Foundation, Concrete and Earthquake Engineering

What is Moisture Movement in Building?

Materials used in building construction have pores, especially manufactured materials like cement or lime concrete, mortar, some types of stones, timbers, and bricks from clays. Such porous materials inherently absorb moisture from many sources and expend which is followed by shrinkage on drying. These moisture movements are of two types

• Reversible movements
• Irreversible movements

The reversible movements are those which have reverse movement on absorbing moisture and subsequent drying. Such movement is of cyclic nature which is due to change in inter-pore pressure controlled by moisture change. The degree of movement depends on porosity and molecular structure of a material.

Pile foundation

Earthquake

What is Pounding? Structural and Foundation Issues of Pounding














Foundation design



Contact Pressure Distribution Beneath Footing

Concrete Properties

Concrete Roof and Floor Surfaces

Relation Between Compressive and Tensile Strength of Concrete

Quality of water in concrete Mix

Storage of Cement

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General Safety Guideline for Operation of Concrete Pump

Familiarization of pump & booms used in concrete work is very important prior to operation of pump without any supervision. Now what is safe operation of concrete pump?


Safe operation includes:

•  Inspection of pump before leaving equipment yard,
•  Driving safely to jobsite
•  Operation of concrete pump safely after setting up in the jobsite
•  Clean up of pump and driving it back safely to yard or drive to other pumping job.


Personal protective equipment:


Before entering jobsite, pump operator should have personal protective equipment stated below:

• Safety helmet
• Comfortable work cloth
• Safety glasses or goggles
• Gloves
• Heavy duty shoes
• Rubber gloves and boots (while cleaning)
• Breathing mask (when there have possibility of exposed to any types of dust).


It is important to ensure availability of all required tools related to pumping. Sometimes personal protective equipment are required like

• Hearing protection
• Steel-toe shoes
• Shirt having long sleeves


Before staring of engine, operator should check

• Engine oil
• Hydraulic oil
• Radiator water
• Yell of engine


All safety covers, grates, gauges, tires, outriggers should be inspected for proper operational safety. Appropriate clean out equipment for particular site should be ensure and pump operator should check adequate safety slings, chains or cables are available for particular system that may drop or fall.


Safety sling should be anchored to boom but not to pipeline. All clamps when required must be pinned including boom. The operator should go through operation manual of manufacture before operating any pumping equipment and if not understand any term or procedure, should contact with manufacturer. Only one selected person can guide pump operate for moving boom and should be predefined. The hand signals (14 safety signal), used in safety regulation should be learnt by all pump operators. The job site should be kept free of alcohol and drug.


Sometimes electrical wires may obstruct free movement of boom and American Concrete Pumping Association recommended to ensure a position of spotter who will be a dedicated person to guide operator to avoid accident. When voltage of electrical wires exceeds 350 kv and electrical wire is within 50 ft of concrete poring area, the spotter is essential.


When voltage is ≤350 kv the distance reduced to 20 ft. The spotter monitors boom movement and warn operator about possible accident. No amendment is valid for this rule form local, state or other requirement of national law.


It is expected to work on clean job area and should be kept clean. Never go into valve or water box; this will prevent serious injury. The water box should be kept covered while machine is in operation (ensure cover of box to remain in place). When the cover has to be removed, transmission should be cut off and accumulator bled should have zero pressure.


A professional concrete pump operator should have following qualification:

• Good driver-knowledge of pump operation
• Mechanically sound
• Good public relation skill
• Good judgment on safe operation of pump.

Why are Index Properties of Soil Required in Addition to Engineering Properties?

Engineering properties of soil that are frequently used in geotechnical engineering are:
b. Compressibility


Permeability:


This is the property of soil that facilitates water to flow through them. The main importance of this property is to measure seepage discharge (either water or soil particles with it) throughout earth masses.


