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This paper is about mix design and fresh properties of concrete. The mix design procedure is a process by which the concrete mix proportions such as cement, water, and aggregates are calculated. This is done by filling the concrete mix design form and going through the five design stages in sequence. The five design stages lead to mix proportions, and from that, the batch weights can be obtained. The fresh properties of concrete consist of testing and noticing concrete properties such as the slump, plastic density, bleeding, segregation, cohesiveness, compactability, and finishability in terms of aggregate type, cement type, amount of fine material, water/cement ratio, and superplasticizer amount required for workability. The manuscript presents experimental testing results of the concrete properties using four different concrete mixes. These are 100% gravel, 100% recycled aggregate, 30% fly ash, and 10% silica fume. The experimental testing was carried out in the concrete laboratory at the University of Dundee.

The BS 1881: Part 125: 1983 – Testing concrete – Methods for mixing and sampling fresh concrete in the laboratory is set to consider the methods of mixing concrete in terms of sample preparation, mixing, and testing. The testing process included is slump and plastic density test.

In the first section of this report, the mix design procedures will be described along with its five stages and group results. The calculations of the mix design will be shown step by step for the 100% gravel of a 0.45, 0.6, and 0.75 water/cement (W/C) ratio, to obtain the mix proportions for concrete mixes and the batch quantities allowing for absorption.

The mix proportions for concrete mixes of the 100% gravel, 100% recycled aggregate, 30% fly ash/gravel, and 10% silica fume (SF)/gravel will be shown in the form of tables along with the corrected mixes. In addition to this, the batch quantities allowing for absorption tables will be introduced, with some discussion on the reasons of adding extra water.

A step-by-step procedure of the concrete mixing will be presented with the work carried out in the laboratory. Moreover, a brief discussion on the slump and plastic density tests will be introduced in this report. The results of the properties of fresh concrete mix will be commented on, such as aggregate type, cement type, amount of fine aggregate, and W/C ratio.

Finally, as part of the discussion in this report, the yield will be calculated with the corrected mix proportions and difference between design/plastic and volumetric method will be explained.

The main objectives of the experimental testing were as follows:

To achieve a better understanding of the concrete mix design process.

To calculate the mix proportions for concrete mixes and batch quantities allowed for absorption.

To obtain a greater in-depth knowledge of the mixing procedure and tests carried out in laboratory.

To become familiar with the fresh properties of concrete.

To be able to correct mix proportions.

To distinguish between the design/plastic and volumetric method.

To discussing fresh properties in terms of aggregate type, cement type, amount of fine material, W/C ratio, and the SP amount required for workability.

The major points of the testing were as follows:

Carrying out mix design for 0.45, 0.6, and 0.75 W/C ratios for uncrushed gravel 100%.

Calculating batch quantities allowed for absorption.

Mixing concrete according to the building research establishment mix design.

Carrying out the slump and plastic density tests.

Observing the fresh properties of concrete, such as bleeding, segregation, cohesiveness, compactability, finishability, and general overall observation of the mix.

Casting appropriate specimens.

Curing the specimens as required.

Demolding the specimens after 24 h.

The required details for designing concrete mix are cement type, aggregate type, maximum aggregate size, fine aggregate grading, aggregate relative densities, fresh concrete properties, and Hardened concrete properties. The aggregate type used is 100% gravel and three W/C ratios 0.45, 0.6, and 0.75.

The order of concrete mix design procedures consists of five stages as follows:

Stage 1 is set out to find the free W/C ratio.

Stage 2 is set out to find the free water content, through approximate free water contents (kg/m^{3}) required to give various levels of workability.

Stage 3 is set out to find the cement content, from equation (1).

Where,

Stage 4 is set out to determine the total aggregate content. First, through estimated wet density of fully compacted concrete to find the concrete density. Second, using equation (2).

Where, ^{3}), ^{3}), and ^{3}).

Stage 5 is set out to find the fine and coarse aggregate contents. First, through the recommended proportions of fine aggregate according to percentage passing a 600 µm sieve to find the proportion of fine aggregate. Second, using equations (3) and (4), respectively.

Where,

Where,

Each of the above stages deals with a particular aspect of the design, leading to the final mix proportions (kg/m^{3}) and the batch weights (kg). The factor that has been considered during the mix design is fresh concrete workability, durability, and compressive strength.

The 0.45 W/C ratio with a cement type of 52.5N, aggregate type gravel uncrushed, maximum aggregate size 20 mm, aggregate density of 2600 kg/m^{3}, sand density of 2600 kg/m^{3}, fine aggregate grading of 44%, and 180 l/m^{3} water content and with a slump between 30 and 60 mm.

