concrete durability

AUTHOR: AU YONG THEAN SENG

www.madisonvelocity.blogspot.com

TOPICS HIGLIGHTED

DEFINITION CONCRETE: STRENGTH AND WEAKNESS IN TERMS OF DURABILITY THE MILLION DOLLARS QUESTIONS METHODS TO PROLONG THE DURABILITY OF THE CONCRETE COST OF CONCRETE REHABILITATION PREVENTION IS BETTER THAN CURE MIDDLE RING ROAD 2 – KEPONG BRIDGE IS FALLING DOWN ALLOCATION FOR REPAIR WORKS UNDER NINTH MALAYSIA PLAN ALLOCATION FOR THE MINISTRY OF WORK UNDER NINETH MALAYSIA PLAN CONCLUSION



DURABILITY OF CONCRETE DEFINITION Durability is defined as the capability of concrete to resist weathering action, chemical attack and abrasion while maintaining its desired engineering properties. It normally refers to the duration or life span of trouble-free performance. Different concretes require different degrees of durability depending on the exposure environment and properties desired. For example, concrete exposed to tidal seawater will have different requirements than indoor concrete. There are 2 key factors affecting durability: a) Permeability - high permeability will decrease durability - water/cement ratio less than 0.45 and continuous curing improve durability b) Ultimate Compressive Strength - long term strength important for superior durability Concrete durability will remain durable if o the cement paste structure is dense and of low permeability o under extreme condition, it has entrained air to resist freeze-thaw cycle. o it is made with graded aggregate that are strong and inert o the ingredients in the mix contain minimum impurities such as alkalis,

chlorides, sulphates and silt It is therefore some requirements need to be paid attention on for a durable concrete structure Cement content Mix must be designed to ensure cohesion and prevent segregation and bleeding. If cement is reduced, then at fixed w/c ratio the workability will be reduced leading to inadequate compaction. However, if water is added to improve workability, water/cement ratio increases and resulting in highly permeable material. Compaction The concrete as a whole contain voids can be caused by inadequate compaction. Usually it is being governed by the compaction equipments used, type of formworks, and density of the steelwork Curing



It is very important to permit proper strength development aid moisture retention and to ensure hydration process occur completely Cover Thickness of concrete cover must follow the limits set in codes Permeability It is considered the most important factor for durability. It can be noticed that higher permeability is usually caused by higher porosity .Therefore, a proper curing, sufficient cement, proper compaction and suitable concrete cover could provide a low permeability concrete



CONCRETE: STRENGTH AND WEAKNESS IN TERMS OF DURABILITY High Humidity and Wind-Driven Rain Concrete is resistant to wind-driven rain and moist outdoor air in hot and humid climates because it is impermeable to air infiltration and wind-driven rain. Moisture that enters a building must come through joints between concrete elements. Annual inspection and repair of joints will minimize this potential. More importantly, if moisture does enter through joints, it will not damage the concrete. Good practice for all types of wall construction is to have permeable materials that breathe (are allowed to dry) on at least one surface and to not cover concrete between two impermeable surfaces. Concrete will dry out if not covered by impermeable treatments. Ultraviolet Resistance The ultraviolet portion of solar radiation does not harm concrete. Using colored pigments in concrete retains the color in concrete long after paints have faded due to the sun’s effects.

Inedible Pests and insects cannot destroy concrete because it is inedible. Some softer materials are inedible but still provide pathways for insects. Due to its hardness, pests and insects will not bore through concrete. Gaps in exterior insulation to expose the concrete can provide access for termite inspectors. Resistance to Freezing and Thawing The most destructive weathering factor is freezing and thawing while the concrete is wet, particularly in the presence of deicing chemicals. Deterioration is caused by the freezing of water and subsequent expansion in the paste, the aggregate particles, or both. With addition of an air entrainment admixture, concrete is highly resistant to freezing and thawing. During freezing, the water displaced by ice formation in the paste is accommodated so that it is not disruptive; the microscopic air bubbles in the paste provide chambers for the water to enter and thus relieve the hydraulic pressure generated. Concrete with a low water-cementitious ratio (0.40 or lower) is more durable than concrete with a high water-cementitious ratio (0.50 or higher). Air-entrained concrete with a low water-cementitious ratio will withstand a great number of cycles of freezing and thawing without distress.



