Ultra High Performance Concrete

Ultra high Performance Concrete (UHPC) is a new generation concrete which has a wonderful combination of high compressive strength and excellent ductility. UHPC’s revolutionary technology is rapidly outperforming the conventional concrete counterpart. By changing the fundamental ideas of conventional concrete; UHPC has the potential to bring concrete industry to a whole new level. Basically, UHPC is a steel fiber reinforced concrete consisting of an optimized gradation of fine powders and a very low water to cementitious materials ratio. Its durability properties prolong the lives of concrete structures. This far exceeds conventional concrete ability to do so.

TABLE OF CONTENTS 1 _

1 Introduction

2 Methods

2.1 Material selection

2.1.1 Cement

2.1.2 Silica Fume

2.1.3 Quartz Fines

2.1.4 Fine aggregates

2.1.5 HRWR

2.1.6 Steel fibres

2.2 Mix design

2.3 Mixing method

2.4 Curing regime

3 Properties Analyses of UHPC

3.1 Fresh properties

3.1.1 Rheology

3.1.2 Shrinkages

3.2 Mechanical properties

3.1.1 Compressive Strength

3.1.2 Residual flexural tensile strength

3.3 Durability

4 Economics of UHPC

5 Special applications

6 Conclusion

7 References

List of Abbreviations and Symbols

UHPC Ultra High Performance Concrete

W/C Ratio Water Cement Ratio

W/B Ratio Water Binder Ratio

HRWR High Range Water Reducer

RFTS Residual Flexural Tensile Strength

SLS Service Limit State

ULS Ultimate Limit State

LOP Limit Of Proportionality

CMOD Crack Mouth Opening Displacement

fR1K Characteristics value of RFTS at Serviceability

fR3K Characteristics value of RFTS at Ultimate state

q Distribution Modulus

Dmin Minimum Particle Size

Dmax Maximum Particle Size

P (D) Fraction of Total Solids

1 Introduction

Ultra high Performance Concrete (UHPC) is a new generation concrete which has a

wonderful combination of high compressive strength and excellent ductility UHPC’s revolutionary technology is rapidly outperforming the conventional concrete counterpart By changing the fundamental ideas of conventional concrete; UHPC has the potential to bring concrete industry to

a whole new level

Basically, UHPC is a steel fiber reinforced concrete consisting of an optimized gradation of fine powders and a very low water to cementitious materials ratio Its durability properties

prolong the lives of concrete structures This far exceeds conventional concrete ability to do so

2.1 Material selection

UHPC consists of Cement, Micro silica, silica sand, quartz powder, fibres as main

ingredients No coarse aggregate of any size is added to concrete Because the interface between cement paste and aggregate particles is the weakest zone in concrete, and the use of ultra-fine particles, such as silica fume, is important for densification and for the improvement of the

stability of fresh concrete, thus, enhancing the overall durability and strength

2.1.1 Cement

The selection of cement is essential for the performance of UHPC Ordinary Portland

Cement (OPC) CEM I 52.5 R is highly preferential Generally, cement should have high C3S and C2S with less content of C3A

2.1.2 Silica fume

A highly amorphous silica fume is an essential ingredient of UHPC The addition of highly reactive material performs the following functions This ultra-fine material reduces voids between components and increases packing density of cementitious matrix This material is of spherical shape and it act as lubricant within fresh mix thereby improving ability to flow

Since it is chemically reactive, it reacts with Ca (OH)2 and generates C-S-H gel This

improves compressive strength and other all desired properties Silica fume that conforms to ASTMC 1240 is suitable for this purpose

2.1.3 Quartz fines

If UHPC is subject to thermal treatment at 900 C for 48 hours, the addition of quartz fines is necessary

2.1.4 Fine aggregates

Fine aggregates of diameter less than 1 mm is mostly preferable But, angularity of fine aggregates is a most important decisive parameter for selection of material Angularity is a description of the degree of roughness and surface irregularities Angular particles require more energy for compaction in comparison with rounded particles Because angular surfaces tend to lock up with one another and resist compaction While smoother and rounded surfaces tend to pass one another allowing for easier compaction Higher the angularity of FA, the more void contend of material The high void content of material in UHPC is undesirable as it needs more cementitious composites to fill voids in order to optimize packing density

