Advanced Concrete Repair Systems (A Recipe for Success)

Himanshu Kapadia, Chief Executive–Construction Chemicals BASF Construction Chemicals (India) Pvt. Ltd. Mumbai

Concrete repairs is not a new phenomenon and a variety of products and application methodologies are in use for a variety of situation. The range of repair mortars can vary from basic sand cement mortar to modern pre bagged mortars which can influence the nanostructures in hydration matrix for durable repairs. The repair methodologies range from cosmetic repairs to structural repairs, rehabilitation and upgradation of the structures. However, the discontent with the effectiveness of repairs has been as common as the repair materials themselves. Inspite of several researches in the field, the need for common understanding of performance requirements for repair materials and suitable methodologies has been felt all along. EN 1504 is one such attempt to bring in the specifications based on performance properties of repair materials and also suggests the repair methodologies for a variety of situations. This paper is a commentary on EN 1504 requirements and how it is influencing the design of performance oriented repair materials.

Introduction

Concrete can deteriorate in a number of ways. The corrosion of reinforcing bars is particularly problematic and is a major limitation on the durability of concrete structures; damage from frost and from alkali-silica reactions is less widespread but still significant. And each year, a large number of buildings suffer fire damage. All of these forms of damage necessitate repair of the structure.

Unfortunately, building owners face uncertainty over the performance of any repairs on their assets due to a history of poor performance and premature failures of some rehabilitated structures.

CONREPNET mentioned in Nov’04 “25% of the structure owners are unhappy with the performance of the repair and protection materials within 5 years after the rehabilitation, 75% are dissatisfied within 10 years!!!”.

On top of all this, the managers of buildings, structures and facilities all over are under pressure to keep their assets operational for minimum cost. These buildings need cost-effective and reliable rehabilitation; something that is not easy in the present circumstances.

This paper describes some of the principles of repairs and material properties required for the performance based repair systems. The new EN 1504 has standardized repair activities and provides an improved framework for achieving successful and durable repair systems. This performance-based approach, whereby the durability and lifetime requirements for the concrete are specified from the outset, is one that is already finding acceptance in the new construction field. It requires a large degree of knowledge of the conditions the concrete will experience and how it will respond during its service.

European Standard EN 1504–Scope of the norm

The European Standard EN 1504 is titled: Products and systems for the repair and protection of concrete structures, and is aimed at all those involved with the repair of concrete. For the first time in the industry, EN 1504 deals with all aspects of the repair and/or protection process including:

• Definitions and repair principles;
• The need for accurate diagnosis of deterioration causes before specification of the repair method;
• Detailed understanding of the needs of the client;
• Product performance requirements and test methods;
• Factory production control and evaluation of conformity
• Site application methods and quality control of the works

When followed, this complex, but comprehensive document, should ensure good quality repair and protection work on the jobsite, which will result in increasing satisfaction of the building owners.

The European standard EN 1504 consists of 10 parts, each covered by a separate document. This provides a resource which helps specifying engineers, contractors as well as material manufacturing companies. It will give the structure owner an increased level of confidence as, for the first time, all issues of concrete repair and protection are addressed by a single integrated standard.

Basic Considerations

This part of the EN 1504 European standard specifies the basic principles which shall be used, separately or in combination, where it is necessary to protect or repair concrete structures, above or below ground or water. Successful repair of a structure starts with a correct condition assessment and identification of the cause of degradation.

All other stages in the repair and protection process depend on these matters. ENV 1504, part 9 explicitly stresses the importance of these issues and identifies the following key stages:

• assessment of the conditions of the structure.
• identification of the cause of the deterioration.
• deciding the objectives of protection and repair together with the structure owners.
• selection of the appropriate principle(s) of protection and repair.
• selection of methods.
• definition of properties of the products and systems (described in EN 1504-2 to 7).
• specification of maintenance requirements following protection and repair.

As obvious as it may seem, EN 1504 is to be applauded for clearly stating that any repair project must identify the goals and objectives of the building or structure owners, before work commences. This includes life expectancy, future use and budget consolidation.

Common Causes of Defects

The nature and causes of defects, including combinations of causes, need to be identified and recorded to arrive at the appropriate repair system. Many defects result from inadequate design, specification, execution and materials. Common causes of defects are represented below which than forms the basis for principles and methods.

Product Selection Based on the Principles and Methods

The principles and methods described in the standard are not new to the industry. What is interesting is the systematic compilation of the prevalent principles and further classification of methods of repairs under each of these principles.

This not only makes the job of the client and the specifier easy, but also guides the material suppliers to make appropriate recommendations and follow a common testing procedure to which the products will comply to. An example of principle definition and methods based on principle is as below (Table 1).

Materials for Concrete Restoration

For the first time in the field of concrete repair, product performance can be compared because the European standard EN 1504 not only specifies minimum performance requirements, it also specifies and standardises testing methods. In many situations, it is essential that products have been tested for the correct intended use and that these minimum performance criteria have been met or exceeded.

A closer look at each of the material requirements for the suggested methods reveals that all products being used predominantly will not fit the criterion. This has also influenced innovation in this field. The description of all new product systems and the basis for their design will not be appropriate at this stage. As an example, this paper describes how the core of the repair materials, the concrete restoration systems are upgraded with the new age understanding of nano technology, shrinkage behavior of the repair materials and compliance with the standards.

EN 1504, Part 3–Structural and Non-Structural Repair of Concrete Structures

The European standard specifies requirements for the identification, performance (including the durability of the materials) and safety of products and systems to be used for the structural and non-structural repair of concrete structures. EN 1504, part 3 covers repair mortars and concretes, possibly used in conjunction with other products and systems, to restore and/or replace defective or contaminated concrete and to protect reinforcement, in order to extend the service life of a concrete structure exhibiting deterioration.

