Concrete Surface

Concrete Surface

The impermeability and appearance requirements for concrete surfaces vary widely. Detailed planning and execution of the structure are required to meet these requirements.

Maximum surface impermeability is essential for all durability requirements. The attack always comes from outside to inside. Over vibration or inadequate curing weakens these zones. High quality appearance requirements have led to the production of so-called “fair-faced” concrete.

Appearance of concrete surfaces

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:

Exposed aggregate concrete

Exposed aggregate concrete is a popular surface design feature, e.g. for retaining walls, façade panels, garden features etc.

The aggregate structure is exposed on the surface by washing off and out. This requires surface retardation, which must be effective down to several mm.

In correctly designed exposed aggregate concrete, 2/3 of any single coarse aggregate is still within the hardened cement matrix.

Frost and Freeze/Thaw Resistance

Frost and Freeze/Thaw Resistance

Frost stress

Damage to concrete structures due to frost can generally be expected when they have been penetrated by moisture and are exposed to frequent freeze/thaw cycles in that condition. The damage to the concrete occurs due to the cyclic freezing and thawing of the water which has been absorbed due to capillary suction. Destruction follows due to the increase in volume of the water [ice] in the outer concrete layers.

Essentials for high frost resistance

·     Frost proof aggregates

·     Impermeable concrete structure and/or

·     Concrete enriched with microspores

·     Thorough and careful curing

·     Degree of hydration of the concrete as high as possible (i.e. it is not a good idea to place concrete immediately before periods of frost)

Test methods

·     Frost resistance

This can be estimated by comparing the fillable and non-fillable voids.

Freeze/thaw resistance

Given the extensive use of deicing salts (generally sodium chloride NaCl, intended to lower the freezing point of the water on roads and prevent ice formation etc.), the concrete surface cools abruptly due to heat extraction from the concrete. These interactions between frozen and unfrozen layers cause structural breakdown in the concrete.

Conditions for freeze/thaw resistance

·     Frostproof aggregates

·     Concrete with an impermeable structure enriched with microspores

·     Thorough and careful curing

·     Avoid too much fine mortar enrichment of the surface area

·     Concreting as long as possible before the first freeze/thaw stress so that the concrete can dry out.

Concrete strength

Concrete strength

Many factors influence the rate at which the strength of concrete increases after mixing. Some of these are discussed below. First, though a couple of definitions will be useful:

The process of strength growth is called 'hardening.' This is often confused with 'setting' but setting and hardening are not the same.

Setting is the stiffening of the concrete after it has been placed. A concrete can be 'set' in that it is no longer fluid, but it may still be very weak; you may not be able to walk on it, for example. Setting is due to early-stage calcium silicate hydrate formation and to ettringite formation. The terms 'initial set' and 'final set' are arbitrary definitions of early and later set; there are laboratory procedures for determining these using weighted needles penetrating into cement paste.

Hardening is the process of strength growth and may continue for weeks or months after the concrete has been mixed and placed. Hardening is due largely to the formation of calcium silicate hydrate as the cement continues to hydrate.

The rate at which concrete sets is independent of the rate at which it hardens. Rapid-hardening cement may have similar setting times to ordinary Portland cement.

High Early Strength Concrete

High Early Strength Concrete

High early strength means the compressive strength of the concrete in the first 24 hours after production.

High early strength concrete for precast structures

High early strength is often very important for precast structures.

Higher early strength means

• Earlier striking
• Faster turnaround of the formwork
• Earlier handling of the precast structures
• More economic use of cement
• Less heat energy, etc.

High early strength ready mixed concrete

Diametrically opposed requirements are often involved here. On the one hand, a long working time is often required (for handling/installation), but on the other hand, early strength after 6 hours is required. These requirements can only be met by using modern superplasticizers, hardening accelerators and specially adapted mixes.

