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.

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.

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.

Elements of Geotechnical Investigation of Shallow and Deep Foundation (IBC)

Geotechnical investigation is important for construction of building as owner or his authorized agent have to submit this document to building official during submission of approval requirement and for design suitable foundation system.

The geotechnical report should include at least following information:

      a.  A drawing of site showing location of bore holes for subsurface investigations.

        b. An entire record of soil boring, log of penetration test and also soil samples should be provided in report which facilitates foundation engineer to determine variation in bearing capacity and settlement.

   
c.  A complete record of soil profile.

      d.  In case of water table encountered in project site, the elevation of ground water table should be included. We have published post about determination of ground water level in our previous post.

      e.  Recommendations regarding

1.  Foundation type
2.  Design criteria which should include at least:

•  Bearing capacity of compacted or natural soil
•  Mitigation measures for expansive soils with authentic reference
•  Mitigation measures for liquefaction where liquefaction susceptibility soil and seismic activity are encountered.
•  Differential settlement
•  Variation of soil strength
•  Any possible influences of loads arrived from surrounding structures.

f.  Desired total settlement and differentials settlement 

g.  Special provision for design and also for construction for foundation supported on soils having expansive nature when required.

h.  Installed capacity of deep foundation.

i.  Recommended spacing of foundation elements (say piles or piers) measure by center to center distance.

j.  Installation procedure of deep foundation element 

k. Driving criteria including drilling techniques, drilling fluids, sequences and also special measures for critical conditions

l. Reporting and inspection procedures which facilitates to determine imstalled capacity of foundation elements. 

m. Requirements for load test; we will learn the topics as per IBC.

n. Durability aspects of foundation elements considering surrounding environment.

o. Group action of foundation elements; usually reducing while action, if applicable.

p. When shallow foundation is supported on compacted fill, the following specification should include

•          Site preparation prior test
•          Material
•          Test method (MDD, OMC)
•          Allowable thickness
•          Test method in filed
•          Frequency of testing and reference of dry density

q.      Specification for controlled low-strength material; the specification required 

•          Site preparation prior test
•          Material
•          Test method (MDD, OMC)
•          Allowable thickness
•          Test method in filed
•          Frequency of testing and reference of dry density

Plain Concrete and Masonry Unit Foundation IBC

Dear reader we have discussed many posts about shallow foundation; here our concern is plain concrete and masonry unit foundation. At first we will start with plain concrete; except light-frame structure, the edge thickness of such foundation supporting walls should not be equal or greater than 203 mm (8 inches) when supported on rock or soil.

When such footing support IBC occupancies group R-3, a reduction in edge thickness of up to 152 mm (6 inches) can be provided. But the footing should not be extended from both sides of supporting wall by a distance more than thickness of footing.

Dear reader we will discuss about plain concrete footing elaborately, here we have just included IBC provision.

Regarding dimension of masonry-unit foundation, we can include, as per IBC, that the mortar used in construction should comply with type M and Type S. Following table provides properties of different mortars according to ASTM C270. ASTM C270 is included here as IBC also referred to this standard.

The depth of footing should be equal or more than 2 times of projection beyond column, wall and pier. The width should be equal or more than 203 mm (8 inches) wider than wall supported by them.

Regarding offset, code recommended to provide a maximum of 38 mm (1.5 inches) for each course in foundation walls (stepped up) from footing. 38 mm is considered for laying single course when double courses are applied 76 mm is recommended.

Foundation Damage due to Fungus Rots

Dear reader, still there have many foundation constructed of wood. The main draw-back of timber foundations materials is deterioration under many biological and chemical action; sources are obviously from ambient environment.

Dear reader here we will learn about rotting and we have already discussed about insect attack on wood foundation. Dear reader we like to include a great palace that were founded over 13569 piles and these piles were all timber pile. You may be surprised to hear that this palace was built in 1648 and still it is standing quite good condition. Dear reader we are talking about Royal Palace of Amsterdam. So why are we discussing about rotting of timber foundations?

The palace was renovated in 2005 and left opened for visitor in 2009. But the renovation was only removing asbestos. We know health hazard associated with asbestos.

Timber is locally available foundation material in many regions of the world and historically this has been good and reliable material for constructing home and foundation. Still now it is more affordable material than steel or concrete.

