Fiber reinforced Concrete

Fiber reinforced Concrete

Many different properties of the fresh and hardened concrete can be effectively influenced by adding fibers. There are innumerable different types of fiber with different material characteristics and shapes. Correct selection for different uses is important. As well as the actual material, the shape of the fibers is also a critical factor.

Fiber reinforced concrete is used for

·     Industrial flooring

·     Sprayed concrete

·     Slender structures (usually in precast plants)

·     Fire resistant structures

Properties of fiber reinforced concretes

·     To improve the durability of the structure

·     To increase the tensile and flexural strength

·     To obtain resistance to later cracking

·     To improve crack distribution

·     To reduce shrinkage in the early age concrete

·     To increase the fire resistance of concrete

·     To influence the workability

Concrete production

The fiber manufacturers’ instructions must be followed when producing fiber reinforced concretes. Adding the fiber at the wrong time or mixing incorrectly can cause great problems and even make the fibers useless.

·     Comply with the manufacturer’s adding time and method (i.e. at the concrete plant or in the ready mix truck)

·     Comply with the mixing times (balling/destruction of fibers)

·     Do not exceed the maximum recommended fiber content (considerable reduction in workability)

·     Fibers generally increase the water requirement of the mix (compensate for this with superplasticizer)

Fiber types

·     Steel fiber

·     Plastic fiber

·     Glass fiber

·     Carbon fiber

·     Natural fibers

Mass Concrete

Mass Concrete

Mass concrete refers to very thick structures (> 80 cm). These structures often have a large volume, which generally means that large volumes of concrete have to be installed in a short time. This requires extremely good planning and efficient processes.

Mass concrete is used for:

·     Foundations for large loads

·     Foundations for buoyancy control

·     Solid walls (e.g. radiation protection)

·     Infill concrete

These massive structures create the following main problems:

·     High internal and external temperature variations during setting and hardening

·     Very high maximum temperatures

·     High internal and external temperature variations and therefore forced shrinkage

·   Secondary consolidation (settling) of the concrete and therefore cracking over the top reinforcement layers and also settlement under the reinforcement bars

Risks

All of these problems can cause cracks and cement matrix defects:

So-called “skin or surface cracks” can occur if the external/internal temperature difference is more than 15°C or the outer layers can contract due to their drying out first. Skin cracks are generally only a few centimeters deep and can close again later.

Measures to be taken

·     Use cements with low heat development

·     Low water content (reduction in w/c ratio)

·     Largest possible maximum particle size (e.g. 0–50 instead of 0–32)

·     If necessary, cool the aggregates to obtain a low initial fresh concrete temperature

·     Place the concrete in layers (layer thickness < 80 cm)

·     Retard the bottom layers to ensure that the whole section can be re-compacted after placing of the top layer

·     Use curing with thermal insulation methods

·     Ensure the correct design and distribution of joints and concreting sections, to allow heat dissipation and to accommodate the temperature developments and differences

Fair-faced Concrete

Fair-faced Concrete

In modern architecture concrete is increasingly used as a design feature as well as for its mechanical properties. This means higher specifications for the finish (exposed surfaces). There are many ways to produce special effects on these exposed surfaces:

·     Select a suitable concrete mix

·     Specify the formwork material and type (the formwork must be absolutely impervious!)

·     Use the right quantity of a suitable release agent

·     Select a suitable placement method

·     Use form liners if necessary

·     Consider any necessary retouching

·     Color using pigments

·     Install correctly (compaction, placing etc.)

·     Thorough curing In addition to all of these factors listed, the following are important for the concrete mix:

Waterproof Concrete

Waterproof Concrete

Waterproof Concrete

Waterproof concrete is normally an impermeable concrete. To obtain an impermeable concrete, a suitable particle size distribution curve must be generated and the capillary porosity should be reduced.

Measures to reduce the capillary porosity are as follows:

·     Reduction in w/c ratio

·     Additional sealing of the voids with pozzolanic reactive material The concrete curing process is another parameter affecting the water resistance.

