Concrete Mix Design

Concrete Mix Design

Introduction:

The term concrete is used to represent the mixture of cement, aggregates which are both fine and course, water and admixtures. The Admixtures are mainly used to bring out a particular characteristic that is require for the given project. As a constructor, one needs to ensure that the concrete being made can be easily transported, placed, compacted and well finished. Also the concrete must be able to set well providing the required strength and durability. Concrete mix design looks to demystify the amount of each material that is to be used to come up with the desired concrete. The concrete mix varies depending on the type of construction it will be used. In this assignment, I will at the concrete mix for bridge construction which are required to meet the following specification.

  • Minimum Slump of 100mm

  • Maximum internal temperature, monitored by thermocouples, limited to 60OC

  • Characteristic strength of 40 MPa

Analysis:

There are two mixes I will demystify are:

  1. Concrete whose binder must contain fly ash and cement.

  2. Concrete whose binder is granulated blast, furnace slug and cement

Cement is mixed with fly ash which is a pozzolan which is an additive that does not contain any cementitious properties but at normal temperatures react with moisture to form a compound which has cementitious properties. The amount of fly ash as a percentage of the cementitious material determines the characteristics of the concrete that’s to be produced. 35-50% of the fly ash is used in case of mass construct ion because it helps reduce the amount of temperature rise for example in construction of dams, and huge foundations.

The design for the concrete mix to be used in the construction of a bridge in Australia with fly ash as the admixture.

Cement type

Maximum aggregate size

Ratio of minimum aggregate/ cement

Water/ Cement ratio

Cement content (Min)

400 kg/m3

Fly ash (min)

140 kg/m3 or 0.35

Specific gravity of sand

Specific gravity of cement

Specific gravity of coarse aggregates

DESIGN OF CONCRETE MIX:

Concrete Mix 1

The mean strength targeted for concrete which is at 28 days is given by [ CITATION Tey97 l 1033 ]:

ft = fck + 1.65 S = 40 + 1.65 x 5 = 48.25 MPa where ft represents the targeted strength, fck is the class of concrete.

Water cement ratio

In order to obtain the amount of water required for the concrete mix, a 0.40 water cement ratio is assumed or adopted. The maximum aggregate size is 20 mm while the sand to be used in concreting is that of grading zone III. The amount of water in a cubic metre of concrete is 180 kg whereas the amount of sand which is given as percentage of total aggregate = 25 %. Therefore the amount of water required is 180 kg/m3 + 5.4 kg/m3 = 185.4 kg/m3

The amount of cement is obtained by

Cement = amount of water/water cement ratio= 185.4/0.40 = 463.5 kg/m3

The average temperature in Sydney isConcrete Mix Design, thus proportion of fly ash —
Concrete Mix Design 1
[ CITATION Bou05 l 1033 ]

The cement is combined with fly ash which is 40 % of the cement content and thus the amount of fly ash = 0.40 × 463.5 = 185.4 kg/m3

Therefore the amount of cement per cubic meter = 463.5-185.4= 278.1 kg/m3

The volume of coarse and fine aggregate contents:

) ] x (1/1000)fa/Sa) + (1/p).(fcV = [W + (C/S

)] x (1/1000)ca) + {1/(1-p)} . (Ca/ScV = [W + (C/S

Where V = volume of concrete 

Sc = specific gravity of cement

           W = amount water per cubic metre of concrete

           C = amount of cement per cubic m of concrete

           p = ratio of fine aggregate to total aggregate volume

     fa, Ca = amount of fine and coarse aggregates, per cubic metre of concrete, respectively, kg, and Sfa, Sca = specific gravities of saturated surface dry fine and coarse aggregates, respectively

Fine aggregates = 530.27 kg/m3, and

Course aggregates = 1153.13 kg/m3.

Flyash affects the plastic properties of concrete by improving workability, reducing water demand, controlling bleeding, and lowering heat of hydration. Flyash increases strength development at later ages, reduces corrosion of reinforcing steel, and generally improves resistance to chemical attack and mobility through a reduction in permeability.