2.1.5 HRWR

As UHPC mix is designed with low W/C or W/b ratio, not much free water is available in composition Hence, careful selection of new generation poly carboxylate based super plasticizer plays major role in determining rheological properties of Mix Unlike conventional concrete, this admixture is added in higher dosage Owing to this, retardation effect that delays setting time is natural phenomenon in UHPC But, it can be solved by adding compatible accelerator with it Apart from modifying rheology and setting time, an action to enhance heat of hydration is also anticipated

2.1.6 Steel fibres

UHPC is brittle in nature In order to enhance ductility of UHPC, addition of steel fibres is essential We add cold drawn micro fibres of straight end at the dosage of 2% volume fraction This is 13mm in length, 0.2mm in diameter with minimum tensile strength of 2600Mpa

The addition of steel fibres can also improve toughness, durability, impact resistance, and fatigue and abrasion resistance of UHPC Pre and post cracking behaviour of cementitious system

is mainly based on percentage of volume fraction of steel fibres added In any case, minimum volume fraction of the fibres for structural application must not be less than 0.3%

Fig: 1 Micro fibres

2.2 Mix design

Conventional mix design methods are not adequate to satisfy all multiple stringent

requirements of UHPC In this research, UHPC mix is designed based on optimization of particle

density theory The fundamental concept of this design method is based on the assumption that

the performance of concrete mix can be optimized by maximising the particle density Though

many methods are available, Modified Andreason and Anderson model is best suited for

optimization of mixture composition to achieve higher packing densities The Fraction of total solids is calculated in this method as below

𝑃𝑃(𝐷𝐷) = 𝐷𝐷𝑞𝑞 − 𝐷𝐷𝑚𝑚𝑚𝑚𝑚𝑚𝑞𝑞

Where q – Distribution Modulus

Dmin - minimum particle size (µm)

P(D) - Fraction of the total solids

If q value is higher (q>0.5), the mixture will be coarser It is advisable keep q value below 0.25

The proportions of each material in the mixture are adjusted until an integral resulting curve comes close to target curve

2.3 Mixing process

Mixing time of UHPC is relatively longer than conventional concrete due to high content of cementitious materials Mixer type, admixture type and packing density are the major factors that determine actual mixing time However, batching must be done in following sequence only

2.4 Curing regime

UHPC shall undergo a thermal treatment at 900 C for 48 hours after set for better result Thermal treatment is employed for the purpose of developing more dense micro texture with the formation of crystalline calcium silicate phases within concrete mixture This curing regime usually

Mixing of Dry Materials

Addition of 80% of Water

Addition of Balance Water + Admixture

Addition of Steel Fibres

Final UHPC

Fig 2: Typical Batching Sequence of UHPC

results in a reduction of pores and increases compressive strength compared to the same specimen cured under ambient temperature

Thermal treatment at elevated temperature is highly preferential for cementitious system

containing secondary cementitious materials such as GGBS, Flyash

3 Properties Analyses of UPHC

2 Rapid chloride penetration test (RCPT) 6 coulombs

5 Chloride migration coefficient 0.05 X 10 -12 m 2 /s

6 Acid Soluble Sulphate Content as SO3 0.97 % (By weight of concrete)

7 Acid Soluble Chloride Content 0.01 % (By weight of concrete)

TABLE 1 – Properties of UHPC

3.1 Fresh Properties

3.1.1 Rheology

UHPC is highly flow able and works well with a need of slight external vibration or no vibration The rheological properties are modified by the particle size distribution of materials rather than type of materials Generally, coarser materials increases super plasticizer demand and mix viscosity Higher maximum packing density leads to a reduction in the viscosity of UHPC thereby improving rheological properties Besides packing density, super plasticizer demands also depend on the fineness of composites But, rheological properties can be controlled to certain extend by adjusting dosage of super plasticizer

Fig 2: Flow ability of UHPC

3.1.2 Shrinkages

Early age cracking of UHPC due to autogenous shrinkage is one of the most important

issues This needs to be properly addressed and suitable mitigation measures should be included for the successful formation of UHPC The low W/C ratio, incorporation of silica fume and

enormous amount of cement are the main parameters that contribute autogenous shrinkage

Chemical shrinkage initiates the autogenous shrinkage and chemical and autogenous shrinkages

are very bonded and unable to draw line between them to distinguish When cement hydrates through chemical reaction with water, the volume occupied by the products of hydration is less than original volume of cement, pozzolan and water So, volume changes takes place at this stage

due to chemical shrinkage This occurs in first hours after mixing As hydration process continues,

the more free water available in pores is absorbed by chemical reaction The limited water available due to low water cement ratio and inability to supply water from external source due to

dense structure results in emptying pore structures Thus, autogenous shrinkage occurs at this stage due to self-desiccation This develops tensile stresses in pore structures and cracks form if

tensile strength of concrete is not adequate to withstand it The effects of other parameters on

