Cavity Walls

Definition:
Two walls with a 5 cm to 8 cm comprise a cavity wall having cavity between them. The outer wall also known as outer leaf, consists of a 10 cm (half brick) thick wall and the inner wall is sufficiently thick and strong to carry the imposed load safety. The minimum thickness of the inner wall is restricted to 10 cm (half brick). The provision of a continuous cavity in the wall efficiently prevents prevents the transmission of dampness to the inner wall.

Advantages:

The advantages of cavity walls may be summarised as below:

1. They have good sound insulation property.

2. The layer of air in the cavity being non-conductor of heat reduces the transmission of heat from the external face to the internal one and as such cavity walls are best suitable for a tropical country like India. Tests have revealed that cavity walls have 25% greater insulating value than solid walls.

3. As there is no intimate contact between the two leaves except at the wall ties (which are of impervious material), there is no possibility of the moisture travelling from the outer leaf to the inner.

4. They are economical.

Locating Cavity:

The cavity should start near ground level and terminate near eaves level in case of sloping roof or near coping in case of flat roof with parapet wall. The cavity should preferably start 15 cm below the damp-proof course level. This has the advantage of draining any condensed moisture below the level of damp-proof course. Similarly, to prevent infiltration of moisture, the bottom of cavity should lie at least 15 cm above the out side ground level. The damp-proof course for the two leaves is laid separately, although at the same level. This is necessary to continue the cavity below damp-proof course. The cavity is kept fully ventilated by providing air bricks in the external wall immediately above damp-proof course. The air bricks should be kept 90 cm apart. Similarly, air bricks are provided near the top of the cavity.

Wall Ties:

The two walls are tied together with metallic or terra-cotta ties to obtain structural stability. They are spaced 90 cm apart horizontally and 30 to 45 cm apart vertically in staggered positions.

Quality of water in concrete Mix

Introduction

Concrete is a chemically combined mass which is manufactured from binding materials and inert materials with water. It is most popular construction material due to its unique durability and reasonable strength; more interestingly can be modified and designed for wide range of strength requirements and set under variable environmental conditions. Cement is the most important material of concrete which is produced at the cost of environmental emission of CO₂; to produce 1 tonne cement nearly 900 kg CO₂ is released in the environment. So such an energy intensive materials constitutes concrete which may be seriously affected by (both strength and durability point of view) by relatively available and cheap but essential element water; more precisely impurities in water. Quality of mixing water are mainly considered for performance of concrete in both fresh and harden state.

Impurities in mixing water intervene the setting time of the paste and may produce detrimental effect on strength and durability of concrete also. When impurities are chemically active, they may take part in the chemical reaction contributing significant change in setting, hardening and development of strength of concrete. More over health hazard during handling these water should carefully considered. In this regard past performance of a particular source of water can be used to evaluate suitability of water; if not available, some testing inevitable to evaluate water for setting time, compressive strength and durability.

Function of Water in Concrete

Three water serves the following purpose:

1. To wet the surface of aggregates to develop adhesion because the cement pastes adheres quickly and satisfactory to the wet surface of the aggregates than to a dry surface.
2. To prepare a plastic mixture of the various ingredients and to impart workability to concrete to facilitate placing in the desired position and
3. Water is also needed for the hydration of the cementing materials to set and harden during the period of curing.

The quantity of water in the mix plays a vital role on the strength of the concrete. Some water which have adverse effect on hardened concrete. Sometimes may not be harmless or even beneficial during mixing. So clear distinction should be made between the effect on hardened concrete and the quality of mixing water.

Potable water as mixing water

The common specifications regarding quality of mixing water is water should be fit for drinking. Such water should have inorganic solid less than 1000 ppm. This content lead to a solid quantity 0.05% of mass of cement when w/c ratio is provided 0.5 resulting small effect on strength.

But some water which are not potable may be used in making concrete with any significant effect. Dark color or bad smell water may be used if they do not posses deleterious substances. P^H of water to even 9 is allowed if it not tastes brackish. In coastal areas where local water is saline and have no alternate sources, the chloride concentration up to 1000 ppm is even allowed for drinking. But this excessive amount of alkali carbonates and bicarbonates, in some natural mineral water, may cause alkali-silica reaction.