Concrete Mix 2

The mean strength targeted for concrete which is at 28 days is given by [ CITATION Tey97 l 1033 ]:

Concrete Mix Design 2

Where Concrete Mix Design 3 is targeted mean strength

Concrete Mix Design 4 is concrete class

For
Concrete Mix Design 5 of concrete;-

Water cement ratio —
Concrete Mix Design 6

Max aggregate size —
Concrete Mix Design 7

Slump —
Concrete Mix Design 8

Water content —
Concrete Mix Design 9

Cement content —Concrete Mix Design 10

Proportion of ground granulated blast furnace slag —
Concrete Mix Design 11

Ground granulated blast furnace content —
Concrete Mix Design 12

Actual Portland cement content —
Concrete Mix Design 13

Aggregate content —
Concrete Mix Design 14

Proportion of fine aggregate —
Concrete Mix Design 15

Fine aggregate content —
Concrete Mix Design 16

Coarse aggregate content —
Concrete Mix Design 17

Ground granulated blast furnace slag is used to make durable concrete structures in combination with ordinary Portland cement. It improves the durability of concrete significantly, extending the lifetime of structures [ CITATION BIT12 l 1033 ].

Replacement of Portland cement by granulated blast furnace slag by more than 20% increases the consistency of the concrete mix. The strength of concrete is maintained up to 60% replacement, but the plasticity diminishes, resulting in very brittle failure [ CITATION Vac12 l 1033 ].

Qn 2. Heat of Hydration

When cement is mixed with water, an exothermic reaction takes place. The heat released from this reaction is referred to as the heat of hydration [ CITATION PCA97 l 1033 ].

The use of fly ash reduces the heat of hydration, consequently the risk of thermal cracking when dealing with mass concrete.

The use of blast furnace slag also reduces the heat of hydration, preventing thermal cracking by approximatelyConcrete Mix Design 18. It also reduces the maximum temperature of concrete, as well as prolonging the time taken to reach that maximum value.

Concrete mix 2 (granulated blast furnace slag) is preferred for the bridge due to the following advantages;

  1. It has a longer strength development, exceeding the design strength at 28 days

  2. It provides better protection against attack from chemical compounds

  3. It increases the durability of the structure.

Qn. 3 Self Compacting Concrete

Self- compacting concrete is a highly fluid mix of concrete that is able to consolidate under its own weight. Its flowing nature makes it suitable for placing in difficult placement conditions or in sections with congested reinforcement.

SCC is characterized by a higher percentage of fine aggregate than coarse aggregate, as compared to other conventional concrete mixes, so as to facilitate the flowing nature. Advantages of its use include;

  • Faster construction

  • Easier placement

  • Increased durability

  • Increased freedom in design, giving thinner concrete sections

Concrete Mix Design

The mean strength targeted for concrete which is at 28 days is given by [ CITATION Tey97 l 1033 ]:

Concrete Mix Design 19

Where Concrete Mix Design 20 is targeted mean strength

Concrete Mix Design 21 is concrete class

For
Concrete Mix Design 22 of concrete;-

Water cement ratio —
Concrete Mix Design 23

Water powder ratio —
Concrete Mix Design 24
[ CITATION EFN02 l 1033 ]

Max aggregate size —
Concrete Mix Design 25

Slump —
Concrete Mix Design 26

Water content —
Concrete Mix Design 27

Powder content (self-compacting additive) —
Concrete Mix Design 28

Cement content —Concrete Mix Design 29

Aggregate content —
Concrete Mix Design 30

Proportion of coarse aggregate —
Concrete Mix Design 31

Coarse aggregate content —
Concrete Mix Design 32

Fine aggregate content —
Concrete Mix Design 33

Qn 4 Chlorine – Induced Corrosion

Steel is not a naturally occurring material, and as such, is vulnerable to corrosion. This occurs when the alkalinity of the concrete is reduced or when the chloride concentration in concrete is increased to a certain level, which is the main cause of premature corrosion of steel reinforcement. Seawater has a high level of chloride ions and dissolved chlorides, which can intrude into sound concrete causing corrosion if there is moisture and oxygen to sustain the reaction. However, chlorides have been found to be only directly responsible for the initial corrosion, with other factors controlling the latter rate of corrosion [ CITATION Por151 l 1033 ].

