Philadelphia UniversityFaculty of Engineering

Civil Engineering Department

Nano Silica Fume for High Performance Concrete

This Project Has Been Accomplished ToAcquire the Bachelor’s Degree in Civil

Engineering January, 2016.

Supervised by

Professor Dr. Wail Nourildean Al-Rifaie

Prepared By:

Abdalmjeed Abdallh Alawaneh 201220162

Mohammed Ismail Al bajawi 201110851

Acknowledge

First, praise and thanks be to Almighty Allah for His unlimited help and guidance and peace is upon His last messenger

Mohammed.

We would like to express our very thanks and deep gratitude to,

Professor Dr. Wail for his stimulating supervision, contact advises and constructive criticisms which made the completion of

this work possible.

Our deep thanks to Jordan Modern Ready Mix Concrete-Mnaseer Group, ISIC-International Silica Industries Co. and Ayla for Construction Chemicals Co. for their fully support by providing

the material to accomplish this research project.

Lastly, and in no sense the least, our whole hearted deep thanks to our Families, Colleagues and Friends for their motivation and kind assistance without which this work would have never been

accomplished..

Content

List of tables

List of Figures

Abstract Concrete is one of the essential elements that used in different types of construction these days, but it has many problems when interacts with environmental such as water, air, temperature, dust and humidity. Concrete made with Portland cement has certain characteristics; relatively strong in compression but weak in tension and tends to be brittle. These disadvantages make concrete limited to use in certain conditions. The most common problems appears on concrete are manifested by tearing, cracking, corrosion and spalling, which will lead to some defect in concrete then in the whole construction, so this research aimed to produce new materials that will decrease these problems and resolve it.

Mixture of Silica fume with concrete in our research shows that the strength and hardness are increased. In this research the main purpose is to measure the deference of compressive strength between plain concrete and concrete with silica fume with different additions ratio, and to investigate its effect on the strength of concrete.

To achieve our goals in this research about 18 concrete test sample were prepared to measure it’s compressive strength, all concrete sample has the same mixing ratio and sub-classified to standard , and Silica fume added by the volume of concrete (5%, 10%, 15%, 20% and 30%).

The results show that the recommended addition was 15% of Silica fumes for optimum compressive strength that reaches 74.8 MPa. Also the economy of mixture compare to the market prices makes silica excellent to use as addition filler to concrete.

Preface of Concrete, Materials and Silica

fume.

1-1 Concrete 1-1-1 History and definition

Since the concrete is the oldest manmade building materials used by Romans as early as 509 B.C, concrete has become one of the most commonly used building material [1].

Concrete used as building materials which is composite material made from several readily available constituents (aggregates, sand, cement, water).It is a versatile material that can easily be mixed to meet a variety of special needs and formed to virtually any shape [1].

Concrete mainly consists of cement, fine aggregates, coarse aggregates and water which mixed together. Admixtures added sometimes to change some of concrete properties. It is a widely used construction material because of its ease of construction, Low cost of its Ingredients, Its good durability [1].

1-1-2 Properties of concrete The properties of concrete can be classified as fresh or hardened properties. Properties of fresh concrete are controlled by workability, consistency, bleeding and segregation.

The strict definition of workability is the amount of useful internal work necessary to produce full compaction. Consistency is the fluidity or degree of wetness of concrete. It is generally dependent on the shear resistance of that mass, it also a major factor in indicating the workability of freshly mixed concrete. Segregation refers to a separation of the components of fresh concrete, resulting in a non-uniform mix. The primary causes of segregation are differences in specific gravity and size of constituents of concrete. Moreover, improper mixing, improper placing and improper consolidation also lead to segregation. Bleeding is the tendency of water to rise to the surface of freshly placed concrete. It caused by the inability of solid constituents of the mix to hold all of the mixing water as they settle down. Testing of fresh concrete was only slump test [1].

The strength of concrete is the major way to describe the properties of hardened concrete as compressive strength. Strength usually gives an overall picture of the quality of concrete because it is related to the structure of cement paste [1].

