Advance Concrete Technology A1 (c).docx

HK01- CIVIL ENGINEERING PROGRAMME FACULTY OF ENGINEERING UNIVERSITI MALAYSIA SABAH

COURSE KA42003 ADVANCE CONCRETE TECHNOLOGY

TITLE ASSIGNMENT 1

DATE OF SUBMISSION 21ST MARCH 2019

PREPARED FOR DR. HIDAYATI BINTI ASRAH

PREPARED BY NAME DANIEL SELVARAJ MASITA BINTI MOHD ABAS SURESH RAJAN

MATRIC NO BK15110293 BK15110149 BK13110406

INTRODUCTION

Concrete deterioration or also known as concrete degradation may have various causes such as fire, aggregate expansion, sea water effects, bacterial corrosion, calcium leaching, physical damage and chemical damage. Sabah has always been known as “The Land below the winds” and it still lives up to its name as the constant wind movement at Sabah is always consistent especially the sea side areas. As a state in Malaysia, it is directly located at the Equator of the Earth meaning equal sunshine and rain throughout the year theoretically. Factors such as wind, rain, acid and temperature are one of the main culprits to concrete deterioration. In this study, we are to visually observe two chosen structure/buildings, 1 with the age of more than 5 years and the other less than 5 years. We chose DKP Baru of University Malaysia Sabah as more than 5 years old building and Mr. Abas second house as less than 5 years old building.

OBJECTIVE

This study has the following objectives: a. To study the selected buildings’ history (age, function and location) b. To classify the concrete deterioration found in both buildings and identify the causes. c. To propose remedial action for the listed deterioration.

HISTORY

Dewan Kuliah Pusat Baru (DKP Baru), University of Malaysia Sabah (UMS).

The 2nd lecture hall complex of UMS went to full operation at the year 2011 upon its completion within the same year. That makes the building in its 8 years of service to the university as spots for daily lectures of students and occasionally places for carnivals or celebration open to the public was held here for example: Christmas Carnival. Besides the lecture halls, this complex was also the place for the Academic Administration Division’s building.

Figure 1: Scenery view of the DKP Baru, UMS.

Within the campus, the average maximum temperature throughout the year is approximately 32 °C with a wind speed about 5 km/h to 10km/h. Hence, the studied area is affected with quite a high temperature and a very moderate wind velocity. The new lecture hall of UMS is surrounded relatively close to the ocean which is affected by sea breeze. The figure below shows the cross section of the lecture hall, 2D plan of the ground floor and the first floor and the satellite image from Google Earth.

Figure 2: 3D-view of the DKP Baru, UMS.

Figure 3: 2-D Ground Floor plan of the DKP Baru, UMS.

Figure 4: 2-D First Floor Plan of the DKP Baru, UMS.

Figure 5: Satellite Image of the DKP Baru, UMS through Google Earth.

Mr. Abas 2nd Residence House

The current house is not the first house lived by Mr. Abas and his family. It is the second address which located at Kg Laya-laya Jalan Bolong Tambalang, Tuaran Sabah. The house was built in June 2018 by Mr. Abas’s own hands. Due to the lack of economical and workers, the progress of construction is slow but the main structures such as slab, column and beam were completed in November 2018. The main materials are concrete.

Figure 6: The house during its construction period Tuaran shows the average daytime maximum temperature around 35˚C with wind speed between 13 km/h and 17 km/h. Therefore, the study area is affected with high temperature and gust of wind. The area of Kg Laya-laya is surrounded by Mangrove trees and salt water.

Figure 7: The Residential House area through Google Earth satellite image

Figure 8: Positions of the concrete deterioration throughout the structure

ANALYSIS/RESULTS/FINDINGS/OBSERVATIONS Dewan Kuliah Pusat Baru (DKP Baru), University of Malaysia Sabah (UMS).

Classification of damage/deterioration a) Long-term drying shrinkage

Figure 9: Long-term drying shrinkage at slab and column It was observed that the formation of a horizontal crack throughout the slab and connection to the columns. The gap of the crack is at the range of 11 mm to 14 mm and can be classified as moderately severe. b) Corrosion of reinforcement

Figure 10: Corrosion of reinforcement at the columns

Based on Figure 10, there is a visible horizontal and vertical cracks on the columns of the structure. Each columns have different crack gaps width but it is observed at the range of 7 mm to 12 mm which is also classified as moderately severe.

c) Plastic shrinkage

Figure 11: Plastic shrinkage at the columns

Based on Figure 11, there is a small vertical gap crack alongside the columns. Since the gap of the crack is barely more than 3 mm, therefore it is classified as very minimal.

