High strength concrete

High Strength Concrete

High-strength concrete

Definitions

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The definition of high-performance concrete is more controversial. Mehta Mehta and and Aitcin Aitcin used used the the term, term, highperformance concrete (HPC) for concrete mixtures possessing high workability, high durability and high ultimate strength.


Definitions

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ACI defined high-performance concrete as a concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing, and curing practice.


Typical Classification

Normal Strength

20-50 MPa

High Strength

50-100 MPa

Ultra High Strength

100-150 MPa

Especial

> 150 MPa


Microstructure

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From the general principles behind the design of high-strength concrete mixtures, it is apparent that high strengths are made possible by reducing porosity, porosity, inhomogeneit inhomogeneity, y, and microcrack microcrackss in the hydrated cement paste and the transition zone.


Microstructure 

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The utilization of fine pozzolanic materials in high-strength concrete leads to a reduction of the size of the crystalline compounds, particularly, calcium hydroxide. Consequently, there is a reduction of the thickness of the interfacial transition zone in high-strength concrete. The densification of the interfacial transition zone allows for efficient load transfer between the cement mortar and the coarse aggregate, contributing to the strength of the concrete. For very high-strength concrete where the matrix is extremely dense, a weak aggregate may become the weak link in concrete strength.


Mat ater eria ials ls - Cem emen entt

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Almost any ASTM portland cement type can be used to obtain obtain concr concrete ete with with adequat adequate e rheology rheology and with with compressive strength up to 60 MPa. In order to obtain higher strength mixtures while maintaining good workability, it is necessary to study carefully the cement composition and finenesses and its compatibility with the chemical admixtures. Experience has shown that low-C3A cements generally produce concrete with improved rheology.


Mate Ma teri rial alss -- Ag Aggr greg egat ate e

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In high-strength concrete, the aggregate plays an important role on the strength of concrete. The low-water to cement ratio used in high-strength concrete causes densification in both the matrix and interfacial transition zone, and the aggregate may become the weak link in the development of the mechanical strength. Extreme care is necessary, therefore, in the selection of aggregate to be used in very high-strength concrete.


Mate Ma teri rial alss -- Ag Aggr greg egat ate e 

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The particle size distribution of fine aggregate that meets the ASTM specifications is adequate for high-strength concrete mixtures. If possib possible, le, Aitcin Aitcin recommends recommends using using fine aggregat aggregates es with with higher fineness modulus (around 3.0). His reasoning is as follows: – a) high-s high-stre trengt ngth h concre concrete te mixtur mixtures es alread already y have large large amounts of small particles of cement and pozzolan, therefore fine particles of aggregate will not improve the workability of the mix; – b) the the use use of coar coarse serr fin fine e agg aggreg regate atess requ requir ires es less less water to obtain the same workability; and – c) duri during ng the the mixin mixing g proce process ss,, the coarse coarserr fine fine aggregates will generate higher shearing stresses that can help prevent flocculation of the cement paste.


Guidelines for the selection of materials

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The higher the targeted compressive strength, the smaller the maximum size of coarse aggregate. Up to 70 MPa compressive strength can be produced with a good coarse aggregate of a maximum size ranging from 20 to 28 mm. To produce 100 MPa compressive strength aggregate with a maximum size of 10 to 20 mm should be used. To date, concretes with compressive strengths of over 125 MPa have been produced, with 10 to 14 mm maximum size coarse aggregate.


Guidelines for the selection of materials 

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Using supplementary cementitious materials, such as blast-furnace slag, fly ash and natural pozzolans, not only reduces the production cost of concrete, but also addresses the slump loss problem. The optimum substitution level is often determined by the loss loss in 12- or 24-hou 24-hourr strengt strength h that is cons conside idered red acceptable, given climatic conditions or the minimum strength required. While silica fume is usually not really necessary for compressive strengths under 70 MPa, most concrete mixtures contain it when higher strengths are specified.


