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In the following coursework we are going to take look at the steps that must be taken into account for performing a concrete mix design. When we talk about mix design, we are going to deal with two categories of property requirements: one category specific for fresh concrete (fresh properties of concrete); and another category specific for hardened concrete (Neville 1995). The aim of this coursework is also to analyze the properties of the freshly formed concrete due to the design performed, and not of the hardened concrete. As outlined in Lecture 2, the two types of property categories are not independent from one another, more likely they are directly linked, and failure in compliance with fresh properties will lead to poor concrete quality in hardened state.
Basically, concrete mix design is performed by a careful selection of quantities for concrete ingredients with the aim at producing cost effective concrete having a minimum set of properties such as: workability, compressive strength and durability (Neville 1995).
For the purpose of this coursework, we are going to design concretes which will contain mixes of classic ingredients like ordinary Portland cement as a bonding material and, either 100% Gravel or 100% Recycled Aggregates, as a filler material. Also, we are going to design mixes by replacing a part of the Portland cement with cementitious materials such as Silica Fume (10% SF) or Fly Ash (30% FA) and the reason of using these fine materials is that they will limit the amount of energy consumed in the mixing process, which ordinary Portland cement is not capable of doing.
Having 4 types of concrete mixes, we are going to deal with different properties between these mixes, each set of concrete properties being governed by the properties of the materials used. For example, recycled aggregate presents an increase in porosity compared with gravel, so it is no surprise that concretes with recycled aggregates will demand more water content than ordinary, natural aggregates. Eventually the workability will be affected by the higher water demand (Dhir et al. 1998). Also, as comparison between the effects on concrete determined by SF or FA we must state that concretes with FA will experience a decrease in water demand, reduced bleeding and good cohesiveness, while concrete with SF will bring an increase in water quantity.
Because they were 4 different types of mixes to be designed and tested in the lab, 4 groups, each designing their own mix, were created. Our group had the task of performing the mix design for the 10% SF mix and this was done according to BS 1881: Part 125: 1983 Testing concrete - Methods for mixing and sampling fresh concrete in the laboratory.
Silica fume has certain characteristics that make the handling, when we make the mixing of components, to be a difficult task. This is due to the small particle size and high fineness of the silica fume, so slurry is going to be prepared, by combining silica fume with water, when performing the mixing and also we should make sure that the slurry will be fully dispersed into the mix. By having a high fineness, the SF particles will demand a large surface to be covered with water, bringing an increase in water demand as stated above.
Mixes having silica fume as ingredient will give us high performance concretes., and the presence of SF in the mix design process will not only affect the quantities of the mix proportions to be used as ingredients of concrete, but also will lead to an improvement in the fresh properties of the concrete like: high cohesiveness, little to none bleeding, suitable for pumping (Neville et al. 1995).
In the lab, after we performed the mix, we made some tests in order to have quality control over the product: Slump Test, according to BS EN 12350-2:2000 Testing fresh concrete - Part 2: Slump test, to determine the workability of the concrete and the Plastic Density Test, according to BS 12350-6, 2000 Testing concrete - Method for determination of density of compacted fresh concrete, to determine the actual plastic density of the concrete.
2. Mix Design Procedure and worksheets.
2.1. Procedure principles.
It should be stated that this method is not an exact method of quantity assessment mainly, because of the variability of the parameters affecting the constituents of this procedure. Trial mixes are made with the purpose of guessing which combinations of ingredients will be suitable for the desired concrete properties and we can modify these mixes to correspond to our requirements (Neville 1995).
2.2. Description for performing concrete mix design.
Mix design follows several steps according to BRE Report 331, in which the flow chart of steps takes into consideration all parameters of the mix constituents and also shows us how they are linked together. The results of the computation are going to be written on a Concrete Mix Design Form.
For performing this procedure a number of initial specifications must be given, specifications which include:
Cement type - 52.5 N;
Aggregate type - Gravel;
Maximum aggregate size - 20 mm;
Fine aggregate grading - 44%;
Aggregate relative densities - 2600 kg/m3;
10% Silica Fume
0.45, 0.6 and 0.75 w/c, 180l/m³ water content, Slump of 30-60 mm (achieving S3 with SP)
The stages which govern this procedure, as specified in BRE Report 331 are:
Stage 1 water/cement ratio
Stage 2 water content
Stage 3 cement content
Stage 4 total aggregate content
Stage 5 fine aggregate proportion
Stage 6 trial mixing
Having the 3 free-water - cement ratios, the slump and the maximum size of the aggregates given, we can skip the first stage and the amount of water needed can easily be determined.
As we can see, 3 mixes will be prepared for which we can determine the amount of bonding material needed if the quantity of water is known. In this total amount of bonding material, 10% is Silica Fume, while 90% is ordinary Portland cement.
