Special Concretes
Special Concretes are concretes with special or enhanced characteristics or qualities, which makes them suitable for specific applications.
The advantage of a product can be descriped can be allocated to a certain category we differentate the following groups which are indicated with the respective colors:
|
|
|
|
|
|
|
|
In the Restricted Area of the Portal |
|
Content |
|
|
- Pavement |
|
|
- Frost and Freeze/Thaw Resistant Concrete |
|
This place is
reserved for
YOUR
Banner
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:
|
Composition |
|
||||||||||||||
|
Aggregate |
|
||||||||||||||
|
Cement |
|
||||||||||||||
|
Additives |
Concrete for traffic areas is a special concrete and the following points require special attention:
|
||||||||||||||
|
Admixtures |
|
Floors
Floors is one of the most important application of concrete. There are different types of floors and each of them has special requirements, depending on the different purposes.
- Decorative Floors
- Industrial Floors
- Elevated Slabs—Concrete Floor Systems
- Concrete Floors and Moisture
- Durable Floors
- Radiant-Heated Floors
- Concrete Shrinkage
Slabs
Concrete is the material of choice for driveways, sidewalks, patios, steps, and for garages, basements, and industrial floors. It is relatively inexpensive to install and provides an attractive, durable surface that is easy to maintain. Proper attention to the standard practices and procedures for constructing exterior or interior concrete can yield a concrete surface that will provide long-lasting, superior performance.
Modern tunnelling methods in unstable rock use concrete segments which are immediately load bearing as linings to the fully excavated tunnel section.
Precast concrete units called tunnel segments perform this function.
|
Factors to consider |
|||||||||
|
Production |
Due to the large numbers required and heavy weight (up to several tones each), tunnel segments are almost always produced near the tunnel portal in specially installed precasting facilities. They have to meet high accuracy specifications. Heavy steel formwork is therefore the norm. Because striking takes place after only 5–6 hours and the concrete must already have a compressive strength of >15 N/mm2, accelerated strength development is essential. There are several methods for this. In the autoclave (heat backflow) process, the concrete is heated to 28–30 °C during mixing (with hot water or steam), placed in the form and finished. It is then heated for about 5 hours in an autoclave at 50 – 60 °C to obtain the necessary demoulding strength. |
||||||||
|
Composition |
|
||||||||
|
Placing |
|
||||||||
|
Special requirements |
The newly demoulded segments must be cured by covering or spraying with a curing agent. However, to obtain a combination of maximum durability in variable ground conditions and optimum curing, the segment surfaces are treated more often with a special protective coating immediately after striking. With this additional protection against chemical attack, extremely durable concrete surfaces are achieved for these segments. |
||||||||
|
Admixtures |
|
||||||||
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 a hot topic. Owners desire long service lives so engineers design concrete mixtures for low permeability. These mixtures typically have high cementitious materials contents, which results in high temperatures within the concrete. To avoid cracking and other temperature related damage to the concrete, contractors must control the temperature and temperature difference in the concrete. This can pit the schedule against the service life.
When all involved parties work together, appropriate changes can be made to achieve the desired service life with minimal impacts to the schedule. The key is an understanding of mass concrete. Selection of an appropriate concrete mixture is the first step.
|
Factors to consider |
|||||||
|
Applications |
|
||||||
|
Potential Problems |
|
||||||
|
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 |
|
||||||
|
Measurement of hydration heat in a 160 cm thick ground slab in three levels |
|
||||||
|
Admixtures |
|
||||||
As the name suggests, underwater concrete is installed below the water line.
|
Factors to consider |
|||||||||
|
Applications |
|
||||||||
|
Aggregate |
|
||||||||
|
Cement |
Min. cement content 350 kg/m3 |
||||||||
|
Special requirements |
A reliable method of placing underwater concrete with minimum loss is the tremie process (Contractor method). The concrete is placed directly through a 20–40 cm _ pipe into and through the concrete already installed. The pipe is raised continuously, but the bottom end must always remain sufficiently submerged in the concrete to prevent the water going back into the pipe. Another method also used today is pumping a suitably modified mix through a standard concrete pump. Here again, the end of the delivery pipe must be kept deep enough in the fresh concrete. Other important considerations:
|
||||||||
|
Special underwater concrete |
Previously installed rough stone bags or “gabions” can be filled in later with modified cement slurries (the bag method). |
||||||||
|
Admixtures |
|
||||||||
|
||||||||||||||||||||||||||||||||||||
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.
