simple fact that as the specimens become larger it is more likely to contain an element that will fail at a low load..
Rate of loading as discussed above is quite important to the test compressive strength. In general, the higher the loading rate the higher the measured strength. The reasons for this are not completely clear, however, it is thought that under slow loading rates more sub critical cracking may occur or that slow loading allows more creep to occur which increase the amount of strain at a given load.
Most concrete specimens are tested in a saturated state. Concrete that has been dried shows an increase in strength, probably do to the lack of lubricating effect moisture has on the concrete particles. Higher temperatures at the time of testing will lower the apparent strength of the concrete.
There is as yet no standard test for directly determining tensile strength. However there are two common methods for estimating tensile strength through indirect tensile tests. The first is the splitting test carried out on a standard cylinder specimen by applying a line load along the vertical diameter. It is not practical to apply the true line load to the cylinder because the side are not smooth enough and because it would induce high compressive stresses at the surface. Therefore, a narrow loading strip made of soft material is used.
Another way of estimating tensile strength is the flexural test. A specimen beam 6 x 6 x 20 inches is moulds in two equal layers each rodded 60 times, once for each 2 in2 of top surface area. The beam may be vibrated and should be cured in the standard way. This test tends to overestimate the true tensile strength by about 50%. This can be explained by the fact that the simple flexural formula used is based on a linear stress-strain distribution whereas concrete has a nonlinear distribution. This is an important test because it model how a concrete beam is normally loaded.
Non-destructive Quality Test
These tests are useful to: (i) quality control; (ii) determination of the time for form removal; and (iii) help assess the soundness of existing concrete structures.
Surface Hardness Methods -- One of the oldest non-destructive tests, developed in Germany in the 1930's. Basically, the surface is impacted with a mass and the size of the resulting indention is measured. The accuracy of these types of tests is only 20 to 30%.
Rebound Hardness -- The most common non-destructive test is the rebound test. The test measures the rebound of a hardened steel hammer impacted on the concrete by a spring. This method has the same limitations as the surface hardness tests. The results are affected by: (1) surface finish; (2) moisture content; (3) temperature; (4) rigidity of the member being tested; (5) carbonation of the surface; and (6) direction of impact (upward, downward, horizontal). Most useful in checking the uniformity of concrete.
Pull-Out Test -- Pull-out test determine the force required to pull a steel insert out of concrete which it was embedded during casting. This test is a measure of the shear strength of the concrete which can be correlated with compressive strength. This test is better than those previously discussed, however, the test may be planned in advance and the assembly embedded in the concrete during casting.
Precast, Prestressed Floor Slabs.
In this part of the report we will be talking about the components that will be manufactured under factory conditions and the test that will be carried out on these components before they are bought on site to where the project is being built. I will be providing specification of the concrete that is going to be used with reference to British standards.
Cement Type: For this particular project we will we showing a specification of the cement to be used on this type of project which will have to comply with British standards. According to British Standards (BS EN 197-1:2000) two types of blast furnace cements BIIIA and BIIIB should be used. I will be specifying requirements for the manufacture, marking and provision of information. I will be talking about the standard strength of cement at a period of 28 days (compressive strength) and also the compressive strength of the blast furnace cement at either 2 or 7 days.
The table above shows the compressive strength of the cement being used they differ in their classes of early strength. The initial setting time shall not be less than 60 min when tested in accordance with BS EN 196-3. There are test that are carried out to determine the compressive and flexural strength of cement. I will be talking about the methods used in the determination of the compressive strength of the cement to be used in the construction of the project. The method comprises the determination of the compressive, and optionally the flexural, strength of:
Prismatic test specimens 40 mm Ã- 40 mm Ã- 160 mm in size.