This steel- induced corrosion can be mitigated by [ CITATION Ber15 l 1033 ];

  1. Using ground granulated blast furnace slag as partial replacement of Portland cement. The GGBS offers relatively good protection against ingress of chlorides

  2. Adding steel fibres to the concrete mix. This reduces the electrical resistivity of the concrete mix, which in turn reduces its vulnerability to chloride attack

  3. Reducing the degree of saturation of concrete. This has the effect of lowering the electrical resistivity of the mix, providing some protection from chloride attack

  4. Preventing the formation of cracks on the concrete surface. Cracks facilitate the ingress of chlorides, meaning they can more easily come into contact with the steel reinforcement, causing corrosion

Qn 5 Chloride Resistant Concrete

The location of the bridge near the coastline
Concrete Mix Design 34 subjects it Exposure Condition C – tidal and splash zones.

For high chloride resistant concrete containing grounded granulated blast furnace slag as partial replacement for Portland cement [ CITATION CCA09 l 1033 ];

Minimum cement content —
Concrete Mix Design 35

Maximum water cement ratio —
Concrete Mix Design 36

Mix Design

The mean strength targeted for concrete which is at 28 days is given by [ CITATION Tey97 l 1033 ]:

Concrete Mix Design 37

Where Concrete Mix Design 38 is targeted mean strength

Concrete Mix Design 39 is concrete class

For
Concrete Mix Design 40 of concrete;-

Water cement ratio —
Concrete Mix Design 41

Max aggregate size —
Concrete Mix Design 42

Slump —
Concrete Mix Design 43

Minimum cement content —
Concrete Mix Design 44

Water content —
Concrete Mix Design 45

Cement content —Concrete Mix Design 46

Proportion of ground granulated blast furnace slag —
Concrete Mix Design 47

Ground granulated blast furnace content —
Concrete Mix Design 48

Actual Portland cement content —
Concrete Mix Design 49

Aggregate content —
Concrete Mix Design 50

Proportion of fine aggregate —
Concrete Mix Design 51

Fine aggregate content —
Concrete Mix Design 52

Coarse aggregate content —
Concrete Mix Design 53

Conclusion

The close proximity of the bridge to the coastline (about 50 m) means that its columns (piers) will be subject to exposure of seawater through tidal waves and splashes. This means that the concrete used in its construction will be subject to attack by the chlorides dissolved in the seawater. It is therefore, necessary to protect the concrete from these chlorides, to prevent corrosion of the steel reinforcement.

To provide high chloride attach resistance, it is recommended that a concrete mix with 30% partial replacement of Portland cement by ground granulated blast furnace slag be used for construction of the bridge. This mix also reduces the maximum heat of hydration and its rate of development, which is crucial given the columns of the bridge may be considered mass concrete structures. It also improves the durability of the structure, which gives the bridge a longer lifespan

References

Berrocal, C. G., 2015. Chloride Induced Corrosion of Steel Bars in Fibre Reinforced Concrete, Goteborg, Sweden: Chalmers University of Technology, Departmet of Civil and Environmental Engineering.

BITS, 2012. High Performance Concrete with GGBS and Robo sand, Pilani: Department of Civil Engineering, Birla Institute of Technology and Science.

Bouzoubaa, N. & Foo, S., 2005. Use of Fly Ash and Slag in Concrete: A Best Practise Guide, Ottawa: Government of Canada.

CCAA, 2009. Chloride Resistance of Concrete, Sydney, Australia: Cement Concrete & Aggregates Australia.

EFNARC, 2002. Specifications and Guidelines for Self- Compacting Concrete, Surrey, UK: European Federation of National Associations Representing for Concrete.

PCA, 1997. Concrete Technology Today, Skokie, IL: Portland Cement Association.

Portland Cement Association, 2015. Corrosion of Embedded Metals. Available at:
[Online] http://www.cement.org/for-concrete-books-learning/concrete-technology/durability/corrosion-of-embedded-materials [Accessed 9 June 2016].

Teychenne, D. C., Franklin, R. E. & Erntroy, H. C., 1997. Design of Normal Concret Mixes. 2nd ed. London: Building Research Establishment.

Vaclavik, V., Dirner, V., Dvorsky, T. & Daxner, J., 2012. The Use of Blast Furnace Slag. Metalurgija, 51(4), pp. 461 — 464.