By applying the mix design procedure, stages stated in Section 2.1, for Stage 1, the free W/C ratio is 0.45 for aggregate type uncrushed 100% gravel and the maximum free W/C ratio using the lower value 0.45 W/C ratio. Stage 2, the free water content slump or Vebe time specified slump is between 30 and 60 mm and the maximum aggregate size specified 20 mm. Accordingly, using the table of approximate free water contents kg/m^{3} required to give various levels of workability in the engineered concrete mix design and test methods, the free water content is 180 kg/m^{3}. For Stage 3, the cement content using equation 1 is 400 kg/m^{3} and the modified free W/C ratio is 0.45. Furthermore, Stage 4 gives a relative density of the aggregate of 2600 kg/m^{3} for total aggregate content. Using the appropriate graph for estimated the wet density of fully compacted concrete, the concrete density is 2370 kg/m^{3} and the total aggregate content using equation 2 equals to 1790 kg/m^{3}. Finally, for Stage 5, for the fine and coarse aggregate content, the grading of fine aggregate is 44% for maximum aggregate size of 20 mm, the proportion of fine aggregate equals 37%. Therefore, using equation (3), the fine aggregate content is 662 kg/m^{3} and the coarse aggregate content is 1128 kg/m^{3} using equation (4).

The same mix design procedure has been carried out for 0.6 and 0.75 W/C ratio and the following Tables

Mix proportions for 100% gravel

Mix proportions for 100% RA

Mix proportions for 100% FA

Mix proportions for 100% SF

Batch quantities are the quantities of materials such as cement, fly ash, SF, water, recycled aggregate, and aggregates required for a concrete mix process. The batch quantities for the concrete mixes are presented in Tables

Batch quantities for 100% gravel

Batch quantities for 100% RA

Batch quantities for 100% FA

Batch quantities for 100% SF

Where,

The main reason for adding extra water is for absorption of water by the aggregate and due to the characteristics of the aggregate to produce concrete of a specified slump. This depends on the W/C ratio, cement-aggregate ratio, maximum aggregate size, and the type, shape, grading, surface texture, absorption capability, strength, and stiffness of aggregate particles. The following equation (6) is used to find the additional water required:

Where,

The absorption values for gravel 5/20, sand 0/5, RA 5/10, and RA 10/20 are 1%, 0.5%, 3%, and 4%, respectively.

The mixing of concrete was according to BS 1881: Part 125: 1983 – Testing concrete – Methods for mixing and sampling fresh concrete in the laboratory. A lightly dampen mixer pan and paddles have been used to carry out the mixing. First, the following was added in order: Coarse aggregate, fine aggregate, and sand and mixed for 30 s. After that, half of the mix water was added and mixed for another 1 min. Then, the lightly dampen mixer was left covered for 8 min for the aggregates to absorb water. Afterward, the cement was spread evenly over the aggregates and mixed for 1 min. The next step was cleaning the paddles and mix material carefully by hand. Finally, the remaining water (with added superplasticizers) was added to mix and followed by a further mixing for 2 min. Additional hand mixing has been carried out to ensure homogeneity.

The slump test [

Slump measuring and testing

The plastic density test was done according to BS 12350-6, 2000 – Testing concrete – Method for determination of density of compacted fresh concrete. First, the mass of an empty 10 L steel container has been recorded to the nearest 0.01 kg. After that, the container was filled with water so that no meniscus is above the rim. Then, the container was reweighed to obtain mass of water in container to the nearest 0.01 kg. The next step was to empty the container from water and carefully fill it with concrete in six equal layers. Each of the layers has been compacted fully on the vibrating table. Then, the final layer has been smoothed with a float to level with the top container. The next step was to skim the concrete surface with 300 mm steel rule, and the outside of the container has been cleaned thoroughly and reweighed. Finally, the concrete has been returned to the mixer and remixed for 30 s. Plastic density results are demonstrated in

Results for the plastic density test for 100% gravel (kg)

After carrying out the mixing of concrete, slump and plastic density tests, returning, and remixing the concrete for 30 s, the concrete was casted in 1 cylinder (100 mm × 300 mm in diameter), 1 prism (100 mm^{2} × 500 mm), 6 cubes (100 mm^{3}), and 1 cube (150 mm^{3}) for test specimens. In addition, curing of the test specimens has been carried out to avoid water loss from the concrete. Finally, the mixer has been cleaned thoroughly and the concrete has been removed from paddles. The casting of the concrete specimens is illustrated in

The picture shows the casting of concrete for testing specimens

The calculation for plastic density which was in the concrete laboratory using the formula (7) below:

Where, m is mass of concrete in container (to the nearest 10 g) and

Accordingly, the plastic density for 0.45 W/C ratio is:

To the nearest 10 kg/m³

The yield has been calculated for each of the three W/C ratios 0.45, 0.60, and 0.75 and tabulated in

Density, corrected density, and actual plastic density (kg/m^{3})