|
Factors to consider |
|
|
Applications |
|
|
Properties |
|
|
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.
|
|
Fiber types |
|
Lightweight means concrete and mortar with a low density. Either aggregates with a lower density are used or artificial voids are created to reduce the weight. The method used depends mainly on the lightweight concrete application and its desired properties.
|
Factors to consider |
||||||||||||||||||
|
Applications |
|
|||||||||||||||||
|
Characteristics |
|
|||||||||||||||||
|
Production |
|
|||||||||||||||||
|
Constituents |
|
|||||||||||||||||
|
Density |
Based on the mix and the constituents used, the following density classes and properties are obtainable:
|
|||||||||||||||||
|
Porous concrete |
Expansion causing additives (e.g. powdered aluminium) are mixed with the mortar for porous concrete. Porous concrete is generally produced industrially. Porous concrete is not really a concrete, it is really a porous mortar. |
|||||||||||||||||
|
Admixtures |
|
|||||||||||||||||
Heavyweight concrete is mainly used for radiation protection. The critical properties of a heavyweight concrete are:
- Homogeneous density and spatial closeness of the concrete
- Free from cracks and honeycombing
- Compressive strength is often only a secondary criterion due to the large size of the structure
- As free from air voids as possible
- Keep shrinkage low
|
Factors to consider |
|||||||
|
Aggregate |
Use of barytes, iron ore, heavy metal slags, ferrosilicon, steel granules or shot |
||||||
|
Cement |
Allow for hydration heat development when selecting the cement type and content |
||||||
|
Water content |
Aim for a low water/cement ratio |
||||||
|
Workability |
To ensure a fully closed concrete matrix, careful consideration should be given to the placing (compaction) |
||||||
|
Curing |
Allowance must be made in the curing method for the high heat development due to the likely large mass of the structure |
||||||
|
Admixtures |
|
||||||
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 |
Standard values for finest grain content (content
Recommendation:
Particle size distribution curve: Pumped concrete should be composed of different individual constituents if possible. A continuously graded particle-size distribution curve is important. The 4 – 8 mm content should be kept low, but there should be no discontinuous gradation. |
||||||||||||||||||
|
Cement |
Recommended minimum cement content
|
||||||||||||||||||
|
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 compactability. |
||||||||||||||||||
|
Fresh concrete consistence |
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 |
|
||||||||||||||||||
|
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.
|
||||||||||||||||||
|
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. |
||||||||||||||||||
|
Admixtures |
|
Self-Compacting concrete (SCC) is a high-performance concrete that can flow easily into tight and constricted spaces without segregating and without requiring vibration. The key to creating self-consolidating concrete (SCC), also referred to as self-compacting, self-leveling, or self-placing concrete, is a mixture that is fluid, but also, stable, to prevent segregation.
To achieve the desired flowability a new generation of superplasticizers based on polycarboxylate ethers works best. Developed in the 1990s, they produce better water reduction and slower slump loss than traditional superplasticizers. The required level of fluidity is greatly influenced by the particular application under consideration. Obviously the most congested structural members demand the highest fluidity. However, element shape, desired surface finish, and travel distance can also determine the required fluidity.
Generally, the higher the required flowability of the SCC mix, the higher the amount of fine material needed to produce a stable mixture. However, in some cases, a viscosity-modifying admixture (VMA) can be used instead of, or in combination with, an increased fine content to stabilize the concrete mixture.
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 superplasticizers, produce unique fluidity and inherent compactability. 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
- For 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:
|
||||||||||||||||||||||||||||||||||||||||||||
|
Admixtures |
To adjust these water contents and ensure the homogeneity and the viscosity adjustment, appropriate superplasticizers should be specified.
|
||||||||||||||||||||||||||||||||||||||||||||
|
Installation |
|
|
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 from below. This can be achieved by using tremie pipes etc. |
In the slipforming method, the formwork is moved continuously in sync with the concreting process in a 24-hour operation. The formwork, including the working platform and the hanging scaffold mounted internally or on both sides, is fixed to the jacking rods in the centre of the wall. The hydraulic oil operated lifting jack raises the formwork 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.
