A properly designed, produced and cured concrete is inherently durable to the environments it will be exposed. Besides, a carefully produced concrete with good quality control is capable of maintenance-free performance for decades without the need for protective coatings, except in highly corrosive environments. However, concrete is potentially susceptible to attack in variety of different exposures unless certain precautions are taken. Deterioration of concrete can be caused by the adverse performance of any one of the three major components: aggregate, paste or reinforcement, and can be due to either chemical or physical causes (Table 2.1). ın most of the cases, an individual environment factor initiates distress, then other factors may contribute and aggravate the situation.
A major difficulty in studying durability is predicting concrete behavior several decades in the future on the basis of short-term tests. Most of the knowledge of the durability has been accumulated through a direct study of actual field problems. The prediction of concrete durability under a variety of service conditions, is still a major problem.
Table 2.1. Durability of Concrete
Chemical Attack | Physical Attack |
Leaching and efflorescence (P) | Freezing and Thawing (P, A) |
Sulfate Attack (P) | Wetting and Drying (P) |
Alkali – Aggregate Reaction (A) | Temperature Changes (P, A) |
Acids and Alkalis (P) | Wear and Abrasion (P, A) |
Re-bar Corrosion (R) |
|
Letters in parenthesis indicates the concrete component most affected, in order of importance: A=>Aggregate ; P=> Paste ; R=> Reinforcement
Water is generally involved in every form of concrete deterioration, and in porous solids permeability of the material to water usually determines the rate of deterioration. Therefore, in this chapter the structure and properties of water are described with special reference to its destructive effect on porous materials; then the permeability of cement paste, aggregates and concrete as well as the factors controlling their permeability are discussed.
Physical effects that adversely influence the durability of concrete include surface wear, cracking due to crystallization pressure of salts in pores, and exposure to extreme temperatures. Deleterious chemical effects include leaching of the cement paste by acidic solutions, and expansive reactions involving sulfate attack, alkali-aggregate reaction and rebar corrosion in concrete. The significance, physical manifestations, mechanism, and control of various causes of concrete deterioration are discussed in detail.
Definition: durability is generally considered synonymous with “long service life”. Since durability under one set of conditions does not necessarily mean durability under another, it is customary to include a general reference to the environment when defining durability. According to ACI Committee 210, durability of Portland cement concrete is defined as its ability to resist weathering action, chemical attack, abrasion, or any other process of deterioration that is, durable concrete will retain its original form, quality and serviceability when exposed to its environment.
Generally, as a result of environmental interactions the microstructure and consequently, the properties of materials change with time. A material is assumed to reach the end of service life when its properties under given conditions of use have deteriorated to an extent that the continuing use of the material is ruled either unsafe or uneconomical.
Significance: it is generally accepted now that in designing structures the durability characteristics of the materials under consideration should be evaluated as carefully as other aspects such as mechanical properties and initial cost.
Mostly a substantial portion of the total construction budget is used for the repair and replacement of existing structures arising from material failures. For example, it is estimated that in industrially developed countries, over 40% of the total resources of the building industry are applied to repair and maintenance of existing structures, and less than 60% to new installations. The escalation in replacement cost of structures and the growing demand on life-cycle cost rather than first cost are forcing engineers to become durability conscious. Furthermore, a close relationship exists between durability of materials and ecology. Conservation of natural resources by making materials having longer service life is, after all, an ecological step. Besides, the uses of concrete are being extended to new applications, such as offshore platforms, containers for handling liquefied gases at cryogenic temperatures and high pressure reaction vessels in the nuclear industry.
General Observations: before a discussion of important aspects of durability of concrete, a few general remarks on the subject will be helpful.
1. Water, the primary cause of both creation and destruction of many natural materials, is also control to most important durability problems in concrete. In porous solids, water is the case of many types of physical process of degradation. As a carrier of aggressive ions, water can also be a source of chemical process of degradation.
2. The physico-chemical phenomena associated with water movement in porous solids are controlled by the permeability of the solid. For example, the rate of chemical deterioration would depend on whether the chemical attack is limited to the surface of concrete or whether it is also work inside the material.
3. The rate of deterioration is also affected by the concentration level of ions in water and by the composition of solids. Due to the presence of alkali calcium compounds in hydration products of Portland cement, unlike many natural rocks and minerals, concrete is a basic material. Therefore, acidic waters are expected to be harmful to it.
Most of our knowledge of physico-chemical processes responsible for concrete deterioration comes from case histories of structures in the field, because it is difficult in the laboratory to simulate the combination of long-term conditions normally present in real life. However, in practice, deterioration of concrete is seldom due to a single cause; usually, at advanced stages of material degradation more than one deleterious phenomena are found at work. In general, various causes of deterioration are so closely intertwined and an interacting so that even separation of the cause from the effect often becomes impossible. Therefore, a classification of concrete deterioration processes into neat categories should be treated with some care. Since the purpose of such classifications is to explain, systematically and individually, the various phenomena involved, there is a tendency to overlook the interactions when several phenomena are present simultaneously.