ConMatic IPC, automated soil consolidation system
ConMatic IPC, automated soil consolidation system

Compressibility:


This is the property of soil which defines deformation of soil mass under compressive loads. This property is required to estimate settlement of foundation on soil.



Shear strength:


Shear strength defines capability of soil mass to counteract shear stress. This property helps us to determine following capabilities of soil



• Slope stability
• Bearing capacity
• Pressure on earth retaining structures.


Now what are index properties? –these are properties that are not determined in geotechnical engineering for primary purposes of foundation design and analysis but helps engineers to have idea about engineering properties.


The major index properties for coarse grained soils are

• Particle size
• Relative density


The major index properties for fine grained soil are consistency or atterberg’s limits.

Triaxial shear test set up for shear strength of soil
Now we have defined engineering properties and index properties; why should we go for index properties? Engineering properties are the main properties with which (using one or more) we can design a foundation. Say with shear strength we can determine bearing capacity of foundation soil and with compressibility we can determine settlement criteria for loosely compacted cohesionless soil or time consuming consolidation of cohesive soils. 


The consolidation tests required several days, again shear strength test required sophisticated instrument for both testing and collecting sample (undisturbed) from field. So engineering properties determination tests are laborious, time consuming and elaborate. Geotechnical engineers are often interested on index properties to gather rough knowledge about engineering properties. The simple tests that are conducted in lab or field (index properties) are called classification tests.


Here soil is indentified based on classification and often index properties offer good prediction of engineering properties. Even methods of testing engineering properties are also selected based on index properties; say draining and confining conditions during shear strength tests.


Soils having identical index properties are often found /assumed to have same engineering properties. But the correlations between two sets of properties are not perfect and reasonable factor safety should be employed if index properties are the basis of design. It is not recommended to design important and large structure only depending on index properties.

Requirements of Sheath for Micropile Encapsulation

We know sheath is used for encapsulation purposes; the encapsulation is required for aggressive exposure of micropile, especially when subjected to tension loading.


Sheath is essential part of corrosion protection process for unbounded length, later we will discuss about encapsulation of bond length. Sheath may be smooth or corrugated.

Sheath for Micropile Encapsulation


Smooth sheath can also be used as bond breaker as discussed in last post. When sleeves are used that may shrink with temperature change or sheaths of corrugated tube are used, separated bond breaker has to be used. We have discussed about requirements of bond breaker in last post.


Sheaths fabricated by following materials were found to have satisfactory performance:

  • A tube made of hot-melted extruded polyethylene comply ASTM D1248,Type III

  • A tube made of hot-melted extruded polyethylene comply PP210B55542-II,as per ASTM D4101

  •  A corrugated tube satisfy requirements of encapsulation of bond length


These are the specifications according to which sheathing material is selected. The required properties for sheathing material are as following:

• Should have resistance against chemical attack; the aggressive environment, corrosion inhibition or grout are considered to produce chemical attack.

• Should have resistance against aging from UV-light

• Not harmful to any type of reinforcement

• Should have ability to withstand

 Impact 
 Abrasion
 Bending while handling & installation

• Allow reinforcement enclosed by sheathing to elongate under stressing and testing.

Specification for Corrosion Inhibition and Bond Breaker for Micropile Foundation

We know corrosion is very important factor for subsurface construction and sometimes becomes governing factors in pile construction depending on ground condition. Dear reader we have already discussed about measures to be taken for corrosion protection of micropile.


Corrosion inhibition, if required, the compounds used for this purposes in free length and should be of organic say wax or grease.


The inhibiting compounds should have

Corrosion protection in micropile, Skyline Steel, Nucor Corporation
Corrosion protection in micropile, Skyline Steel, Nucor Corporation
• Self-healing behavior

• Inhibiting additives

• Appropriate method for displacing polar moisture

• Nonreactive with

  • Prestressing steel
  •  Anchor grout
  •  Sheathing materials

• Chemically stable

• Can permanently retain its viscosity


When bond breaker is required, smooth plastic pipe or tube should be used for fabrication of it. The expected properties of pipe or tube are as follows:

• Should have resistance against chemical attack; the aggressive environment, corrosion inhibition or grout are considered to produce chemical attack.

