Strength of Hardened Concrete

In practice, since the cement content cannot be increased too much, the water content has to be reduced in order to achieve low w/c ratios as a consequence, the workability of concrete decreases. Below a certain w/c ratio, which depends on the technique used for compaction, a proper consolidation of fresh concrete cannot be achieved and thus the strength of hardened concrete decreases (as shown by the dashed lines in Figure 12.2). The use of a superplasticizing admixture is therefore necessary to obtain a suitable workability in particular for a low water content. 

South African National Standards. 2006. SANS 5863 2006, Concrete tests - Compressive strength of hardened concrete. South African National Standard. Standards South Africa. Pretoria, Gauteng, Republic of South Africa (2006). 

Figure 1. Results of compressive and splitting tensile strength of hardened concrete samples.

The most common example of a nonmetallic particle system in a nonmetallic matrix, indeed the most common composite material, is concrete. Concrete is particles of sand and gravel (rock particles) that are bonded together with a mixture of cement and water that has chemically reacted and hardened. The strength of the concrete is... 

Splitting tensile strength of cylindrical concrete specimens Microscopic determination of parameters of the air-void system in hardened concrete... 

To improve the quality of hardened concrete such as increased early and long-term strengths and modulus of elasticity, decrease permeability (and hence inhibit corrosion of embedded steel reinforcement) and absorption, increase abrasion resistance and increase bond with reinforcement. 

Properties of hardened concrete Average compressive strength ... 

The mechanical properties of hardened concrete can be classified as (1) short term or instantaneous properties and (2) long term properties. The short term properties can be enumerated as (a) strength in compression, tension and shear and (b) stiffness measured by modulus of elasticity. The long term prop-... 

Concrete in the fresh state should have good workability in order to be properly placed and compacted in the mould so that it can reach the expected properties after hardening. The most relevant properties of hardened concrete are strength, deformation due to loading or thermal and moisture variations, resistance to cracking, and durability. 

A review of research on the performance of concrete containing GRT particles was recently published (Pacheco-Torgal et al., 2012). In particular, this review discussed the effect of GRT treatment, the size of GRT particles, and the replacement volume on the fresh and hardened properties of concrete. A workability of fresh concrete defining its flowability and the compressive and tensile strength, toughness, elastic modulus, thermal and sound properties, and durability of hardened concretes containing the tire rubber waste was also discussed. 

Carbonation can have a positive influence on the strength of concrete, and it has been used for a long time in practice [289,312]. Carbonation is not effective in the case of fresh, poorly hardened concrete because it results in softerring of concrete surface [61], However, the surface carbonation of hardened concrete can irrrprove the resistance to the attack of aggressive media, for example of sulphates [61],... 

The impregnation of hardened concrete or mortar with polymers increases the strength of the material signifieantly forrrfold increases are not rmcommoa At the same time the material attains a more brittle character. The creep of the material is also reduced... 

Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens (ASTM C39-86). Standard Test Method for Making and Curing Concrete Test Specimens in the Field (ASTM C31-85). Standard Test Method for Penetration Resistance of Hardened Concrete (ASTM C803-82). 

In brittle matrices the reinforcement may be either in the form of ductile fibres to increase toughness or of deformable particles and pores to block the crack propagation. Stiff fibres or hard particles are introduced into ductile matrices to increase their strength. Short chopped fibres are used in many cases, but also long continuous fibres and different types of nets and fabrics. The polymer structure introduced to a porous stiff matrix of hardened concrete may be also considered as its reinforcement. 

Figure 6.20 Relationships between volume of capillary pore space and compressive strength of hardened cement paste, mortar and concrete for different pore distributions, after Uchikawa (1988).

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