Concrete Exam 1

4. Explain how water-cement ratio influences the strength of the cement paste matrix and the interfacial transition zone in concrete.

An increase in the w/c ratio increases porosity in the cement paste matrix resulting in decreased strength. In the ITZ, for normal and low strength concretes, the porosity is increased and strength reduced. However, in high strength concretes with w/c ratios less than 0.3 the strength is disproportionately higher due to the formation of smaller crystals in the ITZ with a larger surface area.

How many types of water are associated with a saturated cement paste? Discuss the significance of each. Why is it desirable to distinguish between the free water in large capillaries and the water held in small capillaries?

1. Capillary water- Made up of large voids called free water because its removal does not cause any volume change, while water removal in the small voids causes shrinkage in the system. 2. Adsorbed water- the loss of absorbed water is responsible for the shrinkage of the hydrated cement paste. 3. Interlayer water- A monomolecular water layer between the layer of C-S-H is strongly held by hydrogen bonding. This water is only lost on strong drying. 4. Chemically combined water- Integral part of the microstructure of various hydration products. It is evolved when the hydrates decompose on heating. It is important to distinguish between free water in small and large capillaries because the removal of water in the large voids does not cause any change in volume and the removal of water in small capillaries causes shrinkage.

How many types of voids are present in a hydrated cement paste? What are their typical dimensions? Discuss the significance of the C-S-H interlayer space with respect to properties of the hydrated cement paste.

1. Interlayer Space in the C-S-H- the space can be anywhere from 5 to 25 A which is too small to have an adverse effect of strength and permeability. Leads to creepage 2. Capillary Voids- 10-50 µm in high water cement ratio pastes or 3-5 µm in early hydration. Macropores are voids > 50nm and micropores are voids <50nm. Shrinkage and creepage 3. Air voids- spherical can range from 50-200 µm. Adversely affect strength

calcium sulfoaluminates

15-20% of the solid volume. Favors the formation of ettringite, early in the hydration process. Ettringite then transforms into monosulfate hydrate.

Discuss why the strength of the I ITZ is generally lower than the strength of the bulk hydrated cement paste. Explain why concrete fails in brittle manner in tension but not in compression.

A Water film forms around the large aggregate particles, increasing w/c ratio Ettringite and Calcium hydroxide form large crystals due to the increased w/c ratio As hydration continues the crystals grow to fill the voids and increase strength. In tension, the micro-cracking within the ITZ expands until cracks reach the cement matrix and the concrete fails. In compression, strain hardening prevents micro cracks from expanding until tensile stresses occur.

14. In general, how are the compressive and tensile strengths of concrete related? Is this relationship independent of concrete strength? If not, why? Discuss how admixtures and aggregate mineralogy can affect the relationship.

As compressive strength increases, the tensile strength increases at a decreasing rate around 10%. Water-cement ratio, type of aggregate, and admixtures all effect the tensile-compressive ratio. Factors that decrease the porosity of the matrix and interfacial zone lead to an improvement in compressive and tensile strength. Can add fibers to increase tensile strength. When pozzolanic admixtures or reactive aggregates are used, the formation of large oriented calcium hydroxide crystals is reduced and the tensile strength of the ITZ is improved.

5. In mix designing, why is it desirable to use a minimum amount of water? For a given slump, how can you reduce the amount of water?

Strength is dependent on the water-cement ratio, in addition, a lower water content requires less cement. The water content can be reduced by increasing the maximum aggregate size used. The use of superplasticizers allows for a reduction of water without effecting slump.

10. Why is it important to control the aggregate gradation once the concrete mix design has been selected? How is this gradation control expressed in a specification?

The aggregate gradation will influence the workability of the mixture, which will directly impact the workability. The slump requirements set in the specification will control any changes in gradation.

7. According to the ACI Building Code 318, selection of mix proportions should be bases on the average strength, not the specified strength. Is this justified? Given a specified strength value, what procedures are used to determine the average strength?

The average strength includes an overdesign factor which takes into account variability in the materials and placement of concrete. The average strength is determined from the specified strength plus a factor times the standard deviation in strength.

Describe some of the unique features of the concrete microstructure that make it difficult to predict the behavior of the material from its microstructure?

The two phases of the microstructure are neither homogeneously distributed with respect to each other, nor are they themselves homogeneous. Concrete has many features that make its microstructure, but they are very unpredictable and nonuniform through the structure. Composed of three phases, the aggregates, cement paste, and interfacial transition zone.