• Should have resistance against aging from UV-light

• Not harmful to any type of reinforcement

• Should have ability to withstand

  • Impact
  • Abrasion
  • Bending while handling & installation


Regarding application of bond breaker, we can include, it allows elongation of prestressing steel while testing and also stressing. The tube or pipe discussed above should be made from polyethylene of medium-high density and should conform

• ASTM D1248

• AASHTO M252


Sometimes of polyvinyl chloride (PVC) tubes/pipes are uses which should comply with ASTM D1784, class 13463-B having wall thickness of at least 0.04 in or equivalent materials can be used.

Milling Methods for Concrete Removal to be Repaired

In concrete repair work usually damaged, deteriorated or defective portion have to be removed and in most cases damaged portions are hardly defined. Most recommendation state that deteriorated or damaged material must be removed completely.


It is very difficult to determine that all such portions are removed or excess amount are removed (i.e. some sound concrete is removed). A recommendation is to continuing removal until aggregate (coarse aggregate) particles are broken down under removal process instead of removal of cement matrix (which is relatively simpler than breaking down of aggregate).


But when low strength concrete is damaged, the aggregate may never fractured under demolition. Here we will learn about milling methods of concrete removal. This method is suitable for removing concrete from both vertical and horizontal surfaces; especially when specific amount from large concrete member.
Metal scarifier for removal of top surface of concrete pavement
The depth of removal may range from 3 mm to 100 mm approximately. This method usually produce sound concrete surface free from microcracks.


Usually scarifiers are used in these purposes. It is a cutting tool that uses rotary action with its mass of cutter bits. The rout cutting into concrete surface removes loose fragments of concrete like scaling from blasted surface. Weakened concrete (by any expansive agent) or previously cracked concrete can be successfully removed by this method.


Scarifiers of various sizes are available. Other advantages of this tool are:

  • Controlled removal of concrete to defined limit.
  • Simple in operation
  • Debris produced is relatively small which can be handled easily.
  • Both sound and deteriorated concrete containing wire mesh and ties (from formwork) can be removed.
  • Applicable for decks, slabs and for mass concrete.
  • Broom-mounted cutters can be used to remove concrete from ceiling and walls

Some limitations are as follows:

  • Concrete with reinforcement cannot removed
  • Removal rate becomes very low when applied to sound concrete
  • Noise, dust and vibration are produced
  • Concrete having microcracks can be damaged.

Effect of Accidental Inclusion of Sugar in Concrete

We know many impurities in water used in concrete mix produces different affect on concrete properties. Deleterious substances may also be included in concrete through aggregate (both fine and coarse aggregate). Here we will learn about effect of sugar in fresh concrete.


Though the title of this post is accidental application of sugar in concrete but in some important projects sugar is used intentionally for specific purposes. Sugar has retarding influence on concrete. Now consider about accidental inclusion; sugar mainly included due to use of sugar bags to transport aggregates to laboratory or when fresh concrete is transported by molasses bags.


This effect was discovered when investigation was conducted for very low strength of lab sample with proper mix design, careful sampling and curing procedures. The effects depend on quantity of sugar used/included and confusing results were found in previous tests.


It was found that careful application of sugar in small quantity resulted satisfactory retarding effect. The dose and retarding effects are as follows

Dosage: 0.05% of cement (measured by mass)
Retarding effect: about four hours


The probable cause of retardation is preventing action of sugar to form C-S-H. But actual rate of retardation depends on chemical composition of cementitious materials. Like other retarder, for this reason, performance of sugar as retarder should be ensured by different trail mixes (with actual cement that will be used in this job).