In stage 4 we shall determine the total quantity of aggregate present in the mix. Because we have the grading of the fine aggregate, we can determine the quantity of the coarse aggregates by subtracting from the total quantity of aggregate the fine aggregate quantity.
From the total quantity of coarse aggregate we know that 1/3 is for 10 mm Gravel and 2/3 for 20 mm Gravel. After we have determined all the mix proportions, we have to make specimens from the resulted concrete mix, specimens which will be subjected to different tests and conditions in order to determine the suitability of hardened concrete. These concrete specimens result from trial mixes for which batch weights are computed for a batch size of 0.02 m3.
One aspect hasn't been discussed so far: the use of admixtures in the concrete mix design. Admixtures are regarded as secondary ingredients of concrete, and not in the same class of importance like water or cement. There are different types of admixtures, depending on their effect on concrete, and, also their use is regarded from an economical and an increase in concrete quality point of view. For the purpose of our assignment, the admixture type which we are going to use is Superplasticizer (Glenium 51) and the British standard that regulates and controls their use is BS 5075: Part 3: 1985. Superplasticizers are water reducing admixtures, and their effect on concrete is related to an improvement in fresh properties, mainly an increase in workability.
4. Batch quantities allowing for absorption.
4.1. Porosity, Absorption and their link.
Porosity and absorption are aspects concerning the aggregates, so the following discussion will take into account the way in which the presence of pores will affect the concrete. There are 2 types of pores: internal and external pores, which vary in size- the external pores, can even be seen sometimes with the naked eye. Because of its viscosity, cement paste cannot fully cover the pores of the aggregates, but water is able to do that, which eventually will lead in an increase of water demand (Neville 1995).
It might seem that because it's only related to aggregate, porosity and absorption will not affect concrete, but let's not forget that the weight of aggregates represents approximately 75% of the weight of concrete. Also, by doing the mixing of concrete in the lab, the moisture content of the aggregates decreases, so an adjustment for absorption of aggregates must be done: we fully dry the aggregates in an oven, after which we put the aggregates in water, for 24 hours. An increase in weight occurs, meaning that all the pores are fully saturated (Neville 1995). The absorption is expressed as a ratio between the moisture content increase observed in the dried aggregates to the mass of the dried aggregates. The following absorption values for aggregates are used for our designed concretes:
Gravel 5/20 1.0%
Sand 0/5 0.5%
RA 5/10 3%
RA 10/20 4%
4.2. Adjustments for absorption.
The adjustments for absorption, determined in accordance with BS 812: Part 2, are performed on the aggregate types which are going to be used in the mixture. The aggregates which we are going to use for the 10% SF mix concrete are: sand 0/5 and gravel 5/20. The method allows computing the additional quantity of water required for absorption by the following formula:
Material batch weight (kg) x absorption value (%) = additional water required
Having found the additional water quantity, the adjustment for absorption of aggregates can be performed by:
Aggregate batch weight (kg) - absorbed water (kg) = adjusted aggregate batch weight
5. Mixing procedure and tests carried out in Lab 1.
5.1. Mixing procedure.
Relevant Standards: BS 1881: Part 125: 1983 Testing concrete - methods for mixing and sampling fresh concrete in the laboratory.
The mixing procedure is the combining of all concrete ingredients, with the purpose so that the aggregates surface is covered by cement paste, and it follows 2 steps: a) Sample preparation; b) mixing.
At the previous topic, the adjustment of aggregates for absorption was explained. It's one of the requirements for the sample preparation step, and was the last one that needed to be carried out before the proper mixing of all concrete ingredients can be performed. When discussing about sample preparation, we also must take into account that we should produce at least 10% more quantity of concrete than the required quantity for the tests that have to be done;
Follows a series of tasks: initially, the aggregates should be added in the following order: coarse aggregate, fine aggregate, sand after which we mix for 30 seconds. 1/2 of the water quantity must be added next, mix for another minute, after which we thoroughly mix by hand. Water absorption by the aggregates takes place when we leave covered the mixer for 8 minutes. Silica fume is mixed with water for 1 minute before adding it to the mix, after which we add the cement and mix it for 1 minute. We clean the paddles; we add the remaining water and superplasticizer and mix for other 4-5 minutes, after which we ensure homogeneity by mixing the sample by hand.
5.2. Tests carried out in Lab 1.
Before starting the discussion about what tests must be done, we should note that all these tests on fresh concrete should be carried out within 15 minutes of mixing. Two tests will be done: slump test and plastic density. After the tests are carried out, we must put the used concrete in the mixer and mix for other 30 seconds.