These specimens are cast from a batch of plastic mortar containing one part by mass of cement, three parts by mass of CEN Standard sand and one half part of water (water/cement ratio 0,50). CEN Standard sands from various sources and countries may be used provided that they have been shown to give cement strength results which do not differ significantly from those obtained using other CEN standard sand. Mortar is prepared by mechanical mixing and is compacted in a mould using a jolting apparatus other compaction methods can be used but according to that they do not show different results of cement strength in comparison with the results obtained using the reference jolting apparatus and procedure.
The cement to be tested shall be exposed to ambient air for the minimum time possible. When it is to be kept for more than 24 h between sampling and testing, it shall be stored in completely filled and airtight containers made from a material which does not react with cement. Carry out demoulding taking care not to damage the specimens. Plastics or rubber hammers, or devices specially made, can be used for demoulding. Carry out demoulding, for 24 h tests, not more than 20 min before the specimens are tested. Carry out demoulding, for tests at ages greater than 24 h, between 20 h and 24 h after moulding. These testing procedures have to be done so that they comply with British standard number BS EN 196-1 2005.
The next stage of the testing is to cure the moulds under water. The moulds are then placed submerged fully under water they have to be kept apart so that all six sides of the mould are exposed to the water. The depth of water above the upper faces of the specimens is less than 5 mm in accordance with British standards. The Specimens can be kept under water until they are going to be tested the age of the mould have to be noted before testing is carried out. Carry out strength tests at the
Different ages within the following limits that are going to be listed in the table below.
24 h Â± 15 min
48 h Â± 30 min
72 h Â± 45 min
7 d Â± 2 h
= 28 d Â± 8 h.
Table showing the time limit at which the moulds can be tested in comparison with the age of the mould.
The next stage of testing is to carry out the compressive strength test on the specimens. Each half of the specimen is tested for compressive strength and for flexural strength. The specimens are placed in the testing equipment centre the prism halves laterally to the platens of the machine within Â± 0,5 mm, and longitudinally such that the end face of the prism overhangs the platens or auxiliary plates by about 10 mm. Increase the load smoothly at the rate of (2 400 Â± 200) N/s over the entire load application until fracture.
Rc= Fc / 1600
Rc is the compressive strength, in megapascals;
Fc is the maximum load at fracture, in Newton's;
1 600 is the area of the platens or auxiliary plates (40 mm Ã- 40 mm), in square millimetres.
After the test are carried out the results for the flexural and compressive strength are to be expressed arithmetically with a mean of 6 individual results expressed at least to the nearest 0,1 MPa, obtained from the six determinations made on a set of three prisms. If one result within the six individual results varies by more than Â± 10 % from the mean, discard this result and calculate the arithmetic mean of the five remaining results. If one result within the five remaining results varies by more than Â± 10 % from their mean, discard the set of results and repeat the determination.
The tests for compressive strength can be repeated over a short term period for measure of the precision of the test method when used for validation testing of CEN Standard sand and alternative compaction equipment. Long term testing can also be used to determine precision of the test method when used for the auto control testing of cement or the monthly verification testing of CEN Standard sand and for assessing the maintenance of the laboratory's precision over time. Above I have talked about the tests that have to be carried out on cement moulds before the cement can be used on site. All the tests will be carried out under laboratory conditions and it is important that each test is carried out in agreement to British standards. During the construction of the project many different types of cement may be used I will be talking about briefly the types of cement and tests that will be carried out on the cement specimens.
Types of Portland Cement
Though all Portland cement is basically the same, eight types of cement are manufactured to meet different physical and chemical requirements for specific applications:
Type I- is a general purpose Portland cement suitable for most uses.
Type II- is used for structures in water or soil containing moderate amounts of sulphate, or when heat build-up is a concern.
Type III- provides high strength at an early state, usually in a week or less.
Type IV- moderate heat generated by hydration that is used for massive concrete structures such as dams.
Type V or Sulphate Resistant Cement- is used in projects such as dams that are exposed to high amounts of sulphates. It is also used wherever there are constructions that are in direct contact with clay soil, which contain large amount of sulphate salt, such as foundations and pillars. This type of cement is very resistant to attack from sodium and magnesium sulphates found in ground water.
