Monday, October 17, 2016

Aggregate Proportioning And Specifications


Construction companies move from job site to job site using the materials available in that specific area to design and produce their own concrete. Over the years many different aggregate proportioning techniques have been used to create a concrete mixture. Aggregate can be proportioned by volume, minimum voids, and gradation.  Furthermore, a well designed and economical mixture will effectively use the aggregate to reduce the amount of paste in a mixture, but still achieve the required workability, which is called optimized graded concrete.  Many conflicting reports have surfaced about the success of different aggregate proportioning methods.  Other than field experience, a very limited amount of useful data has been presented on the subject of aggregate proportioning for concrete mixtures due to the inability to measure the workability of concrete for the specific application.
This difficult proportioning task can be very complex because the gradation, shape, and characteristics of the aggregate changes in every the local geology region.  Several theories have been presented over the year and can be categorized into five aggregate proportioning techniques.

Proportioning Methods for Aggregate
  1. Volume or Weight- Proportion a certain volume percent or weight values for aggregate such as 3:2:1 and 60% coarse aggregate/40% fine aggregate.
  2. Combined Gradation- Proportioning using the whole particle distribution of aggregate. These include: Individual percent retained, cumulative percent passing, and coarseness factor chart
  3. Maximum Packing Density- Proportioning aggregate based off the maximum voids content of the aggregate blend. These include dry-rodded unit weight and various analytical models.
  4. Range of Voids Content-Proportion aggregate based off a certain range of voids content.
  5. Surface Area- Minimum amount of specific surface area (SSA) will give the most workable concrete.

Different Proportioning Techniques
The more popular methods for proportioning aggregate in slip formed pavements was used.  These methods include the Shilstone Coarseness Chart, Individual Percent Retained Chart, Power 45 Chart, and 60% coarse aggregate and 40% fine aggregate.  Below is a brief overview of each method.
60/40 method
In general the 60/40 method is the most typical design method for concrete in general.  Similar to the 1:2:3 volume method, or variation of this method, the 60/40 method is based on the percentage of rock and sand volumes, which are roughly 60% coarse aggregate and 40% fine aggregate by total volume.  These volumes can be slightly adjusted for better workability.  However, the theory overlooks the gradation effects on the workability of a mixture and doesn’t account for the intermediate contained in both the fine and coarse aggregates. 

For our research we chose the 60/40 method because it's the most common aggregate proportioning method and also like previously stated, it does not take into account the effects of gradation.   This can really shed light into the determining if the gradation really effects workability.
Picture
Shilstone Workability-Coarseness Chart
Starting in the late 1980s, James Shilstone revealed a mixture design process based on the proportioning of the aggregate using the combined gradation to proportion a group of sieve sizes that can be categorized as coarse, intermediate, and fine aggregate.  He designed a chart and two proportioning equations from 20 mixture designs in Saudi Arabia and confirmed the chart and equations using aggregates from the Dallas, TX area. From Shilstone’s experiences, he determined the workability could be sufficient in certain areas of the chart and divided the chart into zones as shown below.  Also, two different equations were developed for proportioning the aggregate by groups.  The coarseness factor proportions the percentage amount of coarse to intermediate and workability factor proportions the percentage amount of sand plus cement to coarse and intermediate.
















Coarseness Factor (CF) =(Q/R)*100                                   
Workability Factor (WF) = W + (2.5(C-564)/94)                 

                  Q= cumulative % retained on the 3/8 sieve
                  R= cumulative % retained on the no. 8 sieve
                  W= % passing the no. 8 sieve
                  C= cementitious material content in lb/yd³