Sugar, thus, can be used in many untoward situation like when mixer or in some cases agitators become out of order and concrete cannot be placed due to site condition. The important fact is that it is an inexpensive retarder. Molasses was applied in construction of channel tunnel when residual concrete was required to be retarded due to difficulty in washing out underground.
Channel tunnel was constructed between Folkestone, Kent, England and Coquelles, Pas-de-Calais, France. The tunnel was opened on 6 May 1994. Construction began in 1988 and our reference was in 1990. A dose of (0.2-1) % of cement (by mass) will actually prevent total setting of cement.


Now we have to know about final strength of concrete on this application. When this retarder is applied in controlled manner, obviously, early strength becomes very low but after 7 days, the strength is increased by several percent relative to mix that are not retarded. The probable cause is hydrated cement gel becomes denser due to delayed setting.

Handling Precaution of Concrete Aggregate

We are very conscious about handling of concrete; in most cases don’t pay head to handling of aggregate. Handling of aggregate is also important to have designed gradation, to avoid foreign materials in aggregate and also to have ease of batching and economy of the same too.


Stockpiling and handling of coarser aggregate can lead to segregation. While tippling and discharging aggregate usually rolls down along slope. The large size aggregates have more weight and smaller size aggregates are relatively lighter. Thus large size particles remain in the bottom, the lighter particles remains at the top.
Screening aggregate before feeding concrete mixer
These results uniform grading at all layer; following size fraction should be spitted

 3/16” to 3/8”
 3/8” to ¾”
 3/4” to 1½” etc.


Splitting means stockpiling and handling separately; can be remixed during feeding into concrete mixer according to desired proportions. Even after this precaution segregation can be occurred but would be in narrow range within each fraction and even careful handling can reduce this segregation.


When aggregate particles to be handled are greater than 1½ in; there have every possibility to breakdown large aggregate particles. The precautions should be taken in this case are:

 Using rock ladders to lower aggregate into bins
 Not to drop from height.


In important and large projects, extra precaution to avoid breakage and segregation of aggregate is taken by providing rescreening just before feeding into bins. Thus excess of undesired undersize particles are eliminated.


Rescreening controls proportions of various sizes of aggregate effectively but with greater control more cost and complexity of batching operation are added.


Again a denser pack of aggregate leaves less space to take place finer cement particles; thus requirement of cement or cementitious materials are reduced and eventually economy is added against more cost of screening.


The uniformity of concrete also produces more workable mix facilitating placing. Again improper handling can cause contamination by deleterious substances and also by aggregate of low strength, less durable and undesired sizes. Sometimes aggregates are transported by contaminated container or sacks like sugar or salt etc. The affect of presence of salt in concrete has already been described in previous posts and in upcoming post we will learn about effect of adding sugar to concrete.

Pumping Difficulty of Light Weight Aggregate Concrete

Now-a-days pumping concrete to inaccessible distance is very common. Different specifications are established to make concrete workable and successfully used in concrete industry. We will not discuss here about normal-weight aggregate concrete, our concern is here application of light-weight aggregate in pumpable concrete.


The difficulties with using light-weight aggregate in concrete to be pumped are observed when surface of aggregate is not sealed as water demand of the mix increased greatly. When pressure is exerted by pump, the air voids in the aggregate get contracted. Water from the mix enters into the aggregate pores raising water demand.
Less workable concrete mix may block pumping pipe
The mix, in this case, becomes very dry and problem with pumpability arises. We know lightweight aggregate have voids filled with air which render low unit weight to them .


The remedy is to soak both coarse and fine aggregate for a period of two to three days. Alternatively vacuum saturation at dry rapid rate can be applied. The water absorbed in this way do not become a part of free water within the mix but affect in mix proportioning by mass. It was reported that light-weight concrete has successfully pumped vertically up to 1050 ft.