Chemical Resistance Concrete is resistant to most natural environments and many chemicals. Concrete is virtually the only material used for the construction of wastewater transportation and treatment facilities because of its ability to resist corrosion caused by the highly aggressive contaminants in the wastewater stream as well as the chemicals added to treat these waste products. However concrete is sometimes exposed to substances that can attack and cause deterioration. Concrete in chemical manufacturing and storage facilities is specially prone to chemical attack. The effect of sulfates and chlorides is of them. Acids attack concrete by dissolving the cement paste and calcareous aggregates. Surface treatments can be used to keep aggressive substances from coming in contact with concrete. Resistance to Sulfate Attack Excessive amounts of sulfates in soil or water can attack a concrete that is not properly designed. Sulfates (for example calcium sulfate, sodium sulfate, and magnesium sulfate) can attack concrete by reacting with hydrated compounds in the hardened cement paste. These reactions can induce sufficient pressure to cause disintegration of the concrete. Like natural rock such as limestone, porous concrete (generally with a high water-cementitious ratio) is susceptible to weathering caused by salt crystallization. Examples of salts known to cause weathering of concrete include sodium carbonate and sodium sulfate. Sulfate attack and salt crystallization are most severe at locations where the concrete is exposed to wetting and drying cycles, than continuously wet cycles. For the best defense against external sulfate attack, design concrete with a low water to cementitious material ratio (around 0.40) and use cements specially formulated for sulfate environments. Chloride Resistance and Steel Corrosion

Chloride present in plain concrete that does not contain steel is generally not a durability concern. Concrete protects embedded steel from corrosion through its highly alkaline nature. The high pH environment in concrete (usually greater than 12.5) causes a passive and non-corroding protective oxide film to form on steel. However, the presence of chloride ions from deicer or seawater can destroy the film. Once the chloride corrosion reached its threshold, an electric

cell is formed along the steel or between steel bars and the electrochemical process begins.



The resistance of concrete to chloride is good; however, for severe environments such as bridge decks, it can be increase by using a low water-cementitious ratio (about 0.40), at least seven days of moist curing, and supplementary cementitious materials such as silica fume, to reduce permeability. Increasing the concrete cover over the steel also helps slow down the migration of chlorides. Other methods of reducing steel corrosion include the use of corrosion inhibiting admixtures, epoxy-coated reinforcing steel, surface treatments, concrete overlays, and cathodic protection. Resistance to Alkali-Silica Reaction (ASR)

ASR is an expansive reaction between reactive forms of silica in aggregates and potassium and sodium alkalis, mostly from cement, but also from aggregates, pozzolans, admixtures, and mixing water. The reactivity is potentially harmful only when it produces significant expansion. Indications of the presence of alkali-aggregate reactivity may be a network of cracks, closed or spalling joints, or movement of portions of a structure. ASR can be controlled through

proper aggregate selection and the use of supplementary cementitious materials (such as fly ash or slag cement) or blended cements proven by testing to control the reaction. Abrasion Resistance Concrete is resistant to the abrasive affects of ordinary weather. Examples of severe abrasion and erosion are particles in rapidly moving water, floating ice, and vehicle’s tires. Abrasion resistance is directly related to the strength of the concrete. For areas with severe abrasion, studies show that concrete with compressive strengths of 12,000 to 19,000 psi work well.



THE MILLION DOLLARS QUESTIONS Why does concrete crack? Concrete, like most materials, will shrink slightly when it dries out. Common shrinkage is about 1/16 of an inch in a 10-foot length of concrete. The reason contractors place joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint, where concrete cracks due to shrinkage are expected to occur. Control or construction joints are also placed in concrete walls and other structures. Why do concrete surfaces spall? Concrete spalling (or flaking) can be prevented. It occurs due to one or more of the following reasons. 1.) In cold climates subjected to freezing and thawing, concrete surfaces

have the potential to spall if the concrete is not air-entrained. 2.) Too much water in the concrete mix will produce a weaker, more

permeable and less durable concrete. The water-cementitious ratio should be as low as possible (0.45 or less).

3.) Concrete finishing operations should not begin until the water sheen

on the surface and the excess bleed water on the surface has evaporated. If this excess water is worked into the concrete because finishing operations have begun too soon, the concrete on the surface will have too high of a water content and this surface will be weaker and less durable.