Usage of Superp rpla lasstic iciz ize ers Cons Co nsta tant nt w/ w/c: c: In Incr crea ease se in the the workability

Constant workability: Lower w/c

same workability No admixture

MLS

SMF

SC

LOWER WATER CONTENT

Lower w/c

Courtesy from Prof. Gettu


Superpla Supe rplastic sticizer izer-Sili -Silica ca Fume Inter Interacti action on

With Withou outt supe superp rpla last stic iciz izer er,, the the ceme cement nt + wat water er + silica fume sy system tends to coagulate, ma making the use of a superpl superplast astici icizer zer essen essentia tial. l.

REPULSIVE FORCES Silica fume

COAGULATION

Silica fume

DISPERSION Courtesy from Prof. Gettu


Selec ecttion of the Super erp plasticizer

Study of the compatibility

Optimu Optimum m superp superplas lastic ticize izerr dosage dosage

Cost-b Cost-bene enefit fit consid considera eratio tions ns In se several cases, th this order is inverted, result ltiing in costly consequences



Marsh Cone Test: Eva Evaluation of the compatibility and dosage Comparison with yield shear stresses obtained with a viscometer

800-1000 ml 7. 0

25

Cement Cement I 52.5R 52.5R w/c=0.33 Superplasticizer SD1

15.5 cm

20 6. 5

Bingh Bingham am yield yield stres stress s (Pa) (Pa) Mars Marsh h cone cone flow flow tim time (s) (s)

29 cm

6 cm

) s ( e m i t w o l F

) a P (

6. 0

10

0

5. 5 5

Diameter: 8 mm

5. 0

200-500 ml

15

0 0. 5

1. 0

1. 5

2. 0

% sp/c

2. 5

3. 0

3. 5



Practical Significance of the Saturation Point 210 w/c = 0.35 T = 22°C s , e m i t

w o l f e n o c h s r a M

60 min 170

Saturation Point

130

90

50 0.0

5 min

0.4

0.8

1.2

1.6

2.0

2.4

Superpla Superplastici sticizer zer dosage dosage (% sp/c) sp/c)

2.8



Cement Cem ent/Su /Super perpla plasti sticiz cizer er Com Compat patibi ibility lity 200 180 s , e m i t

w o l f e n o c h s r a M

w/c = 0.35 T = 23° C

60 min

160 140 120 60 min

Cement A

100

Cement B 80

5 min

5 min

60 0

0.4

0.8

1.2

1.6

2.0

2.4

Superpla Superplastici sticizer zer dosage dosage (% sp/c) sp/c)

2.8


Sele lecctio ion n of Superplastic iciizer

CBR =

) s ( e m i T

CBR =

w/c = 0.33

CBR =

% sp/c × (cost/kg)× time (s) s.r.

0.25 % sp/c × ( 3 euros/kg) × 5 s = 12.5 0.3 s.r.

1.5% sp/c ×( 1 euro/kg) ×7 s = 26.3 0.4 s.r.



Factors that Affect the Saturation Point

• Type of cement • Wate Water/ r/ce ceme ment nt ra rati tio o • Pre rese senc nce e of mine minera rall ad admi mixt xtur ures es • Mixi Mixing ng sequ sequen ence ce (better to separate the inc nco orporatio ion n of water and superp rpla lasstic iciz ize er by at least 1 minute of mixing)

• Temperature Courtesy from Prof. Gettu


Effect of Temperature on the Loss of Fluidity 20 ) s ( e m i t15

16 c = I 52.5 R sp = SN w/c = 0.33 sp/c = 1%

35ºC

w o l f e n10 o C h s r a 5 M

45ºC

5ºC

15ºC 25ºC

0 5 15

30

45

60

Time (min)

75

90

) s ( e m i t12

w o l f e n 8 o C h s r a 4 M

c = I 52.5 R sp = SC w/c = 0.33 sp/c = 0.3% 35ºC 25ºC 15ºC 45ºC 5ºC

0 5 15

30

45

60

75

90

Time (min)

• Loss of fluidity in the paste is lower for polycarboxylate based superplasticizers.

• There is no clear trend with respect to temperature. Courtesy from Prof. Gettu


Effect of Temperature on the Water Demand of Cement 0.30

c = I 52.5 R sp = SN

) 0.28 c / w ( 0.26 d n a m 0.24 e d r 0.22 e t a W 0.20

• The water de demand of ceme cement nt incr increa ease ses s with an increase in temperature.