As piling material, it is a popular choice but have low load bearing capacity than steel and concrete. Another consideration that should include in designing timber pile is rotting and subsequent insect attack.

Rotting is synonyms to decomposition which defines a process which break down organic substances into new simpler form. Now necessary supply of following elements is required to commence rotting:

a. Water

b. Oxygen

The royal palace of Amsterdam is still surviving today as these numerous piles remain below groundwater table. These piles have sufficient water but lack of dissolved oxygen.

Fungus grow vigorously using nutrient available in wood. But these have also a controlling factor; temperature. The temperature within (0-40)0C is essential for such growth. The wooden foundation elements must have not less than 20% water (in respect of their dry weight) to grow fungus with the wood.

Sometimes a foundation elements that is designed to have below ground water table, may suffer fungus rotting, as ground water is lowered with seasonal or other many man made changes in ambient environment. So if a foundation is suspected to subject water table fluctuation, the wood/timber has to be preserved by pressure treatments. With pile foundation, rafts are also subjected to rotting when water table drops below top of foundation.

We will learn about these preservation method in our upcoming posts

What is the Interference Effect in Closely Spaced Foundation?

We know raft foundation is adopted when bearing area required to support superstructure loading is equal or close to available floor area. But let’s consider-required bearing area is very close to available floor area and we want to avoid raft; what should be foundation design consideration?

This means there are very small gaps between footings or footings have to be placed in very close spacing. Sometimes footing may even touch each other. Now the question arises about interference effect. We will uncover the answer of the doubt about their structural behavior.

This effect of interaction between footings has been considered in many studies of researchers. They found a parameter that defines whether the interference should be considered seriously or not.

The parameter is angle of shearing resistance. When the value of angle of shearing resistance is low, the interference is too low to be considered negligible. In case of high values, this effect is significant. When a foundation surrounded in both sides by foundation, this effect is very significant.

Another parameter that influences interference effect is length to width ratio of foundation. When the length to width ratio tends to unity, this effect is found reduced.

In analyzing punching shear failure, interference effect needs not to be considered. For these reasons, it is recommended not to consider in design and analysis. However, a design professional should be careful about possibility of existence of them in special circumstances.

Why are Timber Piles Substituted by Concrete Piles?

Once timber piles were very popular as deep foundation; still they have importance in foundation construction and many building foundations are also supported by this type of deep foundation, especially light buildings. Now concrete pile is more popular and it becomes a versatile deep foundation element.

The main issue is durability and goes against timber pile, but the economy goes in favor of timber pile. Here we will discuss about a comparison between timber and concrete piles to explain why concrete piles are becoming popular day by day. These are:

a. The main problem with timber pile is durability and durability is also related to availability of water. For both concrete and timber, water is the vital issue about durability.

Except water-logged area, the water found around deep foundation is at or below ground water table. Concrete can be made waterproofed, porous, resistant against harmful chemical, in one word, concrete can be modified to have any degree of corrosion/deterioration protection. Thus durability of concrete piles is not a function of ground water table. Whereas the cut-off level of timber pile must be below ground water table.

Where ground water table means level of permanent ground water level as the level may vary due to seasonal change in ambient environment.

b. The concrete piles can be constructed to any size and shape and large size of these piles offer greater bearing capacity which limits number of piles in a pile group.

c. The embedment required for pile group, known as pile cap, becomes smaller in size as less piles requires for each group, which save concrete and reinforcement.

d. Any length of pile can be constructed with concrete. Pile length depends on bearing stratum and available skin friction. Now piles of more than 30 m length are very common.

e. Concrete piles can be applied in marine environment without having any treatment. The mix design is done carefully to have resistance against exposure to water, here also permanent exposure level to sea water is important as above these oxygen is available to emphasize corrosive environment.

f. Another important fact is connection with pile cap. Concrete piles usually cast monolithically with pile cap. The rigidity of connection is dependent on detailing of pile cap. We will learn these in our upcoming posts. Except pile cap, concrete pile can be corrected by grillage foundation.

In case of timber pile, the connection cannot be done perfectly.

Some advantage of timber piles cannot be ignored but considering durability, constructability and imperfection in connection they become less popular than concrete pile.

Requirements for Steel Grillage Foundation (IBC)

Dear reader in our previous post we have discussed about many types of foundation, but so far grillage foundation has not been discussed. Dear reader we will just discuss here about IBC requirements for steel grillage foundation. At first we like define grillage foundation in few words.