Uses of high early strength ready mixed concrete

For all ready mixed concrete applications where high initial strength is required, including:

• Short striking times, especially in winter
• Early load bearing situations (traffic areas/tunnel invert concrete)
• Slipforming
• Early finishing (e.g. granolithic concrete during the winter)
• Reduced winter protection measures
• Parameters influencing high early strength concrete

The strength development and consistence depend on the following parameters:

• Cement type and content
• Concrete, ambient and substrate temperatures
• Water/cement ratio
• Element dimensions
• Curing
• Aggregate composition
• Concrete admixtures

Hardened Concrete

Hardened Concrete

An important property of hardened concrete is the compressive strength. It is determined by a compression test on specially produced specimens (cubes or cylinders) or cores from the structure.

The main factors influencing compressive strength are the type of cement, the water/cement ratio and the degree of hydration, which is affected mainly by the curing time and method.

The concrete strength therefore results from the strength of the hydrated cement, the strength of the aggregate, the bond between the two components and the curing. Guide values for the development of compressive strength are given in the table below.

Cement strength class	Continuous storage at	3 days N/mm²	7 days N/mm²	28 days N/mm²	90 days N/mm²	180 days N/mm²
32.5	+20°C +5°C	30…40 10…20	50…65 20…40	100 60…75	110…125	115…130
32.5 R; 42.5 N	+20°C +5°C	50…60 20…40	65…80 40…60	100 75…90	105…115	110…120
42.5 R; 52.5 N 52.5 R	+20°C +5°C	70…80 40…60	80…90 60…80	100 90…105	100…105	105…110

1The 28-day compressive strength at continuous 20°C storage corresponds to 100%.

Determination of Fresh Concrete Density

Determination of Fresh Concrete Density

Principle: The fresh concrete is compacted in a rigid, watertight container and then weighed.

The minimum dimensions of the container must be at least four times the maximum nominal size of the coarse aggregate in the concrete, but must not be less than 150 mm. The capacity of the container must be at least 5 liters. The top edge and base must be parallel.

(Air void test pots with a capacity of 8 liters have also proved very suitable.)

The concrete is compacted mechanically with a poker or table vibrator or manually with a bar or tamper.

Testing the Consistence by Degree of Compatibility

Testing the Consistence by Degree of Compatibility

Principle:

The fresh concrete is placed carefully in the steel test container. Compaction must be avoided. When the container is full to overflowing, the concrete is smoothed flush with the edge without vibration. The concrete is then compacted, e.g. with a poker vibrator (max.bottle diameter 50 mm).  After compaction the distance between the concrete surface and the top of the container is measured at the center of all 4 sides. The mean figure (s) measured is used to calculate the degree of compatibility.

Container dimensions                    Base plate        200 x 200 mm     (±2)

                           Height                      400 mm                   (±2)

Concrete in container                                  Concrete in container

before compaction                                       after compaction

Degree of compatibility: c=       h1

                                                   _________    (non dimensional)

                                                      h1 – S

water/cement ratio in concrete

water/cement ratio in concrete

The water–cement ratio is the ratio of the weight of water to the weight of cement used in a concrete mix and has an important influence on the quality of concrete produced. A lower water-cement ratio leads to higher strength and durability, but may make the mix more difficult to place. Placement difficulties can be resolved by using plasticizers or super-plasticizers.

Often, the water–cement ratio is characterized as the water to cement plus pozzolan ratio, w/(c+p). The pozzolan is typically a fly ash, or blast furnace slag. It can include a number of other materials, such as silica fume, rice hull ash or natural pozzolans. The addition of pozzolans will influence the strength gain of the concrete.

Concrete hardens as a result of the chemical reaction between cement and water (known as hydration, this produces heat and is called the heat of hydration). For every pound (or kilogram or any unit of weight) of cement, about 0.25 pounds (or 0.25 kg or corresponding unit) of water is needed to fully complete the hydration reactions. This requires a water-cement ratio of 1:4 often given as a proportion: 0.25. However, a mix with a w/c ratio of 0.25 may not mix thoroughly, and may not flow well enough to be placed, so more water is used than is technically necessary to react with the cement. More typical water-cement ratios of 0.4 to 0.6 are used. For higher-strength concrete, lower water:cement ratios are used, along with a plasticizer to increase flow ability.