Types IA, IIA and IIIA- are cements used to make air-entrained concrete. They have the same properties as types I, II, and III, except that they have small quantities of air-entrained materials combined with them.
Types of Blended Cements
Blended cements are produced by intimately and uniformly intergrading or blending two or more types of fine materials. The primary materials are Portland cement, ground granulated blast furnace slag, fly ash, silica fume, calcined clay, other pozzolans, hydrated lime, and pre-blended combinations of these materials.
IS-Portland blast furnace slag cement
IP and Type P-Portland-pozzolan cement
Type I(PM)-Pozzolan-modified portland cement
Type I(SM)-Slag-modified portland cement
Types Blended Hydraulic cements
Type GU-blended hydraulic cement for general construction
Type HE-high-early-strength cement
Type MS-moderate sulphate resistant cement
Type HS-high sulphate resistant cement
Type MH-moderate heat of hydration cement
Type LH-low heat of hydration cement
Fly Ash- A by-product of the combustion of pulverized coal in electric power generating plants. Fly ash is used in about half of ready mixed concrete. It is commonly used as a partial substitute at 15 to 25% of Portland cement. Improvements to the properties of fresh concrete during placement include enhanced workability, reduced bleeding, and reduced slump loss. For hardened concrete, it can increase the long-term strength, improve the permeability, increase the durability, reduce the potential for sulphate attack, reduce the heat of hydration, and reduce the potential for alkali-silica reactivity. Adding the wrong type or amount of fly ash can be detrimental to the concrete.
Slag Cement- A by-product of the steel industry. It is formed during the liquification of iron in the blast-furnace. Slag cement is commonly used as a partial substitute for Portland cement at a replacement level of up to 50%. This generally improves workability, finish ability and pump ability of concrete during placement. In hardened concrete it can improve compressive and flexural strength, improve permeability, increase resistance to chloride intrusion and corrosion, mitigate severe sulphate attack, and reduce the potential for alkali-silica reactivity. It can also reduce thermal stress in mass concrete through lower heat generation.
Silica Fume- A by-product from the electric arc furnace used in the production of silicon or ferrosilicon alloy. Silica fume is commonly used as a partial substitute for Portland cement at replacement levels of 5 to 7%. This is used in applications where a high degree of impermeability is needed and in high-strength concrete, it can also increase the resistance of concrete to chloride penetration.
We have above shown the different gradings of cement that may be used in the construction of the project. They all have to be to the specification of British standards. They all vary in different compressive strengths, physical properties, chemical properties and other factors in the manufacture of cement.
Aggregate: we will be also showing specifications of the use of aggregate that is going to be used in the concrete that is gong to be used in the construction of the car park. The British standard BS882:1992 specifies the quality and grading requirements for aggregates obtained by processing natural materials for use in concrete. The main aggregate that will be used is coarse aggregate mainly retained on a 5.0 mm test sieve and containing no more finer material than is permitted for the various sizes in this specification. The test of the retained aggregate should be to BS410.
There are many different types of gravel that may be used which I will list below:
- Uncrushed Gravel
- Crushed gravel
- Partially crushed gravel
coarse aggregate resulting from the natural disintegration of rock
coarse aggregate produced by crushing gravel
coarse aggregate produced from a mixture of crushed and uncrushed gravel
Aggregate for concrete shall consist of any types of coarse aggregate and/or any types of sand or of all-in aggregate, aggregate supplied as a mixture of different sizes or types shall be proportioned and mixed in such a way as to ensure reasonable consistency. For the aggregate used there is a shell size that can not be exceeded I have shown a table below from BS882:1992:
Limits on shell content (%)
Fractions of 10 mm single size, or of graded or all-in aggregate that are 20 finer than 10 mm and coarser than 5 mm
Fractions of single sizes or of graded or all-in aggregate that are coarser 8 than 10 mm
Aggregates finer than 5 mm
The Coarse aggregate had to be to a specific size in agreement with British standard BS882:1992: after the sieve test can be carried out in accordance with British standard BS 812-103.1. We have shown a table below of the size limit of the coarse aggregate to be used.
