Engineered Cementitious Composites (ECC)

Prepared By:Mahnoor Abbas

Muhammad TahaMuhammad Saad Ifrahim

SECTION ‘B’

• Procedure Of ECC Mix• Design Basis of ECC• Experimental Briefing

PROCEDURE OF ECC MIX DESIGN

BY MAHNOOR ABBAS

ECC stand for engineered cementitious composite

• Known as bendable concrete

BASIS OF ECC MIX DESIGN:The mix design for ECC Concrete is based on Micromechanics design basis. It captures the mechanical interactions among the fiber.

INGREDIENTS AND PROPORTIONING:

AGGREGATES: aggregates with a particle size bigger than the average fiber spacing leads to greater interaction of fibers between the large aggregate particles, and the effect is greater as the volume and the maximum size of aggregate particles increase. Coarse aggregates are not used because they tend to have negative effects on ductile behavior of the composite. CEMENT: ECC can be produced using ordinary Portland cement. Compared with conventional concrete, ECC contains considerably higher cement content that is typically two to three times higher than normal concrete. Cement is the major component which binds all the ingredients and contributes to the strength.FLY ASH: Two different types of fly ash are used for the preparation of different ECC mixes, they are known as Class F and Class C, both containing SiO2, Al2O3, Fe2O3.PVA FIBER: uses low amounts, usually 2% by volume, of short, discontinuous fiber. The low fiber volume, along with the common components allows flexibility in ECC structures. The fibers used in ECC are tailored to work with the matrix for the purpose of constraining localized brittle fracture and guaranteeing more uniform distribution of micro cracks.

CASTING OF ECC MIXES: A proper and good practice of mixing can lead to better performance and quality of the ECC Concrete. This shows the performance of the ECC Concrete is influenced by the mixing.

PLACING AND COMPACTION OF ECC MIXES:Before placing of concrete, the concrete mould must be oiled for the ease of removing. Once the workability test of ECC Concrete is done. If the concrete are not compacted to a proper manner, the maximum strength of the concrete cannot be achieved.

CURING OF ECC MIXES: It is placed into the curing tank with a controlled temperature for 28 days for the hardened properties test of concrete. It can be put in accelerated Curing Tank due to time limit.

Initially a proportion is set with a standard super plasticizer and water to cementitious material. Then tested to see if required workability achieved or not if not the proportion is adjusted such that PVA fiber increased while other proportions same.

Difference between ECC and normal concrete: • There is high cement usage in ECC as compared with normal concrete. • All the studies so far state that the presence of water is essential to

facilitate healing of the cracks in ECC . Without water, it is impossible for the calcium hydroxide to be leached out of the bulk material into crack while in normal concrete.

• ECC does not use coarse aggregate as it effects the ductility of the mix while in normal it is an important component.

• Their behavior towards strain is also different. This strain hardening ‐behavior of ECC is similar to ductile metal while normal concrete behaves like brittle material.

DESIGN BASIS OF ECC

MIROMECHANICSBY MUHAMMAD TAHA

DESIGN BASIS OF ECC

• The mix design for ECC Concrete is based on Micromechanics design.

• Micromechanics:

Micromechanics is the analysis of composite or heterogeneous materials on the level of the individual constituents that form these materials.

• Some simplifying assumptions have been made to make the model equations tractable, and that the resulting conditions for strain-hardening can be used as guidelines for material component tailoring or selection.

• These conditions are expressed in strength and energy terms, as shown in the equation below.

where σcs= the cracking strength of the matrix. σ0= the maximum fiber bridging capacity. Jtip = the crack tip matrix toughness. Jb’=the complimentary energy of the fiber bridging relation.

• Physically, the strength criterion eq (A)ensures the initiation of microcracks from initial flaw sites in the composite before the tensile load exceeds the maximum fiber bridging capacity.

• The energy criterion Eq. B prescribes the mode of crack propagation once initiated i.e. the flat crack propagation mode.

• Violation of Eq. B results in fracture localization as in the case of FRC, and terminates the multiple cracking process.

• It should be noted that both equations A and B have been arranged so that the left hand sides of the inequality sign contain terms that pertain to fiber and interface properties, while the right hand sides contain terms that pertain to matrix properties, all of which are measurable physical properties.