There are some factors affecting strength such as Water/Cement Ratio, Curing Time, Cement and Aggregate. Since the W/C ratio controls the porosity of concrete, it controls the strength as well. Therefore, in practice, we can assure the strength of properly compacted concrete at a given age by specifying the W/C ratio. It is usually wise to use as low water content as possible, since the more water added to concrete mix the higher the porosity regardless the fact that the w/c ratio is being kept constant. The relation between strength and W/C ratio is shown in this figure 1.1:

Figure 1.1: Relation between strength and W/C ratio.

The ratio between the strengths at curing times depends on: cement type and curing temperature.

This would help to anticipate whether our mix strength will achieve the required strength or not.

The effect of Portland cement on concrete strength depends on the chemical composition and fineness of the cement. Cement Chemical composition like C3S and CS controls early and later strength, respectively [1].

Aggregates Shape and Texture controls the bond between aggregate and cement and contribute to the stress level at which micro-cracking begins. Therefore, crushed aggregate could lead to higher concrete strength than gravel [1].

1-1-2 mixing, handling, placing and compacting concrete:

The mixing operation consists essentially of rotation or stirring, the objective being to coat the surface of all the aggregate particles with cement paste and to blend all the ingredients of concrete into a uniform mass [2].

The usual type of mixer is a batch mixer, which means that one batch of concrete is mixed and discharged before any more materials are put into the mixer [2] .

The size of mixers are made in a variety of size from 0.04 m for laboratory use and for huge mixer for field or mixing plants use = 13 m.

Also it’s important to know the minimum mixing time necessary to produce a concrete of uniform composition and consequently of reliable strength.

Generally, a mixing time of less than 1 to 1 ¼ min produces appreciable non-uniformity in composition and a significantly lower strength.

Table 1.1 gives typical values of mixing times for various capacities of mixers [2].

There are many methods of transporting concrete from the mixer to the site. The choice of method obviously depends on economic consideration and on the quantity of concrete to be transported. In all cases, the important requirements are that the mix should be suitable for the particular method chosen, i.e. it should remain cohesive and should not segregate.

Figure (1.2) shows the discharge of concrete from a mixer that control

segregation [2].

Figure 1.2 Control of segregation on discharge of concrete from a mixer.

The operation of placing and of compacting are independent and are carried our almost simultaneously. They are most important for the purpose of ensuring

requirements of strength, impermeability, and durability of the hardened concrete in the actual structure. As far as placing is concerned, the main objective is to

deposit the concrete as close as possible to its final position so that segregation is avoided and the concrete can be fully compacted. Figure 1.3 [2].

Figure 1.3 control of segregation at the end of concrete chutes

1-2 Materials

1-2-1 Aggregate

Concrete is a mixture of cementious material, aggregate, and water. Aggregate is commonly considered inert filler, which accounts for 60 to 80 percent of the volume and 70 to 85 percent of the weight of concrete. Although aggregate is considered inert filler, it is a necessary component that defines the concrete’s thermal and elastic properties and dimensional stability [3].

Aggregate is classified as two different types, coarse and fine. Coarse aggregate is usually greater than 4.75 mm (retained on a No. 4 sieve), while fine aggregate is less than 4.75 mm (passing the No. 4 sieve) [3].

The compressive aggregate strength is an important factor in the selection of aggregate. When determining the strength of normal concrete, most concrete aggregates are several times stronger than the other components in concrete and therefore not a factor in the strength of normal strength concrete. Lightweight aggregate concrete may be more influenced by the compressive strength of the aggregates [3].

Other physical and mineralogical properties of aggregate must be known before mixing concrete to obtain a desirable mixture. These properties include shape and texture, size gradation, moisture content, specific gravity, reactivity, soundness and bulk unit weight.

These properties along with the water/cement ratio determine the strength, workability, and durability of concrete [3].

The shape and texture of aggregate affects the properties of fresh concrete more than hardened concrete. Concrete is more workable when smooth and rounded aggregate is used instead of rough angular or elongated aggregate [1].

Most natural sands and gravel from riverbeds or seashores are smooth and rounded and are excellent aggregates.

Crushed stone produces much more angular and elongated aggregates, which have a higher surface-to-volume ratio, better bond characteristics but require more cement paste to produce a workable mixture [2].

The surface texture of aggregate can be either smooth or rough. A smooth surface can improve workability, yet a rougher surface generates a stronger bond between the paste and the aggregate creating a higher strength [2].