Slipforming 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 formwork 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 formwork (“plucking”). The main requirement for slipforming 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.
|
Factors to consider |
|||||||||||||
|
Aggregate |
|
||||||||||||
|
Cement |
|
||||||||||||
|
Workability |
The best workability has proved to be a stiff plastic concrete having a flow diameter of 35 – 40 cm and a low water content. |
||||||||||||
|
Properties |
|
||||||||||||
|
Admixtures |
|
||||||||||||
Rolled concrete is concrete which is placed with standard (asphalt) road pavers and is then leveled and compacted with smooth coated vibrating rollers. Rolled concrete is used mainly in the USA (but also in Germany) for the construction of hard standing, large surfaces (car parks) and road surfaces. The concrete composition is similar to standard concrete. Semidry consistence: Crushed material is preferable for good green strength.
The coarse material, sand, binder (standard cement) and water content must be coordinated. In particular, the water content must be kept constant and precise to allow the voids to be filled as fully as possible by rolling.
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
|
Factors to consider |
|||||||||
|
Aggregate |
|
||||||||
|
Cement |
|
||||||||
|
Additions |
Use specific additions for systematic improvement of the concrete properties as required |
||||||||
|
Water |
|
||||||||
|
Placing |
|
||||||||
|
Curing |
|
||||||||
|
Precautions |
|
||||||||
|
Admixtures |
|
||||||||
Colored concrete is produced by adding pigmented metal oxides (mainly iron oxide). The pigments are in the form of powder, low dust fine granulates or liquid.
The dosage is normally 0.5 – 5 % of the cement weight. Higher dosages do not deepen the color but may adversely affect the concrete quality.
|
Factors to consider |
|||||||
|
Typical colors |
|
||||||
|
Heightening of color |
The color of a “colored” concrete can only be reliably assessed in the dry hardened state and depends on the following factors:
|
||||||
|
Admixtures |
|
||||||
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 of any water during freezing conditions).
One of the greatest advances in concrete technology was the development of air-entrained concrete in the late 1930s. Today, air entrainment is recommended for nearly all concretes, principally to improve resistance to freezing when exposed to water and deicing chemicals. However, there are other important benefits of entrained air in both freshly mixed and hardened concrete. Air-entrained concrete contains billions of microscopic air cells. These relieve internal pressure on the concrete by providing tiny chambers for the expansion of water when it freezes.
Air-entrained concrete is produced through the use of air-entraining portland cement, or by introducing air-entraining admixtures under careful engineering supervision as the concrete is mixed on the job. The amount of entrained air is usually between 5 percent and 8 percent of the volume of the concrete, but may be varied as required by special conditions. The use of air-entraining agents results in concrete that is highly resistant to severe frost action and cycles of wetting and drying or freezing and thawing and has a high degree of workability and durability.
|
Factors to consider |
|
||||||||
|
Type, size and distribution of air voids |
Air voids contained in a standard concrete are generally too large (> 0.3 mm) 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:
Fresh concrete with an air void content of 7% or over should only be installed after detailed investigation and testing. |
||||||||
|
Granulometry |
The air voids are mainly formed around the 0.25–0.5 mm sand fraction. Larger particles have no effect on the air entrainment. Ultrafines from the sand constituents or the cements and some admixtures can inhibit air entrainment. |
||||||||
|
Consistence |
Optimum air entrainment is achieved in the plastic to soft plastic range. A concrete that is softened by the addition of extra water might not retain the air voids as well or as long as the original concrete. |
||||||||
|
Temperature |
The air entrainment capability decreases as fresh concrete temperatures rise and vice versa. |
||||||||
|
Delivery |
A change in the air content can be expected during delivery. Dependent on the method of delivery and the vibration during the journey, mixing or demixing processes take place in the concrete. Air-entrained concrete must be mixed again before installation and the critical air content is only then determined. |
||||||||
|
Compaction of air-entrained concrete |
Correct vibration mainly removes the air “entrapped” during placing, including the coarse voids in the concrete. Pronounced overvibration can also reduce the “entrained” air by 10 to 30 %. Concrete susceptible to segregation can then lose almost all of the air voids or exhibit foaming on the surface. |
||||||||
|
Finest grain replacement |
1 % of entrained air can replace approximately 10 kg of ultra-fine material (< 0.2 mm) per m³ of concrete. Air voids can improve the workability of rough, low-fines mixes. |
||||||||
|
Design of air-entrained concrete |
Detailed specifications for strength, air content and test methods must be given. For major projects, preliminary tests should be carried out under actual conditions. During the concreting works check the air content at the concrete plant and before placing.