Acceptable source of water

Besides potable water, various new and existing sources are available for mixing water which can be used for complete and partial replacement of valuable potable water. This includes

• Ground water

• Reclaimed water

• Treated water from municipal sewer

• Waste water of ready-mix concrete plant etc.

In many regions of the world there have scarcity of water like Dubai and Qatar and the local authorities are looking for new sources and reused water. There treated water are used for agricultural requirements and daily needs for construction industry. like washing aggregates, as concrete mixing water and curing of the same. Water from river and sometimes even sea are considered suitable if it is free from brackish matter. In arid regions, brackish groundwater is mixed with desalinated water and considered suitable for concrete production and for concrete slurry too.

Sampling guideline for mixing water

In addition to testing on constituent of concrete like aggregate cementitious materials and admixtures etc., testing of water is an important part of quality control of concrete. A systematic testing schedule for water testing yields higher efficiency of concrete and assure good performance in regard of strength and durability.

Important thing to remember is that water can be changed by chemical, physical or biological reactions; such modification may occur during sampling and at the time of analyzing. So it should be tested before using in concrete.

Collection of sample

The location of sampling should be at mid-stream and extracted from mid depth, as far as possible. When there have obstructions or major discharges are fall into river, the sample should be taken in downstream of discharges by a distance more than 100m in case of small stream; in a word, the site should be such that no change in water in the stream are seen with naked eyes. In case of wide river at least three samples should be taken along the cross section.

Sampling of waste water

When water from narrow effluent channels of treated sewers are to be tested, the sample should be taken from one third depth of water neither skimming the top surface nor scrapping the bottom. It is important to locate site having sufficient flow so that no nearby deposition is occurred. Caution should be taken during sampling to keep in-situ condition of dissolved gas i.e. must not be agitated to liberate dissolved gasses or to cause some degree if aeration.

It was observed that sewage flows are often intermittent which requires to collect samples an hour interval throughout 24 hours. At room temperature waste water generally decompose rapidly, so test set-up for certain parameter should be available at site. These are as follows:

• Dissolved oxygen
• Residual chlorine
• Sulfides
• PH
• Nitrites etc.

For some tests addition of preservatives just after collection of water will be enough.

Can ready-mix concrete washout water be used in water?

It is recommended in AS 1379 and ASTM C94, that water used in washout operation in ready-mix concrete plant can be used in concrete as mixing water. Only requirement is to store it in such a way that contamination from deleterious matters is prevented and water is collected from storage outlet. Water should conform ASTM C 1602; the sources and testing frequencies and other requirements of testing to qualify water sources. According to ASTM C94 water may be water itself or may be ice or any forms of moisture on the aggregate surface and wash water remains in the drum of truck mixer can be used for concreting next batch.

Plant washout water often called recycled ready-mix water. In 2007 experiment results were published by GL Low et al. about the requirements of recycling of cement-slurry water found from ready-mix concrete batching plant. He also examined the performance of concrete casted from reused water without any treatment. This study revealed the effects of application of such water in concrete mix on both fresh and hardened concrete based on the requirements of ASTM C94 and BS4550. The interesting thing was that they used two criteria namely specific gravity and PH; slurry water from such source can meet acceptance criteria based on concrete performance in setting time, compressive strength and drying shrinkage, when specific gravity of recycles water not exceed 1.03 and PH value of water lies between 12.3 to 13.3.

Determination of Suitability of Mixing Water

A simple way of determining the suitability of such water is to compare the setting time of cement and the strength of mortar cubes using the water in question with the corresponding results obtained using known suitable or distilled water. About 10% tolerance is generally allowed. Such tests are recommended when water for which no service record is available containing dissolved solids in excess of 2000 ppm or, in excess of 1000 ppm. When unusual solids are present a test is also advisable.

Quality Parameters	Maimum Limit (ppm)
Chlorides	500
SO3	1000
Alkali Carbonates and Bicarbonates	1000
Turbidity	2000

The effect on concreting for different types of contamination or impurities are described below:

Suspended Solids

Mixing water which high content of suspended solids should be allowed to stand in a setting basing before use as it is undesirable to introduce large quantities of clay and slit into the concrete.

Acidity and Alkalinity

Natural water that are slightly acidic are harmless, but presence of humic or other organic acids may result adverse affect over the hardening of concrete. Water which are highly alkaline should also be tested.