Coarse Aggregate Table
As for the proposed project we will be using a heavy duty concrete floor finish the aggregate specification will be different to the aggregate specification of a normal concrete floor I will also show a table below that shows the overall limit of the size of the grading that can be used.
We have shown most of the different types of grading that can be used and the specification of the sizes of the aggregates that have to be used in different circumstances.
Another important factor is to test the aggregate for its strength this is an important aspect that has to be seen to before the use of any aggregate in any concrete mix. The size and the structural shape of the aggregate being used will effect the overall strength of the concrete the use of uniform sized aggregate will usually cause friction at a few points of contact this is mainly due to the uniform shape of them, they will also have poor interlocking between each other again the main reason will be due to the overall regular shape of them. Another factor that may arise in the use of uniform shaped aggregate is that they will have a high percentage of voids between each other.
The other type of aggregate that is used in the construction of the project is well graded aggregate as this aggregate is not one regular shaped but mostly irregular shaped there will be friction caused at many points of contact causing a reasonable increase in the strength of the mix. Also with the use of irregular shaped aggregate the interlocking between each of them will be strong and as each particle has a different size some being big in size and some small there will be very few voids. The other main aggregate used in fine aggregate this is a mixture of course and fine aggregate which has friction at many contact points and has very good interlocking between each other and the void space between them is almost to a minimal. Another important aspect which helps engineers is that fine aggregate produced economically.
Uniform size aggregateUniform Sized Aggregate
Well-gradedWell Graded Aggregate
Mixture of coarse aggregate and fine aggregate.
I have shown illustration of the aggregate that may be used in a construction project as we can see the use of the right aggregate plays a major role in the overall strength of the concrete mix. The strength of the aggregate increases with the maximum size of the aggregate and also well graded aggregates are much stronger that non graded aggregates, Strength is increased by rough particle surfaces due to the greater friction. And also we know that the shearing strength of the aggregate is increased by compaction, especially by vibration. An important characteristic of aggregates is that the strength created through interlocking is increased by the more flat, broken faces the particles have. Flat faces fit together with more contact and more compactly than if the particles are rounded, however this doesn't imply the particles should be flat, since flat particles result in a lack of strength.
We have talked about the main constituents of a concrete mix and shown the specifications that have to be met in agreement to British standards. We can see that both cement and aggregate play a major role in producing a good concrete mix. I will further talk about the tests that have to be carried out on both hard and wet concrete and also the workability of concrete.
Testing of concrete mixes: we know that it is important to carry out the test on any concrete mix before it can be used on site of even if it is going to be used for a pre cast production. We will show tests that have to be carried out both on wet concrete and hard concrete.
Wet/Fresh Concrete: A main test that should be carried out on any concrete mix is a slump test this test is used extensively in site work all over the world. The slump test mainly measures the consistency of the concrete mix it does not test for the workability of concrete, the slump test is very useful for identifying variations in the uniformity of a mix of given nominal properties. A slump test is a requirement that is listed in BS1881: Part 102:1983. To see the procedure of this test the user can refer to the British Standard BS1881: Part 102:1983. When the slump test is carried out the slump should be even all around as in any other slump test if this does not take place a sheer slump has taken place and the test would have to be repeated. We can see from slump tests that mixes of a high consistency will usually have a low slump or even a zero slump. Where as rich mixes may behave different and have a higher slump being sensitive to variations in workability. The frequency at which a slump test may be carried out on site is important to know hour to hour the variations in the materials that may be fed into the mixer. It is important to know the workability of concrete especially in a wet mix. The workability of any concrete mix will depend on the compaction of the mix and also the method of compaction. There are also many factors that may affect the workability of concrete in this case a main factor being the water/cement ratio but also if the water content and other mix proportions are fixed, workability is governed by the maximum size of the aggregate, its grading, shape and texture.
