TECHNICAL UPDATE

1. INTRODUCTION

It is unquestionable that cement and concrete have marked the way and the form of building in the 20th century. The arch-shaped / vaulted constructions which prevailed along with metallic constructions to bridge gaps, gave way to straight ones or to those with shell like surfaces displaying a broad range of shapes which are based on reinforced concrete and later on, on pre-stressed concrete. Concrete allows the engineer to give to the construction a wide range of shapes, due to its advantage to be able, when in green condition, to cast into moulds with the desired form and shape. Several days later, when the concrete has been partly hardened, the moulds are taken out, leaving a construction of the desired shape, in stiff stone form, resistant to fire and to environmental influences. Combining the relatively high compression strength of concrete with the high tensile strength of steel, which is being incorporated in the mass of concrete in proper positions in the form of cyclical bars (reinforced concrete), the engineer can bridge wide gaps or create constructions of big height, which were impossible to be realized with the materials available at the time (with the exception of steel). The introduction of pre-stressed concrete later on, gave a new boost to the material, multiplying its capabilities. The introduction of the use of chemical additives in concrete and especially of super plasticizers can be considered as substantial progress (1980). These elements, provided the possibility of producing high strength and durability concrete, giving birth to high performance concrete.
Currently rapid developments are occurring and it is really difficult to predict the future. Following, there will be a short reference concerning the developments in cement use, having as a criterion the perspective of being applied in our country. Complementary to new applications, other ones which have been successfully used in other countries, but not yet in Greece, will also be mentioned.

2. THE PRESENT

2.1 Concrete of high strength or of high early strength

Today, concrete with 140 MPa compressive strength is being used in a lot of countries, while in special occasions, concrete of much higher strength has been prepared and used. This concrete is being utilized in the construction of high buildings, in bridges with wide openings and in special constructions (i.e. oil pumping platforms). Regarding the restoration of street or airport pavements, where direct opening to traffic (within 6 to 12 hours) is a must, concrete with high requirements at early ages is applied. The reduced «own» weight relative to the bearing capacity of the structural member, the speed of construction, the limited use of aggregates and therefore the reduced environmental strain, are few of its advantages, which are being combined with the improved durability and often lead to a lower final cost. In a research of the laboratory of the Hellenic Cement Industry Association, conducted in cooperation with the National Technical University of Athens and concerning mix design to produce concrete of very high strength, with ordinary limestone aggregates of Attica, it was found that the 28 days compression strength was 100 MPa without additives, 110 MPa with the additive of ground natural pozzolan and 120 MPa with the addition of silica fume. At another study, an early 12 hours strength of 55 MPa was recorded.

2.2 High performance concrete

With this term, we refer to concrete, which totally «fulfills» the purpose for which it was produced. Nowadays, the prevailing concrete requirements have changed from «high strength concrete» to «concrete of high durability and better behavior through time». Therefore, high performance concrete should combine high strength and high durability in corrosive environment, while at the same time preserving these properties. In other words, the term in this sense can be misleading because such a concrete should have been required as well in the past, which was the case, but the requirements in general were of a lower degree and particularly the one concerning strength. Today, the requirements in many occasions are very high. It has to be mentioned for instance, that in oil pumping platforms of the North Sea, durability is required: against frost, against chlorides and against waves of 31 meters high. In the future, it is foreseen that the concrete produced will be towards the high requirements concrete category, with substantial reduction of the water to cement ratio (W/C < 0.45) and use of blended cements with a high percentage of chemical additives.

2.3 Self- compacting concrete

A main characteristic of self-compacting concrete is the increased fluidity (workability), which helps placing and compacting of the material. Those two works are being carried out without human intervention (also called self-leveling).
With the self-compacting concrete, the non-segregation of materials is guaranteed, through:

1) Proper choice of aggregates
2) The use of fine (dust particles’ size) aggregates
3) The use of special additives

Self-compacting concrete has a low W/C ratio and high durability and is recommended for constructions with dense reinforcement arrangement like contemporary earthquake-resistant constructions.
Self-compacting concrete which started from Japan since the last decade and spread throughout the world, changes the existing situation in the construction sector, as it contributes to:

• Faster application
  
• Higher performance
  
• Reduced casting cost
  
• Improved working conditions
  
• Reduced environmental disturbances

2.4 Self-compacting concrete of low strength

They are self-compacting concretes with a very low cement content and compressive strength, which ranges from 2-8 MPa and accordingly they can have more than one typical application. The most widespread uses are as a landfilling material, as a seating material, draining sewage filling, pavement, square and pedestrian street construction material. The characteristic property of this concrete is that it can be easily and quickly demolished without any particular tools (air compressors, digging machinery e.t.a.)

2.5 Reactive powder concretes



It consists of a revolutionary cement application, which creates a new range of concretes with endless use in the future. The compressive strength of this material ranges from 200 to 800 MPa, its tensile strength from 25 MPa to 150 MPa, the crushing energy amounts to 30,000 J/m2 and the density varies from 2500 to 3000kg/m3. It is based on the creation of a very homogeneous blend with the use of very fine materials and non-continuous gradation of proper mix so as to achieve the highest possible density (the ratio of the diameter of the various fractions is approximately 7). The W/C ratio is close to 0.15 and the workability of these mixes is controlled with high percentages of super-plasticizers, while pressure is being applied during the hardening. To increase the deformation prior to the crushing, steel or carbon fibers are being used. Due to the very small water/cement ratio, a considerable percentage of cement is not hydrated and is being used as an aggregate. In figure 1,, cross-sections from this new material are being compared with respective cross-sections from steel, conventional reinforced concrete and pre-stressed concrete. The most well-known and spectacular use of this material to date is the 6 meters opening pre-stressed pedestrian bridge in Sherbrooke, Canada, with a 200 MPa compressive strength.