This foundation consists of sleeper with steel framework or some sleepers with some steel beams which will transfer loads to ground from support.

The sleepers should be made of wood, concrete or steel.

Generally they are used to support steel columns having heavy imposed load of superstructure on soil poor bearing capacity. This type of foundation is light and economical too; at the same time they can be placed at shallow depth.

To achieve attachments, grillage foundation should be constructed with sleeper support that transfers load (horizontal component) to sleepers. Sleeper supports are fixed rigidly to foundation beams.

International building code provided some requirements for steel grillage foundation; only steel grillage foundation is included in code, as follows:

-Grillage foundation of the structural steel shapes should be kept separate by using steel spacer approved by building officials.

-The spaces left between shapes as discussed above should be entirely filled with cement grout or concrete

-The encasement of concrete should not be less than 152 mm (6”) at bottom portion and no less than 102 mm (4”) at any points.

Water Proofing of Foundation Wall (IBC)

At first we will learn about ground water control according to IBC. Where it is found that ground water table is at equal or more than 152mm below lowest level of floor (bottom level), the floor and foundation wall as well need not to waterproofed i.e. damp proofing is enough according to recommended material and method of IBC.

When not found at desired elevation, proper control system can be adopted to lower ground water level according to following discussion:

Design of ground water control system to lower the water table should be based on standard and approved principles of civil engineering considering at least following:

• Permeability of soil

• Rate of entrance of water to designed drainage system

• Expected and maintained capacity of the pumps

• Head of operation of pumps

• Rated discharge capacity of disposal area In the ground water control system.

Again walls that will be treated or constructed should be of concrete/masonry; should be strong enough to withstand expected hydrostatic pressures, along with other lateral pressures/loads that are active in the walls.

Here hydrostatic pressure is considered as waterproofing term, when hydrostatic pressure condition dominate in the field and design of ground water control not include or not enough to lower expected elevation (as discussed above).

We know due to imperfection of concrete, formwork sealing and improper mixed or proportioned concrete, the surface of concrete member have some defects. We have discussed many posts about such imperfection of concrete work. However, our concern is here surface preparation; we are not discussing much about imperfection.

Before applying water proofing materials on surface of concrete walls, the imperfection like recesses or holes appeared after removal of formwork or any types of cracks (if not structural cracks) should sealed using bituminous materials or other accepted materials or method also can be used in this purposes.

In case of masonry walls, the exterior walls under ground level should be covered with plaster; may be of cement plaster or with other suitable material. The recommended thickness should be equal or more than 9.5 mm (3/8 inch) when mortar of Portland cement is used.

The plaster should used to masonry footing too. Now think about waterproofing, this should be used from top or above of maximum level of ground water table to bottom of wall. The IBC recommended to keep waterproofing more than 12” above of ground water table. The rest portion of wall should be damp proofed according to IBC as discussed in previous post.

Water proofing material (IBC)

Should consists of two ply hot mopped felts, equal or more than 6 mil polyvinyl chloride, polymer modified asphalt of 40 mil, 6 mil thick polyethylene or another approved materials or methods that is capable to bridge non-structural cracks. Sealing and lapping of joints in membrane should be provided according to installation instruction of manufactures.

Geologic and Geotechnical Investigation for Seismic Design of Foundation

Dear reader we have already discussed about seismic site classification and seismic design criteria according to international building code. We know seismic design categories are category A,B,C,D,E and F. At first we will learn about investigations required for foundations that support structures assigned to seismic design categories C to F.

The structures that are assigned to the seismic design category C, D, E and F according to section 1613, IBC 2009, geotechnical investigation should include determination of seismic and geologic hazards as follows:

1. Stability of slope of foundation is located in relevant site condition.

2. Liquefaction potential; dear reader we have discussed many posts about liquefaction hazard, analysis of hazard and some mitigation against this natural phenomenon.

3. Differential settlement

4. Displacement in surface due to lateral spreading or faulting.

Relation between faulting and occurrence of earthquake, fault geometry and paleoseismology have already discussed in previous posts.