The higher the water/cement ratio the finer the grading required for the highest workability. We know that in a practical situation to predict the workability of a mix there are three factors we have to take into consideration water/cement ratio, aggregate/cement ratio and the water content out of the three only two are independent. For instance, if the aggregate/cement ratio is reduced but the water/cement ratio is to be kept constant the water content will increase and so will the workability. If we look at it in another instance if the water content is kept constant when the aggregate/cement ratio is reduced then the water/cement ratio will decrease but the workability of the mix will not be heavily affected. It has not yet been possible to find a test that will measure the workability of concrete.
Another test that is carried out on wet concrete is the compatibility test. In this test the degree of compaction called the compacting factor, is measured by the density ratio i.e. the ratio of the density achieved is the test to the density of the same concrete fully compacted. The compacting factor test is described and shows the method of the test in BS 1881: Part 103:1993 it is appropriate for concrete with a maximum aggregate size of 40mm. the apparatus mainly consists of two hoppers, each of them are in the shape of a frustum of a cone, and one cylinder, the three being above one another. The hoppers have a hinged door at the bottom.
Picture showing an illustration of the equipment used for the compact ability test.
This particular test is more sensitive at the low workability end of the scale rather than the high workability. As very dry mixes usually stick to the sides of the hopper and have to be poked down. We can see that for low workability mixes the actual amount of work required for full compaction depends on the richness of the mix while the compacting factor does not. The compacting factor test undoubtedly provides a good measure of workability.
Hard Concrete: There are different tests also carried out on hard concrete which have to take place before it can be put to use on any construction site. We will give details of the test that may be carried out and why they are carried out. The tests are made for different purposes but the main objectives of testing are quality control and compliance with specifications.
One of the main tests that are used for hard concrete is the compression test which is to test the compressive strength of the concrete mix. The strength test results may be affected by variations in the type of specimen being tested, the size of the specimen, and type of mould, rigidity of the testing machine and also the rate of application of stress. Compressive strength tests are treated in a standard manner which will usually include full compaction and wet curing for a specified period of time give results that show the potential quality of the concrete. The compression test can be carried out on specimens of both cubes and cylinders. The size of the moulds is usually 6inch cubes and if a cylinder 6inch in diameter, 12inches long. The procedure of the test can be accessed from BS 1881: Part 108:1983 and for the cubes in BS 1881: Part 110:1983.
We can see that each test has to be carried out to specification and has to agree with the British standards. If the specifications are not met the tests will have to be carried out again and also the appropriate test has to be carried out on both wet and hard concrete. For pre cast construction the tests will be mainly be carried out under laboratory conditions making sure that the test is successful and also making sure of quality control before any pre cast elements are bought onto a construction site. Making sure that the test is successful and also making sure of quality control before any pre cast elements are bought onto a construction site.
Post tension beams and associated columns
This section of the work I will design the cement to be used in the in-situ concrete which has a 28 day strength of 40N/mmÂ². I will break each of the components of the concrete mix down separately by providing a specification.
In the design specification for this project it says that the 28 day strength of the concrete must be 40N/mmÂ².I will research a couple of possibilities just in case one is not economically viable.
