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THE LATEST GENERATION OF CONSTRUCTION CHEMICALS FOR APPLICATION IN PRECAST TECHNOLOGY

Sunny Surlaker; Head Admixtures Division, MC-Bauchemie (India) Pvt. Ltd.; Mumbai, India

Keywords

Pre-cast Concrete Improvement, PCE Superplasticizers, Compaction Aids, Calcined Clays, Acid Resistant Concrete, Interfacial Transition Zone, Rheology Modifiers, Concrete Goods, Efflorescence Reduction

Abstract

Concrete Construction  without of the use of Construction  Chemicals,  is not possible today. Due to the changing needs of concrete, coupled with construction  schedules and varied availability  of raw materials, puts tremendous  stress on ensuring production, durability and the sustainability of the concrete produced. Precast concrete addresses many of these shortcomings and it is imperative that Construction Chemicals be a vital component of improving precast production to promote sustainable construction.

Apart from offering High Performance Superplasticizers  for Concrete, Construction Chemicals today offer a wide variety of solutions for the Precast Industry. Solutions include:

1. New generation rheology modifiers and compaction aids for Hollowcore Segment Production to improve dimensional stability, compressive strength and improve de-shuttering times
2.  Long Workability, High Early strength Water Reducers
   1. Mineral Additives based on calcined clays for improving Flexural Strengths, chemical resistance, surface finishing and overall durability
   2.  Latest Generation Curing Compounds that can provide protection to segments against weather, heal micro-cracking on surfaces and aid adhesion of segments

That apart other peripheral solutions for Precast Product include:

1.  New Generation Waterproof and Expansive Grout Additives for filling Pre-stress cable ducts
   1. New Generation Concrete Cosmetics that enhance the finish of the segment surface and provide protection against carbonation and weathering
   2.  Impregnation Systems as protective coating on segments to prevent water ingress and carbonation.

This paper reviews the underlying mechanisms of these additives and the latest applications / joint studies undertaken at MC- Bauchemie and other industry partners incorporating these materials in development of ceramic-like UHPCs for use in Precast Construction.  Pertaining  to  the  concept  of  mass  housing,  these  technologies  can  help  improve  durability  and  mitigate maintenance, thus redefining the term affordable over the long run.

Introduction

Over the years pre-cast concrete construction has evolved as an alternative to in-situ concrete to speed up construction times, and achieve better quality in terms of finish and durability. Precast Concrete Units offer this advantage by virtue of being factory finished. A production  process ensures excellent finish, dimensional  tolerances,  required strength, adequate quality controls and consistency.

Fig 1 - Representative Graphic of Evolved PCE Structure.

From  the  precast  manufacturer’s  perspective,  the  requirements  above  makes  it  imperative  that  units  produced  are  of consistently  high  quality.  For  a  manufacturer,  targets  on  optical  finish,  sharp  unbroken  edges,  compressive  and  flexural strengths and durability must be met. At the same time, the primary aim for every manufacturer is the achievement of high early strength to allow early stripping times / shorter cycle times to ensure financial viability.

For the precast manufacturer, a fusion of meeting the requirements above and securing cost benefits, can only be achieved using Construction Chemicals. One of the basic reasons for evolution of Construction Chemicals was to remove the inherent disadvantage,  associated  with cement hydration.  Though  cement is the best construction  material  available  it has inherent shrinkage and cracking mechanisms, which lowers the life cycle of construction / structures.

To remove these defects construction chemicals like concrete additives (plasticizers, superplasticizers, integral water-proofers, polymer dispersions,  etc.) were designed and introduced. All in all durability can be achieved by improving the quality of concrete as well as protecting concrete from deterioration mechanism due to chemical, physical, thermal and environmental attacks. We look at durability as providing both sustainability and affordability (reduced maintenance and repair costs) over the life of the structure.

Construction Chemicals have also evolved in terms of providing more specific solutions for the precast industry right from production of units to installation. This paper reviews the underlying mechanisms of these additives and the latest applications / joint studies undertaken at MC-Bauchemie and other industry partners incorporating these materials in development of Precast Construction.

Solutions for Improvement of Concrete for Precast Units

Construction Chemicals today offer a wide variety of solutions for the Precast Industry ranging from the latest generation of PCE polymers for manipulation of green properties of concrete to mould-release agents and curing compounds to enhance the perfect surface finish and edges for the elements. There are also solutions available for improving green characteristics of dry / semidry  concretes  for use in concrete  products  like pavers,  pipes, etc. Construction  Chemicals  also provide  additives  for enhancing the durability of the elements especially in aggressive or acidic media. The sections below highlight these latest developments.