Water is the most aboundant fluid in nature in the form of seawater, groundwater, rivers, lakes, rain, snow and vapor. Being small, water molecules are capable of penetrating extremely fine pores or cavities. As a solvent, water is able to dissolve more substances than any other liquid. This is due to the presence of many ions and gases in some waters, which in turn, become instrumental in causing chemical decomposition of solid materials.
It may also be noted that eater has the highest heat of vaporization among the common liquids, therefore, at ordinary temperatures it has the tendency to remain in a material in the liquid state, rather than to vaporize and the material dry.
In porous solids, internal movements and changes of structure of water are known to cause disruptive volume changes. For example, freezing water into ice, formation of ordered structure of water inside fine pores, development of osmotic pressure due to differences in ionic concentration, and hydrostatic pressure build up by differential vapor pressures can lead to high internal stresses within a moist solid. A brief review of the water structure will be useful for understanding these phenomena.
The H-O-H molecule is covalently bonded. Due to asymmetric character of water molecule, the charge centers of hydrogen and oxygen are different. Thus the porosity charged proton of the hydrogen ion belonging to a water molecule attracts the negatively charged electrons of the neighboring water molecules. This relatively weak force of attraction, called the hydrogen bond is responsible for the ordered structure of water.
The highest manifestation of the long-range order in the structure of water due to hydrogen bonding is seen in ice (Fig 2.1.a). Each molecule of water in ice is surrounded by other four molecules, one molecule at the center and four molecules at the corners of tetrahedron. In all three directions the molecule and groups of molecules are held together by hydrogen bonds. When ice melts at 0°C ~15% of the hydrogen bonds breakdown in directionality of the tetrahedral bond, thus, each water molecule can acquire more than four nearest neighbors, which causes the density to rise from 0.917 to 1. Upon solidification of liquid water, reverse process occurs, thus expansion forms rather than contraction.
| Fig. 2.1. a) Structure of ice ; b) structure of oriented water molecules in micro pore. (The structure and properties of water affected by temperature and by the size of pores in a solid). |
Compared to the structure of ice, water at room temperature has about 50% of the hydrogen bonds broken. Materials in the broken-bond state have unsatisfied surface charges, which give rise to surface energy. The surface energy in liquids causes surface tension, which accounts for the tendency of a large number of molecules to adhere together. It is the high surface tension of water (defined as the force required to pull the water molecules apart) which prevents it from acting as an efficient plasticizing agent in concrete mixes until suitable admixtures are added.
Formation of oriented structure of water by hydration bonding in micropores causes expansion. In solids the surface energy is more when numerous fine pores are present. If water is able to permeat such micropores, and if the forces of attraction at the surface of pores are strong enough to break down the surface tension of bulk water and orient the molecules to an ordered structure (analogous to the structure of ice), this oriented or ordered water, being less dense than the bulk water, will require more space and will therefore tend to cause expansion (Fig 2.1-b)
Water as a necessary ingredient for the cement hydration and as a plasticizing agent for concrete components is found in the structure of concrete from beginning. Gradually, depending on the ambient conditions and the thickness of a concrete element, most of the evaporable water in concrete, will be lost leaving the pores unsaturated or empty. Since it is the evaporable water which is freezable and also free for internal movement, the concrete will not be vulnerable to water-related destructive phenomena, provided that there is a little or no evaporable water left drying, and provided that the subsequent exposure of the concrete to environment does not lead to resaturation of the pores. The resaturation of the pores to a large extent depends on the hydraulic conductivity, formed as (coefficient of) permeability (K).
Many attempts have been made to relate the microstructural parameters of cement hydration products with either diffusivity (the rate of diffusion of ions through water-filled pores) or permeability. (The rate of viscous flow of fluids through the pore structure).
According to Garboezi as cited by Mehta, for a variety of reasons the diffusivity predictions need more development and validation before their practical usefulness can be proven therefore, in this course only permeability is discussed, implying that this property covers the overall fluid transport characteristic of the material. Permeability is defined as the property that governs the rate of flow of a fluid into a porous solid. For steady-state flow, the coefficient of permeability (K) is determined from Darcy’s expression:
Where 
: rate of fluid flow µ : viscosity of the fluid
ΔH : pressure gradient
A : the surface area
L : thickness of the solid
The coefficient of permeability of a concrete to gas or water vapor is much lower than the coefficient for liquid water, therefore, tests for measurements of permeability are generally carried out using water that has no dissolved air. Besides, due to their interactions with cement paste the permeabilities of solutions containing ions would be different from the water (pure water) permeability.
In a hcp the size and continuity of the pores at any stage during the hydration process would control the coefficient of permeability. The mixing water is indirectly responsible for permeability of the hcp, because its content determines first the total space available for hydration products and subsequently the unfilled space after the water is consumed by either cement hydration reactions or evaporation to the environment. Generally the permeability of hcp is controlled by W/C ratio and degree of hydration as shown below:
***Lets consider the hydration process of 1cc cement, accepting that 1 cc cement upon full hydration produces 2.1 cc hydration products.