In accordance with BS4027, Sulphate- resisting Portland cement is an excellent choice because if you use 42.5R it has the 28 day strength requirement and its limit is well above so safety margins are easily met also it has an early strength of more than 20N/mmÂ² so work could progress upwards and onwards with very little delay, also the sulphate resisting characteristics of the cement will allow it to oppose the corrosiveness of the surrounding environment hence it will last longer and will require less maintenance. The cement clinker constituents would consist of no less than two thirds by mass of calcium silicates (CaO)3.SiO2 and (CaO)2.SiO2), the remainder containing aluminium oxide (Al2O3), iron oxide (Fe2O3) and other oxides. The ratio by mass (CaO)/(SiO2) shall be not less than 2.0. The content of magnesium oxide (MgO) shall not exceed 5.0 % (m/m). The percentage of tricalcium aluminate shall not exceed 3.5 when calculated by the formula (C3A) = 2.65A - 1.69F, where A is the proportion of aluminium oxide (Al2O3) by mass of the total cement when tested in accordance with 13.11 of BS EN 196: Part 2: 1995 (in %), and, F is the proportion of iron (III) oxide by mass of the total cement when tested in accordance with 13.10 of BS EN 196: Part 2: 1995 (in %). Although you could use 32.5N or 32.5R as there strength limit surpass the specification there is no harm in using and much stronger cement.
The constituents which differ it from the other cement are the proportion of reactive CaO which shall be less than 5 % (m/m). The reactive SiO2 content in siliceous fly ash conforming to this British Standard shall be not less than 25 % (m/m) and the loss on ignition shall not exceed 5.0 % (m/m). Minor additional constituent could be used which would be one or more of the following granulated blast furnace slag, natural pozzolana, or filler.
When testing the cement there has to be consistency and one of these are the conditions they are prepared, held for curing and tested. The laboratory where preparation of specimens takes place shall be maintained at a temperature of (20 Â± 2) Â°C and a relative humidity of not less than 50 %. The moist air room or the large cabinet for storage of the specimens in the mould shall be maintained at a temperature of (20,0 Â± 1,0) Â°C and a relative humidity of not less than 90 %, this will be recorded every 4 hours to make sure it is stable.
Apparatus used in this test will have an accuracy of Â± 1,0 % of the recorded load in the upper four-fifths of the range being used when verified in accordance with EN ISO 7500-1.
Testing procedure: Centre the prism halves laterally to the platens of the machine within Â± 0,5 mm, and longitudinally such that the end face of the prism overhangs the platens or auxiliary plates by about 10 mm. Increase the load smoothly at the rate of (2 400 Â± 200) N/s over the entire load application until fracture.
With reference to BS EN 12620:2002, aggregates to go into the concrete mix must meet certain specification these are:
The necessity for testing and declaring all properties specified in this clause shall be limited according to the particular application at end use or origin of the aggregate. When required, the tests specified in clause 4 shall be carried out to determine appropriate geometrical properties.
The grading of the aggregate, when determined in accordance with EN 933-1, shall comply with the requirements of 4.3.1 to 4.3.6 as appropriate to its aggregate size d/D.
When assessing aggregates within a system of factory production control at least 90 % of gradings, taken on different batches within a maximum period of 6 months, shall fall within the limits specified in 4.3.2 to 4.3.6 for tolerances on producers' declared typical gradings.
The necessity for testing and declaring all properties specified in this clause shall be limited according to the particular application at end use or origin of the aggregate. When required, the tests specified in clause 5 shall be carried out to determine appropriate physical properties.
When required the resistance to fragmentation shall be determined in terms of the Los Angeles coefficient, as specified in EN 1097-2:1998, clause 5. The Los Angeles test method shall be the reference test for the determination of resistance to fragmentation. The Los Angeles coefficient shall be declared in accordance with the relevant category specified according to the particular application or end use.
Where required, the impact value determined in accordance with EN 1097-2:1998, clause 6, shall be declared in accordance with the relevant category specified according to the particular application or end use.
Freeze-thaw is a major factor in this project so frost resistant aggregates is a definite requirement the resistance to freezing is determined in accordance with EN 1367-1 or EN 1367-2.
When very large volumes of concrete or large numbers of concrete units are to be examined, take at least ten independent samples and analyse them separately. The results can then be used to identify locations requiring more extensive investigation.