To use the Shilstone Coarseness Chart, one must select a point in the chart and back calculate to find the proportions of aggregate. However, the zone where the point selected reflects the different proportioning properties.  While Zone I is supposed to be gap graded with very little amounts of intermediate, Zone II is supposed to be well-graded and the location of the optimal gradation for a concrete mixture design.  Zone III has a large majority of intermediate and very little coarse aggregate.  The Zone IV and Zone V correlate with the extreme sandiness and rockiness.  A large focus has been to use multiple regions of zone II for certain applications.  Many DOTs use a parallelogram in the middle of zone II for a requirement of aggregate proportioning in slip formed pavements.  However, Shilstone suggested bottom of zone II would best for this application.   Unfortunately, little testing data has been published by Shilstone or others to validate the chart. 
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Power 45 
Started in the concrete industry in 1907 and now used by the asphalt industry, the power 45 curve uses a combined gradation to best-fit to a straight line on the cumulative percent passing chart (Fuller and Thompson 1907). The straight line from the origin to the nominal maximum size has thought to be the maximum density of a combined gradation, which supposedly creates the maximum density and the minimum amount of voids in a mixture.
Picture
Individual Percent Retained Chart
A method for evaluating the each exact distribution of each sieve size has been the individual percent retained chart.  The chart can easily show the excess or lacking sieve sizes of a combined gradation.  From experiences, people have specified a maximum boundary of 18-22% and a minimum boundary of 5-12% retained on each sieve.  No known research has been conducted to prove the limits. 
Evaluating Different Proportioning Techniques
Since an established and quantitative method to proportion aggregate is not out there, our research team investigated some of the more popular aggregate proportioning methods and compared the workability performance of each mixture.   Mixtures were designed with a fixed amount of paste but using different aggregate proportioning methods for slip formed paving and also typical ready-mix concrete.  With three sources of sand, five different sources of crushed stone, and two river gravel sources, we have test over 800 mixtures.  Many monumental findings have been made in the workability effects on the proportioning of aggregates.
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Sunday, October 16, 2016

SHRINKAGE CRACKS IN CONCRETE – TYPES & CAUSES

Shrinkage cracks in concrete occur due to change in moisture of concrete. Most of the building materials like concrete, mortar, burnt clay bricks are porous in their structure in the form of inter-molecular space. They expand when they absorb the moisture and shrink when they dry. This is the main cause the concrete shrinks on drying. Shrinkage of concrete is an irreversible process.

Types of Shrinkage in Concrete

There are two types of shrinkage in concrete:
1. Initial Shrinkage
2. Plastic Shrinkage
Shrinkage Cracks in Concrete

1. Initial Shrinkage Cracks in Concrete

Initial shrinkage cracks in concrete normally occurs in all building materials or components that are cement/lime based such as concrete, mortar, masonry units, masonry and plaster etc. and is one of the main cause of cracking in structure.
Initial shrinkage in concrete and mortar occurs during construction of structural member due to drying out of moisture. The initial shrinkage of concrete is partly reversible if the moisture is maintained in concrete, but it becomes irreversible when concrete becomes dry. During curing, due to subsequent wetting and drying this shrinkage exceeds and crack is developed in concrete.

The extent of initial shrinkage in cement concrete and cement mortar depends on a number of factors namely :

a) Cement content –It increases with richness of mix.
b) Water content – Greater the water quantity used in the mix, greater is the shrinkage.
c) Maximum size, grading & quality of aggregate –With use of largest possible max. size of aggregate in concrete and with good grading, requirement of water for desired workability is reduced, with consequent less shrinkage on drying due to reduction in porosity. E.g., for the same cement aggregate ratio, shrinkage of sand mortar is 2 to 3 times that of concrete using 20 mm maximum size aggregate and 3 to 4 times that of concrete using 40 mm maximum size aggregate.
d) Curing –if the proper curing is carried out as soon as initial set has taken place and is continued for at least 7 to 10 days then the initial shrinkage is comparatively less. When the hardening of concrete takes place under moist environment there is initially some expansion which offsets a part of subsequent shrinkage.
e) Presence of excessive fines in aggregates –The presence of fines increases specific surface area of aggregate & consequently the water requirement for the desire workability, with increase in initial shrinkage.
f) Chemical composition of cement – Shrinkage is less for the cement having greater proportion of tri-calcium silicate and lower proportion of alkalis i.e. rapid hardening cement has greater shrinkage than ordinary port-land cement.
g) Temperature of fresh concrete and relative humidity of surroundings – With reduction in the surrounding temperature the requirement of water for the same slump/workability is reduced with subsequent reduction in shrinkage. Concreting done in mild winter have much less cracking tendency than the concreting done in hot summer months. In cement concrete 1/3rd of the shrinkage take place in the first 10 days, ½ within one month and remaining ½ within a year time. Therefore, shrinkage cracks in concrete continue to occur and widens up to a year period.