But saturation of aggregate in such way affects durability of concrete in respect of freezing-thawing exposure. The recommendation is this case is to protect concrete from exposure to such temperature. We know several insulation techniques are adopted in cold water concreting.


But when ambient temperature is very low dependence on waiting period is not enough to mitigate such problem. Special agent in addition to application of low absorbent aggregate may provide us a proper remedy for increment of water demand and freezing-thawing problems.


The agents stated above, when added to concrete mix, they enter into the pores around aggregate surface. When initial hydration of cement takes place the PH in the system is increased. With increased PH the viscosity of the agent also increased to form a high-viscous layer which isolates water from aggregate and absorption under pumping pressure is reduced.

How to Determine Water/Cement Ratio of Hardened Concrete?

We all are usually concerned about water/cement ratio of fresh concrete; here we will learn about water/cement ratio of hardened concrete. It should be kept in mind that we will determine water/cement ratio from hardened concrete sample but at the time of placing concrete mix i.e. fresh concrete.


This ratio is expressed as a function of cement content. We have already discussed about determination of cement content of hardened concrete. So if we can predict cement content with the determination of actual water content, the parameter can be determined.
Fluorescence Microscope for hardened concrete test
Now question is how actual water can content in fresh concrete be determined from hardened concrete as majority portion of water is used for hydration (depending on maturity) and evaporation during placing and curing.


The water content during placing is expressed as a function of mass of water combined in the cement grains and volume of estimated capillary pores.


Capillary pores contain remaining water (say hydrated) of actual water content. Capillary water is also called non-evaporable water. This water is considered as 23 percent of mass of cement (obviously anhydrous, L.E. COPELAND and J.C. HAYES, 1983). For Type II cement this water content may even lower than 19 percent.


We can determine original water content in laboratory by igniting concrete sample under 10000C and determining drive out water (BS 1881: Part 124). But this method is not applicable for concrete sample containing blended cements (concrete society Report No.32).


Now consider the accuracy of this test method. Regarding this we can include the water/cement ratio determined may be of 10 % of actual water/cement ratio. There have also other methods like Reflected Light Fluorescence Microscopy.

Economical Strength for Prestressed Concrete

We know prestressed concrete need relatively high strength concrete. Now our question is-how much strength is economical? Generally it is found to be economical to produce 4 ksi to 5 ksi concrete for prestressed concrete. Though concrete strength should be unique for each job and should be considered individually, the mixes falls within this range are proved economical; i.e. there have some established reasons.


Strength ranging from 4 ksi to 6 ksi can be produced without excessive cement or labor. As an example, we can include that only 15% (Average) more cost is required to achieve a mix of 6 ksi strength relative to production of 3 ksi concrete; notice that at the cost of 15% more cost we can have 100% more strength which is very crucial for prestressed concrete and often of serious requirement in this type of construction.



No slump concrete
When strength requirement exceeds 6 ksi, the cost involvement becomes excessive, this type of mix have to be designed carefully and extensive care in control of mixing and subsequent placing and curing are required which are never easy to introduce in field.


For 6 ksi concrete, the plant operations are required where quality of concrete production can be monitored and maintained with great accuracy. 6 ksi to 8 ksi strength are sometimes specified for both precast and prestressed concrete beams. More strength may sometimes be required but not a usual practice.


When strength of more than 5 ksi is the target, the water-cement ratio should not be more than 0.45 (by weight). To have ease of placement, 2 to 4 inch slump should be provided, otherwise specialized vibration, other than ordinary vibration is required.


For a concrete mix of water-cement ratio 0.45, to achieve 76mm slump, about eight bags cement/yd3 of concrete is required. Where utmost care is taken during vibration, half inch or no slump concrete can be designed for only 7 bags cement/yd3 of concrete.


We know excessive cement content in concrete is always associated with increased shrinkage. So a lower cement content is always desirable. In one word, proper vibration is essential and where possible appropriate admixtures should be employed to increase workability of mix.

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