The fields of application covered are in accordance with ENV 1504, part 9 as follows:

Principle 3 Concrete restoration

Method 3.1 Applying mortar by hand

Method 3.2 Recasting with concrete

Method 3.3 Spraying mortar or Concrete

Principle 4 Structural strengthening

Method 4.4 Adding mortar or concrete

Principle 7 Preserving or restoring

Method 7.1 Increasing cover to reinforcement passivity with passivity mortar or concrete

Method 7.2 Replacing contaminated concrete

Mortar classification according EN 1504, part 3


The European standard defines 4 classes of repair mortar R4, R3, R2, R1. These are then divided between structural and nonstructural repairs, i.e. those applications where load transfer has to be considered in the design of the repair specification, or alternatively for cosmetic works. Furthermore the standard classifies the repair products for each type of application, in a high strength or high E-modulus and low strength or low E-modulus mortar. This approach has been developed as a result of 30 years experience in the use of cement mortars for concrete repair. It allows the specifying engineer to select the right quality of repair material for the jobsite specific concrete quality, in order to repair “like with like.” It is well known that incompatibilities between repair mortar and host concrete can lead to premature failure, e.g. through differential thermal expansion/contraction. The different classes do not imply bad, mediocre, good or excellent performances of the repair products. All repair materials meeting the norm are of a high quality. The norm only indicates which repair mortar class should be used for which kind of application. e.g.,

• high strength concrete exposed to heavy loads should be repaired with a high strength/high E-modulus repair product, thus a class R4 mortar.
• a lower strength concrete exposed to loads should be repaired with a structural repair mortar with medium strength and /or E-modulus, thus class R3.
• all concretes in a non-structural situation, i.e. where loads are not to be transferred through the repair zone, can be repaired with a higher quality non-structural repair mortar, class R2 In addition to considering the appropriate classes, it is of utmost importance to recognize and specify the exposure conditions to which the product will be exposed. These exposure classes and the relevant repair mortar testing will determine the durability of the applied mortar systems. e.g.,

• a mortar tested for restrained shrinkage/expansion only can not be used on structures exposed to freezing and thawing
• a mortar approved for use in freeze/thaw conditions (including salt exposure) can be used in all conditions

Performance requirement for cementitious structural and non structural repair products are as below.

Noteworthy is the fact that the performance criteria extends much beyond the compressive strength and the stress is on other aspects like adhesive bond, restrained shrinkage/expansion, carbonation resistance, thermal compatibility in different conditions and elastic modulus. In addition, it is mandatory for the manufacturers to do the testing periodically and provide the certificates.

Structural Grade Repair Mortar–Design inputs and Tests

If the performance based criteria is adopted by the specifying agencies, the role of site batched, uncontrolled repair mortars will diminish. The factory made repair mortars complying with the norms will be more in practice and the end result will be a dependable repair system, which in fact will be at lower life cycle cost.

In order to design the product to meet with the requirements a more sophisticated approach is required. The understanding on nano technology and nano structures plays a crucial role.

Cementitious products exhibit poor tensile strengths due to weaknesses at the contact layer between cement and aggregate. These are known as transition effects and the shrinkage of the pores during the early drying of the mortar are the result of this weakness. The answer is improving the quality of the Nanostructures by reducing the pore shrinkage through the use of Shrinkage reducing admixtures. Improving the quality of the Nanostructures in the cement paste transition zones improves tensile strength and therefore reduces cracking.

Shrinkage and Crack Control

It is well understood that cracking is the most obvious sign of shrinkage and this generally leads to a reduction in the lifespan or durability of the repairs.

Cracking is further reduced by using appropriate fibre in the structural repair mortar. In the lab test with over 50 types and lengths of fibre, the unique kidney shaped fibre with rough surface were found to be the best which increases mechanical adhesion. Fibre type Poly Acrylanitrile promotes chemical adhesion between cement paste and fibre and appear to be the best choice. Best binder packing models need to be used by optimizing cement contents and types, filler gradings and shapes, inorganic additives and pore size distribution.

Shrinkage Test

Just testing shrinkage over the short term (28 or 56 days) may not be sufficient to indicate long term cracking patterns. Amongst several methods, two methods seem to simulate real world conditions better and also seem to be able to discern between good, better and best formulations

These test methods are the Coutinho Ring and the DIN V-Channel test

The Coutinho ring consists of a ring mould with two concentric rings with the inner one having a central diameter of 100mm and the outer a diameter of 200mm. Mortar is applied in the gap between the two rings and allowed to set. Once set the outer ring is removed and the test begins.

The inner ring creates a restraint that means that any shrinkage manifests itself in the form of a crack. The test is basically to store the cast rings and wait to see if any cracks develop. High shrinkage products will often crack in the first two weeks. Continuing the test for some months will also show up those with moderate shrinkage and help to determine which formulas give the best long term performance. Picture 3 shows the test results with two different mortars out of which one complying with the test.

The DIN V Channel is 1 meter long channel with 50mm sides. The bottom of the V is a 90 degree angle and is supposed to be analogous to the long edge of a repair. These are filled with mortar and trowelled off. The mortar is allowed to harden and the strip is left in the mould for some months. Again the restraint of the material causes the shrinkage forces to show up in a crack.

Conclusion

Now, engineers, designers and product manufacturers can rely upon a standard which forms the basis for assessment of the actual or potentials causes of deterioration and consideration of the appropriate principles and methods for protection and repairs of concrete structures.

This paper only illustrates some examples on the basis of which appropriate products for all methods can be designed, specified and used by all stake holders for holistic approach to durable concrete repairs for all kinds of structures.

References

• EN 1504–1 to 10
• Simplified illustrated guide for EB 1504 by PCI Augsburg GmbH