Algae

The presence of algae in mixing water causes air entrainments with a consequent loss of strength. The green or brown slime forming algae should be regarded with suspicion and such water should be tested carefully.

Sea Water

Sea water contains a total salinity of about 3.5%(78% of the dissolved solids being NaCl and 15% MgCl₂ and MgSO₄), which produces a slightly higher early strength but a lower long-term strength. The loss of strength is usually limited to 15% and can therefore be tolerated. Sea water reduces the initial setting time of cement but do not effect final setting time.

Chloride

Water containing large amount of chlorides tends to cause persistent dampness and surface efflorescence. The presence of chlorides in concrete containing embedded steel can lead to its corrosion.

Moisture Content of Aggregate

Aggregate usually contains some surface moisture. Coarse aggregate rearly contains more than 1% of surface moisture but fine aggregate can contain in excess of 10%. This water can represent a substantial proportion of the total mixing water indicating a significant importance in the quality of the water that contributes surface moisture in aggregate.

Effect of lead exist in mixing water

An investigation was conducted on behavior of concrete under existence of heavy metal in mixing water by Madhusudana Reddy, B and et al (2011). They examined the effect of presence of lead (Pb+) in mixing water on setting times, compressive strength, soundness and attack of sodium-sulfate on high strength cement mortar. Two types of specimens of cement mortar were used, one was casted with deionised water and others were casted with deionised water with different concentrations of lead. The lead concentration used were 10, 50, 100, 500, 1000, 2000, 3000, 4000 and 5000 mg/liter. 

The results were interesting, as compared to reference specimens, it was figured out from results that specimens having high concentration of lead lost significant strength with a significant increase in setting time of cement in mortar. However, a marginal increase in setting time and compressive strength was found at a concentration of 2000 mg/liter. 

Impurities influencing setting time of concrete 

H. Y. Ghorab and et al (1990) have studied the effect of water (from natural sources) on the setting time of cement and reported that setting time of ordinary portland cement mainly dependent on quality of water. As compared to setting time of concrete cast of tap water, a reduction of 4% was observed when used water from the Nile river and approximately 25% reduction was found in concrete cast with groundwater; same result also found for sea water. 

V. V. Red and et al. studied on the setting time and development of strength in fly ash concrete under alkaline water in laboratory condition. It was found that initial and final setting time of concrete either accelerated or retarded depending on type of alkalinity rendered by sodium carbonate or sodium bi-carbonate. When sodium carbonate exists in mixing water, both initial and final setting times are accelerated when the concentration is 6 gm/liter and 4 gm/liter respectively. In case of sodium bi-carbonate, both initial and final settinh time are retarded when its concentration in mixing water is equal to 4 gm/liter and 6 gm/liter respectively. Compressive strength and tensile strength were found reduced with increase in sodium carbonate and sodium bi-carbonate content in mixing water in excess of 6 gm/liter and 10 gm/liter respectively. 

African Plate

The earth has two kinds of crust. The continents are mostly made of thick granite. When continents pull apart, the gap is filled by thin crust made of basalt. In plate tectonics, a continent is any piece of continental crust surrounded by oceanic crust or plate boundaries. Madagascar is a continent. When Africa and eastern Gondwanaland (India, Antarctica and Australia) began to pull apart about 120 million years ago, Madagascar originally moved as part of eastern Gondwanaland. It was originally attached to Kenya, and drifted south until it reached its present location. Then the crust broke on the east side of Madagascar, leaving it attached to the African Plate.

The westerly side is a divergent boundary with the North American Plate to the north and the South American Plate to the south forming the central and southern part of the Mid-Atlantic Ridge. The African plate is bounded on the northeast by the Arabian Plate, the southeast by the Indo-Australian Plate, the north by the Eurasian Plate and the Anatolian Plate, and on the south by the Antarctic Plate. All of these are divergent or spreading boundaries with the exception of the northern boundary with the Eurasian Plate (except for a short segment near the Azores, the Terceira Rift).

Fig-2 Map of East Africa showing some of the historically active volcanoes(red triangles) and the Afar Triangle (shaded, center) -- a triple junction where three plates are pulling away from one another: the Arabian Plate, and the two parts of the African Plate (the Nubian Plate and the Somali Plate) splitting along the East African Rift Zone (USGS).