Improved Green and Hardened Concrete Properties Using Latest PCE Technology: The use of Poly Carboxylate Ether (PCE) based admixtures  for concrete is steadily gaining ground, due to the various advantages  it provides.  The hindrance  on its widespread use in India has been on account of its higher initial cost. However, PCE Technology is rapidly evolving to the point where the molecular chemistry of the PCE can be adjusted to make it suitable for use in precast technology. One such evolved material (ref. 1) is discussed in this section.

Figure 2 - Comparison of Slump Flow between conventional and new PCE Technology

The latest chemistry has been instrumental in designing a PCE molecule with a comb-type structure having a short backbone chain and short side chains in addition to a high acid content. Figure 1 shows the representative graphic of the structure of this PCE.

This unique structure allows the PCE to adsorb much faster onto the cement particles, provide a very fast plasticization effect on the mix and retain slump for upto one hour. Due to the molecular structure of the PCE molecule, it provides for a “stearic hindrance” effect, which allows for workability retention without hindering the hydration of the cement grain. This allows the concrete to gain strength rapidly once finishing works on the element are over.

Table 1 shows a Mix Design in production use. Figure 2 shows the slump flow variation of this mix design with respect to time. Figures 3a and 3b show the development of early and final strength for the mix design using this PCE Technology. The unique molecular structure of the PCE also improves robustness of the mix (not greatly affected by minor variations in added mixing water) and shows excellent compatibility with a wide variety of OPCs or blended cements.

Economical advantages for the Precast Manufacturer

•  Short mixing period
•  Low dosage
•  No loss in Workability for 60 minutes
  • Robust   performance   and  compatibility   with  different   cements   (greater   manufacturing   flexibility   for  precast manufacturer)
•  A standard recipe can also be used for advanced precast parts
•  Double shift operation possible (8h hydration with 2h heat treatment)

Using Calcined Clays to Improve Durability, Mechanical & Chemical Resistance of Precast Elements: The experience of use of calcined  clays  in  construction  for  mortars  and  concrete  dates  back  centuries.  For  example,  the  water  storage  “Cisterns” unearthed in Jerusalem makes use of a waterproof mortar consisting of fired brick powder (calcined clay) and hydraulic limes. This system, dating back to about 1000 BC is over 3000 years old. This principle was also used in develop cements and cementing mortars in Germany ca. 1400 and Great Britain ca. 1800 AD.

Figures 3a and 3b - Strength Development for the Mixes in Table1.

Calcined clays, following optimal calcination, are able to react as a pozzolan in presence of Calcium Hydroxide in cementitious systems to form additional strength-building C-S-H or C-A-S-H phases. Equations I (a, b, c and d) illustrates these reactions:

AS2 + 6CH + 9H ⇒	C4AH13  + 2 C-S-H	(Ia)
AS2 + 5CH + 3H ⇒   AS2 + 3CH + 6H ⇒	C3AH6  + 2 C-S-H C2ASH8  + C-S-H	(Ib) (Ic)

Calcined Clay + Calcium Hydroxide + Water ⇒C-A-H/C-A-S-H Phase + CSH                                            (Id)

In terms of the reaction process, Figure 4 shows the 4-phases of reaction (example given for reaction of Metakaolin) viz.

Figure 4 - The Phases of reaction of Calcined Clays.

1.  Introduction of Metakaolin into the Matrix;
2.  Dissolution of reactive calcined clay;
3.  Beginning of the hydration;
4. Complete  incorporation  of  the  calcined  clay  into  foil-like  C-S-H,  C-A-H  (calcium  aluminate-hydrate),  C-A-S-H (calcium aluminosilicate-hydrate) and AFm (monosulphate hydrate) phases (ref. 2). 

In terms of Morphology, the primary difference between C-S-H formation in Ordinary Portland Cement and calcined clays is the formation of a more stable, foil like structure in case of calcined clays as opposed to the fibrous structure in case of OPC hydration (ref. 3).

Figure 5 shows the difference between C-S-H phases in OPC hydration and C-A-S-H phases in the Metakaolin Hydration.

Figure 5 - Fibrous C-S-H Phases of OPC Hydration (left) and Foil like C-A-S-H Phases (right) of Metakaolin Hydration Finally in terms of reactivity, calcined clays such as Metakaolin are much higher in Pozzolanic Activity as compared to flyash, slag and microsilica. We ascertained this by means of the “Chapelle Test” developed by Asbridge et al (ref. 3-4). Our results as per the test are given in Figure 6.