The grading or size distribution of aggregate is an important characteristic because it determines the paste requirement for workable concrete. This paste requirement is the factor controlling the cost, since cement is the most expensive component. It is therefore desirable to minimize the amount of paste consistent with the production of concrete that can be handled, compacted, and finished while providing the necessary strength and durability. The required amount of cement paste is dependent upon the amount of void space that must be filled and the total surface area that must be covered. When the particles are of uniform size the spacing is the greatest, but when a range of sizes is used the void spaces are filled and the paste requirement is lowered. The more these voids are filled, the less workable the concrete becomes, therefore, a compromise between workability and economy is necessary [4].

The moisture content of an aggregate is an important factor when developing the proper water/cement ratio. All aggregates contain some moisture based on the porosity of the particles and the moisture condition of the storage area. The moisture content can range from less than one percent in gravel to up to 40 percent in very porous sandstone and expanded shale [3].

Aggregate can be found in four different moisture states that include oven-dry (OD), air-dry (AD), saturated-surface dry (SSD) and wet.

Of these four states, only OD and SSD correspond to a specific moisture state and can be used as reference states for calculating moisture content. In order to calculate the quantity of water that aggregate will either add or subtract to the paste, the following three quantities must be calculated: absorption capacity, effective absorption, and surface moisture [1].

Most stockpiled coarse aggregate is in the AD state with absorption of less than one percent, but most fine aggregate is often in the wet state with surface moisture up to five percent. This surface moisture on the fine aggregate creates a thick film over the surface of the particles pushing them apart and increasing the apparent volume. This is commonly known as bulking and can cause significant errors in proportioning volume [1].

The density of the aggregates is required in mixture proportioning to establish weight-volume relationships.

Specific gravity is easily calculated by determining the densities by the displacement of water. All aggregates contain some porosity, and the specific gravity value depends on whether these pores are included in the measurement. There are two terms that are used to distinguish this measurement; absolute specific gravity and bulk specific gravity [5].

Absolute specific gravity (ASG) refers to the solid material excluding the pores, and bulk specific gravity (BSG), sometimes called apparent specific gravity, includes the volume of the pores [5].

For the purpose of mixture proportioning, it is important to know the space occupied by the aggregate particles, including the pores within the particles. The BSG of an aggregate is not directly related to its performance in concrete, although, the specification of BSG is often done to meet minimum density requirements [5].

For mixture proportioning, the bulk unit weight (a.k.a. bulk density) is required. The bulk density measures the volume that the graded aggregate will occupy in concrete, including the solid aggregate particles and the voids between them [2].

Since the weight of the aggregate is dependent on the moisture content of the aggregate, constant moisture content is required. This is achieved by using OD aggregate. Additionally, the bulk density is required for the volume method of mixture proportioning [1].

The most common classification of aggregates on the basis of bulk specific gravity is lightweight, normal-weight, and heavyweight aggregates. In normal concrete the aggregate weighs (1,520 – 1,680 kg/m3), but occasionally designs require either lightweight or heavyweight concrete.

Lightweight concrete contains aggregate that is natural or synthetic which weighs less than (1,100 kg/m3).

Heavyweight concrete contains aggregates that are natural or synthetic which weigh more than (2080 kg/m3).

Although aggregates are most commonly known to be inert filler in concrete, the different properties of aggregate have a large impact on the strength, durability, workability, and economy of concrete [2].

These different properties of aggregate allow designers and contractors the most flexibility to meet their design and construction requirements [2].

1-2-1-a Properties of aggregate:

To ensure aggregates continually meet the required specification, and thus to ensure the end product is suitable for its intended use, a series of laboratory tests have been devised. A set of British Standards have been established for many years to ensure consistency across the industry and with effect from 1 January 2004 these were combined with standard tests throughout Europe [1].

Table 1.2 shows a full list of the tests established by these new European standards, but some of the more commonly used are described below in more detail which has been used in this research. The appropriate Standard document should be consulted if the full details of the tests and methods are required [1].

-Specific Gravity:

It is defined as the ratio of mass (or weight in air) of a unit volume of arterial to the mass of the same volume of water at the stated temperature. There are three different definitions of specific gravity:

- Absolute specific Gravity: Refer to the volume of the solid material excluding all pores. - The Apparent specific Gravity (ASG):Refer to the volume of solid material including the impermeable pores, but not the capillary ones.