Factors affecting deterioration/cause a) Long-term drying shrinkage Drying shrinkage is defined as the contracting of a hardened concrete mixture due to the loss of capillary water (Nejadi S, 2014). As soon as water is added to the mix, a chemical reaction between water and cement (hydration) is initiated, although its effects may not be apparent for the first few hours. This shrinkage causes an increase in tensile stress, which may lead to cracking, internal warping, and external deflection, before the concrete is subjected to any kind of loading. The time at which shrinkage cracks occur depends on the rate of drying but is usually several months to three or four years after casting (Golden C, 2007). When the stress exceeds the tensile capacity of the concrete, cracks develop. Thin members with a large surface area such as slabs are particularly vulnerable. Drying out occurs from the surface and hence the surface layer is first affected. The surfaces of large cross-section members may crack because movement is restrained by the inner section of concrete. Concrete near to corners and edges is particularly prone to cracking as loss of moisture takes place from the adjacent surfaces. The shrinkage of a particular concrete mix is also affected by additional factors such as temperature history, curing, relative humidity and ratio of volume to exposed surface. Sound aggregates for concrete have low shrinkage and the more quantity of it is present in concrete smaller would be the shrinkage.

Figure 12: Section Showing Cracking Formation

b) Corrosion of reinforcement Corrosion of steel reinforcement in concrete is due to an electrochemical process. Anodic site where ferrous ions (Fe++) pass into solution and due to a secondary reaction with oxygen and water form rust (or the expansive product of steel corrosion). Then, Conductor (steel reinforcement) that provides a conduit for excess electrons (e-) to move to the cathodic site. Cathode where electrons are consumed in the presence of oxygen and moisture (Jaffer, 2008).

Steel reinforcing bars

embedded in concrete do not corrode because the high alkaline conditions in concrete (pH > 13) produce a passive oxide film on the surface of the steel. The passive oxide film prevents corrosion. However, carbonation and/or chlorides in solution can destroy the passive oxide film. Carbonation and loss of passivity can occur when atmospheric carbon dioxide penetrates into the concrete and in the presence of moisture reacts with the calcium hydroxide to produce calcium carbonate. Calcium hydroxide is a cement hydration product that helps create the high alkaline condition in concrete. Due to the formation of calcium carbonate around the steel, the alkalinity (pH) of the concrete falls from above 13 to less than nine. Subsequently, the low alkalinity destroys the passive oxide film. For corrosion to occur, oxygen and moisture must be available at both the cathodic region and along the hydroxyl ion conduit (concrete) between the cathode and anode. Since these two requirements for corrosion are independent of the crack width, it follows that the corrosion rate of the steel reinforcing is also independent of the crack width.

Figure 13: Electrochemical Corrosion process

c) Plastic shrinkage The primary cause of "plastic shrinkage" cracks is the rapid evaporation of water from the surface of the concrete. Immediately after the concrete has been placed, the particles within the concrete begin to settle. When the particles settle, the water within the concrete displaces and rises to the top. This process is better known as "bleeding" (Sivakumar A, 2006). Not all of the water within the concrete displaces. Under most weather conditions, some of the water on the surface of the concrete evaporates. The rate of evaporation depends on factors such as the temperature of the concrete, temperature of the air, relative humidity, and wind velocity surrounding the concrete. The highest evaporation rates are obtained when the concrete and air temperatures are high, when the relative humidity of the air is low, when the concrete temperature is high compared to the air temperature, and when a strong wind is blowing over the surface of the concrete. The rapid evaporation of water at the surface is most associated with placing concrete in hot weather conditions. Plastic shrinkage cracks typically occur on horizontal surfaces exposed to the atmosphere. These cracks are different from other early cracks because they are deeper and wider. Plastic shrinkage cracks are typically two to four inches deep and approximately one-eighth inch wide. They may also extend several feet in length adopting a crow’s-foot pattern. These cracks form before any bond has developed between the aggregate particles and mortar. Therefore, the cracks tend to follow the edges of large aggregate particles or reinforcing bars and never break through the aggregate particles. Although plastic shrinkage cracks usually do not impair the structural performance of the slab, cracks in some building floors have been blamed for leakage.