Why is it not necessary to take into account durability considerations in concrete mix proportioning when the concrete is subject to normal exposure conditions? Give examples of circumstances when durability must be considered in mix designing

Under normal exposure, the concrete durability will be sufficient if the concrete develops adequate strength. In the presence of aggregates that are known to be alkali reactive, sufficient SCMs should be used to improve durability. There are cases when concrete is not subject to the environment so durability doesn't matter. b. Ex: i. When lower w/c ratio is required ii. Where freeze-thaw cycles occur iii. Concrete exposed to chemical attacks from salts, acids, or sulfates

2. You find yourself the project manager for a concrete structure involving several million cubic yards of concrete. Briefly, what tips would you like to pass on to the engineer in charge of mix proportioning on the subject of materials cost reduction? In your answer emphasize the key ingredient in concrete from the standpoint of cost.

a. Cement costs much more than aggregate, so all steps should be taken to reduce the amount of cement being used (to still get the desired or required properties). Get material on site or as close to site as possible. Have a batching plant on site. i. Pozzolanic or cementitious materials can be used to replace percentages of portland cement

6. Describe the significance of workability of concrete and the factors affecting the property.

a. Directly effects the pumpability and constructability because it determines the ease at which a concrete mixture can be handled without harmful segregation b. Consistency - measure of the wetness of the concrete mixture (evaluated by slump) Cohesiveness - measure of the compactibility and finishability (evaluated by trowelability and visual judgement of resistance to segregation)

4. Theoretically derived ideal gradings of aggregates for maximum density should be the most economical, yet the practice is not followed. Can you explain why?

a. It doesn't produce the most workable concrete. It is not economical

1. Explain why the process of proportioning concrete mixtures is still in the realm of art. Have your any ideas on how to make the currently used practice in the United States more scientific?

a. It is an art because there are many different factors that can adjusted for in a mix. Depending on the circumstances or desired final product the mixes will be tweaked and changed. Also, project specific factors like cost, use, and location can creates other challenges that a mix needs to take care of.

9. Briefly state the influence of maximum aggregate size (i.e., 19 mm vs. 38 mm) on the mixing water content and the cement content of a concrete mixture with a given water/cement ratio of 0.5.

a. The maximum aggregate size has a direct relationship with the amount of concrete needed, but an inverse relationship with the water requirement

13. Determine the SSD mix proportions of concrete required for an outdoor pavement subject to frequent freeze-thaw cycles. The following data are given: Specified 28-day compressive strength:20 MPa Slump: 75 mm Coarse aggregate: 25mm max. size; dry-rodded weight volume per unit volume of concrete=0.71 Fine aggregate: 2.8 fineness modulus Specific gravities of cement, coarse aggregate, and fineness aggregate: 3.15, 2.72, and 2.70, respectively.

a. Water = 340 (table) b. w/c = .68 (table) c. Cement = 500 (340/.68) d. Gravel = 1917 (.71x27x100) e. Sand = 1253 (4010 (table) - 340-500-1917)

8. With respect to the ACI 211.1, Standard Practice for Selecting Proportions for Normal Heavy-Height and Mass Concrete, explain the principles underlying the following:

i. Estimation of water content. 1. Dependent on the particle size and whether or not the mixture has entrained air ii. Estimation of coarse aggregate content. 1. Volume of dry-rodded coarse aggregate per unit volume of concrete 2. Max aggregate size and fineness modulus iii. Estimation of the fine aggregate content by the weight content 1. If unit weight of fresh concrete is known, then the required weight of fine aggregate is the difference between the unit weight of concrete a the total weights of water, cement, and coarse aggregate iv. Estimation of the fine aggregate content by the absolute volume method 1. The total volume displaced by known ingredients is subtracted from the unit volume of concrete to obtain the required volume of fine aggregate

C-S-H

makes up 50-60% of the volumes of solids and it provides the majority of the long-term strength and durability of Portland Cement. It can range from poorly crystalline fibers to reticular network. Has a layer structure with high surface area. Its strength comes from Van Der Waals forces.

Calcium Hydroxide

· 20-25% of the volume of solids. Forms large crystals with a distinctive hexagonal-prism morphology. Can be nondescript to stacks of large plates. Has a lower surface area than C-S-H.

7. What is the principal advantage of using fiber-reinforced concrete? Explain how the concrete acquires this property.

· Advantages: improved cracking resistance, toughness, strength, fatigue life, impact resistance · The addition of fiber increases or decreases the elastic modulus of the composite material. Fiber-reinforced concrete exhibits significant postcracking strength and toughness, since the fibers bridge and transfer load across the cracks.

8. At a given water-cement ratio, either a change in the cement content or aggregate grading can be made to increase the consistency of a concrete mixture. Which one of the two options would you recommend? Why is it not desirable to produce concrete mixtures of a higher consistency than necessary?

· As coarse/fine aggregate proportion and cement content of the mixture is increased the consistency and slump will also increase. However, the strength will decrease. High consistency will lead to excessive segregation or bleeding.

11. From the standpoint of concrete strength, which of the two options is undesirable and why?

· Concrete cast at 5°C and cured at 21°C · Concrete cast at 21°C and cured at 5°C · Option B is undesirable. Generally, lower casting temperatures with higher curing temperature result in a more uniform microstructure and higher overall strength·· . Long hydration time.