2. Plastic Shrinkage Cracks in Concrete:

Plastic shrinkage in concrete occurs immediately after concrete has been placed due to settlement of large solid particles by gravity action. Due to this, water in the concrete rises to the surface. This process is also called bleeding of concrete. Bleeding in concrete continues till the layer of water on the surface of concrete has set.
As long as the rate of evaporation is lower than the rate of bleeding, there is a continuous layer of water at the surface known as “water sheen”, and shrinkage does not occur. When the concrete surface looses water faster than the bleeding action bring it to the top, shrinkage of top layer takes place, and since the concrete in plastic state can’t resist any tension, cracks develops on the surface. These cracks are common in slabs.

The extent of plastic shrinkage depends on:

  • Temperature of concrete,
  • Exposure to the heat from sun radiation,
  • Relative humidity of ambient air and velocity of wind.
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AVOIDING COMMON CONCRETING PROBLEMS

There are a number of problems which can occur in concrete as a result of improper mixing, placing, or curing. The following are common problems that are easily avoided if proper procedures are followed.
Segregation is the tendency of the various constituents of a concrete mix to separate, especially the separation of the large aggregate particles from the cement mortar. Segregation can result in rock pockets or   honeycombs in the hardened concrete, sand streaks, porous layers, scaling, laitance, and bond failure at construction joints. Harsh mixes have a tendency to segregate, usually those that are too wet but sometimes those that are too dry. A well-proportioned mix with a slump of 3 to 4 in. resists segregation, but any mix can segregate if it is not properly handled, transported, and placed. Once segregation has occurred, the aggregate cannot be reintegrated and the mix must be discarded. Segregation can be caused by over-mixing or by improper handling during placement operations.
concrete-seggregation