• This observation emphasizes the usefulness of the equations

to help in the fiber, matrix and interface selection or tailoring process, in arriving at practiceable compositions of ECCs.

CONCRETE SPECIMEN

Under compression

Under tension

ECC SPECIMEN

zoom in portion of the ecc slab.

• Physically, the strength criterion eq (A)ensures the initiation of microcracks from initial flaw sites in the composite before the tensile load exceeds the maximum fiber bridging capacity.

• The energy criterion Eq. B prescribes the mode of crack propagation once initiated i.e. the flat crack propagation mode.

• Violation of Eq. B results in fracture localization as in the case of FRC, and terminates the multiple cracking process.

• It should be noted that both equations A and B have been arranged so that the left hand sides of the inequality sign contain terms that pertain to fiber and interface properties, while the right hand sides contain terms that pertain to matrix properties, all of which are measurable physical properties.

• This observation emphasizes the usefulness of the equations

to help in the fiber, matrix and interface selection or tailoring process, in arriving at practiceable compositions of ECCs.

IMPORTANCE OF GENERATION OF

MICROCRACKS

• In ECC the microcracks are filled due to the self healing property of cement composites. Thus , the by-products of hydration fill the gaps in these tiny cracks.

• This self healing is also true in case of concrete but here the cracks are so large that they can not be hold together by the by-products of hydration.

An Experimental Briefing

Comparison between an standard and non-standard ECC mix.

BY MUHAMMAD SAAD IFRAHIM

• Two appropriate cement based ECC mix proportion denoted as ECC-S and ECC-L employing silica sand and lumajang sand respectively were designed through experiments using the principles of micromechanics for studying strain hardening behavior. The use of lumajang sand was based to produce non-standard ECC-mix.

The percentage of PVA fibre is 2% by volume of specimen.

• The fresh ECC matrices were cast into standard Ǿ50 x 100-mm cylinder specimens for compression test. After placing the fresh ECC matrices into the moulds, compactions were done.

The compressive strength of ECC mortar was computed as an average value of three cylinder specimens. The compression tests were performed at 7 and 28 days under moist curing. After 24 hours of casting, all the specimens were demoulded and moisture cured in plastic bag with a controlled temperature of 25 C.

• Tensile tests were conducted to evaluate the behavior of ECC mixture under tension. Cross section 25 X 25-mm dog-bone specimens were prepared. For each mix, the specimens were cured 7 and 28 days. The specimens were tested in uniaxial tension using MTS machine with 5 kN capacity under displacement control at a rate of 0.01 mm/s.

Dog bone mould for tensile test

Findings• It is observed that the density of ECC-S is slightly lower than ECC-L. However, the

composite densities of both ECC are relatively lower compared with conventional concrete. This may be caused to the lack of coarse aggregate.

• It is noticed that the strain of ECC-S and ECC-L is >3% but when measured after 28 days It is slightly lower than 7 days strain capacity. The blue line is representing the findings of ECC when It is cured 7 days while the red is representing when it is cured 28 days.

• Tensile strength increases as days passed. Its shows tensile strength of ECC-L is slightly less than ECC-S. (Shown in table)

It is concluded that the higher ultimate tensile strength and strain capacity of ECC-S indicates its higher complimentary energy (Jb’) compared to that of ECC-L. Thus, it can be said

that the higher complimentary energy, the higher stress-strain capacity can be achieved.

The area enclosed by the inclined line and the

vertical axis is called the complementary energy

In final assessment the performance of ECC material is highly better than conventional concrete.

References• A. Tambusay, P. Suprobo, Faimum, A.A. Amiruddin

(2015).Experimental Study Of Engineered Cementitious Composite Material For Structural Application. Proceeding of the 2nd Makassar

• Li V.C., Kanda T., Engineered cementitious composites for structural application, ASCE J. Materials in Civil Engineering

• Engineered Cementitious Composites (ECC) – Material, Structural, and Durability Performance Victor C. Li University of Michigan, Ann Arbor, MI 48109

• Engineered Cementitious Composites (ECC) – Material, Structural, and Durability Performance. [book auth.] Victor C. Li. s.l. : Michigan University, 2007.