Climate Resilient Concrete
Defining Climate Resilient Concrete
The infrastructure which
1. Is durable
o so that less cement will be used in its life-cycle but initial cement consumption may be higher than traditional practices
o Because 1 ton of clinker production emits approximately 1 ton of green house gas
2. Uses recycled material
o So that earth become free from hazardous materials
o Fly Ash and Slag are two important recycled material for cement
3. Uses local material
o So that green house gas emission by transportation will be less
Enemy of RCC
Porosity >>>>>>> Rebar Corrosion
Result is total disintegration of concrete
Poor quality of concrete?
Water ingress from roof to entire structure?
Harmful materials in ingredients of concrete?
o Chloride attack
o Sulfate attack
o Rebar corrosion
o Acid attack
o Effect of sea water
o Cracking of concrete
o Clear cover
How to Make Durable Concrete
1. w/c ratio: w/c ratio as low as possible
2. Compaction: proper compaction makes concrete durable
3. Curing: inadequate curing keep pores in concrete
4. Clear Cover: more than carbonation depth within lifespan
5. Type of Cement: other than CEM-I
6. Aggregate: Avoid brick chips and local sand, use well
Durability and permeability of concrete are related. Explain – how?
High strength vs normal strength concrete
1. Use high strength concrete
o Small dimension
2. Use stone chips for all concrete exposed to weather
o Fly over
o Footing, pile, grade beam, roof,
o RCC Road pavement
o Concrete in coastal area
o Roof of building and water tanks
Concrete and the Passivating Layer
o Although steel’s natural tendency is to undergo corrosion reactions, the alkaline environment of concrete (pH of 12 to 13) provides steel with corrosion protection.
o At the high pH, a thin oxide layer forms on the steel and prevents metal atoms from dissolving. This passive film does not actually stop corrosion; it reduces the corrosion rate to an insignificant level.
o For steel in concrete, the passive corrosion rate is typically 0.1 µm per year. Without the passive film, the steel would corrode at rates at least 1,000 times higher (ACI 222 - 2001).
o Cement paste contains 25-50 % wt calcium hydroxide [Ca(OH)2], which mean that the pH of the fresh cement paste is at least 12.5.
o The pH of a fully carbonated concrete is about 7.
o The concrete will carbonate if CO2 from air or from water enters the concrete according to:
Ca(OH)2 + CO2 -> CaCO3 + H2O
o The rate of carbonation depends on porosity & moisture content of the concrete.
o Optimal conditions for carbonation occur at a RH of 50% (range 40-90%).
o Normal carbonation results in a decrease of the porosity making the carbonated paste stronger. Carbonation is therefore an advantage in non-reinforced concrete.
o However, it is a disadvantage in reinforced concrete, as pH of carbonated concrete drops to about 7; a value below the passivation threshold of steel.
Acid Attack (CC+RCC)
o Concrete is susceptible to acid attack because of its alkaline nature.
o The components of the cement paste break down during contact with acids.
o Most pronounced is the dissolution of calcium hydroxide which occurs according to the following reaction:
2 HX + Ca(OH)2 -> CaX2 + 2 H2O
(X is the negative ion of the acid)
o The decomposition of the concrete depends on the porosity of the cement paste, on the concentration of the acid, the solubility of the acid calcium salts (CaX2) and on the fluid transport through the concrete.
o An acid attack is diagnosed primarily by two main features:
· Absence of calcium hydroxide in the cement paste
· Surface dissolution of cement paste exposing aggregates
Alkali silica reaction (CC+RCC)
o Alkali silica reaction is a heterogeneous chemical reaction which takes place in aggregate particles between the alkaline pore solution of the cement paste and silica in the aggregate particles.
o Hydroxyl ions penetrate the surface regions of the aggregate and break the silicon-oxygen bonds. Positive sodium, potassium and calcium ions in the pore liquid follow the hydroxyl ions so that electro neutrality is maintained. Water is imbibed into the reaction sites and eventually alkali-calcium silica gel is formed.
o The reaction products occupy more space than the original silica so the surface reaction sites are put under pressure. The surface pressure is balanced by tensile stresses in the center of the aggregate particle and in the ambient cement paste.