Composition

Aggregate

·     Well graded particle-size distribution curve

·     Fines content of the aggregate kept low

·    Adjustment to the binder content is usually necessary to obtain a satisfactory fines content

Cement

·     Conformity with the minimum cement content according to EN-206

Additions

·     Use of pozzolanic or latent hydraulic additions

w/c ratio

·     Low w/c ratio to reduce the capillary porosity

Placing

·     A plastic to soft concrete is recommended to produce water proof concrete

·     Careful and correct compaction of the concrete is important

Curing

·     Immediate and thorough curing is essential

Slipformed Concrete

Slipformed Concrete

In the slip forming method, the form work is moved continuously in sync with the concreting process in a 24-hour operation. The form work  including the working platform and the hanging scaffold mounted internally or on both sides, is fixed to the jacking rods in the center of the wall. The hydraulic oil operated lifting jack raises the form work by 15 to 30 cm per hour depending on the temperature. The jacking rods are located in pipe sleeves at the top and are supported by the concrete that has already hardened. The rods and sleeves are also raised continuously. These works are carried out almost entirely by specialist contractors.

Slip forming is quick and efficient. The method is particularly suitable for simple, consistent ground plans and high structures such as:

·     High bay warehouses, silos

·     Tower and chimney structures

·     Shaft structures

Because the height of the form work is usually only around 1.20 m and the hourly production rate is 20 to 30 cm, the concrete underneath is 4–6 hours old and must be stiff enough to bear its own weight (green strength). However, it must not have set enough for some of it to stick to the rising form work (“plucking”). The main requirement for slip forming without problems is concreting all areas at the same level at the same time, and then the simultaneous setting of these layers. Therefore the temperature has a major influence, along with the requirement for the consistently optimum w/c ratio.

High Strength Concrete

High Strength Concrete

High compressive strength

Concretes with high compressive strengths (> 60 MPa) are classified in the high performance concretes group and are used in many different structures. They are often used in the construction of high load bearing columns and for many products in precast plants.

Conventional high strength concrete mixes

In conventional high strength concrete production, the mix and the constituents require particular care, as does the placing.

·     High strength aggregates with a suitable particle surface (angular) and reduced particle size (< 32 mm)

·     A highly impermeable and therefore high strength cement matrix due to a substantial reduction in the water content

·     Special binders with high strength development and good adhesion to the aggregates (Silicafume)

·     Use of a soft concrete consistence using concrete admixtures to ensure maximum de-aeration

Sample mix:

CEM I 52.5                                      450 kg/m

Silicafume                                       45 kg/m

Aggregates                                     Crushed siliceous limestone0–16 mm

Eq. w/c ratio                                  0.28

Strength after 7 days 95 MPa

Strength after 28 days                110 MPa

Strength after 90 days                115 MPa

Innovative high strength concrete mixes

Many different alternative mixes for high strength concrete (and mortars) are being developed alongside conventional concrete mixes. The search for high strength constituents and a minimum water content is common to them all. Special aggregate particles and gradings with superplasticizers are used to achieve this. Strength development is also boosted by new drying and hardening techniques (such as compression hardening). Concretes produced in this way, which are more usually mortars, can reach strengths of 150 MPa to 200 MPa plus.

Note in particular that:

·     High strength concrete is always highly impermeable

·     Therefore the curing of high strength concrete is even more important than usual (inadequate supply of moisture from inside the concrete)

·     High strength concrete is also brittle because of its strength and increased stiffness (impact on shear properties)

·     By reducing the water content to below 0.38 some cement grains act as aggregate grains because not all of the cement can be hydrated

·     Apart from Portland cement, high strength concrete uses large quantities of latent hydraulic and pozzolanic materials which have excellent long term strength development properties

Frost and Freeze/Thaw resistant Concrete

Frost and Freeze/Thaw resistant Concrete

Frost and freeze/thaw resistant concrete must always be used when concrete surfaces are exposed to weather (wet) and the surface temperature can fall below freezing.

·     Fair-faced concrete façades

·     Bridge structures

·     Tunnel portal areas

·     Traffic areas

·     Retaining walls

By adding air entrainers, small, spherical, closed air voids are generated during the mixing process in the ultra-fine mortar area (cement, finest grain, water) of the concrete. The aim is to ensure that the hardened concrete is frost and freeze/thaw resistant (by creating room for expansion o any water during freezing conditions).

·     Type, size and distribution of air voids

Air voids contained in a standard concrete are generally too large (>0.3mm) to increase the frost and freeze/thaw resistance. Effective air voids are introduced through special air entrainers. The air voids are generated physically during the mixing period. To develop their full effect, they must not be too far from each other. The “effective spacing” is defined by the so-called spacing factor SF.