Too much water will result in segregation of the sand and aggregate components from the cement paste. Also, water that is not consumed by the hydration reaction may leave the concrete as it hardens, resulting in microscopic pores(bleeding) that will reduce the final strength of the concrete. A mix with too much water will experience more shrinkage as the excess water leaves, resulting in internal cracks and visible fractures (particularly around inside corners) which again will reduce the final strength.

Fresh Concrete Temperature

Fresh Concrete Temperature

The fresh concrete temperature should not be too low, so that the concrete gains sufficient strength fast enough and does not suffer damage from frost at an early age.

·     The fresh concrete temperature should not drop below +5°C during placement and installation.

·     The freshly placed concrete should be protected from frost. Freezing resistance is reached at a compressive strength of approximately 10N/mm².

·     On the other hand too high concrete temperatures can result in (cause) placement problems and decline of certain hardened concrete properties. To avoid this, the fresh concrete temperature should not go above 30°C during placement and installation.

Precautions at low temperatures

The concrete should be protected from rain and frost during handling. Concreting is only possible in freezing temperatures if special protective measures are taken. They must be in place from the start of concrete production to the end of curing.

They depend on the outside temperature, air humidity, wind conditions, fresh concrete temperature, heat development and dissipation and the dimensions of the concrete pour.

The fresh concrete must not be colder than +5°C during placement and installation without additional protective measures. The mixing water and aggregates should be preheated if necessary.

Precautions at high temperatures

The concrete should be protected from drying out during handling.

Concreting is only possible at high temperatures if special protective measures are provided. These must be in place from the start of concrete production to the end of curing. They are dependent on the outside temperature, air humidity, wind conditions, fresh concrete temperature, heat development and dissipation and the dimensions of the pour.

The fresh concrete must not be hotter than +30°C during placement and installation without these protective measures.

Pumpability of Concrete

Pumpability of Concrete

The pumpability of concrete depends basically on the composition of the mix, the aggregates used and the method of delivery.

As far as the delivery and installation of pumped concrete is concerned, a significant reduction in the pump pressures and an increase in the output can be obtained by the systematic addition of pumping agents, particularly for use with crushed aggregates, secondary raw materials, highly absorbent aggregates etc.

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.

Fresh Concrete Density

Fresh Concrete Density

Fresh concrete density means the mass in kg per m³ of fresh, normally compacted concrete, including its remaining voids.

Given the same quantity of cement and aggregate, a lower fresh concrete density indicates a lower concrete strength because the density falls as the water and void content increases.

The fresh concrete density falls

• as the water content increases
• as the void content increases

The fresh concrete density increases

• as the cement content rises
• as the water/cement ratio decreases
• as the void content decreases

Set Acceleration/Cold Weather Concrete

Set Acceleration/Cold Weather Concrete

The concrete should be protected from rain and frost during handling. Concreting is only possible in freezing temperatures if special protective measures are taken. They must be in place from the start of concrete production to the end of curing.

They depend on the outside temperature, air humidity, wind conditions, fresh concrete temperature, heat development and dissipation and the dimensions of the concrete pour.

The fresh concrete must not be colder than +5°C during placement and installation without additional protective measures. The mixing water and aggregates should be preheated if necessary.

Problem

Low temperatures retard the cement setting. At temperatures below –10°C the chemical processes of the cement stop (but continue after warming). Dangerous situations arise if concrete freezes during setting, i.e. without having a certain minimum strength. Structural loosening occurs, with a corresponding loss of strength and quality.  The minimum strength at which concrete can survive one freezing process without damage is the so-called freezing strength of 10 N/mm². The main objective must be to reach this freezing strength as quickly as possible.