o At a certain point in time the tensile stresses may exceed the tensile strength and brittle cracks propagate. The cracks radiate from the interior of the aggregate out into the surrounding paste.
o Alkali silica reaction is diagnosed primarily by four main features
· Presence of alkali silica reactive aggregates
· Crack pattern
· Presence of alkali silica gel in cracks and/or voids
· Ca(OH)2 depleted paste
Sulfate Attack (CC+RCC)
o External sulfate attack is a chemical breakdown mechanism where sulfate ions from an external source attack components of the cement paste.
o Such attack can occur when concrete is in contact with sulfate containing water / soil, e.g. seawater, swamp water, ground water or sewage water.
o The often massive formation of gypsum and ettringite formed during the external sulfate attack may cause concrete to crack and scale.
o However, both laboratory studies and examinations of field concrete show that external sulfate attack is often manifested, not by expansion or cracking, but by loss of cohesion and strength.
o The microscopic appearance of concrete suffering from external sulfate attack appears to be quite variable. Some diagnostic features such as
· Surface parallel cracks
· Presence of gypsum and ettringite
· Depletion of calcium hydroxide, and
· Decalcification of C-S-H
Chloride attack (RCC)
o Primary action is corrosion of steel
o Chloride ion destroy the passivating oxide film on steel surface
o In the presence of H2O and O2, corrosion of steel occurs
o The primary corrosion rate-controlling factors are the availability of oxygen, the electrical resistivity and relative humidity of the concrete, and the pH and temperature.
o There is no corrosion in dry concrete (RH < 60%)
o There is no corrosion in concrete which is fully immersed in water
o Optimum RH = 70 – 80% for corrosion of steel inside concrete
o Pore system in hardened concrete is the major influencing factor of corrosion; because electrochemical cell requires connection between anode and cathode through pore water
o Electrical resistivity greatly influenced by it moisture content, ionic composition pore fluid and continuity of pore system
o Two consequences of corrosion
· Cracking, spalling, delamination of concrete due greater volume of product of corrosion reaction
· Reduction of steel cross-section leading reduced capacity of RCC section
Concrete in Sea Water with alternating wetting and drying
o Chloride attack - discussed
o Sulfate attack - discussed
o Salt weathering
o Alternating wetting and drying
o During drying, pure water evaporates, salts are left behind in form of crystals, mainly sulfates
o These crystals re-hydrate and grow upon subsequent wetting and thereby exert expansive force
o High humidity and air-borne salt deposition on concrete also do the same
Cracked Concrete (Flexural Members) and durability
o Relevant research by Hearn (1999) shows also that the micro-cracking due to early shrinkage contributes much more to the water infiltration than concentrated load-induced cracks.
o If that is true, does it make a sense for durability design to limit the load-induced crack width?
o We often design water retaining structures considering uncracked section
o To limit the shrinkage crack smaller and distributed to whole structure maximum spacing of rebar should be limited to 150 mm (my opinion)
o Check serviceability / deflection (crack will be controlled) (my opinion)
Ref: Hearn N (1999) Effect of shrinkage and load-induced cracking on water permeability of concrete. ACI Mater J 96(2):234–241
Cracking of concrete and durability
o Although all concrete structures in service exhibits some crack; cracking can be controlled by appropriate structural design, detailing and construction procedure
o Cracks wider than 0.2 – 0.4 mm are harmful ( Naville, 1995)
o Prestressed concrete is crack free; if good quality concrete is used – it is best; but presstressing steel is more vulnerable to corrosion than other steel
Curing and Durability
o Cement paste contains an interconnected system of pores, when partially hydrated
>> lower strength, higher permeability >> vulnerable to chemical, freezing-thawing
o Pore system become segmented/isolated when degree of hydration is sufficiently high
Clear Cover and durability
o More clear cover more durability of concrete; however, the concrete is porous, clear cover can not increase durability.
