Cement paste plasticity

The rheological characteristics of AB cements are complex. Mostly, the unset cement paste behaves as a plastic or plastoelastic body, rather than as a Newtonian or viscoelastic substance. In other words, it does not flow unless the applied stress exceeds a certain value known as the yield point. Below the yield point a plastoelastic body behaves as an elastic solid and above the yield point it behaves as a viscoelastic one (Andrade, 1947). This makes a mathematical treatment complicated, and although the theories of viscoelasticity are well developed, as are those of an ideal plastic (Bingham body), plastoelasticity has received much less attention. In many AB cements, yield stress appears to be more important than viscosity in determining the stiffness of a paste. 

Consistency, working time, setting time and hardening of an AB cement can be assessed only imperfectly in the laboratory. These properties are important to the clinician but are very difficult to define in terms of laboratory tests. The consistency or workability of a cement paste relates to internal forces of cohesion, represented by the yield stress, rather than to viscosity, since cements behave as plastic bodies and not as Newtonian liquids. The optimum stiffness or consistency required of a cement paste depends upon its application. 

A viscometer can be used to study the yield stress and viscosity of cement pastes (Section 1.3.1). This is carried out by plotting the shear rate against shear stress as shown in Fig. 2.4 for cement pastes of various water cement ratios. These cement pastes are generally considered to exhibit Bingham plastic behavior where the yield value is the intercept on the shear stress axis and is related to cohesion, and the slope of the line is the apparent viscosity which is related to the consistency or workability of the system. The following general observations can be made ... 

Superplasticizers reduce both the yield value and plastic viscosity [24] of cement pastes. At higher dosages (for example 0.8% for SNF) [5, 6], the yield value approaches zero and the system becomes essentially Newtonian. 

At early age of cement paste reaction with water the plastic shrinkage occurs, which consists in evaporation of water from the surface layers of concrete [108]. The volume of fresh concrete is thus reduced. This process is not related to the properties of cement itself however it can also result in the formation of cracks. This situation is the consequence of substantial humidity gradient in concrete. The plastic shrinkage, equal 2.5 mm/m, observed in the 7 cm thick surface layer of concrete, was two times higher than the shrinkage in the core of element [108]. 

The adhesion of steel to cement paste consists primarily in the meclianical interpenetrating of both materials and hence the so-called elastic component of shear strength is produced. At low load the plastic deformation occurs and subsequently... 

Fig. 6.92 Viscosity of cement pastes (w/c = 0.40) with plasticizers added a to the mixing water, b at various time after mixing, (according to [374]) 1—melamine resin, 2—calcium naphthalene sulphonate, 3—sodium lignosulphonate + phosphate ester, 4—sodium lignosulphonate -r sulpho-nated naphthalene (traces), T—no admixture...

The time for which the suspension preserves its flowabilily rrray vary between a few minutes and several hours, and may be controlled by the amoimt of added additives (Harma, 1977 Odler et al, 1978 Satava and Tyle, 1994). In general, the setting time is extended with an increasing amount of lignosirUbnate, whereas the amoimt of carbonate has little effect on this behavior. Unlike the behavior of ordinary Portland cement pastes, the transition from the plastic to the set state occurs very suddenly. 

Bojadjieva, C., and Glavchev, I. (1995) Investigations on the influence of some plasticizers on gypsum-free cement paste. Cement and Concrete Research 25,685-684. 

The rheology of calcium aluminate cement pastes is broadly similar to that of Portland cement pastes (Banfill and Gill, 1986). They show a Bingham-type behavior with slightly lower yield values and a plastic viscosity. If agitated, they exhibit a stmctural breakdown that depends on the intensity and duration of mixing. Their flow curves are characterized by a distinct hysteresis loop. Usually, concrete mixes made with calcium aluminate cement are somewhat more free flowing, which makes it possible to use a lower water/cement ratio. 

Hasegawa et al (1995) developed a new cementitious material by combining calcium aluminate cement with a methanolic solution of a phenol resin precursor. The water-free mix must be processed by high-shear mixing in a twin-roller mill in a way similar to that used in the production of MDF materials. The rheology of the resin-cement paste may be improved by adding to the system small amounts of a modifier such as alcohol-soluble polyamide and a plasticizer. A typical mix proportion is as follows ... 

Unlike these two phases, tricalcium aluminate (C3A) yields significant amounts of ettringite in the early stages of hydration that is, at a time when the cement paste is still plastic, and when this reaction does not cause expansion. After setting, the formation of ettringite continues at a slow rate for an extended period of time, and is not terminated even after the concrete has lost its ability to expand without cracking. 

Plasticity—capacity of a fresh cement paste, mortar mix or eonerete mix being shaped. 

Bleeding. (1) The appearance of water at the surface of freshly placed concrete, hence the alternative name water gain it is caused by sedimentation of the solid particles, and can to some extent be prevented by the addition of plasticizers and or by air entraining (q.v.). A test for the bleeding of cement pastes and mortars is provided in ASTM C243. See also laitance. 

The fracture mechanics equations derived by Griffith after his tests on glass specimens directly concern the brittle behaviour of materials and are certainly better justified for hardened cement paste than for any other cement-based composite. The general application of the fracture mechanics is therefore associated with the additional assumptions that plastic or quasi-plastic effects are negligible, or with appropriate modifications of the linear formulae in LEFM. In that context the linear and non-linear fracture mechanics approach should be distinguished. 

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