- The bulk Specific Gravity (BSG): Refer to the volume of solid material including the Permeable and impermeable pores.

- Porosity and Absorption: The porosity, permeability and absorption of aggregate influence the bond between it and the cement paste, the resistance of concrete to freezing and thawing and chemical stability, resistance to abrasion, and specific gravity.

When all the pores in the aggregate are full, it is said to be saturated and surface-dry.

If this aggregate is allowed to stand free in dry air, some water will evaporate so that the aggregate is air dry.

Prolonged drying in an oven would eventually remove the moisture completely and, at this stage, the aggregate is oven dry.

These various stages including an initial moist stage are shown diagrammatically in Fig (1.5) [2].

Figure 1.5 stages including an initial moist stage.

- Sieve Analysis: It is the process of dividing a sample of aggregate into fractions of same size. Its purpose is to determine the grading or size distribution of the aggregate [4]. The properties of resistance to abrasion and Impact of small size coarse aggregates we use Los Angeles Machine, This test gives an idea of the hardness of the material and the amount of material that breaks up into small granules during impact in the Machine. Finally, to express the resistance to sudden bumps and vibrations we determine the aggregate impact value (AIV) [5].

1-2-2 Water

The quality of the water is important because impurities in it may interfere with the setting of the cement, may adversely affect the strength of the concrete or cause staining of its surface, and may also lead to corrosion of the reinforcement. For these reasons, the suitability of water for mixing and curing purposes should be considered. Clear distinction must be made between the effects of mixing water and the attack on hardened concrete by aggressive waters because some of the latter type may be harmless or even beneficial when used mixing. In many specifications, the quality of water is covered by a clause saying that water should be fit for drinking [3].

The water–cement ratio is the ratio of the weight of water to the weight of cement used in a concrete mix and has an important influence on the quality of concrete produced. A lower water-cement ratio leads to higher strength and durability, but may make the mix more difficult to place. Placement difficulties can be resolved by using plasticizers, super-plasticizers or Hyper-plast [2].

Often, the water–cement ratio is characterized as the water to cement plus pozzolan ratio, W/(C+P). The pozzolan is typically a fly ash or blast furnace slag. It can include a number of other materials, such as silica fume, rice hull ash or natural pozzolan.

Concrete hardens as a result of the chemical reaction between cement and water (known as hydration, this produces heat and is called the heat of hydration) [2].

For every kilogram -or any unit of weight- of cement, about 0.25 kg -or corresponding unit- of water is needed to fully complete the hydration reactions.

This requires a water-cement ratio of 1:4 often given as a proportion: 0.25. However, a mix with a w/c ratio of 0.25 may not mix thoroughly, and may not flow well enough to be placed, so more water is used than is technically necessary to react with the cement [2].

More typical water-cement ratios of 0.4 to 0.6 are used for higher-strength concrete, lower water/cement ratios are used, along with a plasticizer to increase flow ability [2].

Too much water will result in segregation of the sand and aggregate components from the cement paste.

Also, water that is not consumed by the hydration reaction may leave the concrete as it hardens, resulting in microscopic pores (bleeding) that will reduce the final strength of the concrete.

A mix with too much water will experience more shrinkage as the excess water leaves, resulting in internal cracks and visible fractures (particularly around inside corners) which again will reduce the final strength.

The 1997 Uniform Building Code specifies a maximum 0.50 water-to-cement ratio (1:2) when concrete is exposed to freezing and thawing in a moist condition or to deicing chemicals, and a maximum 0.45 water to cement ratio for concrete in severe or very severe sulfate conditions [3] .

1-2-3 Cement

Cement is a binder, a substance that sets and hardens independently, and can bind other materials together.

Modern concrete that was made from crushed rock with burnt lime as binder. The volcanic ash and pulverized brick additives that were added to the burnt lime to obtain a hydraulic binder were later referred to as cement [1].

Ordinary Portland Cement (OPC) as available in the local market was used in this research. The effect of Portland cement on concrete strength depends on the chemical composition of the cement [7].

The chemical properties of cement presented in Table 1.2.