The African Plate comprises several continental blocks or cartons, stable continental blocks of old rocks, which came together to form the African continent during the assembly of the supercontinent Gondwana around 550 million years ago. These cratons are, from south to north, the Kalahari, Congo, Sahara and West African craton. Each of these cratons can further be subdivided into even smaller blocks or terranes, sutured along pre-Gondwanan orogenic belts.

The African Plate is rifting in the eastern interior along the East African Rift. This rift zone separates the Nubian Plate to the west from the Somali Plate to the east. One hypothesis proposes the existence of a mantle plume beneath the Afar region, while an opposing hypothesis asserts that the rifting is merely a zone of maximum weakness where the African Plate is deforming as plates to its east are moving rapidly northward.

The African Plate's speed is estimated at around 2.15 centimeters per year. It has been moving over the past 100 million years or so in a general northeast direction. This is drawing it closer to the Eurasian Plate, causing subduction where oceanic crust is converging with continental crust (e.g. portions of the central and eastern Mediterranean). In the western Mediterranean, the relative motions of the Eurasian and African plates produce a combination of lateral and compressive forces, concentrated in a zone known as the Azores-Gibraltar Fault Zone. Along its northeast margin, the African Plate is bounded by the Red Sea Rift where the Arabian Plate is moving away from the African Plate.

The New England hotspot in the Atlantic Ocean has probably created a short line of mid to late-Tertiary age seamounts on the African Plate but appears to be currently inactive.

The 1960 Valdivia earthquake

The Great Chilean Earthquake is also known as 1960 Valdivia earthquake. This quake was preceded by 4 foreshocks bigger than magnitude 7.0, including a magnitude 7.9 on May 21 that caused severe damage in the Concepcion area. Many aftershocks occurred, with 5 of magnitude 7.0 or greater through Nov 1. It came after a smaller earthquake in Arauco Province at 06:02 on 21 May 1960. Telecommunications to southern Chile were cut off and President Jorge Alessandri had to cancel the traditional ceremony of the Battle of Iquique memorial holiday to oversee the emergency assistance efforts. The government was just beginning to organize help to the affected region when the second earthquake occurred at 14:55 UTC on 22 May in Valdivia.

The second earthquake affected all of Chile between Talca and Chiloé Island, more than 400,000 square kilometers (154,440.9 sq mi). Coastal villages, such as Toltén, disappeared. Later studies argued that the earthquake actually had 37 epicenters through a 1,350 km (839 mi) north-south line that lasted from 22 May to 6 June. At Corral, the main port of Valdivia, the water level rose 4 m (13 ft) before it began to recede. At 16:20 UTC, an 8 m (26 ft) wave struck the Chilean coast, mainly between Concepción and Chiloe. Ten minutes later, another wave measuring 10 m (33 ft) was reported.

The Great Chilean Earthquake or Valdivian Earthquake is the most powerful earthquake ever recorded. The quake occurred in the early afternoon (19:11 UTC) of May 22, 1960, and had a 9.5 rating[1] on the Moment magnitude scale having the epicenter near Cañete (see map) some 700 km (435 miles) south of Santiago, although Valdivia, Chile was the most affected city. The rupture zone is estimated to be about 1000 km long, from Lebu to Puerto Aisen.

On May 24, Volcan Puyehue erupted, sending ash and steam as high as 6,000 m. The eruption continued for several weeks.

Severe damage from shaking occurred in the Valdivia-Puerto Montt area. Most of the casualties and much of the damage was because of large tsunamis which caused damage along the coast of Chile from Lebu to Puerto Aisen and in many areas of the Pacific Ocean. Puerto Saavedra was completely destroyed by waves which reached heights of 11.5 m (38 ft) and carried remains of houses inland as much as 3 km (2 mi). Wave heights of 8 m (26 ft) caused much damage at Corral.

