Materials test 2

What can be done to control bleeding? Three options

-Air-entrainment. -decreasing water to cement ratio by increasing fineness. -adding more cement.

List four different methods of concrete curing.

1. ponding 2. slow evaporation loss. 3. waterproof plastic film. 4. Liquid curing compounds.

What is the typical range of the compressive strength of concrete?

3000-6000 p.s.i.

What is the standard size of the cylinders to be tested for compressive strength?

6" diameter by 12" tall

What % of the final concrete strength is reached at 28 days under normal curing conditions?

80%

The water-cement ratio is important because it influences

ALL of the desirable qualities of concrete.

When does DRYING shrinkage occur?

About 40-80% of the 20-year shrinkage occurs in the first 3 months.

What is the slump test.

An indirect measurement of the characteristics of workability.

Irreversible creep

Another part of the deformation remains permanent and cannot return to the original shape.

How does aggregate size affect tensile strength?

As the aggregate gradation becomes coarser, the tensile strength increases.

What is a construction joint and why is it used?

Construction joints are used to separate a slab from other parts of a structure such as a beam or column and prevent bonding of the slab to the beam or column. In addition, they allow for horizontal and vertical movement of the concrete.

slow evaporation loss

Cover the concrete with wet mats or blankets to slow evaporation loss and supply moisture to the concrete.

The tensile strength of concrete _____________ with the compressive strength of concrete.

Decreases

What is creep?

Deformation over time. When a load is placed on concrete for an extended period of time, deformation occurs and increases with time.

True or False: the lower the ambient temperature, the more rapid the settling of concrete.

FALSE. The HIGHER the ambient temperature, the more quickly the concrete will set

True or False: Fresh concrete should be placed in big piles and then moved horizontally to its final position.

False

True or False: When strength of concrete is less than the strength of the aggregate, the strength of the concrete is mostly a function of the strength of the aggregates.

False

State one reason for false set.

False set could be attributed to unstable gypsum in the cement.

What are the four ingredients of concrete?

Fine aggregate, coarse aggregate, cement, water

The chemical reaction between cement and water that produces a hard, strong and durable concrete is called_______.

Hydration

What is the difference in hydraulic and non-hydraulic cement?

Hydraulic requires water and non-hydraulic does not require water to become a solid.

Explain why hot weather during mixing and curing had a negative effect on concrete. What can be done during the mixing or curing of the concrete during hot weather to prevent problems from occurring?

If it is too hot the water in the mix could dry out and cause cracking or shrinkage. Add additional water as needed.

What will happen if concrete is not kept moist during curing? Why?

If the concrete dries out, not enough water for hydration, hydration stops and the concrete won't reach the desired properties and will shrink.

What are contraction/control joints and why are they used?

In order to reduce shrinkage cracks, surface grooves are formed every 10' or less to accommodate movement from temperature changes, drying shrinkage, and creep. Their purpose is to direct the cracks to the location of the groove.

_________ the amount of cement increases the compressive strength of the concrete.

Increasing

How does WATER and FINE aggregate affect the workability of concrete?

Increasing the amount of BOTH improve workability.

How is consistency measured?

It can be measured using a slump test.

Why is air-entrained concrete used for freeze-thaw resistance?

It has many tiny air bubbles that allow for VOID SPACE in the concrete and give room for particles to expand and contract under variable temperatures.

Discuss the significance of the compressive strength test on concrete.

It is used in the laboratory, on small samples, to estimate the STRENGTH PROPERTIES of concrete used in a structure.

What 3 ingredients are used for the production of Portland cement?

Lime, alumina silica,

What is consistency?

Measure of concrete's WETNESS or fluidity.

waterproof plastic film

Place over concrete as soon as the concrete has hardened and after it has been thoroughly wet. The sheeting should overlap the edges by at least 12".

When does PLASTIC shrinkage occur?

Plastic shrinkage typically occurs within the first 12 hours after placement.

If false set is encountered at the job site, what would you do?

Re-mixed in the field without the addition of water to be restored.

Know how freeze-thaw cycles, affect the durability of concrete.

SCALING-the top thin layer may flake off from freeze-thaw effects.

What is segregation of concrete?

Separation between coarse and fine particles of fresh concrete.

For proper hydration, what water-cement ratio is needed for normal strength concrete?

Should be about 0.35

Why is the extra water necessary?

So that the concrete remains workable

List the major properties of hardened concrete.

Strength Shrinkage Creep impermeability Modulus of elasticity Durability

Discuss the effect of water-cement ratio on the compressive strength of hardened concrete.

The MAXIMUM strength occurs when the mix contains just enough water for hydration.

What is workability?

The ease with which fresh concrete mix can be handled from the mixer to the final structure.

How does age of concrete affect tensile strength?

The older the concrete is, the greater its tensile strength is at any given compressive strength.

Know how alkali-aggregate reaction, affect the durability of concrete.

The products of the reaction form on the surface of the particles and can cause expansion, cracking, POPOUTS, and deterioration of concrete. Reactive silica reacts in the aggregates which reacts with alkalis in the cement.

What is DRYING shrinkage, what makes it occur?

The reduction in volume from drying of hardened concrete. It usually occurs because of the loss of water from the cement gel.