Because the quality of cement is vital for the production of good concrete, a number of tests are performed in the cement plant laboratory to ensure that the cement is of the desired quality. In our study we test the cement by fineness test only [8].

1-3 Silica fume

Silica fume, also known as micro silica is an amorphous (non-crystalline) polymorph of silicon dioxide, silica.

It is an ultrafine powder collected as a by-product of the silicon and ferrosilicon alloy production and consists of spherical particles with an average particle diameter of 150 nm. The main field of application is as pozzolanic material for high performance concrete [11].

It is sometimes confused with fumed silica. However, the production process, particle characteristics and fields of application of fumed silica are all different from those of silica fume [11].

Figure 1.6: Silica Fume

1-3-1 Silica fume Properties

Silica fume is an ultrafine material with spherical particles less than 1 μm in diameter, the average being about 0.15 μm. This makes it approximately 100 times smaller than the average cement particle. [1]

The bulk density of silica fume depends on the degree of densification in the silo and varies from 130 (undensified) to 600 kg/m3.

The specific gravity of silica fume is generally in the range of 2.2 to 2.3. The specific surface area of silica fume can be measured with the BET method or

nitrogen adsorption method. It typically ranges from 15,000 to 30,000 m2/kg.

1-3-2 Application of Silica fume in Concrete

Because of its extreme fineness and high silica content, silica fume is a very effective pozzolanic material. Standard specifications for silica fume used in cementious mixtures are ASTM C1240, EN 13263 [8].

Silica fume is added to Portland cement concrete to improve its properties, such as its compressive strength, bond strength, and abrasion resistance.

These improvements stem from both the mechanical improvements resulting from addition of a very fine powder to the cement paste mix as well as from the

pozzolanic reactions between the silica fume and free calcium hydroxide in the paste [16].

Addition of silica fume also reduces the permeability of concrete to chloride ions, which protects the reinforcing steel of concrete from corrosion, especially in chloride-rich environments such as coastal regions and those of humid continental roadways and runways (because of the use of deicing salts) and saltwater bridges. [8].

Prior to the mid-1970s, nearly all silica fumes were discharged into the atmosphere. After environmental concerns necessitated the collection and landfilling of silica fume, it became economically viable to use silica fume in various applications, in particular high-performance concrete. [9].

1-3-2-a Effects of silica fume on different properties of fresh and hardened concrete

1- Workability

With the addition of silica fume, the slump loss with time is directly proportional to increase in the silica fume content due to the introduction of large surface area in the concrete mix by its addition. Although the slump decreases, the mix remains highly cohesive [15].

2- Segregation and bleeding

Silica fume reduces bleeding significantly because the free water is consumed in wetting of the large surface area of the silica fume and hence the free water left in the mix for bleeding also decreases. Silica fume also blocks the pores in the fresh concrete so water within the concrete is not allowed to come to the surface [15].

1-3-3 Properties of 100 micron silica

100 micron Silica was provided from ISIC-International Silica Industries Co [18].

The prefixes of this silica are in the following tables.3 Description This grade of micronized silica flour is manufactured from super fine silica sand having a SiO2 content of up to 99%.

1-4 Study Objectives

The fundamental objective of this research was to provide information about the hardened properties of concrete achieved by using easily available local raw materials in Jordan to support the practical work with partners in assessing the practicability of the mixes with Silica fume, and to facilitate the introduction of Silica fume concrete (SFC) technology into general construction practice.

Investigate the effect of silica fume in SFC mixtures and on materials properties such as compressive strength. Also to investigate the use of Silica fume in plain cubes concrete to improve it’s compressive to reduce early cracking and inhibit later crack growth.

Test concrete with and without the addition Silica fume under various tests and using charts to compare the results with each other.

Methods and Experiments.

2-1-1 Aggregate

Local aggregate which used in this research is from Jordan Modern Ready Mix Concrete-Mnaseer Group.

Aggregates typically constitute 75% of concrete volume. Hence, aggregate types and sizes play an essential role in modifying the concrete properties as noted in the previous section [1].

2-1-1-a Tests on coarse Aggregate

2-1-1-a1 Sieve analysis

Grain size distribution according to ASTM C 136-01 a standard test method for Sieve analysis of fine and coarse aggregates [4]. Table 2.1 shows grading of coarse aggregate.