For each test on the fresh concrete, and for making any specimens for hardened concrete tests, use the scoop to obtain suitable amounts of concrete from the concrete batch heaped together either in the mixer or on a non-absorbent surface, ensuring that each sample is representative of the concrete batch. When not sampled immediately, protect the fresh concrete against gaining or losing water. Carry out the required operations during a period of not more than 1 h from the addition of the water to the cement.
With regards to BS EN 12350-6:2000, the density test for wet concrete. The sample shall be obtained in accordance with EN 12350-1. The sample shall be re-mixed, using the remixing container and square mouth shovel, before carrying out the test. Use this test on all new concrete to come into the site, if the concrete falls below the requirement then add water or if this is not possible then add and admixture or an additive to gain the required specification.
Another proven test is the BS EN 12350-2:2000, the sample of the concrete shall be obtained in accordance with EN 12350-1:1999. The sample shall be re-mixed using the remixing container and the square mouthed shovel before carrying out the test. This should also be carried on a daily occurrence. If the specification was not met then additives could be used to get them or if it was really un-usable then it could be rejected and sent back to the manufacture.
A good test for hardened concrete especially for this type of project is the initial surface absorption of concrete which refers to BS 1881-5: part 6 mainly because we will not have a roof on our structure and have to make sure no extra water is entering the concrete mix. This test must be performed for every batch that comes to site and if there is any loss in performance then you can just add an additive on top of the finished concrete to cover the concrete and protect it.
Another test for testing concrete strength is BS1881-127: 1990.
Three sets of six cubes shall be made for testing on each machine to be verified and the same number of cubes from the same groups shall be selected for testing on the reference machine and shall comprise:
- Six 150 mm cubes of 70 N/mm2 to 85 N/mm2 mean strength;
- Six 100 mm cubes of 70 N/mm2 to 85 N/mm2 mean strength;
- Six 100 mm cubes of 14 N/mm2 to 19 N/mm2 mean strength.
The base plate or from the mould joints are found, mark the mould and check it for compliance with 3.2.3 of this Part before re-use. The cube made in this mould is permitted to be used if the requirements for density of cubes in 4.8 of this Part are satisfied. When testing the samples follow the procedures given in 5.2 of BS 1881-116:1983. This test should be carried out daily because if a particular batch of concrete does not meet specification then it must be removed from the structure or be reinforced before it goes into service.
There are two procedures for the determination of the resistance of concrete to rapidly repeated cycles of freezing and thawing; these procedures can be used to compare various mixes. In Procedure A, both freezing and thawing take place in water; in Procedure B, freezing takes place in air but thawing takes place in water. Freezing saturated concrete in water is much more severe than in air, and the degree of saturation of the specimen at the beginning of the test also affects the rate of deterioration. BS 5075: Part 2:1982 describes freezing in water.
The deterioration of concrete can be accessed in several ways. The most common method is to measure the change in the dynamic modulus of elasticity of the specimen, the reduction in the modulus after a number of cycles of freezing and thawing expressing the deterioration of the concrete. This method indicates damage before it has become apparent either visually or by other methods, although there are some doubts about this interpretation of the decrease in the modulus after the first few cycles of freezing and thawing.
The durability of concrete can be accessed;
Durability factor = no. of cycles at end of test x percentage of original modulus 300
A durability factor lower than 40 is probably unsatisfactory with respect to resistance to freezing and thawing; 40-60 is the range for concretes with doubtful performance; above 60, the concrete is probably satisfactory, and around 100 it can be expected to be satisfactory.
Another test method determines the dilation of concrete subjected to slow freezing. The effects of freezing and thawing can also be assessed from measurements of the loss of compressive or flexural strength or from observations of the change of length (BS 5075:Part2:1992) or in the mass of the specimen. A large change in length is an indication of internal cracking; a value of 200x10-6 for tests in water is take to represent serious damage.
