Properties of Aggregates for Concrete in Severe Weathering Regions

The location of different weathering regions are shown in the figure below:

The aggregates are designated as below:

a. 1S

b. 2S

c. 3S

d. 4S

e. 5S

1S class aggregates are considered to be used in following conditions

• Footings
• Foundations
• Columns
• Beams
• Floor slab

That is All concrete members are not exposed to weathering and floor slabs essentially interior slab having any types of covering.

2S class aggregates are considered to apply interior floors having no covering.

3S class aggregates are considered to apply in following conditions:

• Foundation walls located above ground
• Retaining wall
• Abutments
• Piers
• Girders and beams exposed to weather

4S class aggregates are considered to apply in following conditions:

• Pavements
• Bridges decks
• Driveways
• Curb
• Walks
• Patios
• Garage floors
• Exposed porches and floors
• Water-front structures; any types of concrete member that are frequently subjected to wetting

5S class aggregates are considered in architectural concrete exposed to weathering. 4S class is expose to abrasion as most application are road, bridge and water-front structures.

In case of abrasion requirements, crushed and air-cooled BFS (Blast furnace slag) is not included. The rodded bulk density or unit weight of this material should not be less than 70 lb/ft3. 

While determining material finer than passing No.200 sieve, expressed as percentage, following conditions are considered:

a. This value may be increased 1.5 times when the material is free from shales or clay.

b. If fine aggregates used in concrete have particles passing No. 200 sieve is less than that specified in ASTM C33, then percentage limit in coarse aggregate can be increased by

 L=percentage limit in coarse aggregate can be increased

P=percentage of sand in concrete over total aggregate

T=allowable limit of fine particles in fine aggregate

i.e. 3% for concrete that subjected to abrasion

     5% for all other concrete

A=actual quantity in fine aggregate

Required Properties of Concrete Aggregate in Negligible Weathering Region; ASTM C33

A region can be said to be under negligible weathering, where concrete hardly exposed to detrimental effect of freezing under presence of moisture. Here we will include limits of deleterious substances and also expected physical properties of aggregate to be used in such weathering regions. Dear reader we have already discussed about deleterious substances that are found in aggregates. Aggregates are classified as 1N and 2N. ‘N’ stands for negligible weather condition, we can easily gaze this.

ASTM do not specified limits of chart having specific gravity less than 2.4 and no information about soundness (tested under magnesium sulfate for 5 cycles). We have discussed many posts about soundness of aggregate; you can read this for more information. We are providing a table to specify the limits and properties:

1N- in this class designation the concrete location or types includes:

• Concrete slabs that are subjected to abrasion due to traffic


• Bridge floor


• Bridge deck and side walk


• Pavements

2N-includes all other concrete; types or location other than for 1N.

In case of abrasion requirements, crushed and air-cooled BFS (Blast furnace slag) is not included. The rodded bulk density or unit weight of this material should not be less than 70 lb/ft3. While determining bulk density the grading of it should identical to grading of aggregates to be used in concrete.

In case of determining abrasion loss of crushed stone, crushed gravel or gravel, the grading of sample aggregates and also sizes of sample aggregates should be similar to or nearly similar to corresponding aggregates to be used in concrete.

When aggregates are composed of different grading, the abrasion limit provided should be applied to each.

While determining material finer than passing No.200 sieve, expressed as percentage, following conditions are considered:

a. This value may be increased 1.5 times when the material is free from shales or clay.

b. If fine aggregates used in concrete have particles passing No. 200 sieve is less than that specified in ASTM C33, then percentage limit in coarse aggregate can be increased by

L=percentage limit in coarse aggregate can be increased

P=percentage of sand in concrete over total aggregate

T=allowable limit of fine particles in fine aggregate

i.e. 3% for concrete that subjected to abrasion

     5% for all other concrete

A=actual quantity in fine aggregate 

Weathering Regions for Concrete Aggregates in United States (ASTM C33)

ASTM C33 provides maximum permissible limit for deleterious substances of coarse aggregates of concrete. This classifies coarse aggregates depending on severity of weathering. We will discuss about classification of coarse aggregates based on this in our upcoming posts; here our concern is weathering regions within United States. The weathering regions according to this standard are shown below:

The following guidance should follow while using this map:

1. This map shows only a guidance for probable severity of weathering for the regions shown in the map and use for coarse aggregate.