Its resulting tsunami affected southern Chile, Hawaii, Japan, the Philippines, eastern New Zealand, south east Australia and the Aleutian Islands in Alaska.Tsunamis caused 61 deaths and severe damage in Hawaii, mostly at Hilo, where the runup height reached 10.6 m (35 ft). Waves as high as 5.5 m (18 ft) struck northern Honshu about 1 day after the quake, where it destroyed more than 1600 homes and left 185 people dead or missing. Another 32 people were dead or missing in the Philippines after the tsunami hit those islands. Damage also occurred on Easter Island, in the Samoa Islands and in California. One to 1.5 m (3-5 ft) of subsidence occurred along the Chilean coast from the south end of the Arauco Peninsula to Quellon on Chiloe Island. As much of 3 m (10 ft) of uplift occurred on Isla Guafo. Many landslides occurred in the Chilean Lake District from Lago Villarica to Lago Todos los Santos.

Energy released by an Earthquake

Among the earthquake generated energy, traveled through the earth producing disruption, the only quantity that can be measured is that which is radiated through the earth. The total energy from an earthquake includes energy required to create new cracks in rock, energy dissipated as heat through friction, and energy elastically radiated through the earth. Of these, the radiated energy that shakes buildings and is recorded by seismographs.

The radiated energy can be obtained in various ways. Historically, the radiated energy was estimated empirically (from observations) from magnitude Ms through the Richter formula, log Es = 4.8 + 1.5Ms, where Es is seismic energy in Joules. In this formula, magnitude is measured first, after which the formula is used to obtain Es. With modern instrumentation, energy can be measured directly from velocity seismograms and converted to a magnitude. If Es is energy in joules, the energy magnitude Me is obtained by Me = (2/3) log Es -2.9. If Me is not available, the seismic moment Mo of an earthquake can provide an empirical estimate of radiated energy. After Mo is measured, it is converted to a moment magnitude Mw by Mw = (2/3) log Mo – 6.0 where Mo is in Newton-meters (Joules). Mw is then used as the magnitude in the Richter formula to obtain an estimate of radiated energy.

It should be noticed that Me and Mw do not necessarily have the same numerical value because they measure different physical quantities. Mw is a magnitude that is derived from low-frequency displacement spectra whereas Me is measured from higher frequency velocity spectra. Mw is a measure of the area of rupture and the average slip across the fault, whereas is Me is a measure of the shaking from an earthquake.

Magnitudes and corresponding energy (Joules and tons of TNT)

Magnitude	Es (from Me)	Es (from Ms or Mw)	Tons of TNT	Nuclear Bomb Equivalence (# of bombs)
4	0.22E+11	0.63E+11	15.	0.00
5	0.71E+12	0.20E+13	475.	0.02
6	0.22E+14	0.63E+14	15023.	0.79
7	0.71E+15	0.20E+16	475063.	25.0
8	0.22E+17	0.63E+17	15022833.	790.6
9	0.71E+18	0.20E+19	475063712.	25,003.3

Once the energy is known in Joules, it can be compared to the explosive energy of TNT. One ton of TNT has an energy of 4.2*10E09 Joules. In July 16, 1945 the first atomic bomb, or A-bomb, exploded on , Alamogordo, N.Mex. It produced an explosion equal to that of 19,000 short tons (17,000 metric tons) of TNT."

Critical Construction Process Requiring Good Workmanship

In reinforced concrete frame construction, it is very important to have qualified work crews with appropriate experience and competent workmanship. It is also very important to have a feasible and well-thought construction sequence to let the crews perform their tasks in a proper and timely manner. The construction crews are the last, but vital link, in the construction process.

The design engineer and the architect play important roles in ensuring that the design is feasible and can be understood by construction crews.

The design engineer should keep the structural configuration and detailing of the structural system and its sub-elements as simple and straightforward as possible. It is good practice to use standard or typical detailing as much as possible. Of course, it is the responsibility of the whole building team --from the architect and the design engineer to the field crews-- to build a good quality building.

The key processes where workmanship is critical in construction are:
1) Steelwork: the steelwork has to result in reinforcement layouts per the specifications given in the structural drawings. Reinforcing elements should be clean and
should not have any dirt or oil on them (see Figure 1).

2) Formwork: to be able to cast reinforced concrete elements properly, good quality forms need to be built. This requires use of clean, leak-proof and tightly constructed formwork systems, characterized by adequate stiffness and strength. Where necessary, proper falsework may need to be incorporated into the formwork construction to support the forms.