Figure 14: Plastic Settlement Shrinkage Formation

Effect and remedy a) Long-term drying shrinkage Preventive measures for long term shrinkage cracks including minimizing water content, use of plasticizer for compensating workability due to lesser water, use of highest possible aggregate content and hence smaller quantity of cements, eliminate external restrains (e.g. smooth polythene sheet on the sub grade for base slab), sufficiently close spaced reinforcement (e.g. generally 15 cm in slabs & walls). In addition, repair method for long-term drying shrinkage can be done through sealing and grouting depending on the width of crack.

b) Corrosion of reinforcement

Minimizing the risk of steel reinforcement corrosion can be done by controlling the quality of concrete by minimizing permeability. Recommendations for minimum depths of cover should be followed according to the codes of practice and are based on exposure conditions and minimum cement contents. Higher cement contents infer lower water cement rations leading to permitted reductions in cover. Furthermore, blended cements made from combinations of PC/PFA and PC/GGBS can lead to significant reduction in chloride penetration. However, in situations where these materials are not cured properly there is a risk of increased carbonation. Care must be taken that all aggregates and admixtures contain limited amount of chlorides.

c) Plastic shrinkage

Remedy that can be done to reduce the risk of experiencing plastic shrinkage cracks include: erecting temporary windbreaks and sun shades (if practical), applying a sprayable evaporation retardant, covering the flatwork with plastic sheeting between finishing passes, and fog spraying the flatwork. Other preventative measures include dampening the base material and forms before placing concrete, lowering the concrete temperature by using chilled water or chipped ice, including microfibers in the concrete mixture to increase the tensile capacity of the plastic concrete, protecting the concrete from evaporation during

construction delays, and curing as soon as possible after finishing. Plastic shrinkage cracks usually create aesthetic concerns, especially for architectural concrete. Crack width and depth and the concrete’s exposure conditions, plastic shrinkage cracks may create durability concerns. The best way to avoid aesthetic and durability concerns related to plastic shrinkage cracks is to understand the susceptibility of the concrete mixture to cracking, monitor the jobsite conditions and take the necessary actions to minimize rapid moisture loss from the surface of the concrete.

Mr. Abas 2nd Residence House SEGREGATION Segregation means separation of designed fresh concrete ingredients from each other resulting in the non-uniform mix. More specifically, this implies the separation of coarse aggregates from the mortar because of differences in size, density, shape and other properties of ingredients in which they are composed. Figure 16 shows that effect of segregation, honey comb is created in the concrete and it basically affects the strength of the concrete and its porosity. In good concrete all ingredients are properly distributed and make a homogeneous mixture.

Figure 15: Segregation on slab

Figure 16: Honey comb created on part of the concrete slab for testing

Causes: 

The difference in the specific gravity of the mix constituents such as fine and coarse aggregates.



The difference in the size of aggregate



Improper grading of aggregate.



Improper handling of aggregate.



Bad practices in handling and transporting of concrete.



Concrete that is not proportional properly and not mixed adequately or too workable mix.



Placing of concrete from a greater height.

Repair: 1. Segregation can be controlled by maintaining proper proportioning the mix. 2. By peculiar handling, placing, transporting, compacting and finishing of concrete. 3. Adding air entraining agents, admixtures and pozzolanic materials in the mix segregation controlled to some extent.

PLASTIC SHRINKAGE Plastic shrinkage is contraction in volume due to water movement from the concrete while still in the plastic state, or before it sets. This movement of water can be during the hydration process or from the environmental conditions leading to evaporation of water that resides on the surface on the wet concrete. So, the more the concrete bleeds, the greater the plastic shrinkage should be.