2. When a construction site lies on the boundary of regions of weathering, specifier should consult with local weather bureau to collect records about

• Amount of precipitation in winter of that region


• Frequency of freezing and thawing cycle

These records provide guidance to examine coarse aggregates under expected identical field conditions.

-When altitude of construction exceed 500 ft above mean sea level, it is considered that more severe weathering will be experienced than shown in corresponding region in the map

-When construction project is located in arid regions, it is considered that less severe weathering will be happened corresponding to region shown in the map.

-When confusion arisen in selecting weathering region, more severe region of weathering is chosen to remain in safer side.

Frequency of Testing of Concrete Aggregate for Moisture Content

We have discussed few posts about moisture content in aggregates; here we will learn about frequency of testing for this parameter. We know accuracy in testing process is important for any concrete aggregates, both in sampling and weighing, but increase of moisture parameter of aggregate, the speed of test have greater importance than refinement in test results.

This is due to requirement of necessary adjustment of moisture content in batching with change in moisture in aggregates. It is expected to measure immediate measurement of moisture content to monitor changes and providing necessary adjustment in water/cement ratio of concrete.

We will provide some information about instantaneous measurement of moisture content in our upcoming posts. The frequency of testing of moisture content of aggregate depends on following factors:

a. Uniformity of supplied aggregate

b. Requirements for the statistical analysis

In normal cases, testing is scheduled two times a day. In case of fine aggregate, sometimes, more testing is required depends on significant changing in conditions.

To minimize or avoid inaccuracy in sampling and weighing, the sample is taken as large as to conveniently handle the moisture testing within time expected for drying.

What is Freezing and Thawing Test for Aggregates for Concrete?

Dear reader we have discussed about ASTM C-33 specification for soundness of aggregates for concrete; where 8.1 and 8.2 defines specific values of weighted average losses of aggregates under sulfate salt solution and when 8.1 not passed the requirements of previous records are specified respectively.

When both 8.1 and 8.2 failed to pass, 8.3 defines that the aggregates sample should pass freezing and thawing test to be selected for using in concrete as sufficient sound aggregates.

Here we will discuss about freezing and thawing test. The German standard DIN 4226 provides a test method for freezing and thawing test as follows:

This standard specifies two methods for testing:

a. Total immersion freezing and thawing test 

b. Air freezing and subsequent water thawing

Total immersion method of freezing and thawing test: 

• The sample of aggregates cooled for duration of (7-10) hours to reach final temperature within (-15 ~ -20)0C
• At the final cooling temperature the sample is kept at least 4 hours.
• Then the sample is left in water of 200C for thawing and kept for 5 hours.
• The above freezing and thawing cycles are applied for ten times.
• Then the sample is dried and passes through next smaller sieve to determine loss in mass as percentage.

Air freezing and subsequent water thawing:

• Aggregate sample is soaked up to two hours
• Then left for drained
• Then they are left in freezing cabinet for 6 hours to reach temperature (-15~-20)0C
• Then the sample is left in water of 200C to thaw up to 1 hour.
• These cycles are repeated for 20 times
• The next procedure is same as that of total immersion method of freezing and thawing test.

Dear reader we are discussing these tests, as a material engineer have to approve constituents of concrete for better concrete having more durability which is sometimes a primary consideration for concrete mix design.

Specification for Soundness of Fine Aggregates of Concrete (ASTM C33)

Dear reader we have discussed about different tests to determine soundness aggregate for concrete according to both ASTM and BS. In all of these tests, the aggregate are examined to loss of their weight under laboratory disruption conditions done by sodium or magnesium salt solution.