METHODS TO PROLONG THE DURABILITY OF THE CONCRETE Surface Repair Repairing the damaged surfaces of concrete can restore the structural function, protect the surface itself or the underlying concrete and reinforcement from aggressive environments, or restore any lost performance requirements including drainage and abrasive resistance. All repairs require initial surface preparation, which might include abrasive or hydro blasting, chipping, milling, sanding or chemical treatments. Systems for repairing surfaces include overlay, resurfacing, formed repairs, hand-toweled mortars, cast-in-place repairs, shotcrete and, in some cases, full section replacement. Waterproofing Waterproofing techniques prevent water from entering or exiting structures through cracks, joints or failed water stops. Systems include replacement joints and sealants, waterproofing membranes and crack grouting

Waterproofing membranes Sealants Strengthening Strengthening is the process of adding or restoring capacity to a member or structure. Techniques include the addition of steel, FRP composite systems, concrete or other special materials to existing members providing for additional strength and capacity of the structure



Strengthening with FRP composites Structural support using steel bracing to reduce the span length

Section enlargement This method of strengthening involves placing additional "bonded" reinforced concrete to an existing structural member in the form of an overlay or a jacket. With section enlargement, columns, beams, slabs and walls can be enlarged to increase their load-carrying capacity or stiffness. A typical enlargement is approximately 2 to 3 inches for slabs and 3 to 5 inches for beams and columns. The figure depicts details of a section enlargement used to increase the capacity of a main. The girder was re-evaluated because of a change in the required loading and found to be deficient in flexure and shear. To correct the deficiency, additional flexural and shear steel were added. The entire beam was then formed and a 4-inch jacket of concrete was cast to enlarge the section. External post-tensioning The external post-tensioning technique has been effectively used to increase the flexural and shear capacity of both reinforced and prestressed concrete members since the 1950s. With this type of upgrading, active external forces are applied to the structural. Because of the minimal additional weight of the repair system, this technique is effective and economical, and has been employed with great success to correct excessive deflections and cracking in beams and slabs, parking structures and cantilevered members. The post-tensioning forces are delivered by means of standard prestressing tendons or high-strength steel rods, usually located outside the original section. The tendons are connected to the structure at anchor points, typically located at the ends of the member. End-anchors can be made of steel fixtures bolted to the structural member, or reinforced concrete blocks that are cast in-situ. The desired uplift



force is provided by deviation blocks, fastened at the high or low points of the structural element.

Example of structure using external prestressing method

Prior to external prestressing, all existing cracks are epoxy-injected and spalls are patched to ensure prestressing forces are distributed uniformly across the section of the member. The option of an external post-tensioning system was more economical, required less time to complete, and allowed for a strengthening system that provided active forces and therefore was more compatible with the existing construction. After all cracks were injected, the sides of the stems were formed and new concrete was cast to restore the integrity of the stems. The strengthening system was then installed, and after the concrete cured the external strands were stressed according to the engineer-specified forces. This structural-strengthening option was fast and effective, saving the owner a considerable amount in construction and operation costs. Protection Protection techniques are designed to extend the life of the structure by protecting it from the attack of an aggressive environment. Systems are available in the form of coatings, sealers, membranes, liners, cathodic-protection and overlays.



COST OF CONCRETE REHABILITATION An estimated $100–200 million a year is spent in Australia on concrete repair work. Much of this money is spent on either re-doing poor quality work or re-visiting sites where repair work has been poorly conceived. In America, over 100 it is estimated that over 500,000,000 cubic yards (cy) of concrete (almost 2 cy/per person) are installed each year to support the America infrastructure (Table 1). The volume of in-place concrete is estimated at 9 billion cy (32 cy/person). Most of this concrete is older than 20 years. Concrete, even if exposed to freeze-thaw cycles, carbonation, chlorides, and other aggressive chemicals, can have a useful life of 30 or more years. More recent developments in the use of low permeability concrete mixes, proper use of air-entrainment, epoxy-coated reinforcement, protective coatings, and corrosion-reducing admixtures have greatly increased the service life of concrete structures beyond 30 years. But some concrete structures being built today may require repairs after as few as five years of service because of improperly use of repair materials are some of the reasons for short service life of structures. More efficient designs may have a lower tolerance for workmanship and design errors, and fast-track construction methods may make it more difficult to incorporate the quality needed for a long service life. As a result, some new structures, in spite of durability enhancements, undergo early-age deterioration and require repair (Figure 1).

Table 1 : Historical of America Ready Mixed Concrete Production

It is estimated that the total cost for repair, rehabilitation, strengthening and protection (including waterproofing) of the concrete structures in the U.S. is $18-21 billion/year (See Table 1). Assuming there are 9 billion cy of concrete in these structures, concrete, the annual cost is between $2.00 and $2.33 per cy of in-place concrete!