3. Abrams established a rule that related the water-cement ratio to strength of concrete. List two additional factors that have a significant influence on the concrete strength.

· Curing conditions, Water/Cement, Characteristics and proportions of materials, Testing Paramaters.

10. What do you understand by the term curing of concrete? What is the significance of curing?

· Curing of concrete involves a combination of conditions that promote the cement hydration, namely times, temperature, and humidity conditions immediately after the placement of a concrete mixture into formwork. · The hydration reaction slows down considerably when the products of hydration coat the anhydrous cement grains. Proper curing conditions allow the hydration to continue

8. Write a short note on the selection of materials and proportioning of mixtures suitable for use in fiber-reinforced concrete, with special attention to how the requirements of toughness and workability are harmonized.

· Fibers in fiber reinforced concrete can be produced from steel, plastic, and glass; they come in various shapes and sizes. When these are added to concrete the workability is compromised; there is a roughly proportional loss in workability with volume of fibers in the concrete. Toughness and workability are inversely related, when you increase toughness you decrease workability. You must not use aggregates greater than 19mm because larger aggregates greatly hinder workability. It is beneficial to use air entrainment, plasticizers, or high cement paste content to increase workability of fiber reinforced concrete. In normal weight concrete, the volume of fiber contents can range from .38% to 2% by volume.

1. (a)Slump loss and floating of coarse aggregates can be major problems with fresh lightweight concrete mixtures. How are they controlled? (b) The compressive strength of conventional lightweight concrete is limited to about 7000 psi (48 MPa). How can this be increased?

· In order to control the floating of coarse aggregate it is recommended to have a maximum slump of 125mm and to help combat slump loss it is recommended to batch the aggregate in a damp condition. Entraining air and keeping the air content between 5 to 7% also helps. The strength can be increased by using a high cement content and good quality lightweight aggregate of small size can produce light weight concrete up to 50 MPa. This strength can be increased even further if lightweight aggregates with controlled microporosity are used. Limiting the maximum size of aggregate is the key.

7. In regard to concrete strength, discuss the two opposing effects that are caused by an increase in the maximum size of aggregate in a concrete mixture.

· Increasing the maximum size of aggregate in a concrete mixture, decreases the needed amount of mixing water, but it also forms a weaker interfacial transition zone with more microcracks. Strength decreases but it depends on the W/C.

5. Why does air entrainment reduce the strength of moderate- and high-strength concrete mixtures but may increase the strength of low-strength concrete mixtures?

· It reduces the strength of moderate and high strength concrete because it increases the porosity of the concrete. It can increase the strength of low-strength concrete because air improves the strength of the interfacial transition zone through creating a well-organized network of smaller voids.

1. Why is strength the property most valued in concrete by designers and quality control engineers?

· Its most valued because it is easy to test and many properties such as elastic modulus, water tightness, impermeability, and resistance to weathering are dependent on strength.

2. (a) Compared to normal-weight concrete of the same water/cement ratio, a structural lightweight concrete would show higher drying shrinkage but less tendency to crack. Can you explain why? (b) In spite of the cellular structure of aggregate, lightweight concretes show less microcracking and excellent durability. Why?

· Lightweight aggregate exhibit higher moisture movement that results in high ultimate drying shrinkage and higher creep. The cracking is offset by the lower modulus of elasticity and higher creep, which accounts for the greater ability to be extended. · It is due to the similarities of the elastic moduli between lightweight concrete and the mortar fraction. Also, due to the pozzolanic reaction between the thermally activated clay minerals in lightweight aggregate particles and calcium hydroxide present in the hydrated cement paste, the ITZ is normally dense and strong

When concrete is exposed to fire, why the elastic modulus shows a relatively higher drop than the compressive strength

· Microcracking which develops in the ITZ reduces the transfer of stresses between the mortar matrix and aggregate resulting in a larger decrease in elasticity than compressive strength.

What is the significance of the microstructure of a material? How do you define microstructure?

· Microstructure is the type, amount, size, shape, and distribution of phases present in a solid. Microstructure of a material is significant because it is now understood that the overall properties of a material come from the microstructure. Properties of a material can be altered by changing the microstructure.

9. Can we use recycled water from industrial operations as mixing water in concrete? What about the use of seawater for this purpose?

· Recycled water from industrial operations can be used, but tests must be done to check that the setting time, 7- and 28-day compressive strength of concrete are with 90% the strength of concrete with clean water. Seawater can be used but it should be avoided in situations of reinforced and prestressed concrete because it increases the chance of corrosion.

6. Compare normal portland cement concrete to shrinkage-compensating concrete. Assume they have the same water/cement ratio (e.g., 0.6).