These requirements are for seismic design category C to F; but when the structure is assigned to the category D to F, that is D, E or F, some additional geotechnical investigations are required; that is for category C only above four investigations are enough. But for category D,E and F more information is required which include

-Lateral earth pressure

-Soil strength reduction

-Recommended mitigation measure

We need to include following additional information to design a foundation supporting structures assigned to these categories:

1. Lateral pressure on retaining wall or foundation walls as a consequence of earthquake motion. There have many record of failure of earth retaining work during excavation and foundation construction work.

2. Liquefaction potential and strength loss of corresponding soils for

–PGA

–Magnitude

–Source characteristics

This parameter must be consistent with design seismic ground motions.

In calculation of peak ground acceleration (PGA), soil amplification factors are derived based on site specific studies. (Reference ASCE7, chap-21). 

Alternatively PGA can be taken as S_DS /2.5, 

where S_DS=Maximum considered seismic spectral response accelerations (for short period, ref: 1613.5.4 IBC-2009).

3. With assessment of liquefaction potential, consequences of liquefaction and subsequent strength loss should be included. Which includes:

–Differential settlement

–Lateral movement

–Lateral loading on foundation

–Reduction in bearing capacity of foundation soil

–Increased lateral pressure on earth retaining wall

–Floating potential of buried foundation element or structures.

4. At last, how to mitigate the consequences of above phenomenon? This discussion of measure to mitigate should include at least –Ground stabilization –Selection of proper foundation depth and types as well –Selection of proper structural system; the objective is to accommodate expected forces and subsequent displacement –Requirement of combination above measures and appropriate way to account in designing foundation and structures.

Controlled Low-Strength Material to Support Foundation (IBC)

Dear reader this is relatively new term in this blog, controlled low-strength material; this are used to establish structural fill/backfill of trench. In United States there have widespread use of it as foundation backfill. The special attention is for backfilling for utility and storm drainage on highway projects.

Dear reader our concern is here foundation backfill, we will have just introductory idea about controlled low-strength material. This will be expressed as CSLM for convenient of discussion.

This is produced by mixing cement, aggregate, fly ash and essentially water. So we can take it as concrete; the difference is very low strength which is at best 1200 psi and mostly less than that value. We know ordinary concrete have strength around 3000 psi.

Due to very low strength, we will not choose it as supporting material for building, bridges etc. rather we will go for backfilling as discussed earlier. The important parameter is it has greater flowability than normal concrete.

International building code provides us some geotechnical investigation required for using this materials as foundation backfilling material. Dear reader, yes, we are talking about shallow foundation and following are the requirements:

1. Specification to prepare site before placing controlled low strength material should be provided in geotechnical investigation.


2. Specification for CLSM


3. Field or laboratory test methods for determining bearing capacity, in other word compressive strength of CLSM


4. Test methods to approve CLSM in field


5. Frequency and number of in-situ tests, to determine acceptance according to Item 4 (above).

Structural Phenomena except Cracking in Black Cotton Soil

Other structural phenomena associated with black cotton soil except cracking are 

•Gilgais

•Slickensides

•Structural sphenoid aggregate

•Self mulching in surface soils.

All of the phenomena stated above are also attributed to shrinkage and swelling of soil under moisture change; but actual mechanism is not understood clearly. 

The soil movement under moisture variation is not found in only vertical or horizontal planes. This will produce sphenoid aggregate or wedge-shape aggregate and due to movement past each other, the peds becomes polished resulting slickensides.

The development of gilgais is the most interesting phenomena associated with black cotton soils. Gilgais is topographic phenomena where alternate depression and mounds occur at soil surface. The intermediate areas between them are called a shelf.

Many mechanism and many forms of gilgais were described considering uneven swelling & shrinkage of soil. The forms are

• Round or nomal

• Mehen hole

• Lattice

• Linear

• Tank

• Stony

The gilgais are formed by repeated swelling cycles of black cotton soil followed by subsequent shrinkage when moisture is lost. Soil becomes cracked and the cracks are filled with loose materials. When the soil mass swells under next rewetting cycle, the pressure in soil cannot be relieved by cracking which exerts forces sideway and results mounds.

The depressions hold water and make soil wetter and suffer more swelling and subsequent mounds and obviously more shrinkage under drying.

The cracks allow water to penetrate more deep into the soil mass leading more swelling and subsequent shrinkage. The increasing swelling and shrinkage results repeated depressions and mounds.

Thus regular heterogeneity occurs which made the mounds generally more alkaline than depression (shelf); but very few data is available for both shelf and mounds.