·     Production/mixing time

To ensure high frost and freeze/thaw resistance, the wet mixing time must be longer than for a standard concrete and continue after the air entrainer is added. Increasing the mixing time from 60 to 90 seconds improves the content of the air voids by up to 100%.

·     Quantity of air voids required

To obtain high frost resistance, the cement matrix must contain about 15% of suitable air voids. Long experience confirms that there are enough effective air voids in a concrete if the results of the test (air pot) show the following air contents:

-       Concrete with 32 mm maximum particle size 3% to 5%

-       Concrete with 16 mm maximum particle size 4% to 6%

Fresh concrete with an air void content of 7% or over should only be installed after detailed investigation and testing.

Self-compacting Concrete

Self-compacting Concrete

Self-compacting concrete (SCC) has a higher fines content than conventional concrete due to a higher binder content and a different particle size distribution curve. These adjustments, combined with specially adapted super plasticizers, produce unique fluidity and inherent compact ability. Self-compacting concrete opens up new potential beyond conventional concrete applications:

·     Use with close meshed reinforcement

·     For complex geometric shapes

·     For slender components

·     Generally where compaction of the concrete is difficult

·     For specifications requiring a homogeneous concrete structure

·     or fast installation rates

·     To reduce noise (eliminate or reduce vibration)

·     To reduce damage to health (“white knuckle” syndrome)

Composition

·     Aggregate

Smaller maximum particle sizes of approx. 12 to 20 mm are preferable, but all aggregates are possible in principle.

·     Binder content

Based on the fines content, the following cement and aggregate contents can be determined, dependent on the concrete quality required and the sands used.

·     Water content

The water content in SCC depends on the concrete quality requirements and can be defined as follows.

·     Formwork facing

The forms for SCC must be clean and tight. The form pressures can be higher than for normal vibrated concrete. The form pressure is dependent on the viscosity of the concrete, the installation rate and the filling point. The full hydrostatic pressure potential of the concrete should be used for the general formwork design.

·     Placing method

Self-compacting concrete is installed in the same way as conventional concrete. SCC must not be freely discharged from a great height. The optimum flow potential and surface appearance are obtained by filling the form. This can be achieved by using tremie pipes etc.

Concrete for Traffic Areas

Concrete for Traffic Areas

Concrete for traffic areas has many applications and is often installed as an alternative to blacktop because of its durability and other advantages.

The uses of concrete for traffic areas:

·     Conventional road building

·     Concrete roundabouts

·     Runways

·     Industrial floors

When concrete is used for these applications, the concrete layer acts as both a load bearing and a wearing course. To meet the requirements for both courses, the concrete must have the following properties:

·     High flexural strength

·     Freeze/thaw resistance

·     Good skid resistance

·     Low abrasion

The composition is a vital factor in achieving the desired requirements. The criteria for selection of the various constituents are as follows:

·     Aggregate

-       Use of low fines mixes

-       Use of a balanced particle size distribution curve

-       Crushed or partly crushed aggregate increases the skid resistance and flexural strength

·     Cement

-       Dosage 300–350 kg/m³, usually CEM I 42.5

·     Additives

-       Silica fume for use in heavily traffic areas or to increase the durability generally

-       Increase in the skid resistance by spreading silicon carbide or chippings into the surface

Concrete for traffic areas is a special concrete and the following points require special attention:

-       Large areas are often installed using paving machines. The consistence must be suitable for the type of machine

-       Improvement in skid resistance by cut grooves or brush finishing

-       Thorough curing is essential

Pumped Concrete

Pumped Concrete

Pumped concrete is used for many different requirements and applications nowadays. A suitable concrete mix design is essential so that the concrete can be pumped without segregation and blocking of the lines.

Composition

·     Aggregate

Max. particle < 1/3 of pipe bore

The fine mortar in the pumped mix must have good cohesion to prevent the concrete segregating during pumping.

·     Cement

Max. particle		Round aggregate		Crushed aggregate
8 mm		380 kg/m³		420 kg/m
16 mm		330 kg/m³		360 kg/m³
32 mm		300 kg/m³		330 kg/m³

·     Water/binder ratio

If the water content is too high, segregation and bleeding occurs during pumping and this can lead to blockages. The water content should always be reduced by using superplasticizers.