· Shrinkage compensating concrete will have more strength due to a denser cement matrix and stronger interfacial transition zone.

In general, discuss how strength and porosity are related to each other.

· Strength and porosity are inversely related. · As porosity decreases, strength increases exponentially

4. What is the significance of superplasticized flowing concrete to the construction industry? What changes may have to be made in the mix proportions for flowing concrete?

· Superplasticizers help with workability in concrete. This allows for concrete to be pumped and troweled easier without having to add extra water

3. (a). In Highrise buildings, what are the advantages of constructing shear walls and columns with high strength concrete? (b) Discuss why the use of water-reducing and pozzolanic admixtures is essential for producing ultra-high-strength concrete. (c) Superplasticized concrete is, in general, prone to slump loss. How can this problem be overcome in construction practice?

· The benefit of using high strength concrete in constructing shear walls and columns is that concrete is much better than steel at damping vibrations, so it lasts longer due to wind loads. High strength concrete allows for the construction of slender columns and increases the strength of columns 4.7 times and only costs 3.0 times more than conventional concrete. · Water-reducing and pozzolanic admixtures are critical for high strength concrete because microstructure inhomogeneities in the portland cement are strength limiting, the addition of pozzolanic admixtures rids the concrete of these inhomogeneities and it also reduces the volume and size of capillary pores. · Adding the superplasticizer right before it is placed will help stop this slump loss.

13. The temperature during the placement of concrete is known to have an effect on later age strength. What would be the effect on the 6-month strength when a concrete mixture is placed at (a) 10°C and (b) 35°C.

· The strength of the 10°C placing temperature will be greater than the strength of the 35°C placing temperature. Low temperature casting creates a relative more uniform microstructure of cement paste. Less thermal cracking, slow formation of hydration.

9. Draw a typical sketch showing how the microstructure of hydration products in the aggregate-cement paste interfacial transition zone (ITZ) is different from the bulk cement paste.

· The structure of hydrated cement paste is comprised of regions of dense hydrated cement paste and pores · The microstructure of hydrated cement paste in the vicinity of aggregate particles is different from the bulk cement paste · The interfacial transition zone (ITZ) between coarse aggregate particles and the bulk cement paste is more porous

When a saturated cement paste is dried, the loss of water is not directly proportional to the drying shrinkage. Explain why.

· There is no shrinkage until the free capillary water is removed.

6. For the ASTM Types I, III, and V of portland cements, at a given water-cement ratio would the ultimate strength values be different? Would the early-age strength values be different? Explain your answer

· Type I- Normal Cement In the middle. · ASTM Type III portland cement has higher fineness, hydrates quicker, so at early ages it will have lower porosity and a higher strength · Type V portland cement typically have portland-slag or portland-pozzolan cements have a lower early age strength because they take longer to hydrate, after 28 days their strength is similar to other cements.

12. Many factors have an influence on the compressive strength of concrete. Briefly explain which one of the two options listed below will result in higher strength at 28 days:

· Water-cement ratio of 0.5 vs. 0.4. i. 0.4 W/C ratio, lower porosity, higher strength. · Moist curing temperature of 25°C vs. 10°C. ii. 25C will produce higher strength due to increased rate of hydration · Using test cylinder of size 150 by 300mm vs. 75 by 150 mm. iii. 75 by 150, it is about 106-108% of the strength · Using a compression test loading rate of 3 MPa/s vs. 0.3 MPa/s. iv. 3MPA/s, concrete yields a higher strength when subject to rapid loading · Testing the specimens in a saturated condition vs. air-dry condition. v. Air-dry will show 15% higher compressive strength and 15% lower Modulus of elasticity. As concrete dries, Van Der Waals forces increase and lead to higher strength.

In a hydrating cement paste the relationship between porosity and impermeability is exponential. Explain why.

· When hydration begins, the voids in the cement particles begin to fill up with hydration products. When a cement is hydrated the connections between pores become nonexistent. The permeability of a fully hydrated cement paste becomes 10^6 times less than that of a young paste.

5. Explain how the concept works for eliminating drying shrinkage cracking by the use of shrinkage-compensating concrete.

· When restrained by reinforcement the concrete will expand an amount equal to or slightly greater than the anticipated drying shrinkage. Compressive stresses are introduced in the concrete during expansion, subsequent drying shrinkage will reduce these stresses. Ideally, residual compression will remain in the concrete, eliminating the risk on shrinkage cracking

Everything else remaining the same, the strength and impermeability of a mortar will decrease as coarse aggregate of increasing size is introduced. Explain why.

· With an increase in aggregate size, there is an increase of water-cement ratio in the ITZ, this makes the concrete more permeable and weaker. With a larger aggregate size there a thicker film of water and the ITZ formed under these conditions is more susceptible to cracking when subjected to the influence of tensile stresses induced by differential movements between the aggregates and hydrated cement paste.