45 ºC

• This demand

35 ºC 25 ºC 15 ºC 5 ºC

0.18 0.0

1.0

2.0

% sp/c

3.0

4.0

decreases due to inco incorp rpor orat atio ion n of superplasticizer until the the satu satura rati tion on poi point nt..


Differences Between NSC and HSC 

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In normal strength concrete, the microcracks form when the compressive stress reaches ~ 40% of the strength. The cracks interconnect when the stress reaches 80-90% of the strength For For HSC, HSC, Irav Iravan anii an and d MacGr MacGreg egor or re repo port rted ed line lineari arity ty of of the the stress-strain diagram at 65 to 70, 75 to 80 and above 85% of the peak load for concrete with compressive strengths of 65, 95, and 105 MPa.


Differences Between NSC and HSC (2)

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The fracture surface in NSC is rough. The fracture develops along the transition zone between the matrix and aggregates. Fewer aggregate particles are broken. The fracture surface in HSC is smooth. The cracks move without discontinuities between the matrix and aggregates.


Mechanical Behavior

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Stress-strain curve is more linear The strain corresponding to the maximum stress increases with strength The post-peak domain gets steeper The ultimate deformation decreases with the increasing strength


Strength

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Based on 289 observations of moist-cured highstrength concrete samples made with Type III cement cement,, Mokhtar Mokhtarzad zadeh eh and Frenc French h obtaine obtained d the following relationship

f cm

t   = f c28  0.89 + 0.97t 


Long-term strength 

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Irav Iravan anii and Mac MacGr Grego egorr sugg sugges ested ted the the foll follow owin ing g streng strength th values for sustained loading: 70 to 75% (of the short-time loading strength) for 65 MPa concrete

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75 to 80% for 95 MPa concrete, without silica fume

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85 to 90% for 105 MPa concrete, with silica fume

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85 to 90% for 120 MPa concrete, with silica fume


Elastic Modulus 

Great care should be taken if using well-established equations developed for normal-strength concrete to estimate the elastic modulus of high-strength concrete. Extrapolation beyond the validity of the equations often leads to overestimation of the elastic modulus.


Elastic Modulus

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for norm normal al weigh weightt concre concrete te with with 21 MPa MPa < fc < 83 MPa wher where e Ec is the the elas elasti ticc mod modul ulus us of conc concret rete, e, fc fc the the compressive strength.

Ec

= 3320

fc + 6900 MPa


Data Da ta fro from m Tomo Tomosa sawa wa an and d Nogu Noguch chii

60000

) a P M ( s u l u d o M c i t s a l E

40000

river gravel Crushed Graywack Crushed Quartz Crushed Limestone Crushed Andesite Blast furnace slag Calcined bauxite Crushed Cobble Crushed Basalt Lightweight CA Lightweight FA + CA Model

20000

0 0

50

100

150

Compressive Strength (MPa)

200


Chemical and Autogeneous shrinkage 

During hydration of the cement paste in a closed system, the volume of the hydration products, , is less than the sum of the volume of water and the volume of cement that is hydrated. This leads to chemical shrinkage whose magnitude can be expressed by

ε ch

V ( =

+ Vw ) − Vh Vci + Vwi

c

where and are the current and initial volume of cement, and and are the current and initial volume of water, respectively.


Early Volume Change

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Before setting, the chemical shrinkage is not constrained and, therefore, it will induce shrinkage of the same magnitude in the cement paste. As a rigid network of hydration products starts to develop, the values of the chemical shrinkage and that of the measured shrinkage in the cement paste start to diverge, since the rigidity of the paste restrains the deformation.


Definition Definit ion of the the autogen autogenous ous shr shrink inkage age accor accordin ding g to the Japanese Concrete Institute 

macroscopic volume reduction of cementitious materials when cement hydrates after initial setting. Autogenous shrinkage does not include volume change due to loss or ingress of substances, temperature variation, and application of an external force and restraint .

High strength concrete

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