FIGURE 1 : ESTIMATED COST FOR REHABILITATION OF CONCRETE STRUCTURES IN AMERICA

1%2%3%5%

5%

5%

11%

22%

43%

2%

1%

PIPE & WHARFS LOCKS & DAMS RESIDENTIALINDUSTRIAL FACILITIES WATER TREATMENT BUILDINGSPIPELINE OTHER STRUCTURES PARKING STRUCTUREROADWAYS BRIDGES

TOTAL : 18.5 BILLION US DOLLAR



PREVENTION IS BETTER THAN CURE

Figure 2: Typical repair cost history diagram

Figure 3: Alternate repair cost diagram



The figures above show the graph of the Cost of Concrete Repair Versus Time. This figure illustrates the three observed phases describing the natural evolution of the concrete deterioration process and the influence of maintenance on this process: Preventive Maintenance Phase: In this phase, the owner may spend a fixed annual maintenance cost to install systems such as protective coatings to slow down the deterioration process. Money spent in this phase will delay the ingress of aggressive materials, thus Figure 2: Typical repair cost history diagram delaying the start of active deterioration Repair Phase: In this phase, the concrete deterioration has begun, and the repair cost curve increases exponentially over time. The reason for the rapid increase in cost is that once aggressive materials that cause deterioration have sufficiently permeated into the concrete (a process that may take 20 to 30 years), the deterioration rate is rapid and irreversible. Replacement Phase: In this phase, a "wholesale" deterioration occurs throughout the structure at such a rapid rate that repair costs may exceed the costs of replacing the entire structure. However, total replacement of the structure may not be an option because of interruption to the function of the structure. Incurring additional costs at early years to ensure well-protected concrete and addressing deterioration problems as soon as they are observed would delay excessive deterioration and may increase significantly the service life of the structure, as shown in Figure 3.



MIDDLE RING ROAD 2 – KEPONG BRIDGE IS FALLING DOWN Major cracking in the Middle Ring Road 2 highway had cause outburst among the citizens in Kepong. When this matter was highlighted in newspaper, the case was brought to Parliament for debate. Soon, the Ministry of Work directed a consultant, Halcrow from United Kingdom to look into the matter. In their report it had pinpointed that the cracking occurred on the bridge was caused by error during the design process which had caused D.E.F. (delayed ettringite formation) in the concrete. DEF is caused by different temperature occurred inside the concrete. Halcrow proposed a RM 18 million cost for the repair works on the cracked pier crosshead. In the same time, the government engaged another expert from Germany, Leonhardt Andre & Friend.

Transverse cracks appear on the crosshead clearly visible

They too submitted a design for repair work that will be done. The total cost for their method was RM 49.9 million. After reviewing both methods by the Board of Engineers, Malaysia, the methods from Leonhardt Andre was adopted. The cost of rehabilitation work will first be borne by the government and the initial contractors, Konsortium Sukmim-Bumi Hiway and Kontraktor Bumiputera Wilayah Persekutuan who responsible for the design error will cover the cost at a later stage. Even though design error is the main culprit for this problem, one thing to bear in mind is the fact that how concrete durability is being affected by man-made mistake, which in this case was in the form of delayed ettringite formation.



External pre-stressing method is being used to repair the crack crosshead

What is Delayed Ettringite Formation?

Generally DEF is seen as a form of internal sulfate attack. A number of factors such as concrete composition, curing condition and exposure conditions influence the potential for DEF. DEF is believed to be a result of improper heat curing of the concrete where the normal ettringite formation is concealed. The sulfate concentration in the pore liquid is high for an unusually long period of time in the hardened concrete. Eventually, the sulfate reacts with calcium and aluminium containing phases of the cement paste. Thus, the cement paste expands. Due to this expansion, empty cracks (gaps) are formed around aggregates. The cracks may remain empty or later be partly or even completely filled with ettringite.

Microscopic appearance

Gaps around aggregate under

green fluorescent light.



ALLOCATION FOR REPAIR WORKS UNDER NINTH MALAYSIA PLAN



ALLOCATION FOR THE MINISTRY OF WORK UNDER NINETH MALAYSIA PLAN



CONCLUSION Much is left to be seen that such allocation will be enough to warrant another major rehabilitation work of current concrete structure in Malaysia. Even though there is allocation properly prepared, it will normally exceed the budget considering the fact the hazard of problematic structure posed to the citizens. Based on this priority, the checkbook needs to be slashed out to tackle the problem. From what had been discussed, it is noticeable that concrete durability issue can be prevented by taking the appropriate measures. However, there are times where even with precaution measure taken, no one can boldly predict the durability of the structure. Dato' Seri S. Samy Vellu was quoted explaining the question on the lesson learnt from the Middle Ring Road 2 debacle. He said that in the world there are at least five or six out of thousand bridges built that faced problem. Sometimes, even though we provide a good design, we still encounter other factors that can ruin it. Everybody wants a design without any problem. But that doesn’t happen always.

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