The Slump test has the aim of analysis of the workability of the fresh concrete, and it regulated by BS EN 12350-2:2000 Testing fresh concrete - Part 2: slump test standard. When performing this test we must have some basic equipment: a slump cone with foot rests and a metal rod (16 mm diameter, 600 mm long). First, we must moisten the slump cone, after which we pour the concrete in the cone, while it is hold firmly into position. The pouring of the concrete must be done in 3 equal layers which are tamped 25 times with the steel road. The excess of concrete at the top and around the slump cone is removed, after which the cone is removed, inverted and placed next to the slumped concrete in order to enable us to measure the vertical distance from the top of the cone to the highest point of the slumped concrete.
The measured vertical distance has to be reported for the nearest 5 mm, and shows us the nature of the slump that we deal with: true, collapse and shear slump.
Fig. 5.2: True, shear and collapse slump (Neville et al. 1987)
If the shear or collapse of the sample concrete occurs, we must perform the slump test once again.
Plastic Density is determined on a compacted concrete sample in the lab, and is regulated by BS 12350-6, 2000 Testing concrete - Method for determination of density of compacted fresh concrete standard. When performing this test we must have some basic equipment: 10 liter steel container (200 mm internal diameter, 320 mm internal height, and 4 mm wall thickness), vibrating table and 300 mm steel rule.
The test is carried out as follows: we measure the mass of the empty container after which we measure the mass of the container filled with 10 liter of water. The concrete is poured in the empty container, in six equal layers which will be compacted on the vibrating table and the excess concrete at the top will be removed. We record the mass of the container with the concrete in it; we return the concrete to the mixer and clean the equipment.
The computation can now be performed with the following formula:
Plastic density, D = m / V
Where, m = mass of concrete in container
(To the nearest 10 g)
V = volume of container
= mass of water in container (from calibration) /1000.
During the mix design we have found a plastic density for each water-cement ratio. Having found that plastic density, the new lab computed plastic density must not differ by more than ±20 kg/m³ then the actual plastic density (mix design).
7. Yield: corrected mix proportions and differences between design/plastic and volumetric method.
Because in the mix design we have taken into account only those parameters that have a major impact on concrete characteristics, and we disregarded those that have a minor role, some errors might occur during the mix design. Such errors might include: faulty maneuvering of concrete ingredients, errors in performing the mixing procedure. These errors are visible when the total weight of the concrete ingredients is different than the lab computed wet concrete density (plastic density). Having computed in the lab the actual plastic density of the mix, we can make corrections to the mix proportion weights so that some of the residual errors might be able to be corrected. One of the ways by which this can be done is adjusting for yield. Yield is the ratio between the actual density and the total density, and the adjustment is performed by multiplying this ratio with the weight of each constituent, giving us corrected mix proportions.
As an example we shall perform the correction of the mix proportions for the 0.45 w/c ratio mix:
During the mix design we have found that the wet density of concrete (plastic density) is 2380 kg/m3.
Cement+10% SF 360+40 kg/m³
Water 180 kg/m³
Sand 650 kg/m³
10mm Agg 385 kg/m³
20mm Agg 770 kg/m³
TOTAL DENSITY 2385 kg/m³
ACTUAL PLASTIC DENSITY 2410 kg/m³ (tested in labs)
The correction is done with the following formula:
Corrected = (Actual density/ Total Density) x each constituent = 2410/2385=1.0105
Cement+10% SF (365+40) kg/m³
Water 180 kg/m³
Sand 655 kg/m³
10mm Agg 390 kg/m³
20mm Agg 780 kg/m³
CORRECTED DENSITY 2410 kg/m³
Another method by which we can adjust the mix proportions is the volumetric method. The main feature of this method is that, when performing the adjustment, we will take into consideration the particle density of each constituent taking part in the mix design. We shall also make an example for the volumetric method of correction of mix proportions.
Cement 360 kg/m³ / 3150 kg/m³ = 0.114
SF 40 kg/m³ / 2000 kg/m³ = 0.02
Water 180 kg/m³ / 1000 kg/m³ = 0.18
Sand 650 kg/m³ / 2600 kg/m³ = 0.25
10mm Agg 385 kg/m³ / 2600 kg/m³ = 0.148
20mm Agg 770 kg/m³ / 2600 kg/m³ = 0.296
The correction formula: Correction factor = (Theoretical/Actual) x each constituent = 0.992
Cement 355 kg/m³ / 3150 kg/m³ = 0.1133
SF 40 kg/m³ / 2000 kg/m³ = 0.020
Water 180 kg/m³ / 1000 kg/m³ = 0.180
Sand 645 kg/m³ / 2600 kg/m³ = 0.2474
10mm Agg 380 kg/m³ / 2600 kg/m³ = 0.1461
20mm Agg 765 kg/m³ / 2600 kg/m³ = 0.2932
The main difference between the two methods which we have shown is that the yield method requires computing the lab plastic density, and comparing it to the plastic density taken into account in the design phase of the mix, while for the volumetric method is not necessary to make that extra effort in order to make the adjustment.