The temperature t of fresh concrete can be estimated by the following equation:

t_concrete= 0.7 × t_aggregate+ 0.2 × t_water+ 0.1 × t_cement

Measures

1. Minimum temperature

The fresh concrete temperature on delivery must not be below +5°C. (For thin, fine structured elements and ambient temperatures of –3°C or below, EN requires a fresh concrete temperature of +10°C, which must be maintained for 3 days!) These minimum temperatures are important for setting to take place at all. The concrete should be protected from heat loss during handling and after placing (see Protetive measures).

2. Reduction in w/c ratio

The lowest possible water content gives a rapid increase in initial strength. Generally there is also less moisture available to freeze. Superplasticizers allow a low w/c ratio while retaining good workability.

3. Hardening acceleration

Gives maximum hardening acceleration when there are high initial strength requirements.

Retardation/Hot Weather Concrete

Retardation/Hot Weather Concrete

The concrete should be protected from drying out during handling.

Concreting is only possible at high temperatures if special protective measures are provided. These must be in place from the start of concrete production to the end of curing. They are dependent on the outside temperature, air humidity, wind conditions, fresh concrete temperature, heat development and dissipation and the dimensions of the pour.

The fresh concrete must not be hotter than +30°C during placement and installation without these protective measures.

Possible problems

Working with non-retarded concrete can become a problem at air temperatures over 25°C.

·     Hydration is the chemical reaction of the cement with the water. It begins immediately on contact, continues through stiffening to setting (initial set) and finally to hardening of the cement paste.

·     Each chemical reaction is accelerated at a higher temperature.

This can mean that correct and complete compaction is no longer possible.

The normal counter measures are the use of retarded super plasticizers or super plasticizers combined with a set retarder.

Retardation terms and dosing tables

Purpose of retardation: To extend the working time at a specific temperature.

Working time: The time after mixing during which the concrete can be correctly vibrated.

Free retardation: The initial set is certain to start only after a specific time.

Targeted retardation: The initial set is started at a specific time.

Workability of Fresh Concrete

Workability of Fresh Concrete

The consistence defines the behavior of the fresh concrete during mixing, handling, delivery and placing on site and also during compaction and surface smoothing. Workability is therefore a relative parameter and is basically defined by the consistence.

Workability requirements

·     Cost effective handling, delivery/placement and placing of the fresh concrete

·     Maximum plasticity (“flow ability”), with the use of super plasticizers

·     Good cohesion

·     Low risk of segregation, good surface smoothening (“finishing properties”)

·     Extended workability                                          ----->Retardation/hot weather concrete

·     Accelerated set and hardening process  ----->Set and hardening acceleration/ cold weather concrete

Granolithic Concrete

Granolithic Concrete

Granolithic concrete pavements are highly abrasion resistant, cementitious industrial floors and traffic areas with a minimum thickness of 20mm. 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.

Composition

Aggregate

–0–4 mm for a layer thickness of up to 30 mm

–0–8 mm for a layer thickness of 30–100 mm

Cement

–400–500 kg/m³

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.

Monolithic 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.

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 Curing agent (Attention !What coating is to follow?) and cover with sheeting.

Notes

·     Check the potential for the use of steel fibers when forming monolithic concrete slabs

·     To improve the finished surface, we recommend the use of Mineral, synthetic and metallic grades which are spread into the surface during the finishing operation

·     Concrete admixtures for extended workability are not generally suitable for monolithic concrete

Tunnel Segment Concrete

Tunnel Segment Concrete

Modern tunneling 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.

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 5hours in an autoclave at 50–60°C to obtain the necessary demoulding strength.

Composition

Aggregate

·     Normally 0–32 mm in the grading range according to EN 480-1

Cement

·     Cement content 325 or 350 kg/m³

·     CEM I 42.5 or 52.5

Placing

·     The fresh concrete mix tends to stiffen rapidly due to the high temperature, making correct compaction and finishing of the surface difficult.

·     Due to the rapid industrialized process, a plastic fresh concrete consistence can be used. The desired initial strength can only be obtained by a low water/cement ratio, which should therefore always be<0.48.