Figure 17: Diagonal plastic shrinkage causing cracking on the slab

Figure 18: Cracking on top of column Causes: Plastic shrinkage cracks occur when there is rapid loss of water from the surface of freshly poured concrete (before it has fully set). This causes the top of the concrete slab to dry more quickly than the bottom, and they pull apart. Weather also can cause the plastic shrinkage due to some factors: 1. High air temperature 2. Low relative humidity 3. High concrete temperature 4. High wind speed 5. High sun expose

Remedy: To avoid plastic shrinkage, the key is to keep the concrete surface moist by covering it with burlap, polyethylene sheeting or plastic. This is especially important in extreme weather conditions such as high wind, hot temperatures or direct sunlight. Be sure to complete this step after the concrete has set in order to assure the coverings do not disturb or leave unwanted impressions on the new concrete. Further, misting the concrete with water a few times a day will help to reduce the rate of evaporation from the surface. It is critical to continue this process for at least the first three days of curing. Properly curing also can reduce the damage. Joint sealants should ensure structural integrity and serviceability. They should also serve as protection against the passage of harmful liquids, gases, and other undesirable substance which would impair the quality of concrete. In the case of repair of a cracked surface, the cracks are first enlarged along their exposed face and are pointed up with the sealants.

Figure 19: Example of Sealant to repair cracked surface

DRYING SHRINKAGE Concrete mixtures require water for proper placement and workability. As excess water evaporates from the concrete slab during the curing process, there is a reduction in volume or shrinkage that can occur.

Figure 20: Drying shrinkage on the slab Causes: When shrinkage is impeded or blocked — by supporting soils, granular fill, adjoining structures or reinforcement within the concrete — tensile stresses develop within the concrete slab. As a result, these stresses act against the weakest points of the concrete material and cause it to crack. Remedy: To help control random cracking caused by shrinkage, control joints should be placed between concrete slabs. These joints create a weak point in the concrete and help guide where cracks will occur, ensuring they happen in a straight versus jagged line for better overall aesthetics.

Crazing Crazing also called as pattern cracking or map cracking, is the formation of closely spaced shallow cracks in an uneven manner. Causes: Crazing occurs due to rapid hardening of top surface of concrete due to high temperatures or if the mix contains excess water content or due to insufficient curing. Remedy: Pattern cracking can be avoided by proper curing, by dampening the sub-grade to resist absorption of water from concrete, by providing protection to the surface from rapid temperature changes.

Figure 21: Map crazing on slab

Blistering Blistering is the formation of hollow bumps of different sizes on concrete surface due to entrapped air under the finished concrete surface. Causes: It may cause due to excessive vibration of concrete mix or presence of excess entrapped air in mix or due to improper finishing. Excessive evaporation of water on the top surface of concrete will also cause blistering. Remedy: It can be prevented by using good proportion of ingredients in concrete mix, by covering the top surface which reduces evaporation and using appropriate techniques for placing and finishing.

Figure 22: Concrete blisters on slab

Figure 23: Concrete blisters on column

Delamination Delamination is also similar to blistering. In this case also, top surface of concrete gets separated from underlying concrete. Hardening of top layer of concrete before the hardening of underlying concrete will lead to delamination. Causes: It is because the water and air bleeding from underlying concrete are struck between these two surfaces, hence space will be formed. Remedy: Like blistering, delamination can also be prevented by using proper finishing techniques. It is better to start the finishing after bleeding process has run its course.

Figure 24: Delamination

Dusting Dusting, also called as chalking is the formation of fine and loose powdered concrete on the hardened concrete by disintegration. Causes: This happens due to the presence of excess amount of water in concrete. It causes bleeding of water from concrete, with this fine particles like cement or sand will rise to the top and consequent wear causes dust at the top surface. Remedy: To avoid dusting, use low slump concrete mix to obtain hard concrete surface with good wear resistance. Use water reducing admixtures to obtain adequate slump. It is also recommended to use better finishing techniques and finishing should be started after removing the bleed water from concrete surface.

Figure 25: Dusting on concrete slab

Figure 26: Dusting on column

REFERENCES 1. Golden C, Myers JJ (2007) Investigation of Long-Term Crack Patterns in FRP and Steel Reinforced Concrete Panels. Undergraduate Research. University of Missouri-Rolla. 2. Jaffer, S.J. Hansson, C.M., The influence of cracks on chloride-induced corrosion of steel in ordinary Portland cement and high performance concretes subjected to different loading Conditions, Corrosion Science Journal, 2008. 3. Nejadi S, Gilbert I (2004) Shrinkage cracking and crack control in restrained reinforced concrete members. ACI Structural Journal 101(6): 840-845. 4. Sivakumar A, Santhanam M. Experimental methodology to study plastic shrinkage cracks in high strength concrete. In: Measuring, monitoring and modeling concrete properties. Springer; 2006.