In terms of measuring the decrease in mass of the specimen; it is only really appropriate when damage takes place mainly at the surface of the specimen, but it is not reliable in cases of internal failure.
Very important to note that if the failure is due to unsound aggregate, it is more rapid and more severe than when the hardened cement paste is disrupted first.
Specification Freeze-Thaw Resisting Concrete:
Before we go into any detail it is important to understand the terms which are involved with freeze-thaw;
D-Cracking - This is a form of cracking which is associated predominantly with paving. In paving, it is characterised by cracks developing unseen towards the base of slabs at edges and joints under permanent high moisture conditions. The cracks may spread inwards and upwards eventually reaching the surface, and in very extreme cases lead to disruption of entire bays of concrete.
Scaling - Scaling is delamination of the concrete surface. All concrete has a paste-rich or mortar-rich layer at the surface. It is created when the concrete is compacted and finished. This layer can become detached if the stresses occurring exceed the tensile or bond strength holding the layer to the substrate. Lamination and delamination may already exist due to factors other than freeze-thaw action but delamination is increased due to such action.
Scaling can develop into internal damage and is often associated with de-icing chemicals as a safety measure for vehicles and people and intended to delay or prevent surface water freezing.
Aggregates: D-cracking is caused by coarse aggregate failure; D-cracking is associated with some sedimentary rocks of high water absorption, and, less frequently with metamorphosed sedimentary rocks. High absorption is only an indicator of possible susceptibility.
D-cracking is also associated with aggregate size, so therefore reducing the maximum aggregate size to 20mm may significantly reduce an aggregates potential for D-cracking. The widespread use of 20mm maximum aggregate size is perhaps one of the reasons why incidents of D-cracking are rarer than elsewhere.
Fine aggregates have an indirect influence on the freeze-thaw performance. When correctly proportioned, the water demand of the concrete will be minimised as will the cement content needed to achieve the required strength (w/c ratio). Fine aggregates assist in the retention of entrained air, which is an important consideration. The nature of the sand has an affect on the freeze-thaw durability, but this is only likely to be significant when specifying a standard procedure for assessing the freeze-thaw performance of a cement or combination.
Cements and Combinations: Portland Cements conforming to BS12 and cements or combinations with a moderate proportion of a second main constituent (CEM II Cements) are regarded as suitable for producing freeze-thaw resisting concrete. Laboratory tests in the past have shown that fly-ash concretes have a lower scaling resistance than Portland cement concretes at high levels of replacement or in low cement content mixes.
High slag cement concretes are prone to scaling in conditions of freezing with de-icing salts. This has been linked to the depth of the carbonated layer and the formation of metastable carbonation products.
Quality and Volume: Reducing the w/c ratio for a given set of materials has a beneficial effect, as it reduces the quantity of freezeable water. Such concretes have a reduced need for entrained-air to achieve freeze-thaw resistance.
Figure: Water Uptake during CF test (Auberg and Setzer, 1997)
Air Void System: the entrainment of a sufficient quantity of air by the use of an admixture is regarded as the most effective way of achieving high resistance to free-thaw attack. The admixture and mixing process gives a range of small bubbles of air in the cement paste fraction of which the size range 300 to 50 micrometers are the most effective in providing freeze-thaw resistance. Entrapped air of the same size is equally effective.
The Air void system is characterised by:
- Air content in the paste or the concrete
- Spacing factor
- Specific surface of air bubbles
The air content is expressed as the percentage volume of air by volume of mortar or concrete. Although the air is distributed in the cement paste, it is normal to express the air content by volume of concrete.
More fundamentally, the important factor is the spacing between adjacent air voids. The "spacing factor" is a calculated parameter related to the maximum distance of any point in the cement paste from the periphery of an air void, measured through the cement paste. For full protection the spacing factor should be a maximum in the order of 0.20 - 0.25 mm.
As the entrained air is not all of one size, a measure of the bubble size distribution is needed. Often this is done by specifying a minimum specific surface.