2-1-1- a2 Specific gravity

According to ASTM C127-88, Standard Test Method for Specific Gravity and Absorption of Coarse Aggregate [5].

Test procedure:

- Take an amount of 2 kg of sample retained on sieve No. 4 wash it and then immerses it in water for 24 hours

- Get out the sample from water and dry it with a cloth to be relieved the moisture from the surface.

- Take an amount of 1 kg of this sample it considered as saturated dry weight surface (Wssd).

- Immerse the sample on water to find its size.

- Sample should dried in oven at temperature of 110 ° C for 24 hours or until the weight remain constant and take the weight to the nearest 0.1 g, it considered as the dry weight of the sample (Wd).

Results:

Table 2.2 and 2.3 show the result of weights and specific gravities of coarse aggregate. Also in tables 2.4 and 2.5 show the results of medium aggregate in the next pages.

Test procedure: - Put the sample stream by the test inside cylindrical can and compacted 25 times on three layers. - Place the sample to the device and allow the hammer to free falling 15 times. - Empty broken aggregate on the sieve 2.36mm. - Repeat the test using the same steps in order to calculate the average impact value of aggregate. Results: Tables 2.6 and 2.7 show the results of two samples. Then, the average was calculated.

Average Of two trials is equal to = 13.73 %

According to ASTM C131 – 96, Standard Test Method for Resistance to Degradation of Small-Size Coarse Aggregate the test procedure is [9]:

- Wash the sample and then dry it at temperature 105°C until the weight is constant. - Put the sample on the L.A machine with the steel balls for 500 round.

- Get out the sample from the machine and wash it on sieve 1.70mm (No.12) to get rid the aggregate that is smaller than the holes of sieve.

- Dry the sample at a temperature of 105° and weighted again to the nearest 1gm. Result: By Abrasion and Impact in the Los Angeles Machine the result shown in table 2.8.

2-1-1-b Tests on fine Aggregate

2-1-1-b1 Sieve analysis

According to ASTM C 136-01a, standard test method for Sieve analysis of fine and coarse aggregates [6]. Table 2.9 shows grading of fine aggregate. Note that the sieve analysis was made for one source of sand (Silica sand).

According to ASTM C128-97, standard test method for specific gravity and water absorption of fine aggregate [5]:

Test procedure:

- Wash the sample on the sieve No.200 to get rid of impurities.

- Immerse sample in water for 24 hours.

- The surface of sample is dried by the drier, the sample is considered here saturated surface dry sample (Wssd).

- Take 295g of sample and put it in the beaker.

- Fill the beaker with water whiles the sample inside it.

- Get the sample out from the beaker and keep it in the oven for 24 hours at a temperature of 110°.

Results: Table 2.10 and 2.11 shows our result of weights and specific gravities of sand.

2-2-2 Mixtures with and without Silica fume and there proportions

The concrete mixes proportions were kept as (1:4:2) (cement: sand: aggregate) and added Silica Fume at (5, 10, 15, 20 and 30 percent of cement weight.

Sand is divided into Sand and Stamp sand (2 for Sand and 1 for stamp sand).

Aggregate is divided into coarse and medium (0.7 for coarse, 1.3 for medium).

According to the mixing ratios that were given to us and water-cement ratio we were able to calculate the weights of materials to be used in the experiment, and it was as follows :

Mix Ratio→1 :2: 4

wc=0.5

In the project we use (15*15*15) cm cube and its weight is eight kilo gram, by using that information and from the mix ratio, the calculation as following:

Component weights for 1cu.m for the mixture

cement weight=8 Kg∗17

=1.143Kg

fineagg .weight=8 kg∗27

=2.286 kg

coarse agg .weight=8 kg∗47

=4.572kg

Weight of water=wc∗cement weight

0.45 * 1.14 = 0.513 kg

Table 2.14: Silica Addition weight for each percent

Silica fume ratio % Silica fume weight (Kg)5 0.057

10 0.11415 0.17120 0.22830 0.342

Results and discussions for hardened concrete

tests.

The properties of hardened concrete can be significantly improved by Silica fume.

As we said before the strength of concrete is the major way to describe the properties of hardened concrete as compressive.