Bleeding occurs when the cement and aggregate in newly placed concrete begin to settle and surplus water rises to the top surface of the concrete. Bleeding continues until the cement starts to set, until bridging develops between aggregate particles, or until maximum settlement or consolidation occurs. Mix proportions, sand grading, sand particle shape, the amount of aggregate fines, the fineness of the cement, water
concrete-bleeding
content of the mix, admixtures, air content, temperature, and depth or thickness of the concrete all influence the rate and total amount of bleeding. A slab placed on a plastic vapor retarder will bleed more than
one placed directly on soil because the soil absorbs some of the surplus water. Some bleeding is a normal part of concrete curing, but excessive bleeding can decrease the durability of the surface, interfere with the
bond of cement paste to reinforcing bars, and increase porosity of the hardened concrete. Air entrainment reduces bleeding, as does a well-graded sand, an increase in cement content, or a reduction in water
content. If changes are made to some ingredient quantities, the mix must be adjusted to maintain the proper proportions required for strength and durability. Bleed water must be allowed to dry naturally,
as there is little way to remove it from the soft surface of the fresh concrete. Excessive bleeding will delay the start of finishing operations.
Plastic shrinkage cracking is usually associated with hot-weather concreting. It is caused by rapid evaporation of surface moisture from a slab or other flatwork. The procedures recommended for hot-weather concreting will alleviate the possibility of plastic shrinkage cracking.
Dusting is the wearing away of hardened concrete surfaces under traffic. Dusting is caused by mixes with too much water, segregation during the placement and consolidation of the concrete, dirty aggregate, applying water to the concrete surface during finishing operations, or premature or prolonged finishing operations which cause the formation of a weak surface layer called laitance. Laitance is a white or light gray substance which appears on the surface of concrete after it is consolidated and finished and which consists of water, cement, and fine sand or silt particles. Laitance prevents good bond of subsequent
layers of concrete and adhesion of other materials to the concrete such as finish flooring. In an exposed slab, laitance will scale and dust off after the floor is in use, and it can contribute to hairline cracking and
checking. Excessive amounts of rock dust, silt, clay and other similar materials can also contribute to laitance. The same measures that are used to reduce bleeding will also reduce the occurrence of laitance.
Scaling is the flaking or peeling away of a thin layer of cement mortar on the surface of concrete. The aggregate below is usually clearly exposed in patchy areas and often stands out from the remaining surface. Scaling can be paper thin or as deep as  14 in.
One type of scaling is caused by the same things that cause dusting and laitance: mixes with too much water, segregation during the placement and consolidation of the concrete, applying water to the concrete surface during finishing operations, or premature or prolonged finishing operations. Another type of scaling is caused by the use of deicing salts on non-air-entrained concrete, and can be prevented by the use of air-entrained cement or air-entraining admixtures.
In false set, concrete appears to set or harden after only a few minutes. This is a temporary condition caused by hydration of unstable gypsum (calcium sulfate) in the cement. It usually disappears with prolonged mixing or remixing and is generally not a problem with ready-mixed concrete. Do not add more water. After a few more minutes, with or without additional mixing, false set will usually disappear on its own.
In flash set, lumps of dry cement are surrounded by a layer of damp or partially hydrated cement, or solid lumps of partially hydrated cement are formed. Flash set is caused by the use of hot-mixing water in cold weather. To avoid this problem, change the batching sequence so that the hot water and aggregates are put in the mixer first and the cement is added after the water has cooled slightly.
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Saturday, October 8, 2016

Tests on Cement at Construction Site To Check Quality of Cement

Quality tests on cements at construction site (also called field tests on cement) are carried to know the quality of cement supplied at site. It gives some idea about cement quality based on colour, touch and feel and other tests.

Tests on Cement at Construction site

The following are the quality tests on cement at construction site:
  • Color test
  • Presence of lumps
  • Adulteration test
  • Temperature test
  • Float tests
  • Strength test
  • Setting test
  • Date of packing

Color Test of Cement

The color of the cement should be uniform. It should be grey colour with a light greenish shade.

Presence of Lumps

The cement should be free from any hard lumps. Such lumps are formed by the absorption of moisture from the atmosphere. Any bag of cement containing such lumps should be rejected.

Cement Adulteration Test

The cement should feel smooth when touched or rubbed in between fingers. If it is felt rough, it indicates adulteration with sand.

Temperature Test of Cement

If hand is inserted in a bag of cement or heap of cement, it should feel cool and not warm.

Float Test

If a small quantity of cement is thrown in a bucket of water, the particles should float for some time before it sinks.

Setting Test

A thick paste of cement with water is made on a piece of glass plate and it is kept under water for 24 hours. It should set and not crack.

Strength of Cement Test

A block of cement 25 mm ×25 mm and 200 mm long is prepared and it is immersed for 7 days in water. It is then placed on supports 15cm apart and it is loaded with a weight of about 34 kg. The block should not show signs of failure.
The briquettes of a lean mortar (1:6) are made. The size of briquette may be about 75 mm ×25 mm ×12 mm. They are immersed in water for a period of 3 days after drying. If cement is of sound quality such briquettes will not be broken easily.

Date of Packing:

Strength of cement reduces with time, so it is important to check the manufacturing date of the cement. Generally, the cement should be used before 90 days from the date of manufacturing.
Read More on Material Testing Guide. Recommended Book:  Concrete Technology
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