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High Strength Concrete

North Harbour Stadium 1997

Influences on Concrete Strength Performance ECS High Strength Concretes Enhancing concrete service under demanding service conditions Curing Technical Data

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High Strength Concrete

Influences on Concrete Strength Performance

The main factor influencing concrete compressive strength is the water-cement ratio of concrete. The less water in a mix, the stronger and less porous the concrete. The permeability of concrete is even more sensitive to water-cement ratio changes than is the compressive strength. It is important to note that there is a practical limit to which batching water can be restricted in concrete mix design to achieve a desired water-cement ratio. Depending on aggregate type, the minimum batching water is in the range 150-170 litres per cubic metre, equivalent to a slump of 40mm. To achieve a lower water-cement ratio in a concrete mix with minimum batching water, cement contents need to be increased and/or superplasticizer admixtures used. Concrete gains strength with time. Provided there is a supply of moisture to allow cement hydration this strength increase can continue for years. Concretes with different cement types gain compressive strength at different rates. Temperature is a key determinant of strength gain. Hydration is a chemical reaction and is controlled by temperature. Below 12ºC cement will be very slow to react, although concrete cured at about 8ºC will ultimately gain more strength than that cured at higher temperatures. Above 40ºC concrete will gain strength very rapidly but ultimate strengths will be compromised. Slag-based cements do not exhibit this characteristic. Figure 1 shows that concrete for three regional centres with the same cement content have different compressive strengths (and different water-cement ratios) due to differing water demand for the various local aggregates. The early strength of concrete containing rounded aggregates (as in Christchurch) is limited by the bond between cement paste and the coarse aggregate.

Typical Variation of Strength and Permeability with water-cement ratio

Strength (MPa) Concrete Strength Concrete Permeability

50

Permeability

x 3-4

x 5-10

15

0.45

water-to-cement ratio (w/c) 0.85

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ECS High Strength Concretes

These concretes are tailored to achieve high early strength and also high ultimate strength.

ECS High Early 1 A concrete with superior early strength development for precast and onsite concrete construction.

Portland cement (Type GP) with superplasticizer/accelerator admixture. 5-10% pricing premium for approximately 100% strength improvement over conventional concrete at 24 hours with accelerated set time.

ECS High Early 1 is a concrete that exhibits excellent high early strength performance. Early strength development is increased approximately 50% when heat is applied, and approximately 100% under normal conditions (Figure 2). For a cost premium of 5-10% ECS High Early 1 enables the fast turnaround of pre-cast moulds or the early lifting of tilt panels. This concrete also has excellent early set times. ECS High Early 1 develops superior (approximately 10% increase) ultimate strength than conventional concrete due to its lower water-cement ratio.

ECS High Early 2 A concrete for precast producers with excellent early strength development and finish characteristics.

High Early (Type HE) Cement. 5% pricing premium for approximately 50% strength improvement over conventional concrete at 24 hours. Excellent set time, good off-form finish and 5% improvement in ultimate strength.

ECS High Early 2 is an economical concrete that exhibits good high early strength development performance. The improvement in early strength gain is approximately 30% when heat is applied, and approximately 50% under normal conditions. For a cost premium of approximately 5% ECS High Early 2 enables the fast turnaround of precast moulds. Furthermore, it has a faster set. ECS High Strength Concrete is designed to achieve compressive strengths in the range 50 to 100 MPa. This level of performance is achieved through the utilisation of high reactivity pozzolan such as Microsilica 600 or Metamax. A compressive strength of 70 MPa is often a maximum strength limit set by design considerations for seismic performance. High reactivity pozzolan in combination with high cement contents create denser concrete due to superior packing and the inclusion of additional binder material. High Strength Concrete has superior performance over conventional concretes. High Strength Concrete has a range of durability attributes; wear and abrasion, low chloride diffusion, high resistivity, reduced permeability and high chemical resistance.

ECS High Strength Concrete A concrete which achieves high strength at early and final age.

Ordinary Portland Cement (Type GP) with High Reactivity Pozzolan Will achieve concrete strengths in the range 50 to 100 MPa at 28 days.

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Enhancing Concrete Performance under Demanding Service Conditions

The compressive strength of concrete is commonly cited as the single most important indicator of concrete durability. Concrete compressive strength and the associated concrete water-cement ratio are linked to concrete's ability to carry load, to withstand abrasion and wear, to prevent the ingress of damaging fluids, and to resist chloride, sulphate and other chemical attack. Apart from compressive strength, other concrete attributes such as cement type and permeability are also important considerations regarding durability. Slag cement-based concretes can outperform conventional higher strength concretes under certain environmental conditions i.e. exposure to chlorides or sulphates. Higher strength concrete enables the design of lighter more slender structures. The cost per MPa of compressive strength is more economical for high strength concrete. With recent advances in composite materials, chemical admixtures and alternative mineral components high strength concrete can now be readily produced from most concrete batching plants. Furthermore, "high early strength concrete" is available which allows fast turnaround of precast moulds and/or savings in construction times.

Curing

The long-term performance of all ECS concretes can only be achieved in association with good concrete practice and controlled curing. Curing will assist in ultimate compressive strength, reduce surface dusting and enhance resistance to abrasion. Curing allows more water to be made available for the hydration reaction of the concrete's cement paste which leads to better strength development. Technical advice on the handling and curing requirements of ECS concretes should be sought before starting construction. (See brochure SC4 Curing)

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Technical Data

Figure 1

Average Strength Results

(Auckland, Wellington, Christchurch concrete) - BRANZ 1999

Christchurch 350kgs cement/m3 Wellington 350kgs cement/m3 Auckland 350kgs cement/m3 Auckland 280kgs cement/m3

6.4 5.0 48 5.6 4.5 46 5.4 4.5 56.5 5.2 4.5 42

Flexural Tensile Strength Tensile Splitting Strength Compressive Strength

1

Figure 2

10

100 MPa (Log Scale)

Comparison of early age strength development at 20oC (350kg cement/concrete m3)

40 40 35 30 25 20 15 10 5 0 24 hour 2 day Control ECS High Early 1 ECS High Early 2

Compressive Strength

35 30

MPa

25 20 15 10 5 0 18 hour

time

Comparison of early age strength development at 42oC

40

40 35 30 25 20 15 10 5 24 hour 0 2 day Control ECS High Early 1 ECS High Early 2

Compressive Strength

35 30

MPa

25 20 15 10 5 0 18 hour

time

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Other topics in this series of brochures include: ECS 1 Marine & Coastal ECS 2 Chemical Resisting ECS 3 Crack Control ECS 5 Industrial & Commercial Floors ECS 6 Abrasion Resisting Also Site Concrete series: SC 1 Ordering Ready Mixed Concrete SC 2 Moving Concrete SC 3 Placing & Compacting Concrete SC 4 Curing of Concrete

Phone: 0800 ECS DATA 0800 327 328 Fax:

The information presented in this brochure is offered in good faith, however, due to differences in specific conditions, environments and materials no responsibility can be taken for the application of the principles and procedures discussed. For advice on your particular project call these numbers.

0800 ECS FAX 0800 327 329

Email: [email protected] www.concrete.co.nz

Edition: August 1999

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