CEG 210
For proper hydration, what water-cement ratio is needed for normal strength concrete?
About 0.35
Why is air-entrained concrete used for freeze-thaw resistance?
Air-entrained concrete has many tiny air bubbles. These air bubbles allow for void space in the concrete that give room for particles to expand and contract under variable temperatures. (pg. throughout)
What is the typical range of the compressive strength of concrete?
3000-6000 psi
What is the standard size of the cylinders to be tested for compressive strength?
6 X 12
What % of the final concrete strength is reached at 28 days under normal curing conditions?
80%
What is consistency and how is it measured?
Consistency is a measure of concrete's wetness or fluidity. It can be measured using a slump test.
What do we mean by curing concrete? What will happen if concrete is not kept moist during curing? Why?
Curing is any procedure that maintains proper moisture and temperature to ensure continuous hydration. If the concrete dries out, hydration stops and the concrete won't reach the desired properties and will shrink. This happens because the water is needed for the chemical reaction between water and cement (hydration). If the water isn't available for the reaction, the reaction will stop.
Discuss the significance of the compressive strength test on concrete.
The compressive strength test, performed on small samples, is used in the laboratory to estimate the strength properties of concrete used in a structure. Most strength properties are dependent on the compressive strength.
What is the purpose of performing the flexure test on concrete? How are the results of this test related to the compressive strength test results?
The flexure test measures the tensile strength of concrete. It produces compressive stresses on the top of the beam and tensile stresses on the bottom of the beam and since the tensile strength is less than the compressive strength, the specimen fails until tensile stress. Typically the tensile strength is about 10-23% of the compressive strength.
How does temperature affect the setting time of concrete?
The higher the ambient temperature, the more rapid the setting.
Discuss the effect of water-cement ratio on the compressive strength of hardened concrete. Explain why this effect happens. Discuss this in terms of void space.
The maximum strength occurs when the mix contains just enough water for hydration. When the amount of mixing water is increased, the void content increases and the concrete strength decreases.
How does the fineness of Portland cement affect hydration?
The rate of hydration increases with increasing fineness
Describe how the slump test is performed and what information is gained from a slump test.
The slump test is an indirect measurement of the characteristics of workability. If multiple trucks are delivering concrete for a job, it is important that all trucks have a slump that is relatively equal to maintain uniformity. The slump measurement may help to estimate the amount of water and fine aggregates necessary to maintain consistency from batch to batch.
What is segregation of concrete? What properties of concrete cause it to segregate more?
The tendency for separation between coarse and fine particles of fresh concrete. Mixes that are very wet or are lacking finer particles tend to segregate.
What is the role of gypsum in the production of Portland cement?
To retard the settling time of cement
Discuss four different methods of concrete curing.
Water - is the ideal curing medium, and water curing can be accomplished by ponding, spraying, or sprinkling Mats or blankets - slows evaporation loss and supplies moisture to the concrete Waterproofing paper or plastic sheeting - should be applied as soon as the concrete has hardened and after the concrete has been thoroughly wet. Liquid membrane-forming compounds - are used most often because of their versatility, ease of application, convenience, and economy.
. What is workability and how do water and fine aggregate affect the workability of concrete?
can be defined as the ease with which a fresh concrete mix can be handled from the mixer to the final structure
What is bleeding? What can be done to control bleeding?
is the process of separation of water from the mix, air-entrainment is an effective method of controlling bleeding
Define initial set and final set.
is when the paste is beginning to stiffen is when it is beginning to harden and able to sustain some loads
. What are the four ingredients of concrete?
Fine aggregate, coarse aggregate, cement, water
What is the difference in hydraulic and non-hydraulic cement?
Hydraulic cement is any cement that turns into a solid product in the presence of water (as well as air), resulting in a material that does not disintegrate in water. Non-hydraulic cement requires no water to transform it into a solid product
What is a false set of Portland cement?
It is a premature stiffening of the paste or rapid development of rigidity without the development of heat.
What 3 ingredients are used for the production of Portland cement?
Lima Silica Alumina
Be able to describe the consistency of concrete that produces a near-zero slump, normal slump, shear slump, and collapse slump.
Near-zero slump - a very dry mix, low water content Normal slump - concrete mixed with the correct proportions of ingredients Shear slump - concrete lacking plasticity and cohesion Collapse slump - very wet concrete which results in a separation of coarse and fine aggregate
Will the strength of concrete be affected by the strength of aggregate if the strength of aggregate is much higher than the desired strength of concrete?
No
If false set is encountered at the job site, what would you do?
Rework the fresh concrete without adding water.
Why is the extra water necessary?
So that the concrete remains workable
List the major properties of hardened concrete.
Strength Modulus Durability Creep Shrinkage Water tightness
What are the five primary types and functions of Portland cement?
Type I - general purpose cement. Used when special properties of other cements are not required. IA-is a Type I cement with an air-entrained admixture. Type II-general purpose cement used when moderate sulfate resistance or moderate heat of hydration is desired. Usually used where sulfate concentration in ground water is higher than normal, but not severe. Can also be used in mass concrete works such as piers and heavy footings since the heat of hydration is general y lower than that of Type I cement. Type IIA-type II cement with an air-entrained admixture. Type III-high early strength cement. Usually used when early strength is required (i.e. when formwork is to be removed as early as possible and the structure is to be brought into service quickly). Can also be used in cold weather applications to reduce the time needed to protect the cement from freezing. Type IIIA-Type III cement with an air-entrained admixture. Type IV-low heat cement. Used when the heat of hydration must be lowered. No longer manufactured in the US. Type V-sulfate resistant cement - used when higher sulfate resistance is needed. Usually used for concrete construction below ground, in lake water or seawater, and around sewage disposal sites.
State one reason for false set.
Unstable gypsum in the cement