|
||||||||
|
Admixtures |
|
0.15 mm freeze/thaw resistantWaterproof 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.
|
Factors to consider |
|||||||||||
|
Aggregate |
|
||||||||||
|
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 |
|
||||||||||
|
Curing |
Immediate and thorough curing is essential |
||||||||||
|
Admixtures |
|
||||||||||
Concrete with enhanced or high fire resistance means concrete which is improved so that it can withstand the defined high heat conditions. Concrete itself cannot burn, but above certain temperatures it loses first its mechanical properties and then its form. Without special measures concrete is normally heat resistant in service up to a temperature of about 80 °C.
Concrete with high fire resistance is used for:
- Emergency areas in enclosed structures (tunnel emergency exits)
- General improved fire resistance for infrastructure
- Fire resistant cladding for structural members
|
Factors to consider |
|||||
|
Properties of concrete with high fire resistance |
|
||||
|
Production of concrete with high fire resistance |
|
||||
|
Constituents for the production of concrete with high fire resistance |
|
||||
|
Mechanisms of behavior in fire |
The capillary and interstitial water begins to evaporate at temperatures around the boiling point of water (100 ºC). Steam needs more space and therefore exerts expansion pressure on the concrete structure. The cement matrix begins to change at temperatures of about 700 °C. The effect of the aggregates is mainly dependent on their origin and begins at about 600 °C. Concrete starts to “melt” at about 1200 °C. |
||||
|
Admixtures |
|
||||
Wear- and Abrasion-Resistant Concrete
Monolithic Concrete
Wear resistant, level concrete floors or decks ready for use quickly. Monolithic concrete has the same high quality throughout and these floor designs are extremely economic.
|
Factors to consider |
|||||||||||||
|
Composition |
The concrete mix must be adapted to any special requirements (waterproof concrete, frost resistant concrete etc.) |
||||||||||||
|
Placing |
Standard placing and compaction with immersion vibrators. Smooth off with vibrating beam. After the stiffening process begins, the surface is finished with power floats. |
||||||||||||
|
Curing |
Start as early as possible, by spraying with Sika® Antisol® (Attention! What coating is to follow?) and cover with sheeting. |
||||||||||||
|
Notes |
|
||||||||||||
|
Admixtures |
|
||||||||||||
Granolithic Concrete
Granolithic concrete pavements are highly abrasion resistant, cementitious industrial floors and traffic areas with a minimum thickness of 20 mm. They are laid over a cementitious substrate (e.g. old concrete) with a bonding layer and have a density of > 2100 kg/m³. If the layer thickness exceeds 50 mm, a light reinforcement mesh (minimum 100 × 100 × 4 × 4) is normally installed.
|
Factors to consider |
|||||||||||||
|
Composition |
|
||||||||||||
|
Substrate/adhesion |
Before placing a bond coat is brushed into the slightly damp (prewetted) substrate. The granolithic concrete is placed “wet on wet” onto the bond coat and carefully compacted, smoothed off and then finished with power floats. The abrasion resistance is further improved by applying dry shakes during the floating operation. Polypropylene fibers included in the mix can also counteract shrinkage cracking. |
||||||||||||
|
Curing |
Always apply a curing agent (which must be mechanically removed if a coating is to be applied at a later date), and/or cover with sheeting, preferably for several days. |
||||||||||||
|
Admixtures |
|
||||||||||||
Self-cleaning buildings and pollution-reducing roadways: These may sound like futuristic ideas, but they are realities of some of today’s concrete. Recently introduced formulations of cement are able to neutralize pollution. Harmful smog can be turned into harmless compounds and washed away. Anything made out of concrete is a potential application, because these cements are used in the same manner as regular portland cements. These products provide value through unique architectural and environmental performance capabilities.
Proprietary technology (based on particles of titanium dioxide) is what makes this cement special—capable of breaking down smog or other pollution that has attached itself to the concrete substrate, in a process known as photocatalysis.
Pervious pavements have been used for years throughout the warmer climates of the United States with excellent results. However, in climates prone to severe freeze-thaw cycles, some are hesitant to use these pavements until it has been proven that pervious concrete can be made to resist freeze-thaw damage.