As far as to which method is more appropriate to be used, from a first look at our example, we can consider that the volumetric method is more appropriate, at least from an economic point of view, because the adjustments made gave us smaller weights of materials than the adjustment for yield weights. But my opinion is that the plastic density method is much more appropriate to be used, because it's more reliable, and you have a superior certainty about the elimination of the errors that appear in the mixing phase and also a better control of quality of the concrete.
8. Comments on fresh properties.
The discussion about the fresh properties of concrete might seem to be not as important as the hardened concrete properties from a structural point of view, but as we have previously noted, there is a direct link between the two types of properties: for example the strength of hardened concrete is greatly improved when a sufficient compaction of the fresh concrete was established (Neville 1995).The objective of this discussion is to see the degree by which different concrete components affect the fresh properties of concrete, and we shall do this by comparing the mixes presenting silica fume, fly ash and recycled aggregate with the ordinary Portland cement mix (100% Gravel).
The influence of aggregate type: by comparing the fresh properties of the 100% Gravel mix and 100% RA mix. Due to the fact RA have a higher porosity, a larger amount of water is needed in the mix which will eventually affect the workability of the fresh concrete. Because we have considered an absorption value for RA which was too high (RA density was larger than 2400kg/m3), the RA mix presented collapse slump, thus poor workability, also a slight bleeding, and segregation of the concrete constituents is present, leading to a poor cohesiveness. In the 30% FA and 10% SF mixes we have used as aggregate, gravel, so the influence of aggregate type on fresh concrete is the same in all 3 mixes, the only differences occur from the influence of other parameters.
The cement type that we have used was either ordinary Portland cement (CEM I 52.5 N) or composite Portland cement, by combining the Portland cement with a cementitious material like silica fume or fly ash. As we can see from the comparison of the composite Portland cement mixes with those having only Portland cement, the 30% FA mix presents no bleeding or segregation of materials, a good cohesiveness and finishability. Fly ash reduces the amount of water to be used in the mix, having a similar effect as a superplasticizer, so at larger water cement ratios we have collapsed slump, poor cohesiveness, and a slight bleeding and segregation is present but the compactibility and finishability is still good, making it suitable for pumping . That is not the case with silica fume. Silica fume increases the water demand, and generally the mixes with silica fume present good cohesiveness, no bleeding and segregation, true slump giving us a good workability. The usage of both silica fume and superplasticizers has a good effect on concrete.
The amount of fine material is directly linked to the cost of both silica fume and fly ash. The cost of production is quite large, so the use of silica fume and fly ash is no longer a cheap, viable solution for Portland cement replacement. As far as their consequences on fresh properties, for the 100% Gravel mix, at lower water-cement ratios we deal with a stiff mix, having less finishability and compactibility than when the amount of Portland cement is decreased, while the cohesiveness, lack of bleeding and segregation is kept the same on all mixes. We could use higher amounts of SF and FA but after a certain amount they cease to have any effect on the fresh properties of concrete.
The water-cement ratio is one of the main factors affecting the concrete fresh properties, simply because water and cement are two of the main constituents of concrete. In the 100% RA mix, because of the use of RA, it was necessary to have a higher amount of water than in the other 3 mixes, which in return gave us a poor cohesiveness, a collapsible slump resulting in a poor workability. Also some segregation and bleeding was observed, which was not present in the other mixes. As a general rule, when the w/c ratio is decreased, the ratio between the other concrete constituents is kept constant, so the workability increases (Neville 1995).
The SP amount required for workability. Superplasticizers greatly affect the slump of a mix. For the 100% Gravel mix we used approximately 0.12% amount of superplasticizers for all w/c ratios, which gave us a true slump. When we look at the 30% FA and 10% SF mix we can see that we have increased the amount to about 0.31% which gave us a collapse in slump when we used FA, so too much SP, and a true slump for SF, because SF works much better with SP compared to FA.
This coursework has to be looked at as being divided in two parts: a part about the concrete mix design and establishing all the mix proportions of the concrete constituents and another part relating to the fresh properties of the resulted concrete. During this coursework we have successfully designed 4 types of concrete, and as a result the fresh properties of the 4 concretes were individually established and assessed. The mix of concrete ingredients, the determination of slump and the computation of plastic density were all done in the lab according to the relevant standard governing each task.
The designed concrete types were: 100% Gravel, 100% RA, 30% FA and 10% SF. By assessing the concrete fresh properties we have seen which mixes had problems and were not suitable for using on site, but excluding some problems found (a high amount of superplasticizer than the one needed in the 30% FA), in general the mixes presented good workability without segregation or bleeding, were suitable for pumping so they could be used on site, in real conditions.