·     Workability

The fresh concrete should have a soft consistence with good internal cohesion. Ideally the pumped concrete consistence should be determined by the degree of compatibility.

·     Pumping agents

Difficult aggregates, variable raw materials, long delivery distances or high volume installation rates require a pumping agent. This reduces friction and resistance in the pipe, reduces the wear on the pump and the pipes and increases the volume output.

·     Pump lines

-  80 to 200 mm (normally  100, 125 mm)

- The smaller the, the more complex the pumping (surface/cross-section)

- The couplings must fit tightly to prevent loss of pressure and fines

- The first few metres should be as horizontal as possible and without bends. (This is particularly important ahead of risers.)

- Protect the lines from very strong sunlight in summer

Lubricant mixes

The lubricant mix is intended to coat the internal walls of the pipe with a high-fines layer to allow easy pumping from the start.

Conventional mix: Mortar 0–4 mm, cement content as for the following concrete quality or slightly higher. Quantity dependent on andline length

 Effect of air content on pumped concrete

Freeze/thaw resistant concrete containing micropores can be pumped if the air content remains < 5%, as increased resilience can be generated with a higher air content.

Concrete for Precast Structures

Concrete for Precast Structures

Precast concrete is used to form structures which are delivered after hardening. Long journeys in the fresh concrete state disappear, which changes the whole production sequence. Concrete used for the production of precast structures requires an industrialized production process,and a good concrete mix design with continuous optimization is essential. The following points are important through the different stages of the process:

Preparation of concrete design

When preparing the design, the concrete requirements must be defined according to the specific elements, their intended use and exposure conditions. The following parameters should normally be defined:

·     Strength requirements

·     Durability requirements

·     Aesthetic requirements

·     Maximum particle diameter

·     Method of placement

·     Placing rate

·     Concrete consistence

·     General boundary conditions (temperature etc.)

·     Handling of the concrete and its placing

·     Definition of test requirements

·     Consideration of the specific concrete element parameters

·     Curing definition

·     Mix design and specification

·     Preliminary testing

·     Mix design adjustment if necessary

Concrete and Ready Mix Production

Add caption

Production is a critical factor for the resulting concrete and consists basically of dosing and mixing the raw materials. The following parameters can affect the concrete properties during mixing:

-       Type of mixer                          - Addition of raw materials

-       Size of mixer                            - Plant quality control

-       Mixing intensity                     - Concrete mixer operator

-       Mixing time                             - Cleaning/maintenance of mixer

Super plasticizers should generally be mixed with the mixing water or added to the mix with it (at the earliest). Further information can be found in the relevant Sika Product Data Sheets.

·     Preparation on site

The preparation on site includes the following:

–Installation of the concrete handling/placing systems

–Preparation of the formwork (including release agent application)

–Reinforcement check

–Formwork check (fixing, integrity, form pressure)

–Supply of tools for compacting (vibrators etc.) and finishing (beams and trowels etc.)

·     Delivery

If the concrete is supplied by ready mix trucks, the following additional criteria must be considered:

–Delivery time (traffic conditions, potential hold-ups etc.)

–Define the necessary drum revolutions during the journey

–Do not leave the ready mix truck standing in the sun during waiting periods

–For a fluid consistence (SCC), define the maximum capacity to be carried

–Do not add water or extra doses of admixture (unless specified)

–Mix again thoroughly before unloading (1 minute per m³)

·     Placing the concrete

The concrete is generally placed within a limited and defined time period. The following factors contribute to the success of this operation, which is critical for the concrete quality:

–Delivery note check

–Use of the right equipment (vibrators etc.)

–Avoid over handling the concrete

–Continuous placing and compacting

–Re-compaction on large pours

–Take the appropriate holding measures during interruptions

–Carry out the necessary finishing (final inspection)

·     Curing

To achieve constant and consistent concrete quality, appropriate and correct curing is essential. The following curing measures contribute to this:

–Generally protect from adverse climatic influences (direct sun, wind,rain, frost etc.)

–Prevent vibration (after finishing)

–Use a curing agent

–Cover with sheets or frost blankets

–Keep damp/mist or spray if necessary

–Maintain the curing time relevant to the temperature