Strength usually gives an overall picture of the quality of concrete because it is related to the structure of cement paste [2].

In our study we measure the strength of concrete by compressive tests.

3-1 Compressive strength test on cubes

According to Bs1881: part 116:1983[15] Method of determination of compressive strength of concrete cubes. List of tables and figures represents compressive strength results of the concrete mixture.

3-1-1 Result Compressive strength result on testing cubes are shown in Table 3.1

3-3 Compressive strength test discussions

The addition of silica fume i.e.10% actually increases the 28 days compressive strength by about (12.85%) and when the volumes of silica fume reaches 15% then the compressive strength increased (18.52%) which is optimum strength.

Otherwise, the addition of silica fume at rates of 20 and 30 percent decreases the 28 days strength by about (14%) and (18.3%) respectively.

Unexpected behavior of the addition of silica fumes at rate of 5 % which gives an inverse curve. That effects the chemical reaction of pure silica fume with water (Si2O4.nH2O) then water get higher that leads to decrease 14 and 21 days to be less than 7 and 28 days which represent in figure 3.8.

The results in table 3.1 seem to indicate that there may be an effective volume threshold for adversely affecting the compressive strength of concrete that is exceeded at 15%.

All things considered, it appears that at dosage rates (0.1 and 0.15 the addition of Silica fume) does not significantly detract from, and even improve the compressive strength.

Higher dosage rates (0.20 and 0.30 the addition of Silica fume) however decrease the strength of concrete matrix due to higher volumes of Silica Fume interfering with the cohesiveness of the concrete matrix [28].

The graphical representation of the tests results is given in Figure 3.2.

Now, we discuss compressive test of the mixture in different age of days in the following charts. Figure 3.3 in seven days, figure 3.4 in fourteen days, figure 3.5 in twenty-one days and figure 3.6 in twenty eight days.

The addition of silica fume on the mixture gives fewer voids which are the weak-point inside concrete after it dry which simple make a path for the load to go inside it and decreases its resistance to compression The figures below are a graphical representation for the effect of silica at specific rates along 28 days tests.

Benefit’s cost study

Cost considered as one of the most important part of any project which plays an important role ensuring the economic success of the project. So, we’re going to demonstrate a Cost-Benefit Analysis of Silica fume additions to the concrete, and whether we will get any benefit from Silica fume or not. This study includes pricing concrete and Silica fume and changing in the physical properties of concrete after different additions of Silica fume.

4-1 Pricing Concrete and Silica Fume Concrete price classified depending on the compressive strength, the higher the strength the higher the price and it changes from a day to another, nowadays concrete price of one cubic meter for strength of 30, 60, 65, 70, 75 MPa are 55, 75, 78, 85, 90 JD respectively.

Silica fume is priced in accordance with weight, one kilograms of Silica fume is about 0.07 JD. Quantity of Silica fume added to the concrete increasing the price of concrete according for its price. We will focus on percent of silica fume which gives a certain compressive strength.

4-2 Silica Fume in concrete

The uses for silica fume in concrete fit into the general categories of enhancing mechanical properties, improving durability, enhancing constructability.

A special use of silica fume is the production of high-performance concrete bridges where the strength and durability properties of the concrete are critical. This use of silica fume is also described [17].

Although increasing concrete strength using silica fume has gained the most attention, more silica fume has actually been used in applications where increased durability is the driving factor [17].

For the purposes of this presentation, the uses of silica fume have been divided into distinct categories. It should be remembered that adding silica fume will affect all concrete properties simultaneously.

For example, reducing the permeability of concrete will usually increase the compressive strength of that concrete. [17]

The amount of increase will depend upon the concrete mixture selected. You should look at all aspects of concrete performance when specifications for silica-fume concrete are being written [17].

4-2-1 Enhancing Mechanical Properties

The most significant improvements can be seen in the compressive strength and modulus of elasticity. Initially, designers took advantage of increases in strength and modulus in high-rise structures. More recently, high-strength silica-fume concrete has been used in substructures of high-performance concrete bridges [17].

4-2-2 Improving Durability

The greatest use of silica fume has been in applications where improvements in durability have been the goal of the designer. Significant improvements can be seen in the permeability, abrasion resistance, and chemical resistance of concrete [17].

