Cement paste with reinforcement

The chemical reactions of cement paste with zinc (galvanized steel) result in the release of zinc to the solution and the formation of calcium zincate, CaZn2(OH)g 2H2O, covering the surface of metal [49], Simultaneously, there is no portlandite crystals in this transition zone because in the presence of zinc alite hydration is delayed (see Sect. 4.1.3.2). The coating of hydrated calcium zincate crystals on the surface of metal increases the bonding forces between paste and reinforcement by the roughness formation on its surface. 

Results of other verification have been published by Laws (1974) following tests on cement paste specimens reinforced with continuous and aligned glass fibres of various volume fractions (Figure 10.12). The tensile strain at fracture of the plain paste was not given nevertheless, the influence of fibres is clearly visible and in general it confirms the ACK theory predictions, if... 

In hardened cement-based composites the transportation of liquids and gases through pore and microcrack systems plays a very important role in many processes, such as hydration of Portland cement, pozzolane effects of microfillers, carbonation, corrosion of cement paste and reinforcement due to reaction with external agents, shrinkage and creep, etc. These processes are partly described in respective Sections 4.1, 4.3, 6.5 and 11.5. Only basic information is reiterated below concerning the flow of liquids and gases through concretes and mortars. 

Strengthening the interfacial zone betv/een steel and cement paste of reinforced concrete with our technology is a effective way to iraprove the durability and mechanical properties under loading. 

To an extent that increases with the w/c ratio, fresh cement pastes exhibit the phenomenon of bleeding, i.e. settlement of the solid particles. The interparticle attractions are sufficiently strong that particles of all sizes settle at the same rate, typically about 2pms . Settlement also tends to increase the w/c ratio at the top and to decrease it at the bottom of the sample. It decreases with increased fineness or increased early hydration rate of the cement. In a concrete, it can produce layers of water beneath aggregate particles or reinforcing bars. 

Since ancient times, natural fibers have been used to reinforce brittle materials. For example, thousands of years ago, Egyptians began using straw and horsehair to reinforce and improve the properties of mud bricks. In more recent times, large-scale commercial use of asbestos fibers in a cement paste matrix began with the invention of the Hatschek process in 1898. However primarily due to health hazards associated with asbestos fibers, alternate fiber types have been investigated and introduced throughout the 1960 s and 1970 s. 

In the case of a concrete structure, the presence of steel reinforcements can further complicate the system by causing a local heterogeneity since they constitute in terms of dose rate (driving of the radiolysis) or by the influence of iron ionic species on radiolysis at the interface with the cement paste. Although still not very well-studied, the resulting radiolysis-corrosion coupling (as a function of temperature) is identified as the only aspect which could possibly affect the concrete durability under radiation [1]. 

Concrete is a composite material made of aggregates and the reaction product of the cement and the mixing water, i. e. the porous cement paste. The structure and composition of the cement paste determines the durability and the longterm performance of concrete. Concrete is normally reinforced with steel bars. The protection that concrete provides to the embedded steel and, more in general, its ability to withstand various types of degradation, also depends on its structure. This chapter illustrates the properties of the most utiHsed cements and the microstructure of hydrated cement pastes. Properties of concrete and its manufacturing are discussed in Chapter 12. 

Hydration of calcium sihcates also produces hexagonal crystals of calcium hydroxide (Ca(OH)2, Porilandite). These have dimensions of the order of a few pm and occupy 20 to 25 % of the volume of solids. They do not contribute to the strength of cement paste. However, Ca(OH)2, as well as NaOH and KOH that are present in small amounts, are very important with regard to protecting the reinforcement, because they cause an alkaline pH up to 13.5 in the pore liquid (Section 2.1.1). 

Adsorbed water. Even when water has evaporated from the capillary pores, some water will still remain adsorbed to the iimer surface in the form of a very thin layer of adsorbed water. This water can be removed if the external humidity falls below 30 % it contributes htde to transport phenomena, thus it is insignificant with regard to corrosion of reinforcement. Its removal, however, causes shrinkage of the cement paste and influences creep behaviour. 

Generally, if the crack width is modest (e. g. it is below 0.3-0.5 mm), after the initiation of corrosion on the steel surface, the corrosion rate is low. Chemical processes in the cement paste and formation of corrosion products may seal the crack near the reinforcement and allow the protective oxide film to form again. For car-bonation-induced corrosion, repassivation can take place when the migration of alkalinity from the surrounding concrete brings the pH of the pore solution in contact with the corrosion products to values above 11.5. Repassivation may have trouble taking place or may not take place at all in the following situations ... 