Figure 6 - Lime Consumption by different pOZZOLANS MEASERED USING Chapelle Test

Improvement in Mechanical Properties:  Calcined clays when used as additives in HPCs and UHPCs, function on the basis of the Micro-filler effect and its Pozzolanic reactivity. Due to this combined mechanism, there is an increase in overall mechanical properties of the concrete and improvement in the denser material structure of the matrix, and in particular, improvement in the Interfacial  Transition  Zone  (ITZ)  between  the  non-reactive  aggregates  and  the  cement  matrix.  Figure  7  shows  this improvement.

Figure 7 - Comparative SEM Images showing improvement in the ITZ for normal OPC and an OPC-Metakaolin blend

In our laboratory tests, the mechanical properties of UHPC based on Metakaolin was evaluated and compared to UHPC based on silica  fume.  The compressive  and  flexural  strengths  of the two  mixes  were  measured.  It was seen  that,  although  the compressive strength of the two UHPCs were comparable, there was a marked difference in the flexural strengths of the two concretes. Subsequent SEM studies showed that the substantially higher flexural strengths were due to the platy morphology of the unreacted Metakaolin.

Figure 8 shows comparison of the compressive and flexural strengths of the two UHPCs (ref. 5).

Figures 8a and 8b - Comparison of Compressive & Flexural Strengths in UHPCs with Silica Fume & Metakaolin

Improvement  in Durability:  Calcined  clays like Metakaolin,  when added to concrete  as an additive  or as replacement  for cement, add to the durability of the concrete. The chemical resistance of concrete is improved due to the denser matrix and ITZ improvement of the calcined clays and reduction of Free Lime (Portlandite) in the cementitious matrix. Increased resistant to Alkali Silica Reaction is achieved through, reaction of the aluminosilicate with alkalis to form stable compounds. These same actions  also account  for better resistance  to chloride  penetration  through  concrete.  Lastly,  addition  of calcined  clays also improved the sulphate resistance of tested mixes. This reaction is based on the highly densified matrix structure, the dense ITZ and conversion reactions of the ettringite (ATf) phases in the cementitious matrix to more stable AFm (monosulphate hydrate) phases.

Figure 9 - Showing Effect of Water Addition on Green Stability of concrecte Goods.

Due to the improvement in mechanical properties and durability that using calcined clays as concrete additives, the material opens up new avenues for production of various new forms of UHPCs. Using this material the concrete matrix structure is reinforced right down to nanoscale, density is improved and compressive and flexural strength as well as abrasion resistance of the concrete is increased. There is also a significant reduction of micro-crack formation, which makes it particularly suitable for the production of precast tunnel segments, concrete facades, ultra thin precast segments, filigree elements and precast element for heavy  loading (ref.  6-8).  Successful  examples  of UHPCs  incorporating  Metakaolin  include  acid resistant  concrete  for Budapest Central Wastewater Treatment Facility and acid resistant concrete for production of precast sewage pipes for Epiton GmbH and Co. KG (ref. 8).

Figure 10 - Effect of water Addition on Strength of Concrete.

Additive Technology for Dry / Semidry Concrete for Pipes, pavers, etc.

Concrete Goods are defined as “small or large goods made of Plain Cement Concrete (PCC) or Reinforced Cement Concrete (RCC)  and  manufactured  on a large  scale”.  These  concrete  goods  include  products  such  as paver  blocks,  concrete  tiles, extruded pipes, manhole covers, and also hollowcore elements. These goods are characterized by their production methods, which includes casting dry / semi-dry concretes into moulds and forming them by application of pressure. This method is designed to provide early setting (due to the semi-dry concrete) and demoulding, and the pressure compaction / extrusion is designed to provide proper form and sharp edges to the goods.

However,  this method of production  has its limitations.  Concrete used to make concrete products has different properties. Semi-dry concrete with lower water content is by nature difficult to compact. In addition products must be free of efflorescence. A particular challenge in the concrete products industry is the achievement  of sufficient early strength to enable the rapid removal from forms. Only a perfectly matched combination of admixtures and casting techniques is able to produce the desired results. Basic requirements of concrete goods are:

•  High green strength (instant formwork removal)
•  Structural stability, no swelling or shrinkage
•  Sharp edge formation, smooth side surface
•  Compact / Dense Structure
•  No bonding on tools, moulds or machinery in green state
•  Free of Cracks and Efflorescence
•  Resistance to Water / frost

To countermand these limitations and meet requirements, a series of additives were developed, which on one hand modify the rheology of the semi-dry mix by lubricating it and densifies the mix on the other hand to provide optimum compaction and hydrophobicity.