The corrected density has been calculated using equation (8) below:

Where,

The reason behind calculating the yield is to find the corrected mix proportions and comparing the corrected density with actual plastic density. Moreover, it is calculated to compare the correction factor of actual and theoretical methods. There are two types of methods, one is calculated design/plastic and the other is volumetric method.[

Volumetric tabulated calculation for 0.45 W/C ratio (100%) gravel

Volumetric tabulated calculation for 0.45 W/C ratio (100%) gravel corrected

The gravel [

Fresh concrete properties for 100% gravel

The volume of concrete produced using the equation (9):

Where, _{c}_{s}_{a}_{w}

Accordingly, the volume of concrete produced (

Where _{b}

Plastic density and volume of concrete produced for 0.45, 0.60, and 0.75 W/C ratio

The absorption value for this recycled aggregate was too high; therefore, it affected the workability by causing high slumps, due to more water in the mix. For the three W/C ratios, the slump collapsed with values of 185, 195, and 195 mm, respectively. However, on the other hand, the use of recycled aggregate resulted in mix with good compactability and finishability. Portland cement (CEM I) was added to this mix.[^{3} assumed in design stage and it is more likely to be 2550 kg/m^{3}.

Fresh concrete properties for 100% RA

General observation/absorption value probably is too high for this RA, therefore, more water in mix causing high slumps and the density of this RA was probably >2400 kg/m³ assumed in design stage. To be more precise the value was about 2550 kg/m³.

The role of gravel on concrete is that it gives the concrete good compactability and finishability. Thus, the compactability and finishability were good for this concrete mix. Portland cement has reduced bleeding and gave a good cohesiveness in this mix for 0.37 W/C ratio, but for 0.45 and 0.6 W/C ratios, there was a slight bleeding and cohesiveness was poor. In general, it is observed that this mix is a cement-rich mix. The amount of fly ash added was 3, 2.3, and 1.8 kg in respect with W/C ratios of 0.36, 0.47, and 0.59. Fly ash is extremely high fine material. Due to its spherical shape and plain surface of particles gives it an advantage in terms of water requirement in concrete mix.[

Fresh concrete properties for 30% FA

In this mix, the gravel has given the fresh concrete good compactability and finishability. Portland cement (CEM I) has worked effectively as a binder between sand and gravel, for a good fresh concrete with good cohesiveness and compactability. The role of SF on concrete is that it gives concrete lower permeability by providing unprecedented reactions in permeability and complete control of bleeding. Silica fume is an extremely fine and very reactive with spherical shape particles. Therefore, there is no sign of bleeding in the mix due to presence of SF. The amount of sand in this mix was as follows: 12.89, 13.61, and 14.04 kg, respectively, for 0.45, 0.60, and 0.75 W/C ratios. These amounts seem to be fine since the results of fresh concrete properties were in accord with the requirement. For 0.45, 0.6, and 0.75 W/C ratio, the slumps were a bit high with 95, 80, and 75 mm. In addition, plastic density was fine and within the acceptable range of 2400 kg/m^{3} ± 20 kg/m^{3}. Moreover, the amount of glenium 51 used was 0.15%, 0.12%, and 0.10%, respectively. These amounts presented reasonable workability with no segregation or bleeding in fresh concrete.

Fresh concrete properties for 10% SF

Having completed this experimental study, new aspects have been learned, such as calculating a concrete mix design to obtain mix proportions and batch weights. Mixing concrete with accordance to BS 1881: Part 125: 1983 – Testing concrete – Methods for mixing and sampling fresh concrete in the laboratory and carrying out slump and plastic density tests. In addition, it is found that the amount of water absorbed by the aggregate depends on many factors such as W/C ratio, cement-aggregate ratio, maximum aggregate size, and the type, shape, grading, surface texture, absorption capability, strength, and stiffness of aggregate particles. Moreover, the volumetric method found to be more effective than design/plastic method because it is more accurate and close to actual. Furthermore, it is observed that the fresh properties of concrete are affected by many factors such as aggregate type, cement type, amount of fine material, W/C ratio, and amount of superplasticizer added to mix.

The results of 100% gravel established a good concrete mix. The slump expressed good workability by presenting acceptable and within range results between 30 and 60 mm. Moreover, the result for plastic density was fine with values of 2405, 2385, and 2365 kg/m^{3}. In addition, there was no bleeding or segregation with an overall good cohesiveness, compactability, and finishability. The aggregate type 100% gravel, CEM I, amount of fine aggregate, W/C ratio, and the right amount of superplasticizer used had a huge impact in maintaining such good fresh properties of this concrete mix. Finally, with some improvements on the tested fresh concrete it can be suitable for the usage in the construction site. Such improvements may be by reducing the amount of cement in the concrete mix.