4-2-3 Enhancing Constructability

Contractors can take advantage of the properties of silica fume concrete to expedite projects or to achieve properties that might otherwise be impossible to achieve. Don't forget that using silica fume in this role will also improve the mechanical and durability properties of the concrete as well as enhance the constructability [17].

4-2-4 compressive strength

The results indicate that incorporation of Silica Fume in concrete can increase compressive strength and improve many properties of concrete. The increase is dependent on the fines of silica fume. The results on our research were good, which increased the compressive strength as shown in table 4.2 [17].

4-3 Results for benefit’s cost study

From our benefit cost study we can summarize the advantages of using silica fume as addition to concrete.

o Silica fume enhance the compressive strength, shear strength and reduce permeability of concrete.

o With silica fume the voids are minimized which leads to reduce the weak-points in the concrete and to increase strength.

o Cracks are reduced when using silica fume because it works as filler and consume water not needed for concrete to complete its reaction [25].

o Increasing durability by preventing the environment attracts.

o Reducing the maintenance cost by reducing cracks.

o The economical of using silica fume to produce high performance concrete makes it good to use in concrete.

Conclusion

From this research the following conclusion have been established:

Silica Fume Concrete (SFC) is a high strength mixture of aggregates, cement, water, and hyper-plast.

SFC is used to manufacture high strength sectioned.

The particle size of silica fume should not be larger than cement particle (less than 75 micron)

After the cost study we improved that adding silica fume to concrete improve its properties also much economically comparing with market prices with the benefits we gain.

Conclusion from the result

Normal compressive strength for plane concrete 63.11 MPa.

Adding 10% silica fume increases compressive strength at 7, 14, 21 and 28 days by 58.1%, 34.28%, 21.5% and 11.26% respectively which reaches 71.22 MPa.

Adding 15% silica fume increases compressive strength at 7, 14, 21 and 28 days by 54.6%, 37.6%, 26% and 18.65% respectively which reaches 74.88 MPa.

Optimum strength as shown before is almost 75 MPa which can be reached when adding 15% of silica fume on the mixture.

Increasing dosage of silica fume in concrete will reduce the strength of concrete and consume much water than needed at low rates.

Silica fume must be uniformly distributed with cement before mixing. References

1. Mehta and Monteiro. (1993) Concrete Structure, Properties, and Materials, Prentice-Hall, Inc., Englewood Cliffs, NJ

2. Kosmatka and Panarese (1994) Design and Control of Concrete Mixtures, Portland Cement Association, Skokie, Illinois

3. A.M. Neville & J.J. Brooks, “concrete technology”

4. ASTM C 136-01a, standard test method for “Sieve analysis of fine and coarse aggregates”

5. ASTM standards: C 127-88, Standard test method for specific gravity and absorption of coarse aggregate.

6. ASTM standards: C 131-96, Standard test method for resistance to degradation of small – size coarse aggregate.

7. According to ASTM C786 / C786M - 10 Standard Test Method for Fineness of Cement and Raw Materials by the 300-μm (No. 50), 150-μm (No. 100), and 75-μm (No. 200) Sieves.

8. ASTM C511 standard

9. According to Bs1881: part 116:1983 Method of determination of compressive strength of concrete cubes.

10. "Chapter 3 Fly Ash, Slag, Silica Fume, and Natural Pozzolans". The University of Memphis.

11. "Silica Fume User's Manual". Silica Fume Association.

12. ACI Committee 226. 1987b. "Silica fume in concrete: Preliminary report", ACI Materials JournalMarch–April: 158–66.

13. Luther, M. D. 1990. "High-performance silica fume (microsilica)—Modified cementitious repair materials". 69th annual meeting of the Transportation Research Board, paper no. 890448 (January)

14. ASTM C1240. Standard Specification for Silica Fume Used in Cementitious Mixtures, http://astm.org

15. EN 13263 Silica fume for concrete. http://www.cen.eu

16. Detwiler, R.J. and Mehta, P.K., Chemical and Physical Effects of Silica Fume on the Mechanical Behavior of Concrete, Materials Journal Nov. 1989

17. ACI 234R-06. Guide to Silica Fume in Concrete, American Concrete Institute.

18. http://silicajo.com/.

19. http://www.dcp-int.com/.