Presented by

Mahmoud Ashraf Tahaa Ahmed

Mechanical properties of high strength concrete(HSC)

• Introduction

• Required materials

• Mechanical properties

• Applications

• Conclusions

Contents

Introduction

Concrete is a composite material composed of inert filling material (aggregates)

Coarse aggregate (gravel or crushed stones), fine aggregate (sand) and bonding

material (cement paste ( hydrated cement)) and it may contain admixtures and/or

fibers.

In the middle of the 20th century concrete with characteristic strength (f’c) of

25MPa was considered high-strength. In the 1980’s, 50MPa concrete was

considered high-strength.

High-strength concrete (HSC) is generally used for concrete with

compressive strength higher than 60 MPa. HSC has sufficiently advanced such

that concretes with compressive strengths of up to about 120MPa are

commercially available.

The production of HSC that consistently meets requirements for workability

and strength development places more stringent requirements on material

selection than that for lower strength concrete. However, many trial batches

are often required to generate the data that enables the researchers and

professionals to identify optimum mix proportions for HSC.

During the past few years, high-strength concrete (HSC) has been

generating increased interest amongst civil and structural engineers.

And divided in 3 categories shown in table (1).

The expanding commercial use of this relatively new construction

material can be explained partially by the life cycle cost-performance

ratio it offers, as well as its outstanding engineering properties, such

as higher compressive and tensile strengths, higher stiffness and

better durability, when compared to the conventional normal

strength concrete (NSC).

Table 1: Categories of HSC

Category Strength

Category I 50–75MPa Manufactured using good quality, but generally used materials,

existing production technology and w/b ratio of about 0.4. Mineral

admixtures are not required. Superplasticizers may be used to

achieve the required workability,

Category

II

75–100MPa High quality, generally used materials are required. Due to the very

low w/b ratio of about 0.25 - 0.30, superplasticizers are required to

achieve adequate workability. The use of mineral admixtures is also

strongly recommended. The coarse aggregates must be round or

cubic in shape.

Category

III

100-

125MPa

Very high quality materials, efficient mixing techniques and

stringent quality control needed. The w/b ratio must be lowered to

0.22-0.25. High dosages of superplasticizers are essential in

conjunction with silica fume.

The use of HSC in the construction industry has steadily increased

over the past years and leads to the design of smaller sections. This in

turn reduces the dead weight allowing the design of longer spans and

more usable area of buildings.

Reduction in mass is also important for economical design of

earthquake resistant structures. Such advantages often outweigh the

higher production cost of high strength concrete associated with

careful selection of ingredients, mix proportioning, curing and quality

control. High-strength concrete is a brittle material.

• Cement

• Water/binder (w/b) ratio and cement content

• Aggregates (coarse , fine )

• Superplaticizers

• Mineral admixtures (Silica fume , Fly ash, Slag )

Required materials

Mechanical properties of HSC

Compressive Strength Decreasing w/c ratio increases the strength of concrete. However, this trend

follows only where strength of hydrated cement is low compared to the strength of

coarse aggregates. When these two strengths become comparable, decreasing w/c

ratio doesn’t increase the strength significantly, and to further increase the strength

of HSC, strength and quality of coarse aggregates need to be increased, in addition

to other factors. Typically, w/c ratios between 0.20.4 are used for HSC. Low w/c

ratio decreases the workability. Super plasticizers are added to increase the

workability in HSC. Shape, texture and maximum size of coarse aggregate affect

the compressive strength of HSC. Smooth river gravel produces weaker concrete.

Smallest size of coarse aggregate produces highest strength concrete owing to its

high specific surface area. Addition of silica fume decreases the requirement of low

w/c to achieve high compressive strength.

When compared to NSC, there is higher rate of strength gain in HSC at earlier

stages, as shown in Figure 2 (Carrasquillo et al., 1981). There is notable gain in

compressive strength of HSC after 28 days strength. 10-15% increase in

strength is obtained at 56 days and 95 days compared to 28 days strength.

Curing of HSC has a strong influence on the strength development because of

its low w/c ratio. Iravani (1996) investigated the effect of curing conditions on

strength gain of HSC at later stages and concluded that drying followed by

moist-curing increased the 147-day compressive strength of HSC relative to

continuously moist-cured concrete tested under moist conditions. Based on

his test results, he also claimed that 3 weeks is a sufficient most-curing period

for HSC. Testing age of HSC specimens depend on construction requirements;

however, considering notable strength gain at later stages, testing age of 56

days or 90 days is often recommended (ACI, 2010).

Figure (2) Figure(1)

Tensile strength

The tensile strength of HSC is significantly greater than that of NSC, though to a

lesser extent than the compressive strength. Fracture surfaces are smooth,

indicating the homogeneity of the material. The densification of the concrete

matrix and the aggregate-matrix transition zone explains the improvement of the

tensile strength.

The characteristic flexural tensile strength, f′cf, and the characteristic principal

tensile strength, f′ct in accordance to various standard procedures are

summarized in Table 2.The recommended values are plotted against f’c in Figure

3,4.

Figure (4) Figure (3)

Stress-strain Behavior in Compression

Stress-strain behavior of concrete for different range of compressive strength is shown in

Figure 5. The ascending branch of stress-strain is more linear and steeper for HSC. Strain at

maximum strength is greater and descending part becomes steeper compared to NSC.

Stress-strain behavior of HSC depends on material parameters such as aggregate type and

experimental parameters that include age at testing, strain rate and interaction between

specimen and testing machine. The stress-strain model used for NSC cannot be extended for

use in HSC as the nature of loading curve changes significantly. Steeper rise and sudden

drop in strength after maximum value presents difficulty in numerical modeling of stress-

strain behavior of HSC. Aītcin (1998) suggests that HSC behaves like a real composite

material and parallels can be drawn to the stress-strain behavior used in rock mechanics.

Carrasquillo et al. (1981) reported that there is less internal microcracking

in HSC than NSC for the same axial strain imposed, as shown in Figure 6.

This also implies that HSC experience less lateral strain, and consequently

effectiveness of confinement on compressive strength of HSC is often

limited compared to NSC.

Figure (5) Figure (6)

Elastic Modulus

Numerous equations have been proposed for estimating the modulus of

elasticity of HSC; however, due to large variations, most of the equations

are representative of only the selected data for that particular expression.

Moreover, ACI-363 (ACI, 2010) recommends that a design engineer

should verify the elastic modulus through a trial field batching or by

documented performance.

Modulus of Rupture

Applications

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Conclusions

Behavior of HSC in compression differs from NSC significantly. Mechanical

properties of HSC are sensitive to the type of coarse aggregates and curing

techniques. Mechanical properties in, including compressive strength, tensile

strength, stress strain behavior modulus of rupture and elastic modulas.

The constituents of HSC have been discussed. It is noted that the necessary

ingredients making the concrete such as fly ash, slag and silica fume are

used to increase strength and durability. This should be considered towards

recognition of HSC as an environmentally friendly material. The main

engineering properties of HSC are reviewed.