Properties of concrete

Since the ultimate goal in the design of any structure is generally safety, there are a number of precautions that needed to be taken when formulating the design approaches used today. Chief among these is the avoidance of brittle failure modes. 

If a structure were to fail in service, people inside that structure would be at great risk if they had insufficient warning to vacate the premises before collapse. If such a failure occurred instantaneously, as in brittle behavior, there would be no warning. Alternatively, if there was a large amoimt of deformation, movement and noise 

Concrete is relatively strong in compression but very weak in tension (5). If the beam were to be made entirely from concrete, it would fail at the bottom surface under a very low load, possibly even its own weight, and that failure would be very brittle in nature. Thus, something must be done to the lower portion of the beam to prevent the tensile stresses from fading the concrete. 

The basic theory behind conventional reinforced concrete beam design is well known. Essentially, steel reinforcement is placed near the bottom of the beam and is used to carry the tensile stresses while the concrete at the top of the beam carries the compressive stresses. To avoid failure of this concrete in compression, the steel is actually underdesigned so that it will fail first. Thus, the concrete never reaches its ultimate capacity (5). 

Furthermore, the concrete in the bottom portion of the beam is not even considered in the design since its strength is very low in tension relative to the steel. Its job is simply to protect the steel from the surroimding environment by acting as a barrier to deleterious 

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In more recent times, naphthalene has been used in condensation products from naphthalene sulfonic acids, utili2ing formaldehyde as additives to improve the flow properties of concrete these are referred to as superplastici2ers. Another newer appHcation is the production of diisopropylnaphthalenes. The mutual depression of the melting points in the mixture gives a Hquid which is used as a solvent for dyes in the production of carbonless copy paper. 

A. M. Neville, Properties of Concrete, 2d (metric) ed.. Pitman Publishing, London, 1973. 

In the middle of the last century, the tensile properties of concrete were improved by the introduction of steel to reinforce the concrete. This practice has developed since then to such an extent that reinforced concrete is now one of the major structural materials used in construction. In general it has proved to be a good durable material with some of the structures erected at the turn of the century still providing satisfactory service in the late 1970s. 

The sensitivity of Magnetic Resonance (MR) to the local concentration, molecular dynamics and molecular environment of these nuclei make it well suited for the study of deterioration processes in concrete materials. Hydrogen (water), lithium, sodium, chlorine and potassium are all MR sensitive nuclei and play an important role in cement chemistry. The ability of MRI to spatially resolve and non-destructively examine test samples as a function of treatment or exposure has the potential to provide new insight to better understand deterioration mechanisms and mass transport properties of concrete materials. 

The properties of concrete can be considered in terms of a number of stages ... 

The major physical attributes of concrete as a construction material are a high compressive strength and stiffness, an ability to protect and restrain steel and, most important of all, to retain these properties over a considerable period of time. The effects that water-reducing admixtures have on these properties can be considered from the point of view of design parameters, i.e. those properties of concrete at a relatively early age (usually 28 days) which are used for structural calculations, and longer-term aspects or durability. 

The three most important properties of concrete used in calculations for load-bearing applications are the compressive strength, the tensile strength and the modulus. However, for certain applications, e.g. water-retaining structures, the permeability or porosity of the concrete will be a relevant design criterion and this is also considered here. 

In order to understand the way in which superplasticizers affect the properties of concrete, studies have often been made on cement pastes in view of the convenience of such investigations. 

Table 6.9 Mechanical properties of concrete containing calcium nitrite (Berke and Weil)...

Table 7.10 Physical properties of concrete with and without the delayed addition of water-reducing set-retarding admixture (Dodson)...

Often it is difficult to adjust the mixture proportions to achieve desired design parameters for all properties of concrete. Consequently the properties of colloidal underwater concrete are controlled by the addition of three chemical admixtures. Minimum water-cement ratios range from 0.36 to 0.40. Cement and fine-aggregate contents are usually higher than corresponding mixes placed on land, and silica fume may be used in conjunction with a superplasticizer or conventional water reducers to reduce segregation. The key to a non-dispersible concrete with self-leveling characteristics is the successful optimization of the VEA with the superplasticizer used to increase the slump. 

Many admixtures affect more than one property of concrete, sometimes adversely affecting desirable properties. 

Mielenz, R.C. (1960). Symposium on the Effects of Water-Reducing and Set Retarding Admixtures on the Properties of Concrete, ASTM SP-266, 161-82. 

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