Special materials for repair & seismic strengthening of buildings
Every year, numerous buildings across the country are affected by natural calamities. In such a scenario, it is very important for builders and constructors to focus on the structural strength of a building while carrying out any kind of construction.
Like any other country, India too has faced a fair share of climatic adversities in the past. While natural calamities are a big concern, a significant proportion of the country’s infrastructure is incapable of withstanding even acts of nature such as heavy rains. Heavy downpours during June-July 2019 in Maharashtra, northern Karnataka caused several walls to collapse across the state resulting in severe loss of life and property.
According to the National Institute of Disaster Management, Government of India, as much as 59 per cent of land area in India is susceptible to seismic hazards or earthquakes. The country has witnessed some highly devastating earthquakes since the late 19th century which have had consequential effects on our buildings and infrastructure.
In such a scenario, it is very important for builders and constructors to focus on the structural strength of a building while carrying out any kind of construction activity. Some such activities include repair, restoration or seismic strengthening of dilapidated buildings. In a building strengthening guideline prepared by Dr Anand S. Arya, Professor Emeritus, Department of Earthquake Engineering, I.I.T Roorkee and advisor to the Ministry of Home Affairs, he enlists eight special materials that are best suited for building repair and strengthening projects.
Here’s the list of eight materials that are most suitable for repair, restoration and seismic strengthening of..
Quick-setting cement mortar
Ferrocement – fibre concrete
Fibre reinforced plastic (FRP)
According to Dr Arya, cement and steel are the most common materials that are used for the repair and restoration of various kinds of buildings. If need be, an appropriate admixture can be added to the cement mortar or cement concrete mix, concrete mix to enhance some of their properties such as non-shrinkage, bond strength, etc. Some common steel products used for such construction activities include bolts, rods, angles, beams, channels, expanded metal and welded wire fabric. Rounds, sleepers, planks, etc. made of wood and bamboo are generally used for temporary support and scaffolding purposes.
Concrete mix to enhance some of their properties such as non-shrinkage, bond strength, etc. Some common steel products used for such construction activities include bolts, rods, angles, beams, channels, expanded metal and welded wire fabric. Rounds, sleepers, planks, etc. made of wood and bamboo are generally used for temporary support and scaffolding purposes.
Non-shrink grout: This is a very suitable material for filling up cracks in masonry structures. Their specialty is that their volume does not decrease upon setting. These days, ready-mix non-shrink grouts are available in the market which comprises non-shrink grout cement, special sands and polymer. The role of the polymer in such mixes is to improve the adhesive and tensile strength of the grout.
Shotcrete: Shotcrete is a type of mortar or concrete mix with coarse aggregates which are not more than 10 mm in size. This type of cement mortar or concrete is sprayed directly to a concrete or masonry surface using compressed air. This mix is sprayed at high velocity through a jet nozzle which helps the shotcrete to produce a compact homogeneous mass on the surface. The process of manufacturing shotcrete is called ‘shotcreting’ and there are two techniques of shotcreting techniques of shotcreting:
Wet mix process
Dry mix process
The wet mix process involves water being added to all the ingredients of the mix before it enters the delivery hose. Whereas, the dry mix process requires the damp mixture of cement and aggregates to be passed through the delivery hose to the nozzle and water is added to the mix in the nozzle. Dry mix shotcrete is most applicable in the repair of concrete elements. In such applications, an apan appropriate epoxy adhesive is also used to assure proper strength between the surface of the damaged structure and the shotcrete mix. Apart from that, the use of shear keys can further ensure proper shear transfer between the existing layer and the new layer of concrete.
Epoxy resin: Epoxy resins are chemical mixtures with high tensile strength which serve as exceptional adhesive agents. Their composition can be changed as per requirement and the ingredients required to produce a suitable epoxy resin are mixed just before they are applied. The viscosity of some epoxy resins is so low that they can be injected into fine cracks as well. On the other hand, epoxy resins which have high viscosity are generally used for surface coating purposes and for filling large cracks or holes. These binding agents can also be effectively used in applications that require attaching steel plates to damaged structures.
Epoxy mortar: This special material is formed by mixing either high or low viscosity epoxy resins with fine aggregates/sand and is used to fill voids or gaps which are very big in size. The compressive strength and tensile strength of epoxy mortar are higher whereas its modulus of elasticity is lower than that of cement concrete. Sand is added while producing epoxy mortar to enhance its modulus of elasticity.
Quick-setting cement mortar: This is basically non-hydrous magnesium phosphate cement comprising a liquid and a dry powder. Quick-setting cement mortar is formed by mixing the liquid with the dry powder just like cement and aggregates are mixed to produce cement concrete.
Mechanical anchor: These are anchors or fasteners which provide anchorage to concrete or masonry structures by using wedging action. Mechanical anchors are used to provide sufficient strength to a structure and some anchors provide shear as well as tension resistance. For applications where mechanical anchors cannot be used, the use of chemical anchors which are attached to drilled pilot holes using polymer adhesives has been advised.
Ferrocement fibre concrete: This building material is applied as a comparatively thin layer of cement mortar over reinforcing materials such as mild steel rods having a diameter of 4.5 to 6 mm spaced at an interval of 75 mm from each other both ways and covered with 19 gauges, 11 mm opening square mesh on each side. The building technique used with this material is simple and can be executed by masons and unskilled labour. In the case of retrofitting applications, ferrocement plates are positioned at an appropriate location on the surface of the masonry structure and directly attached to it. The thickness of the plates can vary in the range of 30 to 180 mm.
Fibre reinforced plastics or polymers (FRP): This recently developed material can be used for the strengthening of reinforced concrete (RC) or masonry structures or as a replacement of steel plate bonding. The strength to weight ratio as well as corrosion resistance of FRP is high. Further, the weight of an FRP plate can be as low as 20 per cent that of a steel plate while it can be 2 to 10 times stronger than an FRP plate. These plates need to be attached to the affected structure using epoxy mortar and it is advised to refer to the manufacturer’s specifications and conduct proper tests before finalising the design of any retrofitting project.
ABOUT THE AUTHOR
Dr SB Hegde is a ‘Global Visionary Award 2020’ winner for his notable contribution to cement field (with 30 years of experience) both in India and Abroad. He is a ‘Expert Panel’ member in renowned international magazines of cement and concrete. Dr Hegde is also a ‘Visiting Professor’ of one of the reputed universities in the US