The general trend of adding water to a semi-dry mix for concrete goods is shown in Figure 9. The figure shows the effect of rising water content on the stability of the mix when compacted under pressure. As can be seen, the mix is very sensitive to the amount of water added. Add Less water (dry branch) and the concrete would not compact properly and add more water (wet branch) and the mix would collapse after demoulding.

Using a compaction aid / densifying additive reduces the amount of water that can be added, while improving compactability of the concrete by lubricating the mix as well as interface between the mix and the mould. This additive also makes the mix more robust by reducing sensitivity of the mix to added water, which allows the manufacturer to produce concrete goods with much more efficiency. In short adding a compacting aid expands the limit of water addition in the mix from a narrow point to a range of values.

Lesser water means better green strengths of the concrete, better stability, better compaction and sharp edges in the concrete goods. Figure 10 shows the effect of water addition on strength development in green stage as well as at 1 day and 28 days.

Advantages of Using Compaction Aids:

•  Improvement of Compactness and Density of Concrete in Produced Good
•  Complete Hydration of Cement Grains
•  Increased Compressive Strength at Demoulding and 28 Days
•  Increased Stability of Produced Goods
•  Enhanced Surface Finish and Edges
•  Decreasing adhesion of concrete to Moulds and Pressure Head
•  No changes in water demand of the mix
•  Decreased Efflorescence and Water Permeability
•  Minimal Dosage at 0.2 to 1.0% by weight of cement
•  Reduced Rejection Rate
•  Better Quality – therefore higher returns on product
•  Improving production efficiency, due to quicker demoulding

Areas of Application:

•  Paver Blocks
•  Manhole Covers
•  Extruded Concrete Pipes
•  Hollowcore Slabs and Walls
•  Concrete Tiles and Roof Tiles
•  Other Miscellaneous Concrete Goods

Conclusion

The review paper above delves into the working mechanisms and applications of the latest construction chemical technologies in improvement  of concrete for precast production. In addition the industry offers a variety of solutions including, but not limited to:

•  Mould Release Agents for improved surface finish
•  Curing Compounds to Improve Surface Finish and Heal Micro-Cracks on precast elements
•  Concrete Cosmetics to mend broken edges, minor blemishes, blowholes, etc.
•  Non-shrink grouting additives to fill pre-stressing conduits
•  Non-shrink grouts and micro-concretes to join precast elements on site
  • Impregnations and clear coatings to impart hydrophobicity and carbonation resistance to precast elements to improve durability

Of course, improved mix design techniques and placing techniques too play a very important role in the production of high quality,  durable,  precast  elements.  Construction  chemicals  do however,  significantly  broaden  the range  of possibilities  of precast production to achieve different aims such as acid resistance (for pipes), improved durability, surface finish, faster cycle times in production and adding robustness to the concrete in terms of production. Having more durable precast elements for the affordable mass housing, would also mean lower life-cycle costs, reduced energy costs, reduced maintenance and of course benefits for the precast manufacturers as well. Therefore, it is imperative that Construction Chemicals be a vital component of improving precast production to promote sustainable construction.

References

1.  Research Notes, MC-Bauchemie Mueller GmbH and Co. KG, Germany.
2.  Prof. Dr.-Ing. Horst-Michael Ludwig, M.Sc. André Trümer, Bauhaus Universitaet, Weimar.
3.  Jayant D. Bapat, (2013), Mineral Admixtures in Cement and Concrete, Chapters 5 and 6.
4. Asbridge,  A.H.,  Walters,  G.V.,  Jones,  T.R.,  (1994),  Ternary  blended  Concretes  – OPC  / GGBFS  / Metakaolin, Concrete across Borders International Conference, Odense, Denmark.
5.  Eugen Kleen, (2010), Technical Director, MC-Bauchemie Mueller GmbH and Co. KG, Germany, Laboratory Testing.
6.  Technical data sheets and in-house testing of Centrilit NC, MC-Bauchemie Mueller GmbH and Co. KG, Germany.
7.  Test Data from Dr. Huettl, TU-Berlin, Germany.
8.  Technical Project References for Centrilit NC, MC-Bauchemie Mueller GmbH and Co. KG, Germany.

Author

Mr. Sunny Surlaker Head Admixtures Division, MC-Bauchemie India) Pvt. Ltd,; Mumbai, India