3) Proper placement of steelwork into the forms: reinforcing steel assemblies need to be placed and secured within the forms in such a way that the design specifications (such as minimum concrete cover thickness) have been met. This would prevent future corrosion of the reinforcement and spalling of the concrete. The steelwork should not be displaced or distorted when fresh concrete is placed into the forms.

4) Concrete work: transportation, handling, placement and consolidation of fresh concreteshould be done properly. Accumulation or loss of water, or segregation of aggregate in the concrete mix should be avoided as much as possible. If such alterations of the concrete matrix take place, the concrete mix should be reconstituted before placing the fresh concrete into forms. Fresh concrete should be poured into the forms and properly distributed (consolidated) within and around the steel reinforcing elements. Use of vibrators or other instruments that enhance consolidation of the concrete within forms is recommended. It is extremely important to have good bond

Storage of Cement

Storage of cement is a noteworthy item, because proper arrangements for storing the cement have to be made in the factories before sale or on large construction projects before use. Proper Storage preserves its quality and fitness for use. To prevent its deterioration it is necessary to protect it from rain, sun, winds and moisture.

Moisture is the first and the greatest danger to be gaurded against. Cement has great affinity for moisture and hence it should be stored well shieled from moisture laden current of air. The exposed cement is attracted by air setting which gradually spreads. It gives to the formation of lumps. If the lumps fromed are so hard that they can not be powered by passing between the fingers, it should be concluded that the cement has been rendered useless for any sound construction.

Requirement for storehouse of cement:

It is very common in construction projects, storehouses or godowns are made at site to store cement for few days. Though these storehouses are a temporary construction, they must be constructed following requirements as below:

• The walls of storehouse must be made to prevent dampness and plastered properly, 

• A proper water proofing must be applied to roof of storehouse, 

• An elevated floor of at least 300 mm above ground level must be constructed to prevent inflow of water.For economy, flooring may be constructed using two courses of bricks (150 mm thick layer of dry bricks) over a consolidated earth layer having thickness of 150 mm above a ground surface.

• For more protection cement bags are usually staked over wooden planking connected by batten. The planks are kept 100-200 mm above floor; to save timber, often concrete are used. 

• If provision for windows is kept, least number of windows having smaller size should be selected. These should be kept closed tightly to prevent moisture movement from outside of store house.

• A new storehouse is usually not used for storing cement; the exception is when structure is dried properly.

Temporary storage of cement:

It is the special case when cement is required to stored open at site for one day or two. In such situation cement bags should be staked on dry wooden platform resting on brick masonry, dry sand or may be aggregates concrete and raise 150 mm above ground surface.

This stack of cement must be covered with polythene sheet or tarpaulin completely to give protection against ambient moisture. There should have sufficient overlap of sheet on each other. But whatever the protection may be no temporary storage for cement in open condition is allowed in wet season.

Storing information:

The following information should be noted at the time of storing cement bags:

• Cement type
• Net weight of cement (approximately)
• Date of manufacture
• Manufacturer’s name or trade mark
• Certification mark of local authority.

Removal of cement from store:

While removing cement bags from storage, bags should not be removed from only one layer at the front rather two or three layer from the back should be taken off. It will produce a stepped back rows which eliminates over-turning possibility of bags.

While removing “first in, fist out” practice is followed during consumption of bags; i.e. the oldest bags are taken out first. Bags stored having separate invoice should be stacked in storehouse separately. So that one can inspect each consignment easily and can be taken out following proper sequence.

Cement should be stored in dry, leak proof and moisture proof sheds. The shed for the storage of cement should have minimum number of windows and close fitting doors. The cement bags should be stacked on wooden planks placed about 150 to 200 mm above the floor of the shed and a space of 450 to 600 mm should be kept all round between the exterior walls and the stacks.

To prevent possibilities of lumping under pressure, the maximum height of stack should not exceed 15 bags. The width of stacks should not be more than four bags or 3 m. In stacks more than 8 bags high, the cement bags should be arranged alternatively lengthwise and cross-wise so as to ensure stability of the stacks and to prevent danger of the bags in stack topping over.

In monsoon or in situations when it is necessary to store cement for unusually long periods, the cement stacks should be completely enclosed by 700 gauge polyethene sheet or some other water-proofing membrane materials.

The stack should be arranged in such a manner that cement bags can be removed on principle of first come first served i.e. oldest cement should be taken out first.