For W/C = 0,40 by weight
Weight of cement èWc = 1 x 3,15 = 3,15 g
Weight of water è Ww = 0,40 x 3,15 = 1,26 g
Volume of water è Vw = 1,26 cc
For 50% hydration
Volume of hydrated cement Vhc = 0.5 cc
Vhp = 0.5 x 2.1 = 1.05 ml volume of hydration products
Vcp = Total volume - Volume of hydration products - Volume of unhydrated cement
Vcp = 2.26 - 0.50 - 1.05 = 0.71 cc
For 100% hydration
Volume of hydrated cement Vhc = 1 cc
Vhp = 1 x 2.1 = 2.1 ml volume of hydration products
Vcp = Total volume - Volume of hydration products - Volume of unhydrated cement
Vcp = 2.26 - 2.1 = 0.16 cc
*** Lets repeat the same problem for W/C = 0.80
Vc = 1 cc
Wc = 1 x 3,15 = 3,15 g
Ww = 0,80 x 3,15 = 2.52 g
Vw = 2.52 cc
For 50 % hydration è Vhp= 0,5.(2,1)=1,05 ;
Vcp = 3,52 - 0,5 - 1,05 = 1,97 cc
For 100 % hydration è Vhp = 1,0 . (2,1) = 2,10 ;
Vcp = 3,52 - 2,10 = 1,42 ml
The coefficient of permeability of freshly mixed cement paste is of the order of 10-4 to 10-5 cm/s with the progress of hydration both the capillary porosity and coefficient of permeability decrease. However there is no direct proportionality between the two. This is because, in the beginning, as the cement hydration process progresses even a small decrease in the total capillary porosity is associated with considerable segmentation of large pores, thus greatly reducing the size and number of channels of flow in the cement paste. Typically, 30% capillary porosity represents a point when the interconnections between the pores have already become so tortuous that a further decrease in porosity of the paste is not accompanied by a substantial decrease in the permeability coefficient.
In general, when W/C ratio is high and the degree of hydration is low, the cement paste will have high capillary porosity; it will contain a relatively large number of big and well-connected pores and, therefore, its coefficient of permeability will be high. As hydration progresses, most of the pores will be reduced to small size (100 nm or less) and will also lose their interconnections; thus the permeability drops. The coefficient of permeability of hcp when most of the capillary pores are small and disconnected is of the order of 10-12 cm/s. It is observed that in normal cement pastes the discontinuity in the capillary network is generally reached when the capillary porosity is about 30%. With 0,40 , 0,50 , 0,60 and 0,70 W/C ratio pastes this generally happens in 3, 14, 180 and 365 days of moist curing, respectively. Since the W/C ratio in most concrete mixtures seldom exceeds 0,70, it should be obvious that in well-cured concrete the cement paste is not the principal contributing factor to the coefficient of permeability.
Compared to 30 to 40% capillary porosity of typical hcps, the volume of pores in most natural aggregates is usually under 8% and rarely exceeds 10%. Thus, it is expected that permeability of aggregate would be much lower than that of typical cement paste. This may not necessarily be the case. It is shown that the coefficient of permeability of aggregates are as variable as those of hcp of W/C ratios in the range of ~0,4 to 0,7. The reason some aggregates, with as low as 10% porosity, may have much higher permeability than cement paste is because the size of capillary pores in aggregate is much larger than cement paste. (Most of capillary porosity in nature hcp is 10-100nm; average pores in aggregate > 10 mm ; 100 – 1000 times greater than pores in hcp).
Theoretically, the interaction of aggregate particles of low permeability into a cement paste is expected to reduce the permeability of the system, because the aggregate particles should intercept the channels of flow within the cement paste matrix. Therefore, compared to neat cement paste, mortar or concrete with the same W/C ratio and degree of maturity, should give a lower coefficient of permeability. However, this is not the case, and the addition of aggregates to cement paste or mortar increases the permeability considerably. The permeability of concrete depends mainly on the W/C ratio and degree of hydration (Which controls the size, volume and continuity of capillary pores) as well as maximum aggregate size (which determines the microcracking in the transition zone between coarse aggregate and cement paste); in fact, the larger the max aggregate size, the greater the coefficient of permeability.
Owing to the significance of the permeability to physical and chemical processes of deterioration of concrete, a brief review of the factors controlling permeability of concrete should be useful. Since strength and permeability are related to each other through the capillary porosity, as a first approximation the factors that influence the strength also affect the permeability (Fig.2.2).
Fig 2.2 Influence of capillary porosity on comp. strength and coefficient of permeability
A reduction in the volume of a large capillary pores (>100 nm pores) in the hcp matrix would reduce the permeability. This would be possible by using low W/C ratio, adequate cement content, proper compaction and curing. Similarly proper attention to aggregate maximum size & grading, thermal & drying shrinkage strains, and avoiding premature or excessive loading are necessary steps to limit the transition zone microcracking which appear to be the major cause of high permeability in concrete. Finally, the thickness of the concrete element is of importance since it determines the tortuosity of the path of fluid flow, which in turn, determines permeability.