o Excessive clear cover will lead to temp and shrinkage cracks
o If cracking occurs, clear cover may be detrimental
o Clear cover should not exceed 80 – 100 mm (Naville, 1995)
o In thin sections, it is difficult to maintain large clear cover
Criticism on MM recommendation
No mix ratio (volumetric or mass) is specified
No w/c ratio is specified
Corrosion inhibitor used – controversial
Exposure class – it is good initiative
w/c is specified – it is good and correct
Mix proportions are not compatible with w/c ratio
Mentioned compressive strength is not compatible with w/c ratio
CEM-I is suggested which is not good for durable concrete
No spec for fine aggregate
Maximum size of coarse aggregate is not specified
Stone chips is specified – it is good and required for durable concrete
Brick chips – post mortem
How brick chips destroying environment
o Brick fields uses soil from agriculture land
o Emit green house gases during burning
o Emit green house gases during breaking
o Utilize 3 – 5 times cement in a 100 years life of a structure. 1 ton cement emit 1 ton CO2
o Life safety is at risk due to unsafe
o Building, bridge, culvert etc
Concrete age > 15 yrs
o The condition of marine concrete structures greater than 15 years old in the exposed coastal Upazillas were found to be severely deteriorating.
o Half-cell potential testing of most of the concrete structures at this age suggest high-severe risk of reinforcement corrosion.
o The visual observations on concrete cores extracted from these structures suggest workmanship issues related to use of poor graded aggregates, non-homogeneous concrete mix and voiding at the interface between deck slab concrete and wearing course layer.
Concrete age = 5 - 15 yrs
o The visual observation of concrete core sample extracted from the culvert at Boalkhali road, Islamabad Union, Cox’s Bazaar, under the 5-15 year age category, suggest that the stone aggregates used in the concrete were poorly graded with high proportion of >25mm particle size aggregates.
o The additional survey of buildings in this age category suggest that concrete with brick aggregates especially in exposed coastal Upazilla showed signs of early deterioration of concrete caused by salt scaling and corrosion of reinforcement.
Concrete age = 1 - 5 yrs
o The newer concrete structures (1-5 years age category) predominantly had stone aggregates in concrete, which provides better durability compared with brick aggregate concrete.
o The inspection of new construction sites suggested that in the case of manual production of concrete workmanship issues related to use of poor graded aggregates, improper compaction of concrete, use of saline water for concrete mixing and lack of quality control testing were observed.
Chloride content in water
o The comparison of salinity of local water samples obtained close to the road structures surveyed in each district suggest that the chloride content in ground water was observed to be low as compared with canal/river water in the exposed coastal Upazillas.
o The chloride content of water sourced from interior coastal Upazillas were observed to be very low/negligible. As the water sampling was done during the rainy monsoon season (July-October), the chloride content of water is expected to be low compared to summer season.
o In-situ concrete strength for most of the structural elements were found to be much lower than the design strength of 20 MPa
o The deterioration process is rapidly accelerated in concrete structures containing brick aggregates especially in exposed coastal districts
The study on use of calcium nitrate corrosion inhibitor suggests that at recommended 3-4% dosage of corrosion inhibitor has accelerated the setting time of cement drastically.
However, with the use of set retarders, the accelerating effect can be counteracted.
The experimental trials at different dosages of corrosion inhibitor and set retarder suggested optimum combination at 3.5% corrosion inhibitor and 1.8% set retarder resulted in acceptable setting time results in cement samples.
Experimental finding on durability
o The durability of brick aggregate concrete mixes was significantly poorer than the stone aggregate concrete mixes
o In the case of concrete with blended cements, there was no relationship between strength and durability performance
o The durability performance of concrete improved with increase in cement content of the concrete. However, in the case of 100% CEM I concrete mix no further improvement in durability performance was observed with increase in cement content from 450 kg/m3 to 550kg/m3.
o Concrete mixes with fly ash addition showed better durability performance in comparison to slag based concrete mix.
o In general, among the different cement types, 100% CEM I concrete mix showed poor durability performance as compared to blended cement based concrete mix.
o Among all the concrete mixes tested in the experimental programme, concrete mix with 30% fly ash as cementitious addition and 550 kg/m3 cement content showed the best durability performance to resist chloride induced corrosion