Autogenous shrinkage are given below

• The concrete composition with high silica fume content shows highest shrinkage

• The addition of steel fibres leads to a general decrease to autogenous shrinkage of between 10 -15% in relation to the correspondent composition without fibre

• The early development of autogenous shrinkage depends on the retardation effect caused

by HRWR

Drying shrinkage is defined as the contracting of a hardened concrete mixture due to the loss of

capillary water The main causes of drying shrinkage are mostly because of the reduction of

capillary water and water in the composite by evaporation As UHPC is subject to thermal

treatment, drying shrinkage will be fully eliminated

3.2 Mechanical Properties

3.1.1 Compressive Strength

High compressive strength and its strength gain behaviour are important characteristics of UHPC Achievement of up to 80 Mpa after 24 hours is also possible with the help of newly developed admixtures UHPC gains strength rapidly after 3 days due to thermal treatment The strength of more than 150Mpa is easily achievable after three days After this point, the rate of strength development decreases But, strength gain continues until 28 days and strength increases

of 10% over three days strength happens at this period

Fig 4: Compressive strength of UHPC

3.2.2 Residual Flexural Tensile Strength

The tensile behaviour of UHPC can be evaluated in terms of RFTS (Residual Flexural Tensile Strength) values at SLS and ULS LOP (Limit of proportionality) value is also important to access concrete behaviour These values can be determined from Load-Crack mouth opening displacement (CMOD) curve and Load –deflection curve obtained by applying centre point load on

a simply supported notched beam (The test procedure is in accordance with BS EN 14651)

LOP value correspondences directly to the strength of concrete irrespective of steel fibre additions In particular, RFTS at SLS is significant for durability and RFTS at ULS is significant for partially or totally substitute conventional reinforcement RFTS at ULS is can easily be enhanced

by increasing volume fraction of steel fibres added In test according to BS EN 14651, fR1K (CMOD

0

50

100

150

200

1 hour 3 days 7 days 28 days

Compressive Strength Test

Results

Mpa

strength Mpa

1 Day 66

28 Days 163

Fig 5: Load – Crack mouth opening displacement curve & Load – Deflection curve

Fig 6: Test setup

3.3 Durability

UHPC has gained more interest in the concrete industry due to its high durability properties The dense and uniform micro structure leads to high performance characteristics UHPC is highly resistant to acid chloride and sulphate penetration which is attributed to the increased density and decreased porosity High durability performance is found to be suitable under any adverse Assessing the permeability of a UHPC becomes important with focus on its performance All of the specimens exhibited chloride ion penetration in the negligible range (RCPT test) The water penetration and water absorption test results of UHPC were 2mm and 0.2% respectively Apart from this, UHPC has shown impressive results in Abrasion resistance, Alkali silica reaction and scaling resistance tests

Residual Flexural Tensile Strength

4 Economics of UPHC

Although a unit price comparison cannot be made between UHPC and conventional concrete, it is fact that the high cost of UHPC outweighs all of its benefits and the economics of UHPC prevents its popularization in construction industry It is essential to take measures to reduce cost that will help in dispensing the benefit to the construction industry worldwide The following steps will help to achieve this goal

Steel fibre is a primary reinforcement in UHPC and its high cost constitutes more than 50%

of total cost By employing a new design approach to use both steel fibres and steel reinforcement

as primary and passive reinforcement in design calculation will help to reduce percentage addition

of steel fibre Owing to this, further savings on cost will be achieved,

The usage of UHPC will bring about considerable reduction in cross sectional dimensions of

up to 40% The reduction in weight of elements due to reduction in dimensions will bring about savings during handling, transportation and erection processes The volume reduction will result in less material consumption as well All these matters should be considered in case studies to justify the cost For an example

Precast girder:

Using UHPC in production of precast girder has following advantages

The mechanical properties of UHPC make it possible to create very slender structures

UHPC girder would not require web shear reinforcement except for an amount required to connect the cast in place deck slab

Fig 7: Dimensional Reduction in Precast Ginder

5 Special Application

The use of UHPC in specialised precast products will open new market to existing one For