Fig. 6.20 Strength of cement paste bond with different metal reinforcement, (according to [51]) 1—brass, 2—copper, 3—soft steel, 4— stainless steel...

In a paste cured for a short period of time, the cracks induced during bending occur in the vicinity of the paste interface with reinforcement, while in the old samples— along the contact plane [51]. This observation is not fulfilled for copper and brass, which proofs a better cement paste adhesion, of chemical nature, to these metals (Fig. 6.20). 

Covering of reinforcing bars with resin, apphed as an anti-corrosion protection, can lower their adhesion to cement paste. Worsening of adhesion occurs in the case of vinyl polychloride, but it does not occirr if the epoxy resins are used. However, some epoxy resins reveal the creep effect [56]. 

In steel-reinforced concrete stractures made with calcium alununate cement and with a sufficiently low water/cement ratio, the reinforcement is sufficiently protected from corrosion. However, in mixes made with too much water, corrosion of the steel may take place, especially after conversion of the hardened paste has occurred, as the cement paste becomes too porous and too permeable for oxygen of the air. Carbonation of the paste, which progresses especially easily in porous mixes, enhances the corrosion process even further, as the pH of the pore solution drops from its original 10-12 to lower values, making the steel susceptible to corrosion. 

Chloride ions present in the cement paste promote the corrosion of the reinforcement in steel-reinforced concrete, by causing a breakdown of the protective oxide film that develops at the steel surface at high pH. Under these conditions iron oxide (mst) is formed at the steel/concrete interface, and the expansion associated with it causes cracking and spalling of the surronnding concrete. 

A typical feature of hardened inorganic cement pastes is their low tensile/ compressive strength ratio. To improve this ratio and to lower the brittleness of the material, cementitious systems may be reinforced with suitable fibers randomly distributed within the cement matrix (Fig. 25.1). 

Table 25.1 summarizes the properties of the fibers most commonly used in fiber-reinforced cementitious composites and compares them with those of ordinary Portland cement paste. 

Carbon fiber has been found to be an effective thermistor [192-194], such as a cement paste reinforced with chopped carbon fiber (about 5 mm long) with silica fume (15 wt% cement). Its electrical resistivity decreased reversibly with increasing temperature (1-45°C), with activation energy of electrical conduction (electron hopping) of 0.4 eV. This value is comparable to semiconductors (typical thermistor materials) and is higher than that of carbon fiber polymer matrix composites. The current-voltage characteristics of carbon fiber reinforced silica fume cement paste were linear up to 8 V at 20°C. 

Long fibres or mats of fibres of glass, carbon or aramld have been used to reinforce cement paste and concrete but are not considered practical for use In the large concrete pours associated with a PCRV. Short chopped strands could be mixed with the concrete but these would not fulfill the main function of reinforcement close to the reactor cavity, namely to control the opening of cracics, particularly at ultimate load. 

Romualdi first proposed fibres as dispersed reinforcement for concrete in his two papers in 1963 and 1964. Later on, Biryukovichs proposed the employment of glass fibres into concrete, which were originally not resistant and durable in the highly alkaline Portland cement paste. It was Majumdar and Ryder (1968) who invented the alkali-resistant (AR) glass fibres with the addition of zirconium oxide (Brandt, 2008). 

Asbestos cements may be considered as one of the oldest man-made fibre-reinforced materials. They were already being produced in 1900 at a time when other fibre-reinforced composites were not known. Natural inorganic asbestos fibres were used only with neat cement paste and a composite material was obtained with high tensile and flexural strength and several other excellent mechanical properties see Section 5.2. 

Wood fibres are produced in the form of chips, which is usually a waste material in the wood industry. Wood chips mixed with cement paste have been used since the 1920s for the production of sheets applied for thermal insulation in housing. The chips are subjected to chemical pre-treatment to avoid any disturbance of cement hydration by organic acids. The application of wood-origin fibres as a reinforcement for minor structural elements has been developing at a local level. 

The main mechanical problems in the brittle matrix composites with textile reinforcement concern the bond between cement paste and fibres and the durability of the fibres (Brandt 1990). These problems are interrelated. If the alkalinity of the cement paste is not corrosive for the material of the fibres, then the chemical bond does not exist and adherence should be ensured other ways for example by ... 

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