Here we will discuss about ASTM specification for unsound aggregates; our concern is fine aggregates. The weighted average loss of fine aggregate, under five alternative immersions and drying cycles, should not more than 10% when solution used for immersion is sodium sulfate. In case of magnesium sulfate these loss should not more than 15%.

We have discussed difference between sodium and magnesium solution in testing conditions. We have to know whether we have to reject aggregates failed to pass above requirements. ASTM C33-8.2 and 8.3 provides recommendation in such circumstances as below:

Supplier have to ensure purchasers/specifier, demonstrating that concrete having compatible properties containing similar aggregates essentially from identical sources has provided satisfactory services under identical weathering condition that is expected to be encountered in present work.

When aggregates failed to pass above requirement, i.e. specific weight loss and not have essential services record should be well documented by supplier that it produce satisfactory concrete to sustain and freezing and thawing test according to C666(ASTM).

Technical Survey and Analysis for Foundation Repair or Retrofitting

At first we like to introduce technical survey and analysis required before taking decision for foundation repair and choosing method of foundation repair. Any types of retrofit work of large scale should be supervised by professional to evaluate structural and geotechnical conditions in and around the foundation. As extensive repair are very expansive we should be very careful about feasibility of repair and also method of repair and a detail survey followed by analysis is required.

A historical survey is required to have information about foundation or foundation walls. The extent of survey and subsequent analysis depend on the following:

• Size of to be retrofitted building 


• Age of building 


• Condition of building foundation 


• Change in use of buildings etc.

From this survey we will evaluate condition of foundation which will define whether a foundation can be retrofitted economically or not. We have already learnt about cost involvement in this process. A tolerable amount of cracks and tilts is considered enough not to retrofitted when settlement is stopped or further settlement is unlikely to happened. The analyses that should be carried before foundation repair are:

a. At first to evaluate soil conditions; to have this we can investigate government archives and construction records.

b. Survey for foundation walls and foundation as well

c. To collect damage report 

d. Visual survey of foundation and if required ground excavation should be carried out.

e. Crack survey

f. Foundation survey which includes drilling and testing etc.

g. Report of time dependent foundation settlement and settlements as well (precision settlement measurement, precision leveling)

h. Determining foundation load considering static system. This is determined from structural analysis of buildings.

i. Measurement of pore-water pressure

j. Determining ground water table

k. Vibration measurements

l. Quality control of material

m. Straightening and leveling of foundation or other walls

n. Measuring stress in existing tension ribbon

What is Difference between Soundness and Chemical Expansion in Concrete?

The title of this post should be unsoundness of aggregate instead of concrete. However let’s consider soundness and chemical expansion of concrete; both produce expansion related problem to concrete. Here we like to differentiate between two terms.

Soundness of aggregate defines the capacity of aggregate against excessive volume changes which is related to variation in physical states of aggregates. Here important keyword is physical state; whereas there may have some volume changes due to chemical reaction between aggregate and alkali content of cement.

Now we will try to list physical factors that can changes volume of aggregate. The volume changes we are listing may be permanent or large:

1. Freezing and thawing
2. Wetting and drying cycles
3. Thermal changes when temperatures rise above freezing.

Now we know the physical causes of volume changes; unsound aggregate is that result deterioration in concrete due to volume changes under above physical causes.

Now we like to include some chemical reactions that can cause volume changes. These are:

1. Alkali-aggregate reactions
2. Alkali-carbonate reactions
3. Other types of alkali-silica reactions etc.

These are most common types of chemical causes that can produce expansion, deteriorating the concrete properties and most concerned property is durability.

We were discussing about chemical expansion and soundness of aggregate in concrete. We have already learnt about the differences in the processes under which expansion occurs; they are physical process (unsound aggregate) and chemical process (expansion due to chemical reaction).

