Concrete–Protection, Repair & Rehabilitation

Dr. Surendra P. Bhatnagar, Chairman & Managing Director–Tech-Dry (India) Pvt. Ltd.,Bangalore

Concrete is supposed to be the most versatile material used in modern construction. It is a very durable material, but its properties and performance depend on so many factors like selection of mix ingredients, mix design, placing, compaction, curing conditions, design and detailing and environmental factors.

It is an irony to note that inspite of the above statement that concrete is a durable material, these benefits of concrete are not always achieved in practice and we would analyse this factor in this paper.

Concrete is a composite material and therefore its quality would largely depends on the basic constituents that are used like coarse aggregate, fine aggregate and/or sand cement and water.

The quality of concrete is generally related to its strength and permeability which in turn, are broadly governed by the amount of water used in the mix.

Thus, for high strength and low permeability, a low water/binder ratio is often required. Careful selection of mix constituents, including chemical and mineral admixtures, proper mix design practices, placing and adequate curing are significant contributory factors to the overall quality of concrete.

Unfortunately, even RMC in India are unable to maintain low water–cement ratio on account of the fact that they use rather very cheap type of plasticizers and the retention of slump loss is not optimum.

Porosity

Water in excess of that needed for cement hydration is retained in pores within the concrete mass and can be detrimental to concrete durability. Loss of surplus water occurs with the drying of concrete, which leads to an increase in porosity and reduced concrete strength.

Curing

Curing is the most important factor because it is necessary to retain required water by the concrete, which is not always available, and therefore cement hydration is affected.

Correct curing significantly enhances strength and durability. The builder or the client is always in a hurry and therefore correct curing is not achieved which is required for strength and durability.

The durability influenced by the transport of water containing dissolved substances and gas through pore networks and along cracks.

Compaction of the concrete is a key factor, which is normally not properly supervised. The solution contained in the pore structure of concrete is normally highly alkaline as a result of cement hydration, with a pH of about 13 for Portland cements. The alkaline pore solution is particularly beneficial to steel reinforcement as it provides a chemical protection against corrosion.

However, the carbonation if not checked makes the reinforcement vulnerable to corrosion.

• Alkali-silica (alkali-aggregate) reaction
• Sulfate attack
• Carbonation
• Effect of Chlorides

Protective and Remedial Systems

Protective and remedial systems may be applied to concrete in a structure to protect it against chemical or mechanical effects. In principle, such systems are designed primarily to enhance structural durability and extend service life performance. However, improvements or repairs to damaged structures may also be driven by the need to improve aesthetics. Technical considerations are essential to sound decision-making in all cases. To successfully implement a repair or protective system, it is essential to arrest causes, which favor the observed deficiency.

Symptoms often have more than one cause. Therefore, the key to success or failure of a repair system is the proper diagnosis of the problem. This is followed by developing a cost-effective repair or remedial plan, which ensures long-term serviceability and integrity of the structure. Professional expertise is required at every stage of a repair process. This includes design, planning, material selection, placement, installation and inspection of protective systems. As such, it is important that economic considerations do not solely dominate system selection at the expense of performance and eventual structural serviceability.

Cracks

Restraint to deformation is the basic cause of concrete cracking. It is essentially the lack of freedom of concrete members to respond to movements or volume changes.

The primary concerns with cracks are whether they affect structural integrity, whether they have been caused by inappropriate design detailing, or are simply unacceptable aesthetically.

Cracks vary in dimension. Small micro cracks arise from stresses or strains due, for example to restraint of movements as in shrinkage cracking of concrete. There can be fine hairline cracks that may have no apparent durability consequences.

It must be recognized that concrete has an inherent potential to crack, and that crack control is an integral part of reinforced concrete design and construction techniques. In respect of concrete durability, crack patterns can be helpful in diagnosing potential causes of deterioration. Thus, to enable proper repair of cracks, their causes must first be identified.

What kind of cracks can occur and how can they be avoided?

Both plastic and settlement cracking are associated with bleeding of fresh concrete, i.e., water rising to the top of the concrete shortly after compaction. This can be avoided by using 4th generation plasticizers.

Plastic Shrinkage Cracks

Plastic shrinkage cracks occur when moisture from the concrete surface evaporates faster than the rate of bleeding. This can be avoided by using crystallization and curing products.

If the final finishing of the concrete is not good a random, forming a map pattern is created as show in the figure below.

Plastic shrinkage which are sometimes induced by temperature differences as cement hydration are typical.

Some fine cracks occur due to drying shrinkage. These cracks are referred to as ‘surface crazing,’ Crazing is identified as a series of minute closely spaced shallow cracks. This can be avoided by spraying water on the concrete and keeping control of the ambient temperature.

Alkali aggregate reaction can be avoided by washing the jelly and by using alkali aggregate admixtures.

It is important that the reinforcement is coated by repellency products like Silane Cream, which would help the concrete. There are admixtures, which can increase the compressive strength of concrete.

Efflorescence

Normally efflorescence is noticed in concrete and it can be controlled by using anti efflorescence products.

• The quality of water is vital and any chloride content in the water can be dangerous.
• Placing of concreting, selection of reputed RMC suppliers, using your own waterproofingcompounds like admixtures or Salt Retarder can be very useful.
• The proper compaction is required to avoid any voids and honey combs. How to rectify the problems of concrete before waterproofing?

Crack Repair

The key feature for successful crack repair is an understanding of the causes of cracking, whether cracks are dormant or active, and to identify the desired outcome of the repair works.

The objectives of repair work depend largely on the function of the facility. Depending on the nature of the damage and state of the structure, the desired outcome of repair may be structural enhancement, appearance, durability improvement, water tightness, or to prevent the ingress of corrosive substances to reinforcement.

However, in practice it is difficult to repair and therefore we can identify some general methods:

For Wide Cracks/ Thorough Cracks or Cracks in Slab

The best solution is to grout by using injection machine or ordinary grouting depending on the severity of the problem.

For Cracks

The first attempt should be to slow down the rate of seepage or to reduce the void by using products based on natural clays and then use the grouting system. Besides grouting, we should also look into the crack filling products.