2.6 Fiber-reinforced concrete

The deformation prior to the breaking and the respective energy consumed, increase substantially with the use of steel fibers. Due to these properties, the range of usage of concrete is widened a lot also as far as gunite is concerned.

2.7 Cement in road building and in transports





The contribution of cement to transports and particularly to road building is considerable. Bridges, tunnels, safety barriers, concrete pavements as well as soundproof barriers from concrete are several examples of successful cement use. A characteristic of the implementation of cement in the aforementioned applications is the small maintenance cost. In road building, cement is also used for the treatment of aggregates and the stabilization of soils. With the production of cement binding mixtures, there is an upgrading of the bearing capacity of sub grade along with an increase of the bearing capacity of the layers of pavement construction. This increased bearing capacity provides the ability to construct road surfaces of less thickness and thus of a significantly lower cost.
The uses that don’t have yet much application in our country, are briefly mentioned below.




2.7.1 Concrete pavements



Due to their bearing capacity and their characteristic property not to deform under heavy permanent loads, the road surfaces from concrete are being used for:

• Parking aprons and doorways for the takeoff areas in airports
  
• Parking grounds for vehicles of heavy weight
  
• Floors for containers at ports
  
• Industrial floors with high requirements in terms of flatness and durability to abrasion
  
• Road floors of tunnels of big length for fire safety reasons
  
• Floors at toll stations

Concrete floors form an unrivalled type of paving for highways, streets of heavy traffic and simple rural roads. These floors, apart from their ability to withstand permanent deformations and their high bearing capacity, do possess other advantages, which render them attractive in their application:

• A greater degree of safety in driving due to improved visibility. The reason is that obstacles are more visible in concrete grounds due to the bright color of the surface in comparison to asphalt road paving
  
• Small to insignificant maintenance cost
  
• High durability over time
  
• Reduced total cost of the floor (life cost analysis)
  
• Insignificant rolling noise
  
• Low tire wear
  
• Lower overall energy consumption

2.7.2 Soil stabilization with cement or with cement and hydrated lime or with cement and fly ash

Cement can be utilized as well in the construction of pavement layers and for the stabilization of subgrade soil materials with average or small plasticity.
In cases of soil materials with more than average plasticity or in cases of swelling soil materials, a combination of cement and hydrated lime or cement and fly ash can be used with impressive results as far as the improvement of the mechanical properties of these soils is concerned.

In all these cases, the durability of the stabilized materials against environmental impacts is substantially higher than that of the non-stabilized material. In this way, considerable savings are achieved due to the reduced thickness of the road pavements, while the environmental advantages are equally important, since there is no use of materials transported from other areas or use of quarry materials. Constructions of this type are widely applied in countries abroad, while there is no yet wide application in our country.

2.7.3 Recycling of pavement with cement

It is a recent successful evolution, with which restoration and frequently upgrading of worn from traffic road surfaces can be achieved, with the least possible environmental strain and with a very low cost. In these cases, the material of the worn road surfaces in a thickness from surface equal to 20-35 cm (including the asphalt layers) is being milled and pulverised with special machinery together with water and cement. The above materials are fully blended and spread to form a cement stabilized layer on the surface of which one or two asphalt layers are added. With the new recycled road surface, the operating characteristics are restored and the pavement bearing capacity is substantially higher than that of the old pavement. Under this method, the material that exists on the road surface is used on site, without having to be transported to nearby installations or to areas of disposal and without having to transport new material. The environmental benefits are obvious.

3. THE FUTURE

3.1 Self cured concrete

The curing of concrete is a very important and necessary operation, which in many cases is neglected regardless of the well-known side effects that it has on the strength and particularly on the durability of concrete. In such cases, a question should be raised as to what is the benefit of strength or of proper mix design, when the material that is incorporated in the construction is not durable; For this reason, since long ago, there has started an effort to produce «self cured» concrete, which will have no need of curing because either evaporation of the contained water is effectively hindered or the concrete by itself will provide to its mass the extra water needed during hydration. Progress has been achieved in both of the aforementioned ways of producing concrete without need of preservation and it is expected that shortly this issue will be resolved.

3.2 Non- shrinking concrete

Expansive cements of several types are produced which, after setting, tend to swell compensating thus the shrinkage due to drying. These cements are used for the production of shrinkage compensating concrete. The use of these cements for the production of non-shrinking concrete is currently limited but as it looks it will increase substantially in the future.

3.3 High Technology Cement

The technological research concerning the production of cement is an ongoing one, covering a wide spectrum of factors, including among others, increase of strength, reduction of the drying up contraction with the creation of controlled swelling, increase of durability against chemical attacks, reduction of emissions, increase of the energy required during crushing e.t.a.