What is bleeding?

The separation of water from the mix.

What is PLASTIC shrinkage? What makes plastic shrinkage occur?

The settlement of solids and evaporation of water cause the wet concrete to shrink.-When concrete is still wet and moldable, the aggregates settle to the bottom and the air and water rise to the top of the concrete. The water that rises to the top evaporates which draws even more water to the top.

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?

The strength of an aggregate may not influence the strength of concrete if the desired strength of concrete is significantly lower than the aggregate strength. Rather, the limiting factor will be the strength of the paste or cement and its bonding capacity.

What is the role of gypsum in the production of Portland cement?

To slow the settling time of cement

Explain why too much water in the mix decreases strength.

Too much WATER causes increased VOID CONTENT and the concrete STRENGTH DECREASES.

True or False: The finer the cement (more fine particles), the faster the rate of hydration.

True

True or False: To preserve quality, concrete should be deposited within 90 minutes or before the drum has revolved 300 turns after adding water to the dry ingredients.

True

What are the five primary types of Portland cement?

Type 1- general purpose Type 2-MODERATE SULFATE Type 3-high early strength Type 4-low heat cement. Type 5-sulfate resistant

How are the results of the flexure test related to the compressive strength test results?

Typically the tensile strength is about 10-23% of the compressive strength.

Describe how the slump test is performed.

Using a slump cone dampened on the inside, it is filled with fresh cement in three separate layers. The concrete is rodded 25 times after each layer and the sides of the cone are tapped four times. Once filled the rod is used to remove the protruding cement from the top of the cone. the cone is then lifted vertically and the cement's form is observed and measured.

Do wet mixes or dry mixes have more PLASTIC shrinkage?

WET mixes settle over a longer period of time than dry mixes and, thus, shrink more.

Explain why temperatures below 23 degrees is detrimental to the curing of concrete,

Water in the mix could freeze and would prevent proper setting.

What is reversible creep?

When concrete endures a force for a long period of time and it is unloaded, part of the deformation immediately returns to its original shape.

Be able to describe the consistency of concrete that produces a near-zero slump.

a very dry mix, low water content- sand castle shape

Liquid curing compounds

can be applied as soon as the concrete is finished. It stays on for about a month and seals in the moisture

A good quality concrete has a sufficient amount of

cement, well-graded aggregates, and minimum mixing water.

Be able to describe the consistency of concrete that produces a Shear slump.

concrete lacking plasticity and cohesion- slides to one side

Know how reinforcement corrosion affect the durability of concrete.

corrosion of the reinforcing steel REDUCES ITS CROSS SECTIONAL AREA reducing the strength. In addition, the corrosion products take up more room than the original reinforcing steel which generates enough pressure to cause cracking and spalling of the concrete.

What are the primary functions of Portland cement Type I?

general purpose cement that is used when special properties of other cements are not required.

What information is gained from a slump test measurement?

helps to estimate the amount of water and fine aggregates necessary to maintain consistency from batch to batch.

What are the primary functions of Portland cement Type III?

high early strength cement.

The maximum compressive strength occurs when the water to cement ratio contains __________ water than is needed for hydration.

just enough

ponding

kept under a layer of water until the desired properties are achieved.

What are the primary functions of Portland cement Type IV?

low heat cement. No longer manufactured in the US.

What do we mean by curing concrete?

maintains proper MOISTURE and TEMPERATURE to ensure continuous HYDRATION

What is the purpose of performing the flexure test on concrete?

measures the tensile strength

Be able to describe the consistency of concrete that produces a Normal slump.

mixed with the correct proportions of ingredients- maintains general shape but drops a few inches.

false set

premature stiffening of the paste without the development of heat.

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.

What are the primary functions of Portland cement Type V?

sulfate resistant cement - used when higher sulfate resistance is needed.

Know how sulfate attack, affect the durability of concrete.

sulfates in soil and SEA WATER react with free calcium hydroxide and aluminates in the cement gel to produce compounds that have a greater volume than the initial volume which causes expansion and cracking in the concrete.

What information is gained from the split cylinder test?

tensile strength

What are the primary functions of Portland cement Type II?

used when MODERATE SULFATE resistance or moderate heat of hydration is desired.

Be able to describe the consistency of concrete that produces a Collapse slump.

very wet concrete which results in a separation of coarse and fine aggregate.- puddle like

What properties of concrete cause it to segregate more?

very wet or LACKING finer particles

final set

when the cement is beginning to harden and can sustain some loads

initial set

when the paste is beginning to stiffen

What can be done to minimize DRYING shrinkage?

• Increase the mass of aggregates • Decrease the quantity of cement • Increase the maximum size of coarse aggregate • Reduce sand content • Use harder aggregates • Use aggregates with lower absorption capacity

What can be done to minimize PLASTIC shrinkage cracks?

• Revibration prior to floating • Sparying the coarse aggregate pile with cold water • Using cold water for mixing • Any procedure that reduces evaporation loss of water • Water reducing or air-entrained admixtures

A concrete structure will usually remain durable if:

• cement paste is dense and of low permeability • Has entrained air to resist freeze-thaw cycles • made with graded aggregates that are strong and inert •Mix contains minimum impurities