Aggregate in concrete generally take (70-80) % volume in concrete and therefore draw a significant attention from engineers. While we are designing a mix, we are very much conscious about strength and regarding aggregate properties some physical tests are done often ignoring its chemical compositions.

The strength of concrete might not affected by chemical compositions, but chemical composition may have severe impact on concrete durability. In this part our concern is chemical reactions. Let’s discuss this point.

Alkali-aggregate reaction:

Some aggregate have silica which can react with alkalis of cement paste, resulting expansion. The impact is formation of overall cracks in structures, we will learn about this reaction elaborately in upcoming posts.

Alkali-carbonate reaction

The aggregate containing carbonates react with alkali of cement and results expansion.

Other types of alkali-silica reactions

In some sand-gravels, found in river, have very high reactivity against alkali in cement matrix which also produces expansion resulting cracking in concrete.

What are the Mechanical Methods of Concrete Compaction?

In concrete construction, widely accepted concrete compaction method is vibration, this is a mechanical method of removing void from concrete. Vibration techniques may be applied in internally or externally and sometimes both methods are applied for special purposes.

In case of precast concrete, power tamper can be used to compact concrete of low water-cement ratio. Low water-cement ratio means high strength but less workability (stiff concrete) seeking more effort for compaction. As controlled environment is available in precast concrete industry optimum parameter for producing maximum strength and high durable concrete are provided.

In this process with the temping or ramming effect for compaction, additional effort is provided by simultaneous vibration of low frequency.

In some precast member, mechanical driven tamping bar is used for stiff concrete mixture which includes concrete blocks.

Dear reader here we are emphasizing over stiff concrete or concrete having low slump. This is due to in high slump mix, the coarse particles have natural tendency to sink, and with the inclusion of tamping segregation of aggregate may occur.

ACI 309R-14 provides necessary information about different types of compaction tools. Dear reader in this blog we will discuss many aspects of concrete vibration according to recommendation by this committee. In our last post we have discussed about manual method of concrete compaction.

What is the Consequence of Concrete Form Leakage?

We know formwork is an essential element of concrete construction. One of the most important advantages of concrete is derived with its ability to take shape any form. To render section to desire shape, concrete formwork is required that represents architectural shapes.

In fresh concrete, water with fine particles have tendency to leak in normal condition and greatly enhanced with compaction method especially when mechanical method of compaction is provided. This requires a leak free joint in formwork.

 

The form itself should be non-absorbent to keep desired water cement ratio. For any types of concrete consolidation joints in form must be mortar tight and especially taped in regions where appearance is an important requirement of construction. So that leakage is prevented.

 

As soon as, vibration is provided for consolidation, it converts mortar into fluid consistency. This results hydrostatic pressure which will find its way to release through defects in forms like-

• Holes
• Open Joints
• Cracks

The mortar loss in this way results sand streak or rock pockets at the location of leak. Sometimes air may be sucked at the points of such leakage into the form, which also produce additional voids and easily visible when forms are removed.

The imperfections stated above have serious affect on surface appearance requiring rework and often they result a weak structure.

Some defects are can be repaired and some may be ignored even under a careful inspection and repaired surface never becomes as perfect as of perfect one.

How to Compact Concrete in Sloping Surface?

Dear reader usually we have cast concrete in either vertical or horizontal or combination of both. But in some case we have to establish a slopping surfaces for concrete. We will learn how to compact or consolidate concrete in slopping surfaces.

It is very common in building construction that providing slope in roof and toilet/ bathroom etc. where water has to guide to certain direction. In pavement construction we also have to provide slope to facilitate vehicle movement and also to allow run off of water. We know water is the main enemy of pavement either of concrete or asphalt.

But the slopes we have discussed are commonly of very mild slope, exception in highway pavement there may have steep slope in special cases. In mid slopes the normal process of consolidation concrete is enough; but in steep slope like an embankment or any other slope having 1:4 or steeper special consideration is required. Dear reader 1:4 means 1 vertical to 4 horizontal.

In this case slip form screed or temporary-holding form is required, which prevent flow or sag of concrete mass during vibration. This facilitates consolidation by eliminating strike off top surface.

To avoid surface blemishes before reaching final set holding form is removed carefully and worked by hand.

But when sloping forms have difficulty in removing before setting, they are removed just after filling to avoid blemishes.

Manual Method of Concrete Consolidation

Dear reader we all know consolidation is an essential element of concrete placement. Here we will discuss about manual consolidation. Concrete inherently have some flowability and due to gravity some consolidation of concrete is achieved while deposition of it in form.

But how much consolidation is occurred naturally without external effort? In stiff mix, the consolidation is negligible and in well proportioned mixture designed for flowing behavior less external consolidation attempt is required.

The techniques that we know are rodding and spading. Considerably flowable or plastic mixtures are consolidated with rodding techniques.

Spading is also sometimes applied at surface of forms. Now what is spading? Near from insertion of flat tools and subsequent withdrawn is done in this techniques. Repeated insertion and withdrawn reduce air pockets near surface of member.

In spading coarse aggregates are moved away from form which facilitates movement of water and air to top surface of member but bugholes near concrete surface is reduced both in quantity and size.

Hand tamping can be used for stiff mixer. Here concrete is initially placed in a thin layer and tamped or rammed carefully, then subsequent layers are provided and each layer is worked carefully by tamping. The quality of consolidation is not bad but the effectiveness is reaped at the cost of many labors.

Whatever the effectiveness, manual methods of consolidation, now-a-days, are only used in non-structural and small concreting operation.

Influence of Aggregate Permeability on Concrete

Dear reader in last post we have discussed about permeability of different rocks in relation to concrete durability; the coefficient of permeability of basalt, dense granite, traprock, diorite, marble are generally lies between 1X10^-12 to 10X10^-12 cm/s.

But some types of limestone, sandstones, granite and cherts have much higher co-efficient of permeability and this is of the two orders of value provided above. Now question may arise having maximum 10% porosity how do they show such higher permeability?

The answer lies in size of the capillary pores of aggregate. They have much larger size than cement paste. The pore size found in aggregates is greater than about 10µm (average value).

The capillary pores in matured cement paste have size of the range of (10-100) nm i.e. much smaller pore size are available in matured cement paste.

Thus being less porous, due to large size of capillary pores aggregate shows much permeability than cement paste.

In some aggregate there have finer pores in distribution of pores thus lead to less permeability. But this aggregate do harm to concrete by increasing permeability; how is it possible?

These aggregate sometimes suffer expansion which may produce cracks; thus again allowing moisture movement through concrete increasing permeability of concrete decreasing durability of the same. Here moisture movement is slow which results hydrostatic pressure. The example is

• Some cherts
• Limestones

How do Fluctuating Loads Result Long-Term Foundation Settlement in Cohesionless Soil?

Dear reader in previous post we have learnt that cohesionless soil usually not susceptible to long-term settlement in foundation soil. But these have some exceptions; we, here, will discuss about influence of fluctuating loads on foundation settlement on cohesionless soil.

Say a storage tank like oil tanks which have numerous filling and emptying cycle over their life. Consider a foundation, soil sand but in loose state over which this cycles are done. What will happen? The loading unloading cycles result slow and gradual densification of sand.

Thus long-term settlement is observed. These may happen in silos that are used for food grain, coal and raw materials for industrial production. Dear reader we will discuss about structural design of silo of both concrete and steel in our upcoming post.

The structures that impose low fluctuating loads on foundation, it is observed that after 30 years of construction settlement under footing may be of up to 1.5 times that of immediate after completion of structure.

The structures imposing heavy fluctuating loads, suffers up to 2.5 times that of immediate after completion of structure (post construction settlement)

The values provided above are upper limit of settlement, the actual value of settlement depend on following factors:

1. Number of filling emptying cycles
2. Magnitude of fluctuating load